PROCESS INSTRUMENTATION I. Module Code: EIPIN1 PREVIOUS EVALUATION AND ASSESSMENT VUT. Vaal University of Technology 2/10

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1 PROCESS INSTRUMENTATION I Module Code: EIPIN1 PREVIOUS EVALUATION AND ASSESSMENT VUT Vaal University of Technology 2/10

2 Process Instrumentation I EIPIN1 Unit 1 First assessment March 2005 Page 1 Question 1 Define measurement of a process variable. (2) Question 2 State the elements that may be identified in an instrument. (5) Question 3 Define the accuracy of a measurement. (2) Question 4 Define an error of non linearity in an instrument. (2) Question 5 Identify the instrument function and mounting Figure 1 LRC method, from the instrument symbol in Figure 1. (2) Question 6 Define pressure and give the SI unit for pressure. (2) Question 7 The container in Figure 2 has a cross sectional area of A meter 2 and is partially filled with a liquid of density kilogram/meter 3, to a height of h meter. Starting with the expressions for the volume and mass of the liquid, show that the Figure 2 P h pressure exerted by the liquid on the bottom of the container, is given by P = hg, where g is the gravitational acceleration in meter/second 2 A. (4) Question 8 With the aid of a sketch, derive the expression that is used to determine pressure with a well type manometer. All symbols used, must be shown very clearly on the sketch. (4) Question 9 Draw a labelled sketch to show how a bellows element may be used to measure gauge pressure. (2) Question 10 A dead weight tester has a primary piston with a diameter of 1 cm. The mass of the platform and primary piston together, is 500 gram. Calculate the mass m, of the mass pieces, that must be placed on the platform to check a gauge at 100 kpa. (3).../2

3 Process Instrumentation I EIPIN1 Unit 1 First assessment March 2005 Page 2 Air supply Restriction Flapper and nozzle Question 11 In Figure 3, a pneumatic flapper and nozzle arrangement Vent is shown, with associated pilot relay. Draw a labelled sketch of Pilot Relay the pilot relay, to show its Pivot internal components and construction. (3) Feedback Figure 3 bellows Output Question 12 Define viscosity of a liquid and give the SI unit for viscosity. (2) Question 13 State Bernoulli s law (in words). (3) Question 14 A flow rate meter uses a restriction in the flow stream, to measure the flow rate of a liquid in a horizontal pipe. When the pressure difference across the restriction is 400 pascal, the flow rate is m 3 /second. Calculate the flow rate when the pressure difference across the restriction is 900 pascal. (2) Question 15 Draw a labelled sketch of a venturi tube flow meter. Show the dimensions and relative sizes of the instrument, clearly on your sketch. (4) Question 16 Sketch the configuration, including dimensions and labels, when radius taps are used for flow rate measurement with an orifice plate. (3) Question 17 a) Describe the operation of a magnetic flow meter (magmeter). (3) b) Give the operational equation that is used to calculate the flow rate q from the measurements made with a magnetic flow meter. Define each symbol that appears in the equation. (2) ---ooo000ooo--- Total: 50

4 Process Instrumentation I EIPIN1 Unit 1 First Assessment Memorandum March 2005 Process Instrumentation I EIPIN1 Unit 1 First Assessment Memorandum March 2005 Page 1 1. Measurement is defined as the determination of the existence [1] or magnitude [1] of a variable for [2] monitoring and controlling purposes. 2. Primary element [1] Data transmission element [1] Secondary element [1] Manipulation element [1] [5] and Functioning element [1] 3. The accuracy of a measurement is the closeness [1] with which the reading approaches the [2] true value [1] of a variable. 4. Non-linearity is the maximum deviation [1] from a straight line connecting the zero and full-scale [2] calibration points. [1] 5. Level recorder controller, [1] mounted behind board. [1] [2] 6. Pressure is defined as the force exerted over a unit area [1]. The SI unit is newton per square meter [2] (N/m 2 ) or pascal (Pa). [1] 7. Volume of the liquid = V = Ah. [1] Mass of the liquid = m = Ah. [1] Weight of the liquid = w = mg = (Ah)g. [1] P Pressure on the bottom of container due to weight of the liquid = wa = Ahg/A = hg [1] A [4] P = hg 8. P 1 = P 2 + (h+d)g [1]. (1) A 2 P 1 A 1 d = A 2 h d = h [1].... (2) A 1 Sketch: 1 mark h A (2) in (1): P 1 = P 2 + h 2 h ZL A g [1] 1 d A [4] P 1 P 2 = hg 1 2 A 1 A 1 A 2 9. Atmospheric pressure [½] 10. Bellows [½] Weight of masspieces weight of platform and primary piston Pressure = Area of primary piston m 9.81 ( ) [1] = (110-2 ) 2 4 P 1 (Gauge pressure) [½] ( )( ) = 9.81m Pressure indication [½] 9.81m = [2] 9.81m = m = kg. [2] [3] = gm h P 2

5 Process Instrumentation I EIPIN1 Unit 1 First Assessment Memorandum March 2005 Page Spring [½] Diaphragm [1] [3] Supply valve (ball) [½] Valve stem [½] Exhaust valve (cone) [½] 12. Viscosity is a measure of a fluid's resistance to flow. [1] [2] The SI unit is poiseuille (PI) or pascal-second (Pa-s). [1]. 13. If an (incompressible fluid is in a streamlined flow with no friction,) [½] the sum [½] of the [3] pressure energy, [½] the kinetic energy [½] and potential energy [½] per unit volume, is constant [½]. 14. q 1 = k h = k 400 k = [1] [2] q 2 = k h 2 = = = m 3 /s [1] High pressure tap Low pressure tap (upstream tap) [½] (downstream tap) [½] d [½] High pressure tap [½] D [½] ½D [½] Low pressure tap [½] Flow [½] D [½]Inlet cone (19º-23º) [½] d [½] Throat [½] Outlet cone (5º-15º) [½] Flow [½] D [½] [3] D/2 [½] d/2 [½] [4] /5 17. a) A magnetic flow meter generates an emf e, proportional to the flow rate, based on Faraday s law of electromagnetic induction (e = Bv). [1] Current carrying coils on the outside of the meter provides the magnetic field strength B tesla [1] The flow plays the role of the conductor moving through the magnetic field lines with speed v (proportional to q) [1]. (3) b) q = k D [1] e B D = distance between electrodes (meter) [½], B = magnetic field strength (tesla) [½], [5] e = measured emf (volt) [½], k = calibration constant [½] (2) /3

6 Process Instrumentation I EIPIN1 Unit 2 First assessment May 2005 Page 1 Question 1 Draw a labelled sketch of a flexure tube (torque tube) displacer level meter. (4) Question 2 The level of a liquid in an open container, is to be measured with the aid of a U-tube manometer, as shown in Figure 1. The relative densityof the liquid in the container is 3. The manometer H = 3 fluid is mercury with a relative 0.6 m density of 13,6. The zero line of the manometer, is 0.6 meter below Zero level h=0.8m the bottom of the container. Calculate the level H, of the liquid in the container, if the manometer = 13.6 reading h, is 0.8 meter. (4) Figure 1 Question 3 Define the top fixed point on the temperature scale and give this temperature value in degrees Fahrenheit. (2) Question 4 Draw a labelled sketch of a liquid in glass thermometer. (4) Question 5 The relationship between the resistance R t (ohm), of a certain resistance thermometer (RTD), and the temperature t ( C), is shown in Figure 2, and may be assumed to be linear (a straight line). a) Calculate the temperature coefficient of resistance (TCR) of this thermometer. (2) b) Calculate the resistance of the thermometer when the temperature is 40 C. (2) c) Calculate the temperature when the resistance of the thermometer is 110. (2) R t () Figure t ( C).../2

7 Process Instrumentation I EIPIN1 Unit 2 First assessment May 2005 Page 2 Question 6 Draw a labelled circuit diagram to illustrate the three wire method that is used to compensate for ambient temperatures when measuring temperature with a resistance thermometer. (4) Question 7 State the thermocouple law of intermediate temperatures. (2) Question 8 Give the positive element material, the negative element material and the temperature range of a type J thermocouple. (3) Question 9 Define the following concepts with respect to control systems: a) Controlled variable. (1) b) Manipulated variable. (1) c) Disturbance variable. (1) d) Measured value. (1) e) Desired value. (1) f) Error value. (1) Question 10 Distinguish between direct acting and reverse acting control. (2) Question 11 A process is controlled by a proportional controller. The controller has a positive gain and proportional band (PB) of 50 %, a set point (S) of 50 % and a bias (R) of 50 %. The system error is given by E = S M. a) Calculate the proportional gain K P of the controller. (1) b) If the measured value (M) of the process is indicated as 42 %, calculate the output of the controller at that instant. (2) Question 12 Draw a labelled sketch of a pneumatic proportional controller. (4) Question 13 Draw a labelled sketch of a reverse acting pneumatic control valve. (6) Total: ooo000ooo---

8 Process Instrumentation I EIPIN1 Unit 2 First Assessment Memorandum May 2005 Page 1 Process Instrumentation I EIPIN1 Unit 2 First Assessment Memorandum May Torque tube [1] H Torque tube flange [½] X Y Level indicator [½] P atm (H )9.81 Torque rod [½] = P atm [1] 29430(H+1) = H + 1 = Torque arm [1] [4] H = meter. [3] Chain [½] 3. Top fixed point: The temperature Displacer [1] of (distilled water that boils) [½1] at (standard atmospheric pressure of 760 mm. mercury.) [½] Value of top fixed point on the [4] /5 [2] Fahrenheit scale is 212 F [1] a) R o = 100 R 0 R Scale = (etched) [1] = / C (2) b) R t = R o (1 + o t) R 40 = 100( ) Bore [1] = 100( ) Lens front capillary = 120 (2) tube (stem) [1] c) R t = R o (1 + o t) 110 = 100( t) Liquid column [1] t = t = 0.1 [6] t = 20 C (2) [4] /5 6. E Bulb [1] R 2 R 3 V Wheatstone bridge (R 1, R 2, R 3 and E) 1 mark R 1 3 lead wires 1 mark Voltmeter connected to middle R lead 1 mark [4] 7. The law of intermediate temperatures states that the sum [½] of the emf developed by a thermocouple with its junctions at temperatures T 1 and T 2, [½] and with its junctions at temperatures T 2 and T 3, [½] [2] will be the same as the emf developed if the thermocouple junctions are at temperatures T 1 and T 3. [½] R lead R lead R lead RT RTD 1 mark

9 Process Instrumentation I EIPIN1 Unit 2 First Assessment Memorandum May 2005 Page 2 8 Type Positive element Negative element Temperature range ( C) [3] J Iron [1] Constantan [1] -200 to 750 [1] 9. a) Controlled Variable: Process output variable that is maintained between specified limits. (1) b) Manipulated Variable: Process input variable that is adjusted, to steer the controlled variable towards the desired value. (1) c) Disturbance Variable: Process input variable that can cause the controlled variable to deviate from the desired value. (1) d) Measured value: Actual value of the controlled variable, as determined by the instrumentation. (1) e) Desired value: Required value of the controlled variable (set point). (1) [6] f) Error value: The difference between the desired value and the measured value. (1) 10. Direct acting control: A control arrangement in which the controller output increases if the measured value rises above the set point. [1] Reverse acting control: A control arrangement in which the controller output increases if the [2] measured value drops below the set point. [1] 11. a) K P = 100 = 100/50 = 2. (1) PB [3] b) C = K P (S M) + R = 2(50 42) + 50 = 66 %. (2) 12. Set point S [½] Proportional (feedback) Beam [½] bellows [½] Reset bellows [½] Flapper and nozzle [½] Pilot relay [½] Controller output C [½] Air supply [½] [4] /5 13. Gland and packing [½] Bonnet nut [½] Measured value M [½] Bias value R [½] Spring nut [½] Spring [½] Diaphragm plate [½] Diaphragm [½] Stem connector [½] Travel indicator [½] Stem [½] Yoke [½] Actuator (Motor) [½] Bonnet [½] Gasket [½] [6] /8 Plug [½] Seat [½] Valve body [½]

10 Process Instrumentation I EIPIN1 Unit 1 Final Assessment June 2005 Page 1 Question 1 Define the international standards of measurement. (2) Question 2 State the functions an instrument may perform.. (3) Question 3 Define the precision of a measurement. (2) Question 4 Define an error of hysteresis in an instrument. (3) Question 5 Identify the instrument signal in Figure 1. Figure 1 (1) Question 6 Define the density of a substance and give the SI unit for density. (2) Question 7 Convert a pressure of 33 centimetre water (cm. H 2 O), to a pressure expressed in millimetre mercury (mm. Hg). (3) Question 8 A u-tube manometer, shown P 1 P 2 in Figure 2, is filled with = 3 two liquids, one liquid with 0.9 m a relative density of 3 and 0.3 m the other with a relative density of Calculate the pressure difference, P 1 - P 2, applied across the =13.6 manometer. Figure 2 (2) Question 9 With the aid of a sketch, derive an expression that can be used to determine pressure with an inclined limb manometer. All symbols used, must be shown very clearly on the sketch. (4) Question 10 a) Draw a labelled sketch of a pneumatic differential pressure transmitter. (6) b) Give an expression for the output pressure P 0 (kpa), of a calibrated pneumatic differential pressure transmitter in terms of the range wheel adjustment ratio m, and the differential input pressure P 1 -P 2. (1).../2

11 Process Instrumentation I EIPIN1 Unit 1 Final Assessment June 2005 Page 2 Question 11 Identify the forms of energy, involved in a liquid in motion. (3) Question 12 In Figure 3, a restricted horizontal flow line is shown. The pressure difference p 1 - p 2 is measured by taking the reading h, as shown in Figure 3. Use Bernoulli s theorem and the principle of mass flow continuity, to derive the flow equation, q = k h. (7) q p 1 p 2 A 1 A 2 v 1 h Figure 3 v 2 Question 13 State the advantages and disadvantages of a venturi tube flow meter. (2) Question 14 Sketch a segmental orifice plate flow meter and explain its use. (2) Question 15 a) Draw a labelled sketch of an electronic target flow meter. (3) b) State the principles on which the operation of this flow meter is based. (2) b) Give the operational equation that is used to calculate the flow rate q from the measurements made with a target flow meter. Define each symbol that appears in the equation. (2) Total: ooo000ooo---

12 Process Instrumentation I EIPIN1 Unit 1 Final Assessment Memorandum June 2005 Page 1 Process Instrumentation I EIPIN1 Unit 1 Final Assessment June 2005 Memorandum 1. International standards are defined by international agreement, [1] representing units of [2] measurements to the best possible accuracy [1] allowed by measurement technology. 2. Indicating function [1] Recording function [1] Controlling function [1] [3] 3. Precision is the (closeness with which repeated measurements) [1] of the (same quantity [2] agree with each other.) [1] 4. Hysteresis is the difference [1] between the readings obtained when a (given value of the measured [3] variable is approached from below) [1] and when the (same value is approached from above.) [1] 5. Electromagnetic, sonic or radioactive signal [1] 6. Density of a substance is defined as the mass of a unit volume [1] of a substance. [2] The SI unit is kilogram per cubic meter (kg/m 3 ). [1] 7. P = hg = 1000( )9.81 = 3237 Pa [1] en P = hg 3237 = 13600h9.81 h = m. [1] [3] 33 cm H 2 O = mm Hg [1] [of 33 cm H 2 O = ( H2O / Hg )(330 mm H 2 O) = (1/13.6)330 = mm Hg.] 8. P = P P = P P 1 -P 2 = [2] = Pa [2] (=62.39 kpa) a) P 2 Restriction [½] Sketch Air supply [½] 1 mark Pilot relay [½] d X A 1 P 1 ZL A 2 c Y Equating pressures in the XY plane: P 1 = P 2 + (h+d)g [1]... (1) and with mercury incompressible: A A 1 d = A 2 L d = 2 L [½].... (2) A 1 Also in triangle abc: sin = h/l h = Lsin [½].... (3) b L a h Nozzle [½] Flapper [½] Pivot point (range wheel adjust) [½] Range bar [½] Output P o [½] Feedback bellows [½] Zero adjust [½] Pivot and seal Force bar [½] 0.9 P 1 =3 0.6 P m =13.6 Cross flexure [¼] Capsule Diaphragm flexure [¼] (2) en (3) in (1): P 1 =P 2 + A capsule [½] Lsin 2 L g A Low pressure (P 2 ) High pressure (P 1 ) 1 input [½] input [½] A [4] P 1 P 2 = Lg sin 2 [1] (6) /8 A 1 [7] b) P 0 = m(p 1 P 2 ) + 20 kilopascal (1)

13 Process Instrumentation I EIPIN1 Unit 1 Final Assessment Memorandum June 2005 Page Potential energy [1] Kinetic energy [1] Pressure energy [1] [3] 12. From Bernoulli s law: ½v 2 1 +p 1 =½v p [1] (a) Flow continuity demands: q = A 1 v 1 = A 2 v 2 v 1 = (A 2 /A 1 )v [1] (b) h (b) in (a): ½(A 2 /A 1 )v 2 2 +p 1 =½v 2 1 +p 1 v 2 2 = 2(p 1 -p 2 )/[1-(A 2 /A 1 ) 2 ] q v 2 = 2(p p 1 p 2 1 p 2 )/ [1-(A 2 /A 1 )2].. (c) A 1 v A 2 1 v But p 2 1 p 2 = hg [1] (d) (d) in (c): v 2 = 2gh/[1 -(A 2 /A 1 )2].... (e) Also q=a 2 v [1] 2... (f) From (f) and (e): q = A 2 2gh/[1 -(A 2 /A 1 )2]... (g) [note: 1 mark for either simplified v 2 in Eq (e) or for simplified q in Eq (g)] Defining k=a 2 2g/[1 (A /A )2] in Equation (g): 2 1 [7] q = k h. 13. Advantages: Pressure loss is small [½] Operation is simple and reliable [½] [2] Disadvantages: Highly expensive [½] Occupies considerable space [½] 14. The segmental orrifice plate is used in systems where solid particles [½] [2] are present in the liquid medium or if the medium is in pulpy form. [½] [1] 15. a) Electronics housing [½] Strain gauge [½] Flow [½] Target [1] Pivot and seal [½] Force bar [1] (3) /4 b) The target is positioned at right angles to the fluid flow and the approaching stream exerts a drag force [1] on the target. This force is transmitted via a force bar [1] to a bonded strain gauge bridge for electrical output (or flapper and nozzle arrangement for pneumatic output). (2) c) 2 2 (D d ) q = 8F 4 2 d [1] D = pipe diameter (m) [½], d = target diameter (m) [½], F = drag force (N) [½] [7] = fluid density (kg/m 3 ) [½] (2) /3

14 Process Instrumentation I EIPIN1 Unit 2 Final Assessment June 2005 Page 1 Question 1 Draw a labelled sketch of a chain float level meter. (3) Question 2 State the essential advantage that the flexure tube displacer (torque tube) level meter, has over other float type level meters. (1) Question 3 The level of a volatile liquid in a closed container, is measured with the aid of a U tube manometer, as shown in Figure 1. The maximum level of the liquid in the container is 5 meter. The relative density of the liquid in the container is 0,9. The manometer liquid H = m 5 m is mercury with a relative density of 13,6. The zero level of the manometer, is 0.5 meter below the bottom of the container. Calculate the level H, of the liquid in the container, if Zero line = m the manometer reading is 0.25 meter. Figure 1 (4) Question 4 Define temperature and give the SI unit for temperature. (2) Question 5 Convert 40 C to degrees Fahrenheit, Kelvin and Rankine. (3) Question 6 Draw a labelled sketch of a mercury in steel thermometer. (3) Question 7 A Wheatstone bridge is used to measure temperature with a resistance thermometer 10V V RT, as shown in Figure 2. When the temperature around the thermometer is 0 C, + the resistance of RT is 100 and the bridge 100 RT voltage reading is V=0 volt. The temperature of RT is increased to 80 C. Calculate the Figure 2 reading V on the voltmeter when the temperature of RT is 80 C. Assume that the temperature coefficient of resistance of RT is / C and that the resistance change of RT with temperature, is described by the equation: R t = R o (1 + o t). (5) Question 8 Define the Seebeck effect. (2).../2

15 Process Instrumentation I EIPIN1 Unit 2 Final Assessment June 2005 Page 2 Question 9 The relationship between emf and temperature for a certain thermocouple, is described by the relation: v = ½t 2, where v is the generated thermocouple emf in microvolt (V), and t the temperature difference in C, between the hot junction and 0 C. If the thermocouple emf reading is 2000 microvolt and the temperature of the cold junction is 20 C, use the law of intermediate temperatures to calculate the temperature of the hot junction. (3) Question 10 State the principal advantage of thermistor thermometers over resistance thermometers. (1) Question 11 Draw a fully labelled block diagram of a feedback control system. (5) Question 12 Define the on-off control strategy. C (%) (1) Question 13 The output C, of a proportional only controller, changes with the system error Figure 3 E, as shown in Figure 3. Calculate the E (%) proportional band of the controller (3) Question 14 A proportional and integral (PI) controller is adjusted for a proportional gain (K P ) of 2 and a reset time (T R ) of 5 minutes. Calculate the integral gain (K I ) of the controller. (2) Question 15 Give the equation that describes the output C, of a proportional, integral and derivative (PID) controller, in terms of the error signal E and controller gains. (3) Question 16 Draw a labelled sketch of a pneumatic proportional and integral controller. (4) Question 17 Draw a circuit diagram of the integral (I) block of an electronic proportional, integral and derivative (PID) controller. Give the expression of the output of the I block, in terms of the error input signal E. (3) Question 18 Calculate the required characteristic flow coefficient C vc, for a control valve that must control a maximum flow rate of 100 gallons per minute for a liquid with specific gravity of 0.8 (G = 0.8). Assume that the pressure drop across the valve is 10 pound per square inch at maximum flow. (2) ---ooo000ooo--- Total: 50

16 Process Instrumentation I EIPIN1 Unit 2 Final Assessment Memorandum June 2005 Process Instrumentation I EIPIN1 Unit 2 Final Assessment Memorandum June 2005 Page 1 1. Level 3. indicator [½] Drum [½] H =0.9 5 m Chain [½] 0.5 m Weight [½] Zero line 0.25m X Float [1] =13.6 Y P X = P Y [3] 900(H )g g = g 900(H+0.375) = Minimum movement 900H = [1] [4] H = m 4. Temperature is defined as the degree of heat [1] of a body. The SI unit for temperature is Kelvin (K). [1] [2] 5. F= 5 9 C+32= =72+32=104 F [1] [3] R t = R o (1 + o t) RT = 100( ) = [1] Bourdon tube [1] Pointer and scale [½] Steel tube [½] Mercury [1] [3] /3½ Steel bulb [½] R=F+460= =564 R [1] K=C+273=40+273=313 K [1] I = 10/( ) = A V AC = RTI = = V [1] V BC = (100/200)10 = 5 V V = V AC V BC = [5] = V [3] 10V V BC B 100 V AB = 1 V Seebeck effect: If two dissimilar metals [½] are joined together to form a closed loop, [½] and if one junction is kept at a different temperature from the other, [½] an electromotive force is generated [½] [2] and electric current will flow in the closed loop. 9. Emf corresponding to (25 0) C = ½20 2 = 200 V [1] 0 C 20 C 20 C T Total emf (T - 0) = = 2200 V [1] 200V 2000V According to the law of intermediate temperatures: 0 C T Hot junction temperature T = 2v = 4400 [3] = C [1] 2200V 10. Higher sensitivity. [1] V AC C I A RT

17 Process Instrumentation I EIPIN1 11. [5] /7½ Comparator [1] Measured value [½] Unit 2 Final Assessment Memorandum June 2005 Disturbance variables [½] Desired Error Manipulated value [½] value [½] Control variable [½] Process [1] unit [1] Sensor [1] Controlled variable [½} Output [½] Page On-off control: A control strategy in which the controller output switches the final control element [1] fully on or off [1] to keep the controlled variable near set point. 13. K P = C/E = 100/80 C (%) 14. T R = 5 min = 300 sec [1] = 1.25 [1] 100 K I = K P /T R = 2/300 PB = 100/K P = 100/1.25 [2] = [1] = 80% [2] 50 E (%) Or by inspection: PB is the 15. C=K P E [1] +K I Edt [1] de +K [1] D range of E that results in [3] dt 100% change in C [3] = 80% 16. Set point S [½] Proportional (feedback) Beam [½] bellows [½] Reset bellows [½] Flapper and nozzle [½] Pilot relay [½] Restriction [½] Controller output C [½] Air supply [½] [4] /6 Measured value M [½] Automatic reset R [½] Needle valve [½] Reset time adjust [3] E R I [1] C I [1] V I =- V I 1 [1] R C I I Edt [2] C VC = = Q Rated P Full G = [1] [1]

18 Process Instrumentation I EIPIN1 Unit 1 First Assessment September 2005 Page 1 Question 1 State the SI units for electric current and luminous intensity. (2) Question 2 A liquid filled thermometer is shown in Figure 1. Identify parts A, B and C and state for each of parts A, B and C, the fundamental instrument element, represented by that part. (3) A Figure 1 Question 3 Define the repeatability of an instrument. (2) Question 4 Output voltage (mv) The input output relationship for a 40 displacement measuring instrument, is shown in Figure 2. Calculate the sensitivity of the instrument. (2) 0 Input displacement (cm) Question Figure 2 Draw the instrument symbol for a flow indicator transmitter, mounted on board. (2) Question 6 Define the relative density of a substance. (2) Question 7 Both legs of a mercury (relative density = 13.6) U tube manometer, are open to an atmospheric pressure of 100 kpa, with the zero line, 1 meter below the top of the manometer, as shown in Figure 3(a). The right hand leg is now sealed off, airtight, and the air inside the sealed chamber, is trapped at a pressure of 100 kpa, as shown in Zero line = kpa 100 kpa 1 m B C 100 kpa 100 kpa P 1 m h = 1 m (a) (b) (c) Figure 3 Figure 3(b). The pressure applied to the left hand tube is increased to a new higher value of P pascal, that results in a manometer reading h, of 1 meter, as shown in Figure 3(c). Calculate the applied pressure P. (4).../2

19 Process Instrumentation I EIPIN1 Unit 1 First Assessment September 2005 Page 2 Question 8 The reading h on a well type mercury ( = 13.6) manometer is 1 meter, when measuring a pressure of 135 kpa. a) Calculate the ratio (A 2 /A 1 ) of the tube area (A 2 ) to the well area (A 1 ). (3) b) Determine the change in level (d) that the well mercury experiences. (2) Question 9 Discuss mercury as a manometer liquid, with respect to its relative density, applications, advantages and disadvantages. (4) Question 10 a) Draw a labelled sketch of a foil type strain gauge. (3) b) State the purpose of a strain gauge. (1) Question 11 Define flow rate of a fluid and give the SI unit for flow rate. (2) Question 12 With reference to the energy content of a liquid in motion, define the following quantities and give an expression for the energy per unit volume, in each case: a) Pressure energy. (2) b) Kinetic energy. (2) Question 13 a) Make a labelled sketch of a Pitot tube flow meter. (2) b) Give a formula from which the velocity v of a liquid may be calculated, when measuring flow rate with a Pitot tube. (1) Question 14 State the various methods of positioning the high pressure and low pressure tap-points, that may be used when measuring flow rate with orifice plates. (5) Question 15 a) Draw a labelled sketch of a Doppler flow rate meter. (3) b) Give a formula with which the flow speed v may be calculated, when a Doppler flow meter is used to determine the flow rate of a stream. Define the variables in the equation. (3) Total: ooo000ooo---

20 Process Instrumentation I EIPIN1 Unit 1 First Assessment Memorandum September 2005 Process Instrumentation I EIPIN1 Unit 1 First Assessment Memorandum September Current-Ampere [1] [2] Luminous intensity Candela [1] 2. A Tube [½] Transmission element [½] B Bourdon tube [½] Secondary element [½] [3] C Link and gears [½] Variable manipulation element [½] 3. Repeatability is the closeness [½] of the instrument readings when the (same input is applied [2] repetitively) [½] over a short period of time [½] with the same conditions. [½] 4. Sensitivity = output/input [1] = 40/5 = 8 mv/cm. [1] [2] 5. [2] FIT Symbol [1] FIT [1] Page 1 6. Relative density of a substance is defined as the ratio of the density of the substance [1] to the [2] density of water. [1] 7. According to Boyle s law: ( )(1A) = P X (0.5A) P X = Pa [2] Equating pressures on the XY line: P = P X P = [4] P = kpa [2] Zero line = kPa 100kPa 1m X P 1m P X Tube cross sectional area is A meter 2 8. a) P 1 P 2 = hg(1 + A 2 /A 1 ) [1] = [1 + (A 2 /A 1 )] 135kPa = [1 + (A 2 /A 1 )] 1m 1+(A 2 /A 1 ) = / = Zero line (A 2 /A 1 ) = [2] d (3) b) d = (A 2 /A 1 )h [1] d = A 1 A 2 [5] = m = 11.9 mm. [1] (2) 9. Relative density: 13.6 [1] Applications: Pressure measurements in compressed gas, and in water and steam applications. [1] Advantages: High density. Can be easily seen. Mercury does not: i) evaporate, ii) mix with other liquids, iii) wet sides of tubes. [1] [4] Disadvantages: Expensive. Mobility and density are affected by contamination. [1] 10. a) b) Convert pressure or force to an electrical signal. (1) 0.5m Y [4] Alignment marks [1] Backing material [1] Grid [1] Solder tabs [1] (3) /4

21 Process Instrumentation I EIPIN1 Unit 1 First Assessment Memorandum September 2005 Page Flowrate is the volume of a liquid or gas passing a given point per unit time, [1] and is measured [2] in cubic meter per second (m 3 /s). [1] 12. a) Pressure energy is the energy which a liquid has by virtue of its internal pressure. [1] A liquid under pressure p pascal, possesses pressure energy per unit volume equal to p joule. [1] (2) b) Kinetic energy is the energy a liquid has by virtue of its motion. [1] A liquid with density kilogram/meter 3 and moving at velocity v meter/second, possesses kinetic energy per [4] unit volume equal to ½v 2 joule. [1] (2) 13. a) Static pressure p stat [½] Stagnation (impact) pressure p stag [½] [3] b) v = v [½] 2(p stag Impact hole [½] p stat ) (2) (1) 14. Corner taps [1] Flange taps [1] Radius taps (D&D/2 or throat taps) [1] Vena-contracta taps [1] Pipe taps [1] [5] 15. a) Piezoelectric crystals [½] Receiver [½] Transmitter [½] v [½] or flow [½] Bubbles or solid particles [½] (3) f b) v = c R f T [1] 2f T cos f R = received frequency [½] f T = transmitted frequency [½] [6] c = speed of sound in medium [½] = incidence angle [½] (3)

22 Process Instrumentation I EIPIN1 Unit 2 First assessment October 2005 Page 1 Question 1 Draw a labelled sketch of a magnetic float level meter (magnetic coupled float and follower). (4) Question 2 The level of a liquid in an open container, is measured with the aid of a well type Figure 1 manometer, as shown in Figure 1. The ratio of the tube area to the well area is 0.01 (A 2 /A 1 = 0.01). The relative densityof the liquid in the container is 1 (=1) and the manometer liquid is mercury with a relative H = 1 density of 13.6 (=13.6). The zero level of 1 m the manometer liquid, is 1 meter below the h=0.3m bottom of the container. Calculate the Zero line level H, of the liquid in the container, if = 13.6 the manometer reading h, is 0.3 meter. (4) Question 3 Define the following fixed points on the international temperature scale: a) The oxygen point. (2) b) The silver point. (2) Question 4 a) Make a labelled sketch of a gas filled thermometer. (4) b) State which gas is normally used in a gas filled thermometer. (1) c) Give the equation that describes the gas law for ideal gasses. (1) d) With reference to the gas law, state the basic principle underlying the operation of the gas filled thermometer. (1) Question 5 A Wheatstone bridge is used to measure temperature with a resistance thermometer RT, as shown in Figure 2. The V three fixed resistors have a resistance of 100 each, 10V and the bridge is powered by a 10 V battery. When the temperature around the thermometer is 0 C, the resistance of RT is 100 and the bridge voltage reading RT is V = 0 volt. When the temperature of RT is increased, the voltmeter reading V, increases to 1 volt. Assume that the Figure 2 temperature coefficient of resistance of RT is / C and that the resistance change of RT with temperature, is described by the equation R t = R o (1 + o t). Calculate the temperature of RT when the voltmeter reading is 1 V. (5) Question 6 a) State the thermocouple law of intermediate metals. (2) b) State the practical benefit derived from the law of intermediate metals. (1).../2

23 Process Instrumentation I EIPIN1 Unit 2 First assessment October 2005 Page 2 Question 7 Give the positive element material, the negative element material and the temperature range of a type T thermocouple. (3) Question 8 A simple float and lever, water level control system with proportional controlled valve regulated inflow, is illustrated in Figure 3. a) Identify the quantities in Figure 3 that represent the following: i) Controlled variable. (1) ii) Manipulated variable. (1) iii) Disturbance variable. (1) b) Identify the components in Figure 3 that evaluate/determine the following: i) Measured value. (1) ii) Desired value. (1) iii) Error value. (1) Float arm Float Figure 3 Pivot H Lever Valve Water outflow Water inflow Question 9 Define the following concepts with respect to proportional only control systems: a) Proportional control law. (1) b) Proportional gain. (1) c) Proportional band. (1) d) Offset. (1) Question 10 The set point of a proportional only control system M Disturbance is 50%. The behaviour of the measured value M, 50% after a disturbance, is shown in Figure 4. Sketch a graph of the possible behaviour of the measured value, if the disturbance occurred while the controller s integral control function, was also active. 40% Figure 4 Time (2) Question 11 Draw a circuit diagram of the derivative (D) block of an electronic proportional, integral and derivative controller. Give the expression of the output of the D block, in terms of the error input signal E. (3) Question 12 Explain the difference between direct and reverse acting valve actuators. (3) Question 13 Explain the term valve throttling. (2) Question 14 Draw a labelled sketch of a valve positioner that helps the actuator of a pneumatic valve, to position its valve stem in the required position, as dictated by the value of the instrument signal. (6) ---ooo000ooo--- Total: 56 Q L

24 Process Instrumentation I EIPIN1 Unit 2 First Assessment October 2005 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 2 First Assessment October 2005 Memorandum Level indicator [1] P A H = 1 Indicator rod [1] 1 m P 1 h=0.3m Non-magnetic ZL dip tube [1] d=(a 2 /A 1 )h=0.003m = 13.6 Ignore d and equate pressures on the zero line: Doughnut float P A (H+1) g = P A g [1] with H = 3.08 m [3] outer magnet [1] Ignore d, calculate P 1 on zero line and use well type manometer equation: P 1 = P atm (H + 1)g and P 2 = P atm Follower with P 1 P 2 =1000(H+1)g & P 1 P 2 =hg(1+a 2 /A 1 ): inner magnet [1] 1000 (H+1) g = g(1+0.01) [1] H= m [3] Include d and equate pressures in line with mercury meniscus in well: P A +1000(H )g = P A ( )g [1] H = m [3] Include d and use well type manometer equation: [4] /5 [4] P 1 -P 2 =1000(H+1.003)g=13600(0.3)g(1+0.01) [1] H = m [3] P 2 P A 3. a) The oxygen point: The boiling point of liquid oxygen [1] : 4. a) C. [1] (2) b) The silver point: The melting point of silver [1] : [4] C. [1] (2) 4. b) Nitrogen gas. (1) c) PV = nrt. (1) d) V is constant, therefore P T [½] P is detected by the Bourdon tube [½] (1) [7] Bourdon tube [1] Pointer and scale [1] Steel tube [1] Gas [1] (Nitrogen) Steel bulb [1] (4) /5 5. V BC = (100/200)10 = 5 V and V AB = 1 V V AC = 6 V [1] RT 10 = 6 10RT = 6(RT+100) RT 100 RT = 150 [2] 150 = 100( t) t = 1.5 [5] t = 0.5 t = C [2] 10V V BC B V AB = 1 V V AC C I A RT 6. a) The law of intermediate metals states that a (third metal may be inserted into a thermocouple system without affecting the emf generated,) [1] (if, and only if, the junctions with the third metal are kept at the same temperature.) [1] (2) [3] b) A measuring instrument may be inserted into the thermocouple circuit. (1)

25 Process Instrumentation I EIPIN1 Unit 2 First Assessment October 2005 Memorandum Page 2 7. Type Positive element Negative element Temperature range ( C) [3] T Copper [1] Constantan [1] -200 to 350 [1] 8. a) i) Controlled Variable: Water level H. (1) ii) Manipulated Variable: Water inflow Q. (1) iii) Disturbance Variable: Water outflow L. (1) b) i) Measured value: Float vertical position. (1) ii) Desired value: Length of float arm (or vertical position of pivot). (1) [6] iii) Error value: Lever rotation. (1) 9. Proportional control: A control strategy in which the controller output is proportional to the magnitude of the error (C = K P E + R). (1) Proportional gain: Ratio of controller output change to error value change (K P = C/E). (1) Proportional band: The error range that causes 100 % change in controller output (PB = 100/K P ). (1) [4] Offset: The steady state difference between the set point and the measured value. (1) 10. M Disturbance M Disturbance 50% or 50% [2] 40% Time 40% Time Direct acting actuator: In a direct-acting actuator, an increase in the R [1] D pneumatic pressure applied to the diaphragm E extends the valve stem (for a normally seated valve this will close the valve and is called C [1] D Op-amp V D air to close ) [1½] de V D =-R D C [1] D dt Reverse acting actuator: In a reverse acting actuator an increase in the pneumatic pressure applied to the diaphragm lifts the valve stem (in a normally seated valve this [3] [3] will open the valve and is called air to open ) [1½] 13. Throttling occurs 14. when the valve stem position is between closed and open (0 > > 1) and the valve is busy regulating the [2] flow stream. Elastic forcebalance beam [1] Cam [1] Valve stem [½] Pivot [½] Instrument bellows [1] Instrument signal [½] Valve actuator [½] Actuator Output [½] [6] /8 Flapper and nozzle [1] Pilot relay [½] Restriction [½] Air supply [½]

26 Process Instrumentation I EIPIN1 Unit 1 Final Assessment November 2005 Page 1 Question 1 Discuss the significance of working standards in the hierarchy of instrument standards. (2) Question 2 Explain the recording function of an instrument. (2) Question 3 Define the precision of an instrument. (2) Question 4 A static calibration test was performed on Output pressure reading (kpa) a gauge pressure meter. An error of 10 % was recorded for each test input, as shown Figure 1 in Figure 1 where the results obtained, were Applied input plotted on a graph. State the type of error that 0 pressure (kpa) the instrument is suffering from and suggest a possible remedy to cure the problem. (2) Question 5 Figure 2 shows one section of a larger instrumentation schematic drawing. Identify items 1 and 2 and comment on the function 1 2 FIC performed by this segment of the system. Figure 2 (3) Question 6 Define atmospheric pressure and give the assigned standard value. (2) Question 7 Convert a pressure of 50 kpa to a pressure expressed as millimeter mercury. (2) Question 8 P 0 Pa The left hand tube of a u-tube mercury ( = 13.6) manometer, shown in Figure 3, = m is filled up with a liquid with relative 1 m density of 0.8 (=0.8). The right hand tube is sealed and forms a chamber with a perfect vacuum (0 Pa). Calculate the pressure P, applied to the left hand tube of the manometer. (2) Figure 3 =13.6 Question 9 With the aid of a sketch, derive the expression that is used to determine pressure with a well type manometer. All symbols used, must be shown very clearly on the sketch. (4).../2

27 Process Instrumentation I EIPIN1 Unit 1 Final Assessment November 2005 Page 2 Question 10 The inclined limb of an inclined limb manometer, forms an angle of 30 degrees with the horizontal plane. The relative density of the manometer fluid is 0.8 (=0.8). The internal diameter of the well is 3 cm and the internal diameter of the inclined limb is 12 mm. a) Calculate the maximum applied pressure for a maximum scale reading L, of 100 cm on the scale attached to the inclined limb. (3) b) The range of the above inclined manometer must be extended so that the maximum pressure that can be applied to the manometer, is increased by 1000 pascal. Calculate the relative density of the manometer fluid that is required. (2) Question 11 Make a labelled sketch of a C-type Bourdon tube pressure gauge. (4) Question 12 Define a streamlined flow and a turbulent flow of a stream. (4) Question 13 A venturi tube, shown in Figure 4, is p 1 - p 2 = 375 pascal used to measure the flow rate of water (=1). The cross sectional p area of the throat is m 2 1 q p 2 and the cross sectional area Pipe area=0.002 m 2 Throat area=0.001 m 2 of the pipe is m 2. The pressure difference measured across the high and low pressure Figure 4 taps, is 375 pascal. Calculate the flow rate q, of the water, using the flow equation, q = A 2 2(p 1 p 2 ) [1(A 2 /A 1 )2]. (3) Question 14 a) Draw a sketch of a concentric orifice plate and indicate on the sketch where a vent hole and drain hole may be inserted. (2) b) Explain the purpose of a vent hole. (2) c) Explain the purpose of a drain hole. (2) Question 15 State the main disadvantage of an orifice plate flow meter. (1) Question 16 a) Draw a labelled sketch of a vortex flow meter. (3) b) Give the operational equation that is used to calculate the flow rate q, from the measurements made with a vortex flow meter. Define each symbol that appears in the equation. (3) ---ooo000ooo--- Total: 50

28 Process Instrumentation I EIPIN1 Unit 1 Final Assessment Memorandum November 2005 Process Instrumentation I EIPIN1 Unit 1 Final Assessment Memorandum November Workplace standards are used to calibrate instruments used in industrial applications and instruments used in the field, [1] for accuracy and performance. Working standards are [2] checked against secondary standards [1] for accuracy. 2. An instrument may provide the information of the value of a quantity under measurement [2] against time [1] or some other variable, in the form of a written record, [1] usually on paper. 3. Precision is the closeness [1] with which (repeated measurements of the same quantity) [1] [2] agree with each other. 4. Range error [1] (systematic error because of drift, miscalibration, mishandling, etc.). [2] Possible cure: recalibration. [1] 5. 1: Pneumatic valve [1] 2: Venturi tube [1] [3] Function: regulating the flow rate (FIC) of the stream by means of a pneumatic control valve [1] 6. Atmospheric pressure is the absolute pressure caused by the weight of the earth s atmosphere. [1] [2] Standard atmospheric pressure at sea level is kpa. or 760 mm. mercury. [1] 7. P = hg = 13600h9.81 h = meter [1] [2] 50 kpa mm Hg. [1] 8. P = [2] P = Pa. 9. P 1 = P 2 + (h+d)g [1]. (1) A 2 A 1 d = A 2 h d = h [1].... (2) A 1 A (2) in (1): P 1 = P 2 + h 2 h A g [1] 1 [4] P 1 P 2 = hg A 1 2 A 1 d =0.8 Sketch: 1 mark P 1 P h =13.6 P 2 ZL A 1 A 2 Page a) P 1 P 2 = Lg(sin+A 2 /A 1 ) [1] 11. = [sin30 + (12/30) 2 ] = 7848( ) = = 5180 Pa [2] (3) b) (P 1 P 2 ) new = = 6180 Pa [1] P 1 P 2 = Lg(sin+A 1 /A 2 ) 6180 = = 6180/6.475 = kg/m 3 [5] new = [1] (2) Pointer and scale [½] Bourdon tube [1] Adjustable link [½] Range adjust [½] Pivot point [½] Sector gear [½] Pinion gear [½] Hairspring [½] [4] /5 Pressure connection [½]

29 Process Instrumentation I EIPIN1 Unit 1 Final Assessment Memorandum November 2005 Page Streamlined flow: In a streamlined flow, all the particles in the liquid, flow in the same direction and parallel to the walls of the pipe, and the streamlines are smooth. [2] Turbulent flow: In a turbulent flow, the particles in the stream, flow axially as well as [4] radially, and the streamlines are in a chaotic pattern of ever changing swirls and eddies. [2] 13. q = A 2 [2(p 1 -p 2 )/{[1-(A 2 /A 1 ) 2 ]}] = [2375/{1000[1-(1/2) 2 ]}] [3] = (750/750) = m 3 /sec. 14. a) b) Vent holes are provided to prevent Vent hole (top [1] ) Drain hole (bottom [1] ) [6] (2) 15. High pressure loss. [1] (gasses when transporting liquids) [1] to accumulate at the top [1] the pipe on the upstream side of the orifice plate. (2) c) Drain holes are provided to prevent (solid particles in liquids) [½] and (condensate in gasses) [1] to accumulate at the bottom [½] of the pipe on the upstream side of the orifice plate. (2) 16. a) Bluff body (vortex generator or shredder bar) [1] Eddies (vortices, whirls, swirls or Von Karman vortex street) [1] v [1] d Heat sensors in bluff body or ultrasonic sensors [1] (3) /4 b) q = A fd [1] S t A = unblocked flow area [½] f = measured vortex frequency [½] d = width of bluff body [½] [6] S t = Strouhal factor [½] constant (3)

30 Process Instrumentation I EIPIN1 Unit 2 Final Assessment November 2005 Page 1 Question 1 Draw a labelled sketch of a magnetic float switch. (3) Question 2 The level of a liquid in a closed container, is measured with the aid of a well type manometer, as shown in Figure 1. The ratio of the tube area to the well area is 0.01 (A 2 /A 1 = 0.01). The relative densityof the liquid in the container is 1 (=1) and the manometer liquid is mercury with a relative density of 13.6 (=13.6). The H = 1 1 m 5 m zero level of the manometer liquid, is h=0.15m Zero line 1 meter below the bottom of the container. Calculate the level H, of the = 13.6 liquid in the container, if the Figure 1 manometer reading h, is 0.15 meter. (5) Question 3 Define the bottom fixed point on a temperature scale and give this temperature value on the Fahrenheit scale. (2) Question 4 Convert 50 F to degrees Celsius, Kelvin and Rankine. (3) Question 5 a) Draw a labelled sketch of a bi-metal thermometer. (3) b) State the principle on which the operation of the bi-metal thermometer is based. (1) Question 6 A platinum resistance thermometer has a resistance of 100 at 0 C and a resistance of at 100 C. Using the linear approximation R t = R o (1 + o t), calculate the resistance of the thermometer at 140 C. (3) Question 7 Draw a labelled circuit diagram to illustrate the four wire method that is used to compensate for ambient temperatures when measuring temperature with a resistance thermometer. (3) Question 8 Give the positive element material, the negative element material and the temperature range of a type E thermocouple. (3).../2

31 Process Instrumentation I EIPIN1 Unit 2 Final Assessment November 2005 Page 2 Question 9 Table 1 A type E thermocouple is C used to measure the temperature of a medium. An excerpt from the type E thermoelectric voltage table (in microvolt) for temperatures in degrees Celsius, is given in Table The emf reading obtained from the thermocouple, is microvolt (V). If the temperature of the reference junction (cold junction) is 20 C, use Table 1 to calculate the temperature of the hot junction. (3) Question 10 a) Distinguish between feedback control and feedforward control. (2) b) What essential control strategy, feedback or feedforward control, is used by a person trying to catch a ball. (1) Question 11 Name the two kinds of delays (lags) that may be identified when a system is subjected to a step input. (2) Question 12 Sketch a graph of the typical behaviour of a controlled variable, when controlled by an on-off controller, between 60% upper limit and 40% lower limit. (2) Question 13 The output of a proportional only controller, changes by 15% when the error changes by 10%. Calculate the proportional band setting of the controller. (2) Question 14 Define reset time with reference to integral control action. (2) Question 15 Draw a circuit diagram of the proportional (P) block of an electronic proportional, integral and derivative (PID) controller. Give the expression of the output of the P block, in terms of the error input signal E. (3) Question 16 Name the linear movement valve types. (5) Question 17 Sketch a graph of the inherent valve characteristic for an equal percentage valve. (2) Total: ooo000ooo---

32 Process Instrumentation I EIPIN1 Unit 2 Final Assessment Memorandum November 2005 Process Instrumentation I EIPIN1 Unit 2 Final Assessment Memorandum November Float magnet [1] 2. Non-magnetic housing [1] Magnetic reed switch [1] Swivel pin [½] Float [1] Page 1 P 1 =P t = P t [3]/4½ and P 2 =P t +1000(H+0.85)9.81 =P t +9810(H+0.85) 3. Bottom fixed point: The temperature of ice (prepared P 1 P 2 = (H+0.85) [1] from distilled water) mixed with distilled water [½], at But P 1 -P 2 =hg(1+a 2 /A 1 ) standard atmospheric pressure of 760 mm. mercury. [½] = (1+0.01) [2] Temperature value on the Fahrenheit scale is 32 F [1] =20213 [1] 4. C= 9 5 (F 32)= 9 5 (50-32)= = 10 C [1] H 1 m ZL P t = 1 h= m P 2 P t P (H+0.85) = [5] H = 3.09 m [3] =13.6 R=F+460=50+460=510 R [1] { or including d=(a 2 /A 1 )h= = [3] K=C+273=10+273=283 K [1] P 1 -P 2 = (H+0.85) 20213= H 8337 H = m } 5. a) Pointer and scale [1] R 6. 0 = 100 R = = [1] R R t = R o (1 + o t) [3] R 140 C = 100( ) = [2] Socket [½] Bearing [½] Shaft [1] Guide [½] Stem [½] 7. [3] Current source I 1 mark I M Voltmeter return across RT - 1 mark a b R lead R lead 4 leadwires plus RTD 1 mark c d R lead R lead RT Helical bi-metal element [1] (3) /5 [4] b) Different temperature coefficients of expansion of two metals bondedtogether in a helix. (1)

33 Process Instrumentation I EIPIN1 8. [3] Type Positive element Negative element Unit 2 Final Assessment Memorandum November 2005 Temperature range ( C) E Chromel [1] Constantan [1] -200 to 800 [1] 9. Emf corresponding to (25 0) C = 1192 V [1] Total emf (T - 0) = = 5848 V [1] [3] From table, hot junction temperature T = 93 C [1] 0 C Page 2 0 C 1192V 20 C 20 C 4656V T 5848V 10. a) Feedback control: Measure the controlled variable to determine the control strategy. [1] Feedforward control: Measure disturbance variables to determine the control strategy. [1] (2) [3] b) Feedforward control. (1) 11. Dead time [1] and first order lag. [1] [2] 12. Measured value 60% T 40% [2] Time 13. K P = C/E = 15/10 = 1.5 [1] [2] PB = 100/K P = 100/1.5 = % [1] 14. Time taken for integral action to equal [1] proportional action under the influence of a constant error. [1] [2] 15. R [1] PF 16. i) Globe valve [1] ii) Gate valve [1] E iii) Needle valve [1] R [1] PI Op-amp V P iv) Pinch valve [1] [5] v) Diaphragm valve [1] [3] V P = - R PF E [1] R PI 17. Inherent =% characteristic f(x) 1 =% [2] x Valve travel

34 Process Instrumentation I EIPIN1 Unit 1 First Assessment 10 March 2006 Page 1 Question 1 Define the primary standard of measurement. (2) Question 2 Define the span of an instrument. (2) Question 3 Define the sensitivity of an instrument. (2) Question 4 Define the error of drift that may occur in an instrument. (2) Question 5 Identify the instrument function and mounting TIR method, from the instrument symbol in Figure 1. Figure 1 (2) Question 6 An electronic transmitter with an output of 4-20 ma, is calibrated for a pressure range of kpa. What pressure is represented by a 16 ma signal? (2) Question 7 Define relative density. (2) Question 8 Convert a pressure of 100 centimeter water to a pressure expressed in millimetre mercury. (2) Question 9 With the aid of a labelled sketch, derive the expression that is used to determine pressure with a well type manometer. (4) Question 10 Discuss Bromoform as a manometer liquid, with respect to its relative density, applications, advantages and disadvantages. (4) Question 11 Draw a labeled sketch of a force-balance gauge calibrator ( dead weight tester ). (6) Question 12 Define volumetric flow and give the SI unit for volumetric flow. (2) Question 13 a) Define a streamlined flow of a stream. (2) b) The Reynolds number for a flow condition is determined as Is the flow streamlined or turbulent? (1) c) Explain the function of a flow straightener (straightening vane) in a pipe. (1) Question 14 In Figure 2, a restricted horizontal flow line is shown. The pressure difference p 1 - p 2 is measured by taking the reading h, as shown in Figure 2. Use Bernoulli s theorem and the principle of mass flow continuity, to derive the flow equation, q = k h. (7) Figure 2 q p 1 p 2 Question 15 Sketch the configuration, including dimensions and labels, when flange taps are used for flow rate measurement with an orifice plate. (2) Question 16 a) Draw a labelled sketch of a rotameter (variable area flowmeter). (4) b) Why does the displacer in a rotameter move upwards with increasing flow rate? (1) A 1 ---ooo000ooo--- Total: 50 v 1 h A 2 v 2

35 Process Instrumentation I EIPIN1 Unit 1 First Assessment 10 March 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 1 First Assessment 10 March 2006 Memorandum 1. Primary standards are maintained at institutions in various countries. [1] The main function [2] is to check the accuracy of secondary standards. [1] 2. The span of an instrument is the arithmetic difference [1] between the minimum and maximum [2] range [1] values, used to describe both the input and the output. 3. Sensitivity is the rate of change [1] of the output [½] of a system with respect to input [½] changes. [2] 4. Drift is the change in instrument indication over time [1] while the input and ambient conditions [2] are constant. [1] 5. Temperature indicator recorder, [1] mounted on board. [1] [2] 6. P = 70 + (12/16) (150 70) = 130 kpa [2] 150kPa 130kPa 70kPa 4mA x 80 16mA 20mA 7. Relative density of a substance is defined as the ratio of the density of the substance [1] to the [2] density of water. [1] 8. P H2O = P Hg H2O h H2O g = Hg h Hg g 1000 ( ) 9.81 = h Hg 9.81 [2] = h Hg h Hg = m = mm. 9. P 1 = P 2 + (h+d)g [1]. (1) A 2 A 1 d = A 2 h d = h [1].... (2) A 1 A (2) in (1): P 1 = P 2 + h 2 h A g [1] 1 [4] P1 P 2 = hg(1 + A 2 /A 1 ) Sketch: 1 mark P 1 80/16=x/12 5=x/12 x=60 P 2 ZL d A 1 A Relative density: 2.9 [1] Applications: Useful where pressure measurement demands manometer liquid with density between water and mercury. [1] Advantages: Density that falls between water and mercury. [1] [4] Disadvantages: Density uncertain. Poisonous. Freezes easily. Subject to attack. Attacks rubber. [1] Volumetric flow is the total volume Gauge under Mass pieces [1] Platform [1] of a liquid or gas passing a given point test [1] over a certain period of time [1], and is Primary piston [1] [2] measured in cubic meter (m 3 ). [1] Secondary 13. a) Streamlined flow: In a streamlined flow, all the particles in the liquid, Oil [1] piston [1] Screw [1] flow in the same direction and parallel to the walls of the pipe, and the streamlines are smooth. (2) [6] / 7 b) Turbulent. (1) [4] c) To streamline a flow. (1) h

36 Process Instrumentation I EIPIN1 Unit 1 First Assessment 10 March 2006 Memorandum Page From Bernoulli s law: ½v 1 2 +p 1 =½v p 2 [1].... (a) Flow continuity demands: q = A 1 v 1 = A 2 v 2 v 1 = (A 2 /A 1 )v 2 [1] (b) (b) in (a): ½(A 2 /A 1 )v 2 2 +p 1 =½v 2 2 +p 2 v 2 2 = 2(p 1 -p 2 )/[1-(A 2 /A 1 ) 2 ] q p 1 p 2 A 1 A 2 v 2 = 2(p 1 p 2 )/ [1-(A 2 /A 1 )2].. (c) But p 1 p 2 = hg [1] (d) (d) in (c): v 2 = 2gh/[1 -(A 2 /A 1 )2].... (e) Also q=a 2 v [1] 2... (f) From (f) and (e): q = A 2 2gh/[1 -(A 2 /A 1 )2]... (g) [note: 1 mark for either simplified v 2 in Eq (e) or for simplified q in Eq (g)] Defining k=a 2 2g/[1 (A /A )2] in Equation (g): 2 1 [7] q = k h mm [½] 25mm [½] 16. a) Flow [½] High pressure tap [½] Low pressure tap [½] Scale [1] v 1 h v 2 [2] / 2½ Float (displacer) [1] 16 b) When the flow rate increases, the pressure difference across the float will increase, [½] Tapered tube [1] which will tend to push the float upwards. As the float moves upwards, the restricted flow area will increase [½] due to the tapered tube. This will allow the pressure difference to decrease to its original value where the float [5] will remain suspended in its new position. (1) Flow [1] (4)

37 Process Instrumentation I EIPIN1 Unit 2 First Assessment 5 May 2006 Page 1 Question 1 Distinguish between point level and continuous level metering. (2) Question 2 Draw a labelled sketch of a magnetic float sight glass level meter with isolation (shut off) capability. (3) Question 3 The level of a liquid in an open container container, is to be measured with the aid of a U-tube manometer, as shown in Figure 1. The relative density H = 1.5 of the liquid in the container is 1,5. The manometer 0.4 m fluid is mercury with a relative density of 13,6. Zero level h=0.5m The zero line of the manometer, is 0.4 meter below the bottom of the container. Calculate the level H, of the liquid in the container, if the manometer reading h, is 0.5 meter. (4) = 13.6 Figure 1 Question 4 Convert -40 C to degrees Fahrenheit, Kelvin and Rankine. (3) Question 5 Draw a labelled sketch of a vapour pressure thermometer. (4) Question 6 Name three common metals used in resistance thermometers. (3) Question 7 A Wheatstone bridge is used to measure temperature with a resistance thermometer RT, as shown in Figure 2. When the thermometer temperature 10V V is 0 C, the resistance of RT is 100 and the bridge + voltage reading is V = 0 volt. Calculate the reading on 100 RT the voltmeter when the temperature of RT is: Figure 2 a) 20 C and b) 50 C. Assume that the temperature coefficient of resistance of RT is / C and that the relationship between RT and temperature t, is given by: R T = R o (1 + o t). (6) Question 8 State the essential difference between the principle of operation of a resistance thermometer and a thermocouple thermometer. (1) Question 9 Give the positive element material, the negative element material and the temperature range of a type K thermocouple. (3) Question 10 Define the following concepts with respect to control systems: a) Controlled variable. (1) b) Manipulated variable. (1) c) Disturbance variable. (1) d) Measured value. (1) e) Desired value. (1) f) Error value. (1) Question 11 Define dead time (transportation lag) in a control system. (2) Question 12 A process is controlled by a proportional controller. The controller is programmed for a positive gain and proportional band of 80 %, a set point of 50 % and a bias of 50 %. If the measured value of the process is indicated as 65 %, calculate the output of the controller at that instant. (4) Question 13 State the fundamental advantage offered by integral control. (1) Question 14 Draw a labelled sketch of a pneumatic proportional plus integral plus derivative controller. (5) Question 14 Draw the graphs for the inherent valve characteristics for a quick opening valve, a linear valve and an equal percentage valve. (3) ---ooo000ooo--- Total: 50

38 Process Instrumentation I EIPIN1 Unit 2 First Assessment 5 May 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 2 First Assessment 5 May 2006 Memorandum 1. Continuous level metering devices measure level on a constant basis, displaying or transmitting the actual level of the liquid as it changes. [1] Point-level devices measure liquid at specific points [2] within the tank. [1] Isolation valve [1] Sight glass [1] H X 0.5 Y P atm (H )9.81 = P atm [1] 14715(H+0.65) = H = [3] /4 [4] H = meter. [3] 4. F = 5 9 C + 32 = 5 9 (-40)+32 = = -40 F [1] [3] K = C = = 233 K [1] Magnetic float [1] Metallic flaps [1] R = F = = 420 R [1] Platinum, [1] Copper [1] and Nickel [1] [3] 7. a) 20 C: R T = R o (1 + o t) = 100( ) = [1] Bourdon tube [1] Pointer and scale [½] Steel tube [½] Vapour [1] V AB = 1 V 100 I I = 10/( ) 10V = ma B A V AC = RTI + = = V [1] V BC 100 V AC RT and V BC = (100/200)10 = 5V C V = V AC V BC = = V [1] (3) b) 50 C: R T = R o (1 + o t) = 100( ) = [1] I = 10/( ) = ma Steel bulb [½] Volatile [4] liquid [1] V AC = RTI = = V [1] and V BC = (100/200)10 = 5V [6] V = V AC V BC = = V [1] (3) 8. A resistance thermometer produces a changing resistance with changing temperature [½] while a [1] thermocouple generates a changing emf with changing temperature. [½] 9. Type Positive element Negative element Temperature range ( C) [3] K Chromel [1] Alumel [1] -200 to 1200 [1] 10. a) Controlled Variable: Process output variable that is maintained between specified limits. (1) b) Manipulated Variable: Process input variable that is adjusted, to steer the controlled variable towards the desired value. (1) c) Disturbance Variable: Process input variable that can cause the controlled variable to deviate from the desired value. (1) d) Measured value: Actual value of the controlled variable, as determined by the instrumentation. (1) e) Desired value: Required value of the controlled variable (set point). (1) [6] f) Error value: The difference between the desired value and the measured value. (1)

39 Process Instrumentation I EIPIN1 Unit 2 First Assessment 5 May 2006 Memorandum Page Delay due to the time it takes information or material to be transported [1] from one point to another. [1] [2] 12. K P = 100/PB = 100/80 = 1.25 [1] E = = -15 % [1] C = K P E+R = 1.25 (-14) + 50 = 31.25% [2] [4] 13. Offset is eliminated. [1] 14. Set point S [½] Proportional (feedback) Beam [½] bellows [½] Reset bellows [½] Needle valve rate time adjustment [½] Flapper and nozzle [½] Pilot relay [½] Restriction [½] Controller output C [½] Air supply [½] [5] /6½ Measured value M [½] Automatic reset R [½] Needle valve [½] Reset time adjust 15. f(x) 1 Quick [1] Linear [1] [3] 0 =% [1] x 0 1

40 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 21 April 2006 Page 1 Question 1 Give the SI units for time and amount of substance. (2) B Question 2 A liquid filled thermometer is C shown in Figure 1. Identify parts A, B and A Figure 1 C, and state for each of parts A, B and C, the fundamental instrument element, represented by that part. (3) Question 3 Define the precision of an instrument. (2) Question 4 Define an error of hysteresis in an instrument. (3) Question 5 An item appears on an industrial schematic drawing that is labelled with the letters LCV. Identify this element. (2) Question 6 Define pressure and give the SI unit for pressure. (2) Question 7 Assuming that the density of the atmosphere is a constant value of 1.2 kg/m 3 and that the atmospheric pressure at sea level is 760 mm. mercury, calculate the height of the atmosphere above sea level. (3) Question 8 You are requested to design a scale plate for a U-tube manometer that uses mercury, with relative density of 13.6, as manometer liquid. You are told that the maximum differential pressure to be measured, will be 100 kpa. From the zero line upward, the following values must be marked off on the scale plate: 25 kpa, 50 kpa, 75 kpa and 100 kpa. Calculate the distances from the zero line to each marking on the scale, and sketch the designed plate. (4) Question 9 Draw labelled sketches to show how a bellows element may be used to measure: a) differential pressure (2) b) absolute pressure. (2) Question 10 Draw a labelled sketch of a pneumatic differential pressure transmitter. (6) Question 11 Define flow rate of a fluid and give the SI unit for flow rate. (2) Question 12 State Bernoulli s law (in words). (4) Question 13 State the advantages and disadvantages of the venturi tube. (4) Question 14 State three methods of positioning the high pressure and low pressure tap-points, that may be used when measuring flow rate with orifice plates. (3) Question 15 Water flows through a horizontal pipe with cross sectional area of m 2. A circular object, facing Pipe area = m 2 the stream with an area of m 2, is placed in the flow, as shown in Figure 2. The force on the object is measured p 1 p 2 as 0.5 newton. Calculate the flow rate q of the water, q F=0.5 N 2(p p ) if the flow rate is given by: q = A 1 2 2, 1 (A /A ) Object area = m 2 where p 1 -p 2 is the pressure difference across the object, A 2 is the restricted flow area, A 1 is the unrestricted flow Figure 2 area (pipe area) and = 1000 kg/m 3 (for water). (3) Question 16 Draw a labelled sketch of a transit time (transmissivity) flow meter. (3) ---ooo000ooo--- Total: 50

41 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 21 April 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 21 April 2006 Memorandum 1. Time: second [1] Amount of substance: mole [1] [2] 2. A Bulb [½] Primary element [½] B Bourdon tube [½] Secondary element [½] [3] C Pointer and scale [½] Functional element [½] 3. Precision is the closeness [1] with which (repeated measurements of the same quantity) [1] [2] agree with each other. 4. Hysteresis is the difference [1] between the readings obtained when a (given value of the measured [3] variable is approached from below) [1] and when the (same value is approached from above.) [1] 5. Level control valve [2] 6. Pressure is defined as the force exerted over a unit area [1]. The SI unit is newton per square meter [2] (N/m 2 ) or pascal (Pa). [1] h atm g = g h atm = 8613 m [3] 8. P 1 P 2 = hg. for P 1 -P 2 =100 kpa: = 13600h9.81 h = 750 mm. [1] Distance from zero line to 100 kpa marking = 375 mm. [1] [4] 9. a) Differential Low pressure 10. Bellows [½] (P 2 ) [½] [4] High pressure (P 1 ) [½] b) Absolute Bellows [½] High pressure (P 1 ) [½] Pressure indication [½] Vacuum [½] (2) (2) Pressure indication [½] [6] / 8 Restriction [½] Air supply [½] Pilot relay [½] Nozzle [½] Flapper [½] Pivot point (range wheel adjust) [½] Range bar [½] Output P o [½] Feedback bellows [½] Zero adjust [½] Pivot and seal [½] Force bar [½] Diaphragm capsule [½] 375mm [½] 281.3mm [½] 187.5mm [½] 93.74mm [½] Zero line Low pressure (P 2 ) input [½] 100 kpa 75 kpa 50 kpa 25 kpa 0 kpa Cross flexure [¼] Capsule flexure [¼] High pressure (P 1 ) input [½] 11. Flowrate is the volume of a liquid or gas passing a given point per unit time, [1] and is measured [2] in cubic meter per second (m 3 /s). [1] 12. If an (incompressible fluid is in a streamlined flow with no friction,) [1] the sum [½] of the [4] pressure energy, [½] the kinetic energy [½] and potential energy [½] per unit volume [½], is constant [½].

42 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 21 April 2006 Memorandum Page Advantages: Pressure loss is small [½] Operation is simple and reliable [½] [2] Disadvantages: Highly expensive [½] Occupies considerable space [½] 14. Corner taps [1] Flange taps [1] Radius taps (D&D/2 or throat taps) [1] [3]/5 Vena-contracta taps [1] Pipe taps [1] 15. p 1 p 2 = F/A object = 0.5/210-3 = 250 Pa. [1] and A 2 = [1] = 310 2(p p ) q = A = ( ) 2 1 (A /A ) (3 10 / 5 10 ) 2 1 [3] = (310-3 ) (0.6) =( ) 0.64 =(310-3 )0.8839= m 3 /s [1] 16. Ultrasonic transceiver 1 (Piezoelectric crystals) [½] Flow [1] [1] L [½] [3] Ultrasonic transceiver 2 (Piezoelectric crystals) [½]

43 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 26 May 2006 Page 1 Question 1 Distinguish between direct methods and indirect methods of level measurement and give an example of each method. (4) Question 2 State the principle on which level measurement with the flexure tube displacer (torque tube) level meter is based. (1) Question 3 Draw a labelled sketch of a bubble meter (gas flushing system) for level measurement (using a manometer) in an open container. (4) Question 4 Define temperature and give the SI unit for temperature. (2) Question 5 Define the following fixed points on the international temperature scale: a) The boiling point. (2) b) The sulphur point. (2) Question 6 Name three liquids used in a liquid in glass thermometer. (3) Question 7 Draw a labelled sketch of a resistance thermometer construction with protective tube and terminal head. (4) Question 8 A Wheatstone bridge is used Figure to measure temperature with a type PT100 platinum resistance thermometer RT, as shown in 10V V Figure 1. The resistance of the thermometer is given + by: RT = 100 [1 + ( )t ( ) t 2 ], where RT is measured in ohm and t in degrees Celsius. 100 RT Calculate the reading V on the voltmeter, if the temperature of RT is 150 C. (4) Question 9 In Figure 2, a type K Table 1 thermocouple is shown with its C hot junction at 55 C and its cold junction at 18 C. An excerpt from the type K thermoelectric voltage table (in microvolt) for temperatures in degrees Celsius, is given in Table 1. Use Table to determine the reading V, on the voltmeter. (3) C 55 C V Figure Question 10 A ship s automatic steering control system, must steer the ship in an exactly northerly direction, independent of the influences of the sea currents and winds. For this purpose, the control system monitors the ship s compass reading, and position the ship s rudder accordingly. Identify the following aspects regarding this specific control system: a) Controlled variable. (1) b) Manipulated variable. (1) c) Disturbance variable. (1) d) Measured value. (1) e) Desired value. (1) f) Error value. (1) Question 11 Define on-off (bang-bang) control. (2) Question 12 Give the equation that describes the output C, of a proportional, integral and derivative (PID) controller, in terms of the error signal E and controller gains. (3) Question 13 Draw a circuit diagram of the integral (I) block of an electronic proportional, integral and derivative (PID) controller. Give an expression for the output of the I block, in terms of the error input signal E. (3) Question 14 Draw a labelled sketch of a reverse acting pneumatic control valve. (7) ---ooo000ooo--- Total: 50

44 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 26 May 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 26 May 2006 Memorandum 1. Direct methods involve direct measurement of 3. the fluid level as such. [1] Examples: dipstick, overflow pipe, float or sight glass [1] Indirect methods involve measuring another variable that is related to the fluid level. [1] Examples: Pressure exerted by the fluid, echo time of ultrasonic signals [4] beamed to the fluid surface. [1] 2. Archimedes s Principle. [1] [4] /5 4. Temperature is defined as the degree of heat [1] of 7. [2] a body. The SI unit for temperature is Kelvin (K). [1] 5. a) The boiling point: The boiling point of pure water: [1] 100 C. [1] b) The sulphur point: The boiling point of pure [4] sulphur: [1] C. [1] 6. Mercury [1], Alcohol [1], Pentane [1], Toluene [1], [3] /5 Creosote [1] 8. RT = 100 [1 + ( )t ( ) t 2 ] = 100 [1+( )150 ( ) ] = 100 ( ) = = [2] V RT = [158.2/( )] 10 = V [1] V = [100/( )] 10 10V V RT [4] / 5½ [4] = volt [1] 9. E 0-55 C = E 0-18 C + E [1] C 2230 = V V = = 1512 V [2] [3] 10. a) Controlled Variable: Sailing direction. (1) 14. b) Manipulated Variable: Rudder position. (1) c) Disturb. Variable: Sea currents & winds. (1) d) Measured value: Compass reading. (1) e) Desired value: Northern direction. (1) f) Error value: Degrees difference between [6] north and compass direction. (1) 11. On-off control: A control strategy in which the controller output switches the final control element fully on or off [1] to keep [2] the controlled variable near set point. [1] 12. [3] 13. E C=K P E [1] +K I Edt [1] de +K [1] D dt R I [1] C I [1] V I 1 [1] V I =- R C Edt [3] I I [7] /8 V RT Valve body [½] Actuator (Motor) [½] Bubbler sight glass [1] Filter [1] Pressure regulator [1] Dip tube [1] Terminal Cap [1] Connector conduit [1] Socket [½] Stem (protective tube) [1] Leads [1] Resistor bulb (resistance winding [1] Air/gas supply [1] Spring nut [½] Spring [½] Diaphragm plate [½] Diaphragm [½] Stem connector [½] Travel indicator [½] Stem [½] Yoke [½] Gland & packing [½] Bonnet nut [½] Bonnet [½] Gasket [½] Plug [½] Seat [½]

45 Process Instrumentation I EIPIN1 Unit 1 First Assessment 8 September 2006 Page 1 Question 1 Define the working standard of measurement. (2) Question 2 Define the range of an instrument. (2) Question 3 Define the resolution of an instrument. (2) Question 4 Identify the following power supply symbols: WS, AS, ES, GS and SS. (5) Question 5 Give the density of the following substances: Water, Mercury, Air and Transformer oil. (4) Question 6 Define Gauge Pressure. (2) Question 7 With the aid of a labelled sketch, derive the expression that is used to determine pressure with a well type manometer. (4) Question 8 The reading h, on a well type mercury manometer, is 73 cm. when measuring a pressure of 100 kpa. (a) Calculate the ratio of the well diameter to the diameter of the tube. (4) (b) Determine the change in level of the mercury in the well of the manometer. (2) Question 9 Describe the operation of a C-type Bourdon tube gauge. (4) Question 10 Give a formula used for Poiseuille s law and explain each symbol used in the formula. (3) Question 11 A flow rate meter, uses a restriction in the flow stream, to measure the flow rate of a liquid in a horizontal pipe. The pressure difference across the restriction is determined by allowing the liquid into two vertical tubes installed on top of the pipe and on both sides of the restriction. When the flow rate is 0,1 cubic meter per second, the level difference of the liquid in the tubes is 0,3 meter. Calculate the flow rate when the level difference of the liquid in the two tubes is 0,6 meter. (4) Question 12 Give the advantages and disadvantages of orifice plates. (5) Question 13 Explain the operation of the following meters: (a) Target meter (b) Reciprocating piston PD meter. ---ooo000ooo--- Total: 50

46 Process Instrumentation I EIPIN1 Unit 1 First Assessment 8 September 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 1 First Assessment 8 September 2006 Memorandum 1. Workplace standards are used to calibrate instruments used in industrial applications and instruments used in the field, for accuracy and performance. [1] Working standards are checked [2] against secondary standards for accuracy. [1] 2. The range of an instrument is the minimum and maximum values [1] of the measured variable that [2] the instrument is capable of measuring. [1] 3. Resolution is the smallest variation [1] in the measured variable that can still be measured. [1] [2] 4. WS - Water supply [1] AS - Air supply [1] ES - Electric supply [1] GS - Gas supply [1] [5] SS - Steam supply [1] 5. Water: 1000 kg/m 3 [1] Mercury: kg/m 3 [1] Air: 1.2 kg/m 3 [1] Trafo oil: 864 kg/m 3 [1] [4] 6. Gauge pressure, is the difference between the absolute pressure in a medium and local atmospheric [2] pressure [1], when the pressure in the medium is higher than atmospheric pressure. [1] 7. P 1 = P 2 + (h+d)g [½]. (1) P 1 A 1 d = A 2 h [½] A 2 d = h [½]... (2) A 1 Sketch: 2 marks h (2) in (1): P 1 = P 2 + A h 2 h g ZL A 1 P d 1 P 2 = hg(1 + A 2 /A 1 ) [½] [4] A 1 A 2 8. P 1 P 2 = hg(1 + A 2 /A 1 ) [1] = 13600( )9.81[1 + (A 2 /A 1 )] [1] 1 + (A 2 /A 1 ) = /[13600( )9.81] (A 2 /A 1 ) = [1] (D 2 /D 1 ) 2 = D 2 /D 1 = [1] (4) b) d = (A 2 /A 1 )h [1] = [6] d = mm. [1] (2) 9. Bourdon tube pressure gauges are usually used where relatively large static pressures are to be measured. [½] The Bourdon tube pressure gauge consists of a C-shaped tube with one end sealed. [½] The sealed end is connected by a mechanical link to a pointer on the dial of the gauge. [½] The other end of the tube is fixed and open to the pressure being measured. [½] The inside of the Bourdon tube experiences the measured pressure, [½] while the outside of the tube is exposed to atmospheric pressure. [½] Therefore, the tube responds to changes in P measured P atm. [½] Increasing this pressure will tend to straighten out the tube and move the pointer to a higher scale position. [½] [4] 10. Poiseuille s law: q = R 4 (p 1 p 2 ) [1] 8L where q is the liquid s flowrate (m 3 /s) [½], R is the radius of the pipe (m) [½], is the viscosity of the fluid (PI) [½], L is the length of the pipe (m) and p 1 p 2 is the pressure differential across [3] the pipe (Pa). [½] P 2

47 Process Instrumentation I EIPIN1 Unit 1 First Assessment 8 September 2006 Memorandum Page q 1 = k [4] q 2 = k h [½] 0.1 = k 0.3 k = 0.1/ 0.3 [½] = [1] 1 h [½] = [½] = m 3 /s [1] Advantages Orifice plates are cheap and easy to install. [1] Orifice plates are reliable and require a minimum amount of maintenance. [1] Orifice plates can easily be changed to accommodate widely different flow rates. [1] Disadvantages The orifice meter has a large permanent loss of pressure. [1] [5] The higher pressure loss may be associated with higher cost. [1] 13. (a) The target meter (also called a drag plate meter), uses a flat disk or target positioned at right angle to the fluid flow. [1] The drag force exerted on the target [1] by the approaching stream, is transmitted via a force bar to a bonded strain gauge bridge (or differential pressure arrangement for pneumatic [1] output). The strain gauge converts the mechanical stress caused by the target, into an electrical signal, representing the flow rate. [1] (4) [7] (b) In the reciprocating piston flow meter shown, the piston is pushed upwards by the incoming fluid and when it reaches the top of its stroke, a slide valve opens the top piston chamber to the inlet port while the outlet is connected to the bottom chamber. The piston is now forced downwards and when it reaches the bottom of its stroke, the slide valve shifts to its initial position and the cycle repeats. [2] During each cycle, the meter dispenses a precise amount of fluid and therefore the total volume of fluid is represented by the number of cycles in a given period. [1] (3)

48 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 October 2006 Page 1 Question 1 A DP transmitter must be calibrated to measure the level of a liquid in an open tank. The density of the liquid is 1000 kg/cubic meter. The DP transmitter will be mounted one meter below the bottom of the tank. The tank is full when the height of the liquid in the tank is 5 meter and it is empty when there is only liquid in the high pressure line connected to the DP transmitter. Determine the necessary calibration specifications for an output signal of 4 to 20 ma. (4) Question 2 Describe the operation of the following level meters: 2.1 Chain float level meter. (4) 2.2 Magnetic float switch. (4) 2.3 Flexure tube displacer (torque tube) level meter. (4) Question 3 To set a temperature scale, at least two fixed points are needed. What is the distance between these two fixed points called? (1) Question 4 Define the following fixed points on the international temperature scale: 4.1 The sulphur point. (2) 4.2 The boiling point. (2) 4.3 The gold point. (2) Question Sketch and describe the operation of a bi-metal thermometer. (5) 5.2 State the thermocouple law of homogeneous circuits. (2) Question 6 Explain the following terms: 6.1 Feedback and feedforward control. (4) 6.2 Direct acting control and reverse acting control. (4) Question 7 Draw a circuit diagram of the proportional (P) block of an electronic proportional, integral and derivative controller. Also give the expression of the output of the P block, in terms of the error input signal E. (3) Question 8 A process is controlled by a proportional controller. The controller is programmed for a positive gain (reverse acting controller) and proportional band of 70%, a set point of 50% and a bias of 50%. Calculate the output of the controller when the measured value is 70%. (4) Question 9 Give an equation that describes the output C of a proportional and derivative (PD) controller in terms of the error signal E and controller gains. (3) Question 10 Define derivative control. (2) ---ooo000ooo--- Total: 50

49 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 October 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 October 2006 Memorandum Question 1 1. Empty: P 1 = P atm + hg = P atm = P atm P 1 P 2 = 9810 Pa Full: P 1 = P atm + hg = P atm = P atm P 1 P 2 = Pa 1 m 4 ma P atm Empty P 1 P atm DP cell P 2 5 m Full 1 m 20 ma P atm P 1 P atm DP cell P 2 Calibration specification: Output = 4 ma when input = 9810 Pa (empty condition) [4] Output = 20 ma when input = Pa (full condition) Question Chain Float: This type of float is linked to a rotating drum, by means of a chain. [1] The chain engages a sprocket, which turns the drum, and with it the level indicator. [1] A tape, that wraps around the drum, is also used, instead of a chain. [1] A weight is attached to the other end of the chain or tape, to keep the chain pulled straight while the float moves up or down with the changing level. [1] (4) 2.2 Magnetic Float Switch: The magnetic float switch is a point level device. When the level reaches a certain point, the float magnet activates the magnetic reed switch. [1] The electric contacts are safely isolated from the inside (wet side) [1] of the container by non-magnetic material that allows magnetic interaction between the float magnet and magnetic switch. [1] The contacts may be used to switch a pump on or off, to sound an alarm or for other control purposes. [1] (4) 2.3 Flexure Tube Displacer (torque tube) Level Meter: The displacer type liquid level measuring instrument is not a float as such, for the displacer is heavier than the process fluid and the displacer moves very little during changes in tank level (a definite advantage over other float types). [1] According to Archimedes s law, the apparent weight of the displacer when immersed in a liquid, is its nominal weight in air minus the weight of the displaced liquid. [1] The weight of the displacer will thus vary linearly from its weight in air (when the tank is empty) to its apparent weight when fully immersed in the liquid (when the tank is full). [1] The weight of the displacer acting on the torque arm, will cause an angular displacement of the free end of the flexible torque tube and this movement will be transmitted to the outside world, by the torque rod. [1] (4) [12] Question 3 3. The fundamental interval. [1] Question The sulphur point: The boiling point of pure sulphur [1] C [1] (2) 4.2 The boiling point: The boiling point of pure water [1] 100 C [1] (2) 4.3 The gold point: The melting point of gold [1] C [1] (2) [6]

50 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 October 2006 Memorandum Page 2 Question A bi-metal thermometer uses a bi-metal [½] strip, shaped in a helix [½] or spiral form, as shown in the sketch. The one end is fixed and the other end is free to rotate [½] as the helix curls in or out with changing temperature. [½] A shaft and pointer is linked to the rotating helix, to indicate the temperature. [½] The stem is filled with silicone fluid, to provide damping and thermal conductivity between the stem and bi-metal strip. [½] (5) Sketch 2 marks Pointer and scale [½] Shaft [½] 5.2 If two thermocouple junctions are at T 1 and T 2, then the thermal emf generated is independent and unaffected by any temperature distribution along the wires. (2) [7] Stem [½] Helical bi-metal element [½] Question Feedback control: Measure the controlled variable to determine the control strategy. (2) Feedforward control: Measure disturbance variables to determine the control strategy. (2) 6.2 Direct acting control: A control arrangement in which the controller output increases if the measured value rises above the set point. (2) Reverse acting control: A control arrangement in which the controller output increases if the measured value drops below the set point. (2) [8] Question 7 E R PF [1] R PI [1] Op-amp V P [3] V P = - R PF E [1] R PI Question 8 C = 100/PB(S M) + R [1] = (100/70) (50 70) + 50 [1] = 21.43% [2] [4] Question 9 C = K P E [1] + K D de/dt [1] + R [1] [3] Question 10 A control strategy in which the controller output is proportional to the derivative of the error. [2]

51 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 6 October 2006 Page 1 Question Define the secondary standard of measurement. (2) 1.2 Explain the controlling function of an instrument. (2) 1.3 Define the reproducibility of an instrument. (2) 1.4 Identify the following instrumentation symbol: (2) TRC Question A column of liquid in a vertical tank is 0,01 meter high and the liquid has a density of 1100 kg/cubic meter. The tank has an internal diameter of 1 centimetre. (a) Calculate the pressure exerted by the column of liquid, in pascal. (2) (b) Calculate the force exerted by the column of liquid, in newton. (2) 2.2 With the aid of a sketch, derive an equation that can be used to determine pressure with an inclined manometer. All the symbols that are used, must be shown on the sketch. (4) 2.3 The inclined limb of an inclined manometer, forms an angle of 30 degrees with respect to the horizontal plane. The liquid in the manometer has a relative density of 1,9. The cistern of the manometer has a diameter of 10 centimetre and the inclined tube a diameter of 1 centimetre. The reading (L) on the inclined limb is 45 centimetre. (a) Calculate the applied pressure on the manometer in kilopascal. (3) (b) Calculate the new density of the manometer liquid if the applied pressure must be increased by 500 pascal and the manometer reading must stay the same. (3) 2.4 Give the advantages and the disadvantages of transformer oil, when used as a manometer liquid. (4) 2.5 Calculate the mass of the mass pieces of the hydrostatic test balance with a piston diameter of 2 centimetre, to apply a pressure of 100 kilopascal to a pressure gauge. The mass of the platform together with the piston is 500 gram. (3) Question Define the following flow terms: volumetric flow, rate of flow, potential energy, kinetic energy and pressure energy. (10) 3.2 Sketch and describe the operation of a Doppler flow meter. (5) 3.3 Sketch and describe the operation of a rotameter used to determine the flow rate in a flow line. (6) ---ooo000ooo--- Total: 50

52 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 6 October 2006 Memorandum Page 1 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 6 October 2006 Memorandum 1.1 Secondary standards are employed in industry [1] as reference for calibrating high-accuracy equipment and components. Calibration and comparison are done periodically by the involved industries against the primary standards maintained in the national standards labs. The main function of the secondary standards is to verify the accuracy of working standards. [1] (2) 1.2 This is one of the most important functions of an instrument, especially in the field of industrial control processes. In this case, the information provided by the instrument is used by the control system to control the original measured quantity. (2) 1.3 Reproducibility is the closeness of the instrument readings when the same input [1] is applied under different conditions over a long period of time. [1] (2) 1.4 Temperature recording controller, [1] mounted on board. [1] (2) [8] 2.1 a) Pressure = gh = = Pa. (2) b) P = F/A F = PA = [ ( ) 2 /4] = mn. (2) 2.2 Equating pressures in the XY plane: P 1 = P 2 + (h+d)g [½]... (1) P and with mercury incompressible: 2 Sketch A A 1 d = A 2 L d = 2 L [½].... (2) 2 marks A P 1 1 a Also in triangle abc: L sin = h/l h = Lsin [½] h.... (3) (2) en (3) in (1): ZL A b c P 1 =P 2 +Lsin 2 L d g X Y A 1 A 1 A 2 A P 1 P 2 = Lg sin 2 [½] A (4) a) P 1 P 2 = Lg(sin + A 1 /A 2 ) [1] = (sin30 + 1/100) [1] = 4278 Pa. [1] (3) b) = (P 1 P 2 )/[Lg(sin + A 1 /A 2 )] [1] = 4778/( ) [1] = 2122 kg/m 3 [1] (3) 2.4 Advantages: Low density for measuring small pressure differences. [1] Unaffected by ammonia. [1] Can be easily seen. [1] Does not readily evaporate. [1] Disadvantages: Tends to cling to inside of tubes. [1] Density of transformer oil varies. [1] (4) 2.5 Pressure = [21] = of masspieces weight of platform and primary piston Area of primary piston Weight [½] m 9.81 (50010 ( ) - 2 ) [½] ( )( ) = 9.81m m = = m = kg [2] (3)

53 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 6 October 2006 Memorandum Page Volumetric flow is the total volume of a liquid or gas passing a given point over a certain period of time, and is measured in cubic meter (m 3 ). (2) Rate of flow is the volume of a liquid or gas passing a given point per unit time, and is measured in cubic meter per second (m 3 /s). (2) Potential energy is the energy that a liquid has by virtue of its height above a given plane. The potential energy per unit volume, equals gh joule. (2) Kinetic energy is the energy a liquid has by virtue of its motion. The kinetic energy per unit volume equals ½v 2 joule. (2) Pressure energy is the energy which a liquid has by virtue of its internal pressure. The pressure energy per unit volume equals p joule. (2) 3.2 Doppler flow meters use the well known Doppler frequency shift effect, Piezoelectric crystals to determine the flow velocity of a stream. [½] In order to operate, the Receiver Transmitter liquid must contain some small particles or bubbles. [½] Ultrasonic sound waves are transmitted at an angle into the flow, by an ultrasonic v transmitter, [½] and reflected back by the moving bubbles or particles in the stream. [½] The receiver picks up the reflected waves at a higher frequency Bubbles or solid particles than the transmission frequency [½] and the flow velocity is a function of the frequency difference between Sketch: 2 marks the received and transmitted frequencies. [½] (5) 3.3 The rotameter (also called a variable area flow meter) consists of a gradually tapered transparent tube, [½] mounted vertically in a frame with the large end up. [½] The fluid flows upward through the tube and a metal displacer or float, is suspended in the fluid. [½] The float is the indicating element and the reading is taken on the scale in line with the top of the float. [½] The position in the tube where the float reaches equilibrium, depends on the flow rate of the fluid. [½] The greater the flow rate, the further up the tube the float rises. [½] The tube is often made of high strength glass to allow for direct observation of the float position, [½] but if greater strength is required or if the liquid is very dark or dirty, a metal tube is used and the float position detected externally. [½] Scale Float (displacer) Tapered tube Sketch: 2 marks [21] Flow (6)

54 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 3 November 2006 Page 1 Question Distinguish between contact and non-contact level metering. (2) 1.2 Sketch and describe the operation of a ball float level meter. (5) 1.3 The level of a volatile liquid in a closed container, is measured with the aid of a U tube manometer, as shown in Figure 1. The maximum level of the liquid in the container is H =0.9 5 m 5 meter. The relative density of the liquid in the container is 0,9. The manometer liquid is mercury with a 0.5 m relative density of 13,6. The zero Zero line 0.25 m level of the manometer, is 0.5 meter below the bottom of the container. =13.6 Calculate the level H, of the liquid in the container, if the manometer Figure 1 reading is 0.25 meter. (4) 1.4 State the disadvantages of a bubbler system used for level measurement. (4) Question Define the following fixed points on the international temperature scale: (a) Ice point. (b) Silver point. (4) 2.2 Sketch and describe the operation of a mercury in steel thermometer as used for the measurement of temperature. (5) 2.3 Draw a labelled circuit diagram to illustrate the three wire method that is used to compensate for ambient temperature when measuring temperature with a resistance thermometer. (3) 2.4 A Wheatstone bridge is used to measure temperature with a resistance thermometer RT, as shown in Figure 2. The three fixed resistors have a resistance V 10V of 100 each, and the bridge is powered by a 10 V battery. When the temperature around the + thermometer is 0 C, the resistance of RT is RT and the bridge voltage reading is V = 0 volt. When the temperature of RT is increased, the voltmeter Figure 2 reading V, increases to 1 volt. Assume that the temperature coefficient of resistance of RT is / C and that the resistance change of RT with temperature, is described by the equation R t = R o (1 + o t). Calculate the temperature of RT when the voltmeter reading is 1 V. (5) 2.5 State the thermocouple law of intermediate metals. (2) 2.6 Give the positive element material, the negative element material and the temperature range of a type E thermocouple. (3)... / Page 2

55 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 3 November 2006 Page 2 Question Define the following concepts with respect to a proportional only control system: (a) Proportional control law. (1) (b) Proportional gain. (1) (c) Proportional band. (1) (d) Offset. (1) 3.2 Name the two kinds of delays (lags) that may be identified when a system is subjected to a step input. (2) 3.3 The set point of a proportional only control system is 50%. The behaviour of the measured M Disturbance value M, after a disturbance, is shown in 50% Figure 3. Sketch a graph of the possible 40% behaviour of the measured value, if the Time disturbance occurred while the controller s Figure 3 integral control function, was also active. (2) 3.4 The output of a proportional only controller, changes by 20%, when the error changes by 15%. Calculate the proportional band setting of the controller. (2) 3.5 Draw a circuit diagram of the integral (I) block of an electronic PID controller. Give the expression of the output of the I block in terms of the error input signal E. (3) ---ooo000ooo--- TOTAL 50

56 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 3 November 2006 Memorandum Page 1 Question A contact type device, such as a float, makes physical contact with the liquid in the container, in order to determine the level. [1] A non-contact device, such as ultrasonic or radar, does not require contact with the material in the container to measure the level. [1] (2) Level 1.2 Ball float: The float is attached to a indicator rod and rotary shaft, operating through a packing and bearing in Rotary shaft the container wall, to the level Packing and indicator as indicated in the sketch. bearing Practical considerations, limit the shaft rotation to ± 30º from the Float horizontal, and therefore the range [2] of the instrument as well. [3] (5) 1.3 P X = P Y 900(H )g g = g 900(H+0.375) = H = H = m (4) H =0.9 5 m 1.4 Disadvantages of the bubbler system: a) the liquid s density, influences measurement accuracy. [1] b) the need for compressed air or gas. [1] c) the end of the dip tube may become plugged or clogged. [1] and the dip tube must periodically be purged. [1] (4) [15] 0.5 m Zero line X = m Y Question a) The ice point: The melting point of pure ice [1] - 0 C. [1] (2) b) The silver point: The melting point of silver [1] C. [1] (2) 2.2 The mercury in steel thermometer system, allows for rugged construction and is used extensively in industrial applications. The thermometer consists of a steel bulb, a steel capillary tube and Bourdon tube, as shown in the sketch. An advantage of the mercury in steel thermometer is that measurements can be taken a distance away from the application, as the steel tube can be made fairly long and flexible. The whole system is completely filled with mercury under pressure and sealed off. When the Pointer temperature around the bulb increases, the mercury inside the bulb will expand. The effect of the mercury trying to increase its Bourdon and scale [½] volume within a confined space, will be an increase in pressure, tube [1] Steel tube [½] transmitted via the capillary tube, to the coiled Bourdon tube. The increase in mercury volume and pressure inside the coiled Bourdon tube, will result in the Bourdon tube starting to uncoil, proportional to the temperature. A pointer, linked to the free end of the Bourdon tube, will subsequently move over the scale to [2] Mercury [1] Steel bulb [½] indicate the temperature. [3] (5)

57 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 3 November 2006 Memorandum Page R 1 Wheatstone bridge (R 1, R 2, R 3 and E) Voltmeter connected to middle R lead 1 mark E R 2 V R lead R lead R lead RT R 3 3 lead wires 1 mark RTD 1 mark (3) 2.4 V BC = (100/200)10 = 5 V and V = V AB = 1 V V AC = V AB + V BC = = 6 V [1] V RT 10V 10 = 6 10RT = 6(RT+100) B A RT RT = 150 [2] V BC V AC 150 = 100( t) t = RT t = 0.5 t = C [2] C (5) 2.5 The law of intermediate metals states that a (third metal may be inserted into a thermocouple system without affecting the emf generated,) [1] (if, and only if, the junctions with the third metal are kept at the same temperature.) [1] (2) 2.6 Type Positive element Negative element Temperature range ( C) [22] E Chromel [1] Constantan [1] -200 to 800 [1] (3) Question a) Proportional control: A control strategy in which the controller output is proportional to the magnitude of the error. {C = K P E + R = (100/PB)(S-M) + R} (1) b) Proportional gain: Ratio of controller output change to error value change C/E. (1) c) Proportional band: The error range that causes 100 % change in controller output. (1) d) Offset: The steady state difference between the set point and the measured value. (1) 3.2 Dead time lag [1] and first order lag. [1] (2) 3.3 M 50% 40% Disturbance Time or M 50% 40% Disturbance Time (2) 3.4 K P = C/E = 20/15 = [1] 3.5 PB = 100/K P = 100/1.333 = 75% [1] (2) E R I [1] C I [1] V I [13] V I =- 1 [1] R C I I Edt (3)

58 Process Instrumentation I EIPIN1 Unit 1 First Assessment 16 March 2007 Page 1 Question 1 State the base SI units for mass, current and luminous intensity. (3) Question 2 Name the instrument parts specified as variable conversion elements. (2) Question 3 Define the repeatability of an instrument. (2) Question 4 Define random errors that may occur during measurement. (3) Question 5 Identify the measured variable and instrument PRC Figure 1 function, for the instrument symbol shown in Figure 1. (2) Question 6 Draw the instrument symbol for an orifice plate. (1) Question 7 Define atmospheric pressure and give the standard value (pascal). (2) Question 8 The container in Figure 2 has a cross sectional area of A meter 2 and is partially filled with a liquid of density kilogram/meter 3, to a height of h meter. Starting with the expressions for the h P volume and mass of the liquid, show that the pressure exerted by the liquid on the bottom of the container, is given by P = hg, Figure 2 A where g is the gravitational acceleration in meter/second 2. (4) Question 9 Convert a pressure of 15 kpa into a pressure expressed in meter water. (2) Question 10 P 1 P 2 A mercury (relative density of 13.6) u-tube = m manometer, is shown in Figure 3. The left hand 0.6 m leg is filled to 0.6 m and the right hand leg to 0.2 m with a liquid with relative density of 1,6. =13.6 Calculate the pressure difference, P 1 P 2, Figure 3 applied across the manometer. (3) Question 11 Draw a labelled sketch of pneumatic differential pressure transmitter. (6) Question 12 Give an expression for the gauge factor (GF) of a strain gauge. (2) Question 13 Define flow rate of a liquid and give the SI unit for flow rate. (2) Question 14 Explain the significance and importance of the Reynolds number. (2) Question 15 In Figure 4, a restricted horizontal flow line is shown. The pressure difference, p 1 - p 2, is measured h Figure 4 by taking the reading h, as shown in q p Figure 4. Use Bernoulli s theorem and 1 p 2 A 1 the principle of mass flow continuity, v A 2 1 v 2 to derive the flow equation, q = k h. (7) Question 16 Sketch the configuration, including dimensions and labels, when radius taps are used for flow rate measurement with an orifice plate. (3) Question 17 Draw a labelled sketch of a magnetic flow meter (magmeter). (4) ---ooo000ooo--- Total: 50

59 Process Instrumentation I EIPIN1 Unit 1 First Assessment 16 March 2007 Memorandum Page 1 [3] 1. Mass: kilogram [1] Current: ampere [1] Luminous intensity: candela [1] [2] 2. Primary element (or bulb) [1] Secondary element (or Bourdon tube) [1] 3. Repeatability is the closeness [½] of the instrument readings when the same input [½] is [2] applied (repetitively over a short period of time) [½] with the same conditions. [½] 4. Random errors occur because of unknown and unpredictable variations [1] that exist in all measurement situations. This results in slightly different values [1] obtained for each [3] repeated measurement (scattered evenly about the mean value) of the same [1] input. [2] 5. Pressure [1] Recording controller [1] PRC [1] Atmospheric pressure is the absolute pressure caused by the weight of the earth s [2] atmosphere. [1] Standard atmospheric pressure at sea level is pascal. [1] 8. Volume of the liquid = V = Ah. [1] Mass of the liquid = m = Ah. [1] h P Weight of the liquid = w = mg = (Ah)g. [1] Pressure on the bottom of container due to weight of the liquid: A [4] P = wa = Ahg/A = hg [1] P 1 P 2 [2] 9. P = hg = 1000h 9.81 h = m [2] = m 10. P = P [3] P = P P 1 -P 2 = = Pa [3] (=47.09kPa) = Restriction [½] Air supply [½] 15. Pilot relay [½] h Nozzle [½] Flapper [½] Pivot point (range wheel adjust) [½] Range bar [½] Output P o [½] Feedback bellows [½] Zero adjust [½] Pivot and seal Force bar [½] [6] Low pressure (P 2 ) High pressure (P 1 ) /8 input [½] input [½] [2] 12. GF = (R/R)/(/) = (R/R)/, 13. Flowrate is the volume of a liquid or gas passing a given point per unit time, [1] and is measured [2] in cubic meter per second (m 3 /s). 14. For R e under 2000 the flow is streamlined [1] while at [7] [2] R e over 3000, the flow becomes fully turbulent. [1] High pressure tap [½] Flow [½] [½] Diaphragm capsule [½] D [½] ½D [½] Cross flexure [¼] Capsule flexure [¼] Low pressure tap [½] D [½] q Flow v [1] p 1 p 2 A 1 A 2 v 1 From Bernoulli s law: ½v 1 2 +p 1 =½v p 2 [1]... (a) Flow continuity demands: A 1 v 1 = A 2 v 2 v 1 = (A 2 /A 1 )v 2 [1].... (b) (b) in (a): ½(A 2 /A 1 )v 2 2 +p 1 =½v 2 2 +p 2 v 2 2 = 2(p 1 -p 2 )/[1-(A 2 /A 1 ) 2 ] 2 v 2 = 2(p1 p 2 )/ [1- (A 2/A1) ] [1].. (c) But p 1 p 2 = hg [1] (d) 2 (d) in (c): v 2 = 2gh/[1- (A /A ) ] [1].(e) Also q=a 2 v 2 [1]..... (f) From (f) and (e): 2 q = A 2 2gh/[1- (A 2 /A1) ]... (g) [note: 1 mark for either simplified v 2 in Eq (e) or for simplified q in Eq (g)] 2 Defining k=a 2 2g/[1 (A 2/A1) ] in Equation (g): q = k h. B [1] D [1] 2 v 2 1 [1] [3] [4] /5 Magnet coils [1] Electrodes [1]

60 Process Instrumentation I EIPIN1 Unit 2 First Assessment 4 May 2007 Page 1 Question 1 Draw a labelled sketch of a flexure tube (torque tube) displacer level meter. (5) Question 2 The level of a volatile liquid in a closed container, is measured with the aid of a U tube manometer, as shown in Figure 1. The maximum level of the liquid H =0.8 5 m in the container is 5 meter. The relative density of the liquid in the container is 0,8. The manometer liquid is mercury with a relative density of 13,6. The zero 0.5 m level of the manometer, is 0.5 meter below the Zero line 0.2 m bottom of the container. Calculate the level H, of =13.6 the liquid in the container, if the manometer reading Figure 1 is 0.2 meter. (4) Question 3 Convert 50 degrees Fahrenheit to Rankine, degrees Celsius and Kelvin. (4) Question 4 Define the silver point and gold point on the international temperature scale. (2) Question 5 Draw a labelled sketch of a vapour pressure thermometer. (4) Question 6 A resistor thermometer measures 100 at 0 C, and 145 at 100 C. Assuming that the relationship between resistance and temperature is described by the linear equation, R t = R 0 (1 + 0 t), calculate: a) The resistance of the thermometer at 150 C. (4) b) The temperature when the resistance of the thermometer is 200. (2) Question 7 State the thermocouple law of intermediate metals. (2) Question 8 Give the positive element material, the negative element material and the temperature range of a type S thermocouple. (3) Question 9 A water level control system that aims to keep the water level in the container half full (50%) by regulating the inflow to compensate for changes in the outflow, is shown in Figure 2. Explain the following concepts regarding the system in Figure 2: Inflow valve Inflow Level a) Controlled variable. (1) detector b) Manipulated variable. (1) and Container c) Disturbance variable. (1) controller d) Desired value. (1) Water inlet Outflow e) Measured value. (1) f) Error value. (1) Figure 2 Question 10 Define feedback and feedforward control. (2) Question 11 Sketch a graph of the output C (in %) versus the input error E (in %), for a reverse acting proportional controller with proportional band of 50% and bias of 50%. (2) Question 12 A digital PI controller is programmed with a proportional gain K P = 1, an integral gain K I = 0.1, a set point S = 50% and a sampling period of T = 1 second. The controller obtains the following sequence of samples for the measured variable M, M(0) = 40%, M(1) = 30%, M(2) = 20% and M(3) = 30%. a) Calculate the corresponding error values E(0), E(1), E(2) and E(3). (2) b) Calculate the proportional part (P) of the controller output after M(3) was sampled. (1) c) Calculate the integral part (I) of the controller output after M(3) was sampled. (2) Question 13 Name the types of valves, that use a linear movement to operate. (5) ---ooo000ooo--- Total: 50

61 [5] /6 Process Instrumentation I EIPIN1 Unit 2 First Assessment 4 May 2007 Memorandum Page Torque tube [1] Torque tube flange [½] Level indicator [½] Torque rod [1] Torque arm [1] Chain [1] Displacer [1] [4] H P X = P Y 3. C = 5/9[F 32] F = 5/9 [50 32] = 5/9 18 = 10 C [2] [4] R = F = = 510 R [1] K = C = = 283 K [1] = m Zero line X = m 0.2m 800(H )g g = g [1] 800(H+0.4) = H = H = 1440 H = 1.8 m [3] [2] 4. Silver: melting point of silver [½] C [½] Gold: melting point of gold [½] C [½] a) o = (R 100 -R 0 )/100R 0 = ( )/ = / C [2] Bourdon tube [1] Pointer and scale [½] Steel tube [½] Vapour [1] Steel bulb [½] [6] 8. R T = R o (1 + o t) = 100( ) = [2] (4) b) R T = R o (1 + o t) 200 = 100( t) t = 1 t = C (2) 7. The law of intermediate metals states that a (third metal may be inserted into a thermocouple system without affecting the emf generated,) [1] (if, and only if, the junctions [2] with the third metal are kept at the same temperature.) [1] Type Positive Negative Temperature element element range ( C) 90% Platinum 10% Rhodium [1] Platinum [1] 0 to 1500 [1] Volatile [3] S [4] liquid [1] /4½ 9. a) Controlled variable: Water level (1) b) Manipulated variable: Water inflow (1) [6] c) Disturbance variable: Water outflow (1) d) Desired value: Level = 50% (1) e) Measured value: Current water level (1) f) Error value: Desired val. measured val. (1) [2] [2] 10. Feedback control: Measure the controlled variable to determine the control strategy. [1] Feedforward control: Measure disturbance variables to determine the control strategy. [1] 11. [½] 100% -25% [½] 0% C (%) 25% [½] [½] R=50% E(%) 12. a) E(0) = S M(0) = = 10% [½] E(1) = S M(1) = = 20% [½] E(2) = S M(2) = = 30% [½] E(3) = S M(3) = = 20% [½] (2) b) P = K P E(3) = 1 20 = 20% E (1) c) I = K I [TE(0) + TE(1) + TE(2)] [1] 30 M(2) 20 M(1) M(3) = 0.1 [ ] 10 M(0) = = 6% [1] t [5] (2) Y [5] 13. Globe valve [1] Gate valve [1] Needle valve [1] Pinch valve [1] Diaphragm valve [1]

62 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 20 April 2007 Page 1 Question 1 Define measurement of a process variable. (2) Question 2 State the basic functions of an instrument. (3) Question 3 Define the sensitivity of an instrument. (2) Question 4 Define the error of hysteresis in an instrument. (3) Question 5 Identify the instrument signal shown in Figure 1. (1) Figure 1 Question 6 Define the density of a substance and give the SI unit for density. (2) Question 7 With the aid of a labelled sketch, derive the expression that is used to determine pressure with a well type manometer. (4) Question 8 Name three manometer liquids and give their respective relative densities. (3) Question 9 Draw a labelled sketch of a C-type Bourdon tube pressure gauge. (4) Question 10 Draw a labelled sketch to show how a bellows element may be used to measure gauge pressure. (2) Question 11 A dead weight tester has a primary piston with a diameter of 1 cm. The mass of the platform and primary piston together, is 500 gram. Calculate the mass m, of the mass pieces, that must be placed on the platform to check a gauge at 100 kpa. (3) Question 12 Define viscosity of a liquid and give the SI unit for viscosity. (2) Question 13 State Bernoulli s law. (3) Question 14 Draw a labelled sketch of a Pitot tube flow meter and give the equation for flow velocity when using a Pitot tube. (4) Question 15 A cylindrical object, suspended in a horizontal flow stream, has a cross sectional area of m 2, which is positioned perpendicular to the flow. If the object experiences an upstream pressure of 500 Pa and a downstream pressure of 300 Pa, calculate the force exerted on the object, by the flow. (3) Question 16 Name the three main types of orifice plates used. (3) Question 17 a) Draw a labelled sketch of an electronic target flow meter. (4) b) Give the operational equation that is used to calculate the flow rate q, from the measurements made with a target flow meter. (2) ---ooo000ooo--- Total: 50

63 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 20 April 2007 Memorandum Page 1 1. Measurement is defined as the determination of the existence [1] or magnitude [1] of a variable for [2] monitoring and controlling purposes. [3] 2. Indicating function [1] Recording function [1] Controlling function [1] [2] 3. Sensitivity is the rate of change [1] of the output [½] of a system with respect to input [½] changes. 4. Hysteresis is the difference [1] between the readings obtained when a (given value of the measured [3] variable is approached from below) [1] and when the (same value is approached from above.) [1] [1] 5. Pneumatic signal. 6. Density of a substance is defined as the mass of a unit volume [1] of a substance. [2] The SI unit is kilogram per cubic meter (kg/m 3 ). [1] 7. P 1 = P 2 + (h+d)g [1].. (1) 8. Sketch: P 1 A 2 A 1 d = A 2 h d = h [1] 1 mark.. (2) A 1 h (2) in (1): A ZL P 1 = P 2 + h 2 h g [1] d A 1 [3] [4] P 1 P 2 = hg(1 + A 2 /A 1 ) A 1 A 2 /6 [4] /5 9. Pointer and scale [½] Bourdon tube [1] Adjustable link [½] Range adjust [½] Pivot point [½] Sector gear [½] Pinion gear [½] Hairspring [½] Pressure connection [½] 10. P 2 Bellows [½] P 1 (Gauge pressure) [½] Transformer oil [½] = [½] Aniline [½] = [½] Dibutylphathalate [½] = [½] Carbon Tetrachloride [½] = [½] Tetrabromoethane [½] = [½] Bromoform [½] = 2.9 [½] Mercury [½] = 13.6 [½] Atmospheric pressure [½] [2] Pressure indication [½] 11. P = Weight masspcs, platfrm & prim. piston Area of primary piston - 3 m 9.81 ( ) [1] = ( /4) (110 ( )( ) = 9.81m m = m = m = kg. [2] = gm [3] [2] 12. Viscosity is a measure of a fluid's resistance to flow. [1] SI unit: poiseuille (PI). [1]. 13. If an (incompressible fluid is in a streamlined flow with no friction,) [½] the sum [½] of the [3] pressure energy, [½] the kinetic energy [½] and potential energy [½] per unit volume, is constant [½]. 14. Static pressure [1] Stagnation pressure [1] 17. a) v [½] Electronics housing [½] ) Strain gauge [1] [4] /4½ Impact hole [1] v = 2(p stag p stat ) [3] 15. F = PA = ( ) ( ) [1] = 0.8 N [2] 16. Concentric, [1] eccentric [1] & [6] [3] segmental [1] orifice plates. [1] b) q = Flow [½] 2 2 (D d ) 4 Force bar [1] Target [1] 8F 2 d Pivot and seal [1] (4) /5 (2)

64 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 25 May 2007 Page 1 Question 1 Draw a labelled sketch of a chain float level meter. (4) Question 2 During calibration of a Flexure tube displacer (torque tube) displacer type level meter, it was found that the torque registered by the meter was 30 N-m, when the tank was empty. If the length of the torque arm is 0.15 m, calculate the torque that will be measured when the displacer, with a volume of m 3, is fully immersed in the process fluid with density 1000 kg/m 3. (4) Question 3 Define temperature and give the SI unit for temperature. (2) Question 4 Convert 537 Rankine to Kelvin. (4) Question 5 Draw a labelled sketch of a liquid in glass thermometer. (4) Question 6 A platinum resistance thermometer has a resistance of 100 at 0 C and a resistance of at 100 C. Calculate the temperature coefficient of resistance (TCR) of the thermometer. (2) Question 7 Draw a labelled circuit diagram to illustrate the four wire method that is used to compensate for ambient temperatures when measuring temperature with a resistance thermometer. (3) Question 8 Define the Seebeck effect. (3) Question 9 In Figure 1, a type K thermocouple is shown, which is used to measure an unknown temperature T. The voltmeter reading is 2390 microvolt, and the temperature of the thermocouple cold junction is 24 C. An excerpt from the type K thermoelectric voltage table (in microvolt) for temperatures in degrees Celsius, is given in Table 1. Use Table 1 to determine the temperature T of the hot junction. (3) 24 C 2390 V T Figure 1 Question 10 A heat exchanger that uses steam to heat cold water and aims to deliver hot water at a fixed temperature of 60 C, is shown in Figure 2. Identify or explain the following control variables and values, with reference to the system in Figure 2: Table 1 C Steam supply Steam control valve Cold water Steam outlet TIC Thermometer Hot water Figure 2 (For example: Controlled variable: Temperature of hot water delivered) a) Manipulated variable. (1) b) Disturbance variable. (1) c) Desired value. (1) d) Measured value. (1) e) Error value. (1) Question 11 Define open loop and closed loop control systems. (2) Question 12 Define integral control reset time. (2) Question 13 Draw a labelled sketch of a pneumatic proportional plus derivative controller. (4) Question 14 Explain the valve term: throttling. (2) Question 15 Draw a labelled sketch of a reverse acting pneumatic control globe valve. (6) ---ooo000ooo--- Total: 50

65 [4] /5 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 25 May 2007 Memorandum Page Temperature is defined as the degree of heat of a body. [1] The SI unit is Kelvin (K). [1] [4] [2] R = 100 R [1] 7. Scale R (etched) [1] = [2] = / C [1] Bore [1] [4] /5 [5] [2] 10. a) Manipulated variable:steam flow. (1) 15. b) Disturb. variable:hot water demand (steam pressure., ambient temp.). (1) c) Desired val.:required temp.-60 C. (1) d) Meas. value:thermometer reading. (1) e) Error value:difference between req. temp. and thermometer reading. (1) 11. Open loop system: The input to the system is not determined by the output. [1] Closed loop system: The input to the system is determined by the output. [1] 12. Reset or repeat time:time taken [¼] for the integral control action [½] to equal [¼] the proportional control action [½] under [2] the influence of a constant error. [½] 13. Set point S [½] [4] /5½ Lens front capillary tube (stem) [1] Liquid column [1] Bulb [1] Proportional (feedback) bellows [½] Beam [½] Reset bellows [½] Measured value M [½] Level indicator [1] Drum [1] Chain [1] Weight [1] Float [1] [3] [3] d 8. Seebeck effect: If two dissimilar metals [1] are joined together to form a closed loop, and if one junction is kept at a different temperature [1] from the other, an electromotive force [1] is generated and electric current will flow in the closed loop. Flapper and nozzle [½] Bias value R [½] [4] 4. R = F+460 F = R 460 = = 77 F [1] C = 5/9(F 32) = 9/5(77 32) = 25 C [2] K = C = = 298 K [1] Needle valve rate adj. [½] Pilot relay [½] Current source I 1 mark I Voltmeter return across RT - 1 mark Controller output C [½] Air supply [½] M a b c R lead R lead 4 leadwires plus RTD 1 mark R lead R lead [3] C 960 V [1] E T = = 3350 V [1] T = 82 C [1] [6] /8 2. Weight of displacer in empty tank: T = Fr F = T/r = 30/0.15 = 200 N [1] Weight of displacer in full tank = Weight of displacer in empty tank Weight loss of displacer in full tank = = = N [2] Torque measured in full tankt full = = N-m [1] [2] Plug [½] Seat [½] RT Spring nut [½] Spring [½] Diaphragm plate [½] Diaphragm [½] Actuator [½] Stem connector [½] Travel indicator [½] Stem [½] Yoke [½] Gland and packing [½] Bonnet nut [½] Bonnet [½] Gasket [½] Valve body [½] 14. Throttling occurs when the valve stem position is between closed and open (0 > x > 1) and the valve is busy regulating the flow stream.

66 Process Instrumentation I EIPIN1 Unit 1 First Assessment 02 September 2011 Page 1 Question 1 Give the SI units for time and amount of substance. (2) Question 2 Explain the recording function of an instrument. (2) Question 3 Define the error of drift that may occur in an instrument. (2) Question 4 State the elements that may be identified in an instrument. (5) Question 5 Define pressure and give the SI unit for pressure. (2) Question 6 Assuming that the density of the atmosphere is a constant value of 1.2 kg/m 3 and that the atmospheric pressure at sea level is 760 mm. mercury, calculate the height of the atmosphere above sea level. (3) Question 7 You are requested to design a scale plate for a U-tube manometer that uses mercury, with relative density of 13.6, as manometer liquid. You are told that the maximum differential pressure to be measured, will be 100 kpa. From the zero line upward, the following values must be marked off on the scale plate: 25 kpa, 50 kpa, 75 kpa and 100 kpa. Calculate the distances from the zero line to each marking on the scale, and sketch the designed plate. (4) Question 8 Draw labelled sketches to show how a bellows element may be used to measure: a) differential pressure (2) b) absolute pressure. (2) Question 9 Draw a labelled sketch of a pneumatic differential pressure transmitter. (6) Question 10 Define flow rate of a fluid and give the SI unit for flow rate. (2) Question 11 State Bernoulli s law (in words). (2) Question 12 State three methods of positioning the high pressure and low pressure tap-points, that may be used when measuring flow rate with orifice plates. (3) Question 13 Water flows through a horizontal pipe with cross sectional area of m 2. A circular object, facing the stream with an area of m 2, is placed in the flow, as shown in Figure 1. The force on the object is measured as 0.5 newton. Calculate the flow rate q of the water, q 2(p p ) 1 2 if the flow rate is given by: q = A 2, 1 (A /A ) Pipe area = m 2 p 1 p 2 F=0.5 N Object area = m 2 where p 1 -p 2 is the pressure difference across the object, Figure 1 A 2 is the restricted flow area, A 1 is the unrestricted flow area (pipe area) and = 1000 kg/m 3 (for water). (3) Question 14 Sketch the configuration, including dimensions and labels, when flange taps are used for flow rate measurement with an orifice plate. (2) Question 15 a) Draw a labelled sketch of a rotameter (variable area flowmeter). (4) b) Why does the displacer in a rotameter move upwards with increasing flow rate? (1) Question 16 a)the Reynolds number for a flow condition is determined as Is the flow streamlined or turbulent? (1) b) Explain the function of a flow straightener (straightening vane) in a pipe. (2) ---ooo000ooo--- Total: 50

67 Process Instrumentation I EIPIN1 Unit 1 First Assessment 02 September 2011 Memorandum Page 1 [2] 1. Time: second [1] Amount of substance: mole [1] [2] 2. An instrument may provide the information of the value of a quantity under measurement against time [1] or some other variable, in the form of a written record, [1] usually on paper. 3. Drift is the change in instrument indication over time [1] while the input and ambient conditions [2] are constant. [1] 4. Primary element [1] Data transmission element [1] Secondary element [1] Manipulation element [1] [5] and Functioning element [1] 5. Pressure is defined as the force exerted over a unit area [1]. The SI unit is newton per square meter [2] (N/m 2 ) or pascal (Pa). [1] 375mm [½] h atm g 1 = g 1 h atm = 8613 m 1 [3] 281.3mm [½] 75 kpa 7. P 1 P 2 = hg. for P 1 -P 2 =100 kpa: = 13600h9.81 h = 750 mm. [1] Distance from zero line to 100 kpa marking = 375 mm. [1] 187.5mm [½] 93.74mm [½] [4] Zero line 8. a) Differential Low pressure b) Absolute Bellows [½] (P 2 ) [½] Bellows [½] Vacuum [½] Bellows [½] (2) High pressure (2) High pressure (2) 50 kpa 25 kpa 0 kpa (P 1 ) [½] (P 1 ) [½] [4] 9. (P 1 ) [½] Restriction [½] Air supply [½] Pilot relay [½] Nozzle [½] Flapper [½] Pivot point (range wheel adjust) [½] Range bar [½] Output P o [½] Feedback bellows [½] Zero adjust [½] Force bar [½] Pivot and seal [½] Pressure indication [½] Cross flexure [1/2] Pressure indication [½] [6] Diaphragm capsule [½] Low pressure (P 2 ) input [½] Capsule flexure [1/2] High pressure (P 1 ) input [½]

68 Process Instrumentation I EIPIN1 Unit 1 First Assessment 02 September 2011 Memorandum Page Flowrate is the volume of a liquid or gas passing a given point per unit time, [1] and is measured [2] in cubic meter per second (m 3 /s). [1] 11. If an (incompressible fluid is in a streamlined flow with no friction,) [1] the sum [½] of the [2] pressure energy, [½] the kinetic energy [½] and potential energy [½] per unit volume [½], is constant ½]. 12. Corner taps [1] Flange taps [1] Radius taps (D&D/2 or throat taps) [1] [3]/5 Vena-contracta taps [1] Pipe taps [1] 13. p 1 p 2 = F/A object = 0.5/210-3 = 250 Pa. [1] and A 2 = [1] = 310 2(p p ) q = A = ( ) 2 1 (A /A ) (3 10 / 5 10 ) 2 1 [3] = ( ) 2 =( ) =(310-3 )0.8839= m 3 /s [1] (0.6) mm [½] 25mm [½] 15.(a) Flow [½] High pressure tap [½] Low pressure tap [½] Scale [1] [2] / 2½ 15.(b) When the flow rate increases, the pressure difference across the float will increase, [½] which will tend to push the float upwards. As the float moves upwards, the restricted flow area will increase [½] due to the tapered tube. This will allow the pressure difference to decrease to its original value where the float [5] will remain suspended in its new position. (1) Flow [1] Float (displacer) [1] Tapered tube [1] 16.a)Turbulent. 1 [2]b)To streamline a flow 1, if the flow is turbulent for measurement purposes. 1

69 Process Instrumentation I EIPIN1 Unit 2 First Assessment 14 October 2011 Page 1 Question 1Distinguish between direct and indirect level measurement, and give one example for each method. (4) Question 2Draw a labelled sketch of a magnetic float level meter (magnetic coupled float and follower). (4) Question 3a) Make a labelled sketch of a gas filled thermometer. (4) b) State which gas is normally used in a gas filled thermometer. (1) Question 4State the most frequently used method of level measurement. (2) Question 5The level of a liquid in an open container, is measured with the aid of a well type manometer, Figure 1 as shown in Figure 1. The ratio of the tube area to the well area is 0.01 (A 2 /A 1 = 0.01). The relative density of the liquid in the container is 1 (=1) and the manometer liquid is mercury with a relative density H = 1 of 13.6 (=13.6). The zero level of the manometer liquid, 1 m is 1 meter below the bottom of the container. Calculate the h=0.3m level H, of the liquid in the container, if Zero line the manometer reading h, is 0.3 meter. (4) = 13.6 Question 6Name three common metals used in resistance thermometers. (3) Question 7Give the positive element material, the negative element material and the temperature range of a type T thermocouple. (3) Question 8 Convert 40 C to degrees Fahrenheit, Kelvin and Rankine. (3) Question 9 Sketch and describe the operation of mercury in steel thermometer as used for the measurement of temperature. (5) Question 10 Define dead time (transportation lag) in a control system. (2) Question 11Draw a fully labelled block diagram of a feedback control system. (5) Question 12 Define integral control reset time (2) Question 13 Draw a labelled sketch of a pneumatic proportional plus integral plus derivative controller. (5) Question14 Draw the graphs for the inherent valve characteristics for a quick opening valve, a linear valve and an equal percentage valve. (3) ---ooo000ooo--- Total: 50

70 [4] Process Instrumentation I EIPIN1 Unit 2 First Assessment 14 October 2011 Memorandum Page 1 1. Direct acting method involves measuring the height of the fluid directly. [1] can be dipstick, overflow pipe, float or sight glass [1] Indirect method, another variable is measured that correlate to the liquid level [1] it can be measuring the weight of a substance in the container, pressure exerted on the bottom of the container or transmitting the ultrasonic beam to the level surface [1] 2. Level indicator [1] 3. (a) Indicator rod [1] Non-magnetic dip tube [1] Doughnut float with outer magnet [1] Follower with inner magnet [1] [1] Bourdon tube [1] [4]/5 (b) Nitrogen gas. [1] Pointer and scale [1] [1] Steel tube Gas [1] (Nitrogen) Steel bulb [1] [4] /5 Level indicator [1] 4. Pressure measurement method [2] [2] 5. d=(a 2 /A 1 )h=0.003m Ignore d and equate pressures on the zero line P A (H+1) g = P A g [1] H = 3.08 m [3] Ignore d, calculate P 1 on zero line and use well type manometer equation P 1 = P atm (H + 1)g and P 2 = P atm P 1 P 2 =1000(H+1)g & P 1 -P 2 =hg(1+a 2 /A 1 ): 1000 (H+1) g = g(1+0.01) [1] H= m [3] H ZL P A = 1 1 m Include d and equate pressures in line with mercury meniscus in well: P A +1000(H )g = P A ( )g [1] H = Include d and use well type manometer equation: [4] P 1 -P 2 =1000(H+1.003)g=13600(0.3)g(1+0.01) [1] H = m P 1 P 2 P A h=0.3m = Platinum, [1] Copper [1] and Nickel [1] [3] 7. Type Positive element Negative element Temperature range ( C) [3] T Copper [1] Constantan [1] -200 to 350 [1] F= C+32= 40+32=72+32=104 F [1] [3] 5 5 R=F+460= =564 R [1] K=C+273=40+273=313 K [1]

71 [5] [5] /6½ Process Instrumentation I EIPIN1 Unit 2 First Assessment 14 October 2011 Memorandum Page 2 9. The mercury in steel thermometer system, allows for rugged construction and is used extensively in industrial applications. The thermometer consists of a steel bulb, a steel capillary tube and Bourdon tube, as shown in the sketch. An advantage of the mercury in steel thermometer is that measurements can be taken a distance away from the application, as the steel tube can be made fairly long and flexible. The whole system is completely filled with mercury under pressure and sealed off. When the temperature around the bulb increases, the mercury inside the bulb will expand. The effect of the mercury trying to increase its volume within a confined space, will be an increase in pressure, transmitted via the capillary tube, to the coiled Bourdon tube. The increase in mercury volume and pressure inside the coiled Bourdon tube, will result in the Bourdon tube starting to uncoil, proportional to the temperature. A pointer, linked to the free end of the Bourdon tube, will subsequently move over the scale to indicate the temperature. [3] 10. Delay due to the time it takes information or material to be transported [1] from one point to another. [1] [2] Disturbance 11. Comparator [1] variables [½] [5] 12. Reset or repeat time: Time taken [¼] for the integral control action [½] to equal [¼] [2] the proportional control action [½] under the influence of a constant error. [½] 13. Desired Error Manipulated value [½] value [½] Control variable [½] Process [1] unit [1] Measured value [½] Set point S [½] Proportional (feedback) Beam [½] bellows [½] Reset bellows [½] Measured value M [½] Sensor [1] Needle valve rate time adjustment [½] Flapper and nozzle [½] Automatic reset R [½] Needle valve [½] Reset time adjust Pilot relay [½] Bourdon tube [1] Restriction [½] 14. [2] Controlled variable [½} f(x) 1 Controller output C [½] Air supply [½] Quick [1] Pointer and scale [½] Steel tube [½] Mercury [1] Steel bulb [½] Output [½] Linear [1] [3] 0 =% [1] x 0 1

72 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 23 September 2011 Page 1 Question 1 Define measurement of a process variable. (2) Question 2 State the basic functions of an instrument. (3) Question 3 Define the sensitivity of an instrument. (2) Question 4 Define the error of hysteresis in an instrument. (3) Question 5 Identify the instrument signal shown in Figure 1. (1) Figure 1 Question 6 Define the density of a substance and give the SI unit for density. (2) Question 7 With the aid of a labelled sketch, derive the expression that is used to determine pressure with a well type manometer. (4) Question 8 The reading h on a well type mercury ( = 13.6) manometer is 1 meter, when measuring a pressure of 135 kpa. a) Calculate the ratio (A 2 /A 1 ) of the tube area (A 2 ) to the well area (A 1 ). (3) b) Determine the change in level (d) that the well mercury experiences. (2) Question 9 Draw a labeled sketch of a C-type Bourdon tube pressure gauge. (4) Question 10 A dead weight tester has a primary piston with a diameter of 1 cm. The mass of the platform and primary piston together, is 500 gram. Calculate the mass m, of the mass pieces, that must be placed on the platform to check a gauge at 100 kpa. (3) Question 11 Define viscosity of a liquid and give the SI unit for viscosity. (2) Question 12 Define a streamlined flow and a turbulent flow of a stream. (4) Question 13 Draw a labelled sketch of a Pitot tube flow meter and give the equation for flow velocity when using a Pitot tube. (4) Question 14 (a) Explain the purpose of a vent hole. (1) (b) Explain the purpose of a drain hole. (1) Question 15 Name the three main types of orifice plates used. (3) Question 16 a) Draw a labelled sketch of an electronic target flow meter. (4) b) Give the operational equation that is used to calculate the flow rate q, from the measurements made with a target flow meter. (2) ---ooo000ooo--- Total: 50

73 [2] [3] [2] [3] [1] [2] Process Instrumentation I EIPIN1 Unit 1 Final Assessment 23 September 2011 Memorandum Page 1 1. Measurement is defined as the determination of the existence [1] or magnitude [1] of a variable for monitoring and controlling purposes. 2. Indicating function [1] Recording function [1] Controlling function [1] 3. Sensitivity is the rate of change [1] of the output [½] of a system with respect to input [½] changes. 4. Hysteresis is the difference [1] between the readings obtained when a (given value of the measured variable is approached from below) [1] and when the (same value is approached from above.) [1] 5. Pneumatic signal. 6. Density of a substance is defined as the mass of a unit volume [1] of a substance. The SI unit is kilogram per cubic meter (kg/m 3 ). [1] 7. P 1 = P 2 + (h+d)g [1/2].. (1) A A 1 d = A 2 h d = 2 h [/21].. (2) A 1 (2) in (1): A P 1 = P 2 + h 2 h g [1]...(3) A 1 [4] P 1 P 2 = hg(1 + A 2 /A 1 ) [1]...(4) 8. a) P 1 P 2 = hg(1 + A 2 /A 1 ) [1] = [1 + (A 2 /A 1 )] = [1 + (A 2 /A 1 )] 1+(A 2 /A 1 ) = / = (A 2 /A 1 ) = [2] b) d = (A 2 /A 1 )h [1] d = [5] = m = 11.9 mm. [1] d A 1 P 1 h P 2 ZL A 2 Sketch: 1 mark 9. Pointer and scale [½] [4] /5 Bourdon tube [1] Adjustable link [½] Range adjust [½] Pivot point [½] Sector gear [½] Pinion gear [½] Hairspring [½] Pressure connection [½] [2] 11. Viscosity is a measure of a fluid's resistance to flow. [1] SI unit: poiseuille (PI). [1]. 12. Streamlined flow: In a streamlined flow, all the particles in the liquid, flow in the same direction and parallel to the walls of the pipe, and the streamlines are smooth. [2] Turbulent flow: In a turbulent flow, the particles in the stream, flow axially as well as [4] radially, and the streamlines are in a chaotic pattern of ever changing swirls and eddies. [2] 10. [3] P = Weight masspcs, platfrm & prim. piston Area of primary piston - 3 m 9.81 ( ) [1] = ( /4) (110 ( )( ) = 9.81m m = m = m = kg. [2] = gm )

74 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 23 September 2011 Memorandum Page Static pressure [1] Stagnation pressure [1] 16. a) Electronics housing [½] Strain gauge [1] v [½] [4] /4½ Impact hole [1] v = 2(p stag p stat ) ρ [1] Flow [½] Pivot and seal [1] Target [1] Force bar [1] (4) /5 [2] [3] b) q = [6] 14. a) Vent holes are provided to prevent (gasses when transporting liquids) [1/2] to accumulate at the top [1/2] the pipe on the upstream side of the orifice plate. b) Drain holes are provided to prevent (solid particles in liquids) and (condensate in gasses) [1/2] to accumulate at the bottom [½] of the pipe on the upstream side of the orifice plate. 15. Concentric, [1] eccentric [1] & segmental [1] orifice plates. 2 2 π(d d ) 4 8F 2 d (2)

75 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 28 October 2011 Page 1 Question 1 Give three types of level indicators which can be used with the float in level measurement. (3) Question 2 Mention the advantages and disadvantages of using the differential pressure method to measure level in a container. (4) Question 3 Explain the term wet leg system, used in a closed container to determine level. (2) Question 4 Differential pressure level measurement in a closed vessel is fundamentally different from level measurement in an open vessel. Mention those differences. (4) Question 5 Draw a labelled sketch of a bubble meter (gas flushing system) for level measurement (using a manometer) in an open container. (4) Question 6 When designing a temperature measuring device, the two fixed points must be defined. Mention and explain those two fixed points. (4) Question 7 The resistance thermometers are based on what principle. (2) Question 8 The thermocouples are used to measure temperature. Name the three laws of thermocouples. (3) Question 9 The following equation is provided for a type K thermocouple, to calculate temperature (t in C) from emf (v in μv) in the temperature range 0 C to 500 C. Use the equation below to calculate the temperature when the emf is 2602 μv t = ( )v + ( )v 2 ( )v 3 + ( )v 4 ( )v 5 + ( )v 6 ( )v 7 + ( )v 8 ( )v 9 (4) Question 10 Mention the main advantage of using compensating leads with thermocouples to measure temperature. (2) Question 11 There are three main values associated with the controlled variable in process control. Mention them. (3) Question 12 With examples, show how direct acting and reverse acting can be used in process control. (4) Question 13 Most processes exhibit time delays between their input and output. Mention and explain first order lag in a process. (2) Question 14 Explain the following terms in process control, when using proportional control: Bias, Proportional gain and Proportional band. (3) Question 15 Draw a circuit diagram of the derivative (D) block of an electronic proportional, integral and derivative controller. Give the expression of the output of the D block, in terms of the error input signal E (4) Question 16 What is the main function of the control valves as used in control systems. (2) ---ooo000ooo--- Total: 50

76 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 28 October 2011 Memorandum Page 1 1 Any three Chain float, Ball float [1], Magnet float(magnetic coupled float and follower) [1], [3] Magnetic float switch [1] and Flexure tube level meter [1] 2. Advantages: (a) The equipment measuring the differential pressure, can be externally installed or retrofitted to an existing vessel [1]. (b) It can be also be isolated safely from the process using block valves for maintenance and testing [1]. Disadvantages: a) Leak paths could be the cause of many problems [1] b) Measurement errors occur due to changes or change of product, these variations must [4] be compensated for, to maintain accurate measurements [1] 3. Wet system: when the outer leg [1] is allowed to be completely filled with the same process fluid [2] as in the tank. [1] [4] 4. With the closed vessel the high pressure will be connected to the outer leg [1], but with the open vessel the high pressure is at the bottom of the container [1] With the closed vessel the differential pressure is zero when the tank is full [1/2] and maximum when the tank is empty [1/2], but with open tank the pressure is maximum when the tank is full [1/2] and zero when the tank is empty [1/2] 5. Bubbler sight glass [1] Filter [1] Pressure regulator [1] Air/gas supply [1] [4] Dip tube [1] [4] [2] [3] [4] [2] 6. Top fixed point: The temperature of (distilled water that boils) [1] at (standard atmospheric pressure of 760 mm. mercury.) [1] Bottom fixed point: The temperature of ice (prepared from distilled water) mixed with distilled water [1], at standard atmospheric pressure of 760 mm. mercury. [1] 7. Resistance thermometers are based on the principle that the resistance [1] of a metal increases with temperature [1] 8. a) Law of homogeneous circuits [1] b) Law of intermediate metals [1] c) Law of intermediate temperatures [1] 9. t = ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) [1] t = [1] = C [2] 10. Compensating leads are cheaper than thermocouple leads [1] and are used to connect the thermocouple to the measuring device at the reference junction a distance away. [1] [3] 11. a) Measured value [1], b) Desired value (setpoint value) [1] c) Error value [1]

77 Process Instrumentation I EIPIN1 Unit 2 Final Assessment 28 October 2011 Memorandum Page 2 [4] [2] [3] 12. Reverse acting: heating system [1] as temperature decreases, the demand on controller output will increase [1] Direct acting: cooling system [1], as temperature increases the demand on the controller output will increase. [1] 13. First order lag: delay due to the time it takes energy [1] to be transferred from one point or form to another [1] 14. a) Bias: the control effort that is maintained when the error is zero [1], b) Proportional gain: ratio of controller output change [1/2] to error value change [1/2], c) Proportional band: the error range that causes 100% change in controller output [1]. 15 R D [1] E C D [1] Op-amp V D [1] de V D =-R D C [1] D dt [4] [2] 16.a) Control valves are used to regulate the flow rate of a medium [1] and serve as the correcting element in many control systems. [1]

78 Process Instrumentation I EIPIN1 Unit 1 First Assessment 02 March 2012 Page 1 Question 1 Give the SI units for current and the amount of substance. (2) Question 2 Define the significance of working standards in the hierarchy of instruments standards. (2) Question 3 Define reproducibility and resolution. (4) Question 4 Non linearity and Dead band are some of the typical instrument errors. Explain these typical instrument errors. (4) TRC Question 5 Identify the following instrumentation symbol: (2) Question 6 The glass U-tube mercury manometer is 300 millimeter (mm) long and has a bore of 5 mm. The scale is movable for zero adjustment, and is calibrated from 0 to 250 mm. With the aid of a sketch calculate the maximum differential pressure which can be applied to the manometer. (4) Question 7 With the aid of a sketch, derive an expression that is used to determine pressure with an inclined limb manometer.. (4) Question 8 Draw a labeled sketch of a force-balance gauge calibrator ( dead weight tester ). (5) Question 9 The absolute pressure and vacuum pressure are two types of pressure. Explain what is absolute pressure and vacuum pressure. (4) Question 10 Draw a labelled sketch of a foil type strain gauge. (3) Question 11 Draw a labelled sketch of a venture tube flow meter. Show the dimensions and relative sizes of the instrument clearly on your sketch. (4) Question 12 State the advantages and disadvantages of the venturi tube. (4) Question 13 Give a formula used for Poiseuille s law and explain each symbol used in the formula. (3) Question 14 Sketch and describe the operation of a transit time flowmeter. (5). ---ooo000ooo--- Total: 50

79 Process Instrumentation I EIPIN1 Unit 1 First Assessment 02 March 2012 Memorandum Page 1 [2] 1. Current: amperes and amount of substance: mole 2. [2] Workplace standards are used to calibrate instruments used in industrial applications and instruments used in the field, [1] for accuracy and performance. Working standards are checked against secondary standards [1] for accuracy. 3. Resolution is the smallest variation [1] in the measured variable that can still be measured. [1] Reproducibility is the closeness of the instrument readings when the same input [1] is applied Dead band is the largest change of input to which the [2] under different conditions over a long period of time. [1] 4. Non-linearity is the maximum deviation [1] from a straight line connecting the zero and full-scale calibration points. [1] P 1 P 2 [4] instrument does not respond due to friction or backlash effects [2] 5. Temperature recording controller, [1] mounted on board. [1] P1-P2 = hg [1] P1-P2 = 13600(250/1000)9.81 [1] [4] P1-P2 = kpa [1] 7. Equating pressures in the XY plane: 1 Mark for sketch HG P 1 = P 2 + (h+d)g [½]... (1) and with mercury incompressible: [4] 8. A A 1 d = A 2 L d = 2 L [½].... (2) A 1 Also in triangle abc: sin = h/l h = Lsin [½].... (3) (2) en (3) in (1): P 1 =P 2 + A Lsin 2 L g A 1 P 1 P 2 = Lg A sin 2 [½] A 1 8. Gauge under Mass pieces [1] Platform [1] 9. Absolute pressure (total pressure) test [1] is the pressure measured from Primary piston [1] absolute zero pressure. [2] Vacuum pressure is the difference between Secondary local atmospheric pressure and the piston [1] Oil [1] absolute pressure in a medium [1], Screw [1] when the pressure in the medium is [4] lower than atmospheric pressure [1] [5]/7 d X A 1 P 1 Sketch 2 marks ZL A 2 b L a P 2 h c Y 10. [3/4] Alignment marks [1] Backing material [1] Grid [1] Solder tabs [1]

80 Process Instrumentation I EIPIN1 Unit 1 First Assessment 02 March 2012 Memorandum Page High pressure tap (upstream tap) [½] Low pressure tap (downstream tap) [½] d [½] Flow [½] D [½] Inlet cone Throat [½] Outlet cone (19º-23º) [½] d [½] (5º-15º) [½] [4] D/2 [½] d/2 [½] 12. Advantages: Pressure loss is small [1] Operation is simple and reliable [1] [4] Disadvantages: Highly expensive 1] Occupies considerable space [1] ], 13. Poiseuille s law: q = R 4 (p 1 p 2 ) [1] 8L where q is the liquid s flowrate (m 3 /s) [½], R is the radius of the pipe (m) [½], is the viscosity of the fluid (PI) [½], L is the length of the pipe (m) and p 1 p 2 is the pressure differential across] [4] the pipe (Pa). [½] 14. Ultrasonic transceiver 1 2 MARK FOR SKETCH (Piezoelectric crystals) [½] 3 MARKS OPERATION Flow [1] [1] L [½] Ultrasonic transceiver 2 (Piezoelectric crystals) [½] [5] The transit time flowmeter (also called transmissivity, time of flight or time of travel flowmeter) uses two ultrasonic transducers to beam a high frequency sound wave (ultrasonic wave), alternatively upstream and downstream at an angle θ, across the flow, as shown in[1]. The difference in times required for the sound waves to travel upstream (t12) and downstream (t21), can be used to calculate both the sound speed and the mean fluid velocity along the path followed by the sound[1]. This meter gives accurate results but is only applicable to clean liquids and gasses. It is however not easy to accurately measure the extremely short time intervals that are involved.[1]

81 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 April 2012 Page 1 Question 1 A DP transmitter must be calibrated to measure the level of a liquid in an open tank. The density of the liquid is 1000 kg/m 3. The DP transmitter will be mounted one meter below the bottom of the tank. The tank is full when the height of the liquid in the tank is 5 meters and it is empty when there is only liquid in the high pressure line connected to the DP transmitter. Determine the necessary calibration specifications for an output signal of 4 to 20 ma. (4) Question 2 Sketch and describe the operation of the following level meters: 2.1. Magnetic float switch level meter. (4) 2.2. Flexure tube displacer (Torque tube) level meter. (5) Question 3 Define the following fixed points on the international temperature scale: 3.1.Oxygen points (2) 3.2. Silver point (2) 3.3. The ice point (2) Question 4 Name three common metals used in resistance thermometer. (3) Question 5 Give the positive element material, negative element material and the temperature range of a type J thermocouple. (3) Question 6 Draw a labelled circuit diagram to illustrate the three wire method that is used to compensate for ambient temperature when measuring temperature with a resistance thermometer. (4) Question 7 State the thermocouple law of intermediate temperature. (2) Question 8 What is the main use of compensating leads, as used for temperature measurement. (2) Question 9 Explain the following terms: 9.1. Feedback and feedforward control systems. (4) 9.2. Direct acting control and reverse acting control systems. (4) Question 10 Draw a circuit diagram of the proportional (P) block of an electronic proportional, integral and derivative controller. Also give the expression of the output of the (P) block, in terms of the error input signal E. (3) Question 11 A process is controlled by a proportional controller. The controller is programmed for a positive gain (reverse acting controller) and proportional band of 70%,a set point of 50% and a bias of 50%. Calculate the output of the controller when the measured value is 70%. (4) Question 12 Define derivative control. (2) TOTAL MARKS:50

82 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 April 2012 Memorandum Page 1 [4] Question 1 1. Empty: P 1 = P atm + hg = P atm = P atm P 1 P 2 = 9810 Pa Full: P 1 = P atm + hg = P atm = P atm P 1 P 2 = Pa Calibration specification: 1 m P atm Empty P 1 P atm Output = 4 ma when input = 9810 Pa (empty condition) Output = 20 ma when input = Pa (full condition) P 2 5 m 1 m DP cell 4 ma 20 ma P atm Full P 1 P atm DP cell P 2 Question 2 1. Magnetic Float Switch: Float magnet [1/2] Non-magnetic housing [1/2] Magnetic reed switch [1/2] Swivel pin [½] Float [1/2] The magnetic float switch is a point level device. When the level reaches a certain point, the float magnet activates the magnetic reed switch. [1/2] The electric contacts are safely isolated from the inside (wet side) [1/2] of the container by nonmagnetic material that allows magnetic interaction between the float magnet and magnetic switch. [1/2] The contacts may be used to switch a pump on or off, to sound an alarm or for other control purposes. [1/2] [4] [5] 2. Flexure Tube Displacer (torque tube) Level Meter: Torque tube [1/2] Torque tube flange [½] Torque rod [½] Torque arm [1/2] Chain [½] Displacer [1/2] The displacer type liquid level measuring instrument is not a float as such, for the displacer is heavier than the process fluid and the displacer moves very little during changes in tank level (a definite advantage over other float types). [1/2] According to Archimedes s law, the apparent weight of the displacer when immersed in a liquid, is its nominal weight in air minus the weight of the displaced liquid. [1/2] The weight of the displacer will thus vary linearly from its weight in air (when the tank is empty) to its apparent weight when fully immersed in the liquid (when the tank is full). [1/2] The weight of the displacer acting on the torque arm, will cause an angular displacement of the free end of the flexible torque tube and this movement will be transmitted to the outside world, by the torque rod. [1/2]

83 Process Instrumentation I EIPIN1 Unit 2 First Assessment 13 April 2012 Memorandum Page 2 Question 3 a) T b) The silver point: The melting point of silver [1] C. [1] [6] c) The Ice point: the melting point of pure water [1] at 0 0 C [1] [3] Question 4 Iron, copper and aluminium. Question 5 Type [3] J Question 6 Positive Isolation element colour element Iron [1] Negative Isolation colour Constantan [1] Outer isolation Temperatu re range ( C) -200 to 750 [1] [1/2] [1/2] [1/2] [1/2] [1/2] [1/2] [1/2] [1/2] [1/2] [4] Question 7 The law of intermediate temperatures states that the sum [½] of the emf developed by a thermocouple with its junctions at temperatures T 1 and T 2, [½] and with its junctions at temperatures [2] T 2 and T 3, [½] will be the same as the emf developed if the thermocouple junctions are at temperatures T 1 and T 3. [½] Question 8 Thermocouple thermometers are normally installed some distance away from the voltmeter or computer that measures the emf generated by the thermocouple. For this purpose, cheaper and lower grade [2] [8] thermocouple wires, called extension wire or compensating leads, are used to connect the thermocouple to the measuring device at the reference junction. Compensating leads have the same thermoelectric Question 9 Feedback control: Measure the controlled variable to determine the control strategy. [2] Feedforward control: Measure disturbance variables to determine the control strategy. [2] Direct acting control: A control arrangement in which the controller output increases if the measured value rises above the set point. [2] Reverse acting control: A control arrangement in which the controller output increases if the measured value drops below the set point. [2] Question 10 E R PF [1] R PI [1] Op-amp V P [3] V P = - R PF E [1] R PI Question 11 [4] C = 100/PB(S M) + R [1] = (100/70) (50 70) + 50 [1] = 21.43% [2] Question12 [2] A control strategy in which the controller output is proportional to the derivative of the error. [2]

84 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 23 March 2012 Page 1 Question 1 Define the International standard of measurements. (2) Question 2 Instruments may be classified according to the functions they perform. Name and explain each function. (9) Question 3 Define the error of drift that may occur in an instrument. (2) Question 4 Define the Relative density of a substance. (2) Question 5 Explain the operation of a C-type Bourdon tube pressure gauge. (5) Question 6 Discuss Aniline as a manometer liquid, with respect to its relative density, application, advantage and disadvantage. (4) Question 7 U- tube manometers can be used to measure differential pressure, gauge pressure and absolute pressure. Give the formulae used to calculate those types of pressures. (3) Question 8 A differential pressure transmitter is correctly calibrated for a process variable that varies from 0 kpa to 170 kpa. Determine the output of the DP transmitter when the process variable reaches 90 kpa (5) Question 9 a) Draw a labelled sketch of a vortex flow meter. (3) b) Give the operational equation that is used to calculate the flow rate q, from the measurements made with a vortex flow meter. Define each symbol that appears in the equation.. (3) Question 10 Sketch the configuration, including dimensions and labels, when pipe taps are used for flow rate measurement with an orifice plate. (3) Question 11 Describe the operation of a magnetic flow meter. (6) Question 12 Give the operational equation that is used to calculate the flow rate (q) from the measurements made with a magnetic flow meter. Define each symbol that appears in the equation. (3). ---ooo000ooo--- Total: 50

85 [2] Process Instrumentation I EIPIN1 Unit 1 Final Assessment 23 March 2012 Memorandum Page 1 1 International standards are defined by international agreement, representing units of measurements [1] to the best possible accuracy allowed by measurement technology [1] [9] 2. Indicating function[ 1] an instrument may provide the information about the value of a quantity under measurement, in the form commonly known as an indicating function [2]..Recording function [1] An instrument may provide the information of the value of a quantity under measurement against time or some other variable, in the form of a written record, usually on paper [2]..Controlling function [1] This is one of the most important functions of an instrument, especially in the field of industrial control processes. In this case, the information provided by the instrument is used by the control system to control the original measured quantity [2]. [2] 3. Drift is the change in instrument indication over time [1] while the input and ambient conditions are constant. [1] 4. Relative density of a substance is defined as the ratio of the density of the substance [1] to the [2] density of water. [1] [5] 5.Bourdon tube pressure gauges are usually used where relatively large static pressures are to be measured. [1] The Bourdon tube pressure gauge consists of a C-shaped tube with one end sealed. [½] The sealed end is connected by a mechanical link to a pointer on the dial of the gauge. [½] The other end of the tube is fixed and open to the pressure being measured. [½] The inside of the Bourdon tube experiences the measured pressure, [½] while the outside of the tube is exposed to atmospheric pressure. [½] Therefore, the tube responds to changes in P measured P atm. [½] Increasing this pressure will tend to straighten out the tube and move the pointer to a higher scale position. [1] [4] 6 Aniline Relative density: [1] Applications: Suitable for pressure measurement in low pressure gas or air installations, with the exception of ammonia and chlorine [1]. Advantages: Low density for measuring small pressure differences. Evaporates slowly. Does not mix with water. Can be easily seen [1]. Disadvantages: Attacks paint. Poisonous, penetrates the skin and causes blood poisoning. Aniline darkens on contact with air [1] [3] 7. (a) Differential pressure: P1-P2= ρ.g.h [1] (b) Gauge pressure P1-Patm= ρ.g.h [1] (c) Absolute pressure Pabs= ρ.g.h [1] 8. At 170 kpa Po is 100 kpa [1] then Po= m (P1-P2) + 20 [1] 100 = m ( 170) + 20 [5/6] m = [1] then at 90 kpa: Po = m (P1-P2) + 20 [1] Po = (90) + 20 [1] = kpa [1]

86 Process Instrumentation I EIPIN1 Unit 1 Final Assessment 23 March 2012 Memorandum Page 2 Bluff body (vortex generator or shredder bar) [1] Eddies (vortices, whirls, swirls or Von Karman vortex street) [1] 9 v [1] d [3/4] Heat sensors in bluff body or ultrasonic sensors [1] [3] (b) ) q = A fd [1] A = unblocked flow area [½] S t of bluff body [½] S t = Strouhal factor [½] constant 10. f = measured vortex frequency [½] d = width [3] [6] [3] 11. Magnetic flow meters can measure the flow rate of any conductive liquid while offering no obstructions to the flow stream [1]. Magnetic flow meters are based on Faraday s law of electromagnetic induction (e = B.v), which states that when a conductor is moved through a magnetic field, an emf e (volt) will be generated that is proportional to the velocity v (m/s) of the conductor, the length. (m) of the conductor, and the strength B (tesla) of the magnetic field [1]. The section of pipe that is part of the flow meter, contains the coils through which current is passed to produce the magnetic field [1] as well as the electrodes that produce the voltage that is proportional to the flow rate [1]. This section must be made of a material that is non-magnetic so as not to distort the magnetic field and also a material that is non-conductive so that the electrodes are not short circuited [1]. To ensure that the electrodes make contact with the liquid at all times, they should, preferably lie in a horizontal plane. [1] 12. q = kebd. [1] where D is the distance between electrodes or pipe diameter (meter), [1/2] B the magnetic flux density (tesla), [1/2] e the measured emf (volt) [1/2] and k a calibration constant (dimensionless). [1/2]