Instrumentation & Data Acquisition Systems

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1 Instrumentation & Data Acquisition Systems Section 5 -Flow Robert W. Harrison, PE Bob@TheHarrisonHouse.com Made in USA 1

2 The Perfect Flowmeter Infinite rangeability / turndown with negligible uncertainty Senses flow over entire cross section, insensitive to velocity profile changes Insensitive to temperature, pressure, fluid property changes Easy to install, non-intrusive No calibration drift over time, no wear, no moving parts No pressure drop across meter Not subject to fouling by debris Instantaneous response to flow changes (zero time constant) It doesn t exist! So instead, we need to learn which meter will work in a particular application there will always be compromises 2

3 Flowmeter Categories Flow obstruction with wetted moving parts Utilize high tolerance machined moving parts Subject to mechanical wear Applicable to clean fluids only Examples Positive displacement Turbine Paddlewheel 3

4 Flowmeter Categories Flow obstruction with wetted non-moving parts The lack of moving parts in the flow material is an advantage Subject to mechanical wear Plugged impulse tubing Problems with excessively dirty fluids Examples Differential pressure Thermal Vortex 4

5 Flowmeter Categories Flow obstructionless Allow fluids to pass undisturbed with no obstructions Have wetted parts Maintain performance when handling dirty and abrasive fluids Not applicable for all applications due to limitations Examples Coriolis Magnetic Ultrasonic 5

6 Flowmeter Categories Externally mounted Allow fluids to pass undisturbed with no obstructions No wetted parts Maintain performance when handling dirty and abrasive fluids Not applicable for all applications due to limitations Examples Clamp-on Ultrasonic Magnetic Weir 6

7 Measurement Methods Velocity The flow is determined by multiplying the velocity by the area through which the flow passes Examples Magnetic Turbine Ultrasonic Inferential The flow is inferred by some other physical property (such as differential pressure or level) and correlated to flow Examples Differential pressure Target Variable area Weir 7

8 Measurement Types Volumetric Flow (Q) Volumetric flow is defined as a volume of fluid/gas passing a given point per unit of time (usually in a pipe) Q = A x V A = Cross Sectional Area (ft 2 ) V = Average velocity (ft/sec) Q = Volumetric Flow (ft 3 /sec) Example Positive displacement 8

9 Measurement Types Mass (ṁ) Mass flow is defined as volumetric flow times density Independent of temperature and pressure ṁ= Q x Q = Volumetric Flow (ft 3 /sec) = Density (lbs / ft 3 ) ṁ -Mass Flow (lbs/sec) Examples Coriolis Thermal 9

10 Density Density Fluid (Liquid) Varies inversely proportionally with temperature 1/T Gas Varies proportionally with pressure and inversely with temperature P * 1/T 10

11 Rangeability and Turndown Ratio They are Not the Same Rangeability The ratio of the maximum full scale range to the minimum full scale range of the flowmeter Turndown Ratio The ratio of the maximum flow that the flowmeter will measure within the stated accuracy to the minimum flow that can be measured within the stated accuracy Example: A flowmeter is calibrated for 100 gpm and measures gpm within 1% of the actual flow rate, yet the full scale can be adjusted between 0-50 to gpm Rangeability: 200/50 = 4:1 Turndown: 100/20 = 5:1 11

pipe than at the pipe walls where the flow is slowed The viscous forces are not always

12 Laminar Flow The viscous forces cause the fluid to slow down as it passes near the pipe wall. The flow profile is near parabolic, with more flow traveling at the center of the pipe than at the pipe walls where the flow is slowed The viscous forces are not always equal at both walls resulting in an asymmetrical parabolic profile Flow Profile Velocity Profile 12

therefore more uniform than laminar flow; however, the fluid layer next to

13 Turbulent Flow The inertia forces are much greater than the effect of the viscous forces, so the effect of the pipe wall is reduced The flow profile is therefore more uniform than laminar flow; however, the fluid layer next to the pipe wall remains laminar Most flow is turbulent Flow Profile Velocity Profile 13

14 Reynolds Number (R d ) Dimensionless number: the ratio of the liquid s inertial forces to its viscous (drag) forces R d = DV / D = Inside pipe Diameter V = Fluidvelocity = Fluid density = Fluid viscosity Related to flow types Laminar flow: R d < 4,000 Turbulent flow: R d > 7,000 (Most flow) Transition flow: R d 4,000-7,000 14

15 Flow Meter Considerations Cost Hardware cost Installed cost Maintenance cost Fluid characteristics Conductivity Viscosity Corrosiveness Instrument characteristics Repeatability Rangeability Turndown Pressure limits 15

16 Flow Meter Considerations Instrument characteristics Temperature limits Measurement type Volumetric Mass Sensor position Intrusive Obstructionless (Non-Intrusive) Line size Intrinsic safety/explosion proof requirements Power requirements 16

Variable Area Meter (Rotameter, Float Type) The most common specified, purchased & installed flow meter Consist of a tapered tube and a float For no flow, the float rests on the bottom stop in the

17 Variable Area Meter (Rotameter, Float Type) The most common specified, purchased & installed flow meter Consist of a tapered tube and a float For no flow, the float rests on the bottom stop in the tube As liquid enters the bottom of the tube, the float begins to rise The float is pushed up by the flow and down by gravity The float continues to rise until it hits the upper flow stop The position of the float varies directly with the flow rate The flow rate is read directly on a scale mounted next to the tube Automatic sensing devices can be used to sense the float s level and transmit the flow signal Turndown ratio 15:1 Accuracy: ±0.5% full scale 17

Positive Displacement Meters Separates the incoming fluid into a series of known discrete volumes Totalizes the number of volumes in a known

18 Positive Displacement Meters Separates the incoming fluid into a series of known discrete volumes Totalizes the number of volumes in a known length of time No secondary flow reading devices are necessary Measures volumetric flow Turndown ratio as high as 400:1 Accuracy: ±0.5% rate 18

19 Positive Displacement Meters Nutating flow meter example Most used for water supply 19

20 Positive Displacement Meters Pros Very high precision possible Accuracy independent of media viscosity Accuracy independent of media density High viscosity media possible Bi-directional flow Cons High pressure loss Noisy Lubrication media only No low viscosity liquids Clear liquids only 20

21 Differential Pressure Intrusive flow metering system consisting of a flow obstruction Measure high (upstream) and low (downstream) pressure differential Flow is proportional to the square root of the differential pressure Non compressible flow: Q = k ( p) ½ Compressible flow: Q = k ( p(p/t)) ½ k ( p( )) ½ K = Obstruction coefficient p = Differential pressure p = Downstream pressure (> 8 pipe diameters) t = Fluid temperature = Density Accuracy: 1.0% of maximum flow Generally turndown ratio: 4:1 Service: Liquid, gas, or steam Most common flowmeter in use 21

Differential Pressure - Orifice Most common DP flow measuring device Three shapes Eccentric is preferred due to more accurate and repeatable performance Orifice plates have a problem with slurries

22 Differential Pressure - Orifice Most common DP flow measuring device Three shapes Eccentric is preferred due to more accurate and repeatable performance Orifice plates have a problem with slurries and dirty fluids as the plates are eroded and material builds up around the plate due to sudden changes in contour, sharp corners, and projections into fluid stream Up to 44 pipe diameter of straight run Turndown ratio: 3:1 5:1 22

Differential Pressure - Venturi An elliptical contour approach section No sudden changes in contour, no sharp corners, no projections into fluid stream Can be used for

23 Differential Pressure - Venturi An elliptical contour approach section No sudden changes in contour, no sharp corners, no projections into fluid stream Can be used for slurries and dirty fluids Difficult to inspect in place May be less accurate than an orifice plate unless calibrated in place Up to 22 pipe diameters of straight pipe run 23

Excellent working and transfer standards Uncertainty: ±0.

24 Differential Pressure Critical Flow Venturi Accelerate gas to sonic velocity at the throat of a restriction Advantages Measures mass flow Simple, no moving parts to wear Highly reproducible, stable calibration Excellent working and transfer standards Uncertainty: ±0.1% of reading Down stream pipe length is not a problem Disadvantages Significant pressure drop Turndown ratio 5:1 Flows > 1 L/min Rarely applied to liquid flows 24

Differential Pressure Cone Meter Newer DP flow meter works on same principle as Orifice & Venturi Cone acts as a conditioning device as well as a differential

25 Differential Pressure Cone Meter Newer DP flow meter works on same principle as Orifice & Venturi Cone acts as a conditioning device as well as a differential producer Up to 5 pipe diameters of straight pipe run Welded construction, calibrate prior to service Accuracy: ±0.5% FS Repeatability: ±0.1% FS Turndown ratio: 10:1 25

Differential Pressure - Pitot Tube A cylindrical probe inserted into an air stream Fluid flow velocity at the upstream face of the probe is reduced substantially to zero Velocity head is converted to

26 Differential Pressure - Pitot Tube A cylindrical probe inserted into an air stream Fluid flow velocity at the upstream face of the probe is reduced substantially to zero Velocity head is converted to impact pressure, which is sensed through a small hole in the upstream face of the probe A small hole in the side of the probe senses the static pressure Causes practically no pressure loss in the stream flow Adjust position for proper air flow pickup Averaging preferable Primarily used for low velocity air flow in ventilation systems and aircraft velocity 26

differential pressure ( P) signal that is the algebraic difference between the average value of the high pressure signal (P H

27 Differential Pressure - Annubar Essentially an averaging pitot tube Special two chamber flow tubes with several pressure openings distributed across the flow stream The annular averaging elements are called annubars The sensor produces a differential pressure ( P) signal that is the algebraic difference between the average value of the high pressure signal (P H ) and the low pressure signal (P L ) Turndown ratio: Conventional and most flows 4:1 Diamond and bullet shapes 10:1 Elliptical shape 17:1 27

28 Differential Pressure Advantages Most understood method Easy to install Low cost Easy to service and repair Repeatable measurement Density and viscosity dependencies can be calibrated into the meter at production No moving parts in the flow stream Limitations Low turndown ratio (rangeability) Not designed for dirty, abusive or slurries Requires straight run piping 10 psig minimum line pressure requirement 10 pipe diameter inlet piping requirement 28

(optical) pickup The motor housing must be non magnetic The rotor must also be non magnetic except for

29 Turbine Provides a frequency output signal that varies linearly with volumetric flow Pickup probe converts motor velocity to an equivalent frequency signal Variable reluctance, Hall Effect or inferred (optical) pickup The motor housing must be non magnetic The rotor must also be non magnetic except for the signal source Meter coefficient K = Cycles/Volume Volumetric flow Accuracy: ±1% full scale Turndown ratio: 10:1 29

30 Paddlewheel Similar to turbine meter Provides a frequency output signal that varies linearly with volumetric flow rate over specified flow ranges Bidirectional flow Mechanical and electrical pickup are commonly used Requires fully developed turbulent flow Upstream straight pipe run of 15 pipe diameters With elbows, pipe diameters Downstream, 5 pipe diameters 30

Paddlewheel Meter coefficient K specified on sensor Units are commonly factory calibrated 50 GPM = 20ma 0 GPM = 4 ma Accuracy: ±0.5% full scale Repeatability: ±0.

31 Paddlewheel Meter coefficient K specified on sensor Units are commonly factory calibrated 50 GPM = 20ma 0 GPM = 4 ma Accuracy: ±0.5% full scale Repeatability: ±0.1% full scale Turndown ratio: 20:1 Low cost solution Example: Mounted perpendicular to flow with sensor and transmitter Side or top mounting, not bottom due to sediment 31

Vortex Background Oscillations occur when fluid passes by an object or obstruction Examples in nature high flow rate Whistling caused by wind blowing by tree branches Swirls produced downstream of a

32 Vortex Background Oscillations occur when fluid passes by an object or obstruction Examples in nature high flow rate Whistling caused by wind blowing by tree branches Swirls produced downstream of a rock in a rapidly flowing river Waving of a flag in a wind Oscillations stop at low flow rate Examples in nature low flow rate Whistling stops when wind dies down Water flows calmly around rock when river is not flowing rapidly Flag does not wave in a mild breeze 32

Vortex Vortex shedding is described mathematically by the von Karmen effect As a fluid passes a bluff object, the fluid separates alternately from each side of the bluff object and swirls to form

33 Vortex Vortex shedding is described mathematically by the von Karmen effect As a fluid passes a bluff object, the fluid separates alternately from each side of the bluff object and swirls to form vertices downstream A vortex is one area of swirling motion with high local velocity and lower pressure than the surrounding fluid The frequency is directly proportional to the velocity of the fluid V = k * vortex frequency A piezoelectric element senses the frequency of the vortices and produces a signal that can be measured 33

34 Vortex Measurement Limitations Flowmeter can turn off if operated at near the bottom of flowmeter range Typically 1 ft/sec for liquids, much higher for gases/vapors depending on density Flowmeter becomes nonlinear and turns off as Reynolds number is reduced Do not operate flowmeter below the minimum Reynolds number constraint Oscillations stop at low flow rates Flowmeter turns off and measures zero flow Volumetric flow 34

35 Vortex 35

36 Vortex Service: Gas and liquid, some slurries Accuracy: ±1% of rate or better Turndown ratio: 30:1 Advantages No moving parts Unaffected by fluid density High turndown High temperature Medium price, low installation cost Minimal maintenance required when used when used in clean flow conditions Limitations Straight pipe required Sensitive to high or varying viscosity Susceptible to pipe vibration Can t measure low velocity Low to medium pressure drop 36

strength (B) The fluid is the conductor with a length equivalent to the flowmeter diameter (D) The fluid moves with an average velocity

37 Magnetic Use Faraday s law to obtain flow measurement Relative motion at right angles between a conductor and a magnetic field will develop a voltage in the conductor The induced voltage (E) is proportional to the velocity of the conductor (V) and magnetic field strength (B) The fluid is the conductor with a length equivalent to the flowmeter diameter (D) The fluid moves with an average velocity (V) Through a magnetic field (B) E = kbdv or V = E/kBD k= Meter coefficient The volumetric flow (Q) is proportional to velocity (V) Q E/kBD 37

38 Magnetic Flow Measurement 38

39 Magnetic Service: Electrically conductive liquids or slurries Accuracy: ±0.5% of rate or better Turndown: 30:1 Size: 0.1 to 96 Advantages High accuracy / high turndown Unaffected by fluid density and viscosity Minimum flow obstruction or pressure loss Bi-directional flow Exceptional for slurries, corrosive and abrasive applications No moving parts Low maintenance costs Limitations Must be conductive liquid, no gases Subject to electrode coating Moderate to expensive cost 39

40 Ultrasonic There are two types of ultrasonic flow meters Time of travel Doppler Volumetric flow Accuracy: ±2% rate Turndown ratio: 100:1 40

Ultrasonic Time of travel (transit) flow meters Measure the velocity of sound as it passes through the liquid flowing in a pipe Piezoelectric crystal transmitters send acoustic signals in the fluid

41 Ultrasonic Time of travel (transit) flow meters Measure the velocity of sound as it passes through the liquid flowing in a pipe Piezoelectric crystal transmitters send acoustic signals in the fluid flowing in the pipe to piezoelectric crystal receivers The velocity of the sound ( s ) is increased by the fluid velocity ( ) from a to b ( s + cos ) and decreased by the same fluid velocity from b to a ( s - cos ) The frequency from a to b: f a = ( s + cos )/d, and b to a: f b = ( s - cos )/d Δ f= f a f b = (2 cos )/d, = = fd/(2 cos ) Since and d are constants, the velocity can be obtained by measuring the beat frequency f 41

liquid to be measured The signal hits solids, bubbles (contained in air), or any discontinuity in the liquid

42 Ultrasonic Doppler flow meters Measures frequency shifts caused by liquid flow A doppler consists of a transmitter and receiver usually mounted side by side A pulsed signal of known frequency is sent into the liquid to be measured The signal hits solids, bubbles (contained in air), or any discontinuity in the liquid and reflects back to the receiver input of phase shift This frequency shift is proportional to the liquid s velocity 42

43 Ultrasonic - Transit 43

44 Coriolis Coriolis force: The apparentdeflection of an object observed by a person on the moving earth 44

Coriolis Flow Measurement Measures true mass flowas opposed to volumetric flow Because the mass does not change, the meter is linear without having to be adjusted for changes in liquid properties

45 Coriolis Flow Measurement Measures true mass flowas opposed to volumetric flow Because the mass does not change, the meter is linear without having to be adjusted for changes in liquid properties (temperature, density, pressure) A U: shaped tube that is vibrated at its natural frequency Moving fluid through the tube causes the unit to twist The amount of twist is directly proportional to the mass flow rate Coriolis meters can be used on virtually any liquid or gas flowing at a sufficient rate to operate the meter Typically used for harsh chemicals, low to medium viscosity fluids, foods and slurries 45

46 Coriolis Service: Gases, liquids and slurries Accuracy: ±0.2% of rate or better Turndown: 100:1 Size 1/16 to 8 Advantages Measures mass flow directly High accuracy / high turndown Not affected by temperature, pressure, viscosity Simultaneous density measurement Wide temperature range Low maintenance Fast response Not affected by pulsation No piping requirements Wide range of liquid flow conditions Very hot (molten sulphur) and very cold (liquid nitrogen) liquid flow 46

47 Coriolis Limitations Susceptible to pipe vibrations High pressure drop Subject to corrosion Limited line sizes Sensor is part of the process Moderate to expensive 47

Thermal (Hot Wire) Flow Measurement Based on the cooling effect of a passing fluid on a heated resistance temperature device The flow is measured

from the mass portion in the energy balance equation of the measured fluid Continuous energy consumption Thermal properties of the fluid (specific

48 Thermal (Hot Wire) Flow Measurement Based on the cooling effect of a passing fluid on a heated resistance temperature device The flow is measured by either The change in heating power required to keep its resistance constant The change in temperature reading Measures mass flowinferentially from the mass portion in the energy balance equation of the measured fluid Continuous energy consumption Thermal properties of the fluid (specific heat and heat transfer) must remain constant No moving parts Unaffected by viscosity changes Turndown ratio: as high as 100:1 Accuracy: ±2% full scale 48

construct The V notch has a high capacity range Level

49 Weir A plate with a notch in it (rectangular or V shaped) The rectangular notch is simple and easy to construct The V notch has a high capacity range Level is measured and its value is converted to rate of flow 49

Flume A restriction in a channel A free flow

50 Flume A restriction in a channel A free flow open channel (similar to an open venturi) The entrance section (upstream) converges in a straight section with parallel sides, then the sides diverge Level is measured in the entrance section and its value is converted to rate of flow 50

51 Weirs and Flumes Produce low head loss Only means to handle seifilled pipes Unaffected by viscosity changes Used in open channel clean and dirty water applications Wastewater Advantages of flumes Ease of construction Sturdy build Handle higher velocity flows Self cleaning Advantages of weirs Handle large volumes of liquid More accurate Turndown ratio: 2.5:1 51

Target A force is exerted by the flow on a solid disk in the pipe at right angles to the flow The force is related to the flow The opposite of an orifice plate No

52 Target A force is exerted by the flow on a solid disk in the pipe at right angles to the flow The force is related to the flow The opposite of an orifice plate No moving parts and is inexpensive Mostly used for viscous fluids Hot tarry fluids Sediment bearing fluids Volumetric flow Turndown ratio: 4:1 Accuracy: ±1% full scale 52

53 Flow Meter Summary Meter Obstruct Vol/Mass Accuracy Turndown Ratio Rotameter Y V ±0.5% FS 15:1 Positive Displacement Y V ±0.5% R 400:1 Differential Pressure Y V ±1.0% FS 4:1 to 15:1 Turbine Y V ±0.5% R 10:1 to 35:1 Paddle Wheel Y V ±0.5% FS 20:1 Vortex Y V ±1.0% R 30:1 Magnetic N V ±0.5% R 30:1 Sonic/Ultrasonic N V ±2.0% R 100:1 Coriolis N M ±0.5% R 100:1 Thermal B M ±0.2% R 100:1 Target Y V ±1.0% FS 4:1 Weir/Flume Y V ±1.5% FS 2.5:1 53

FLOW MEASUREMENT. INC 102 Fundamental of Instrumentation and Process Control 2/2560

FLOW MEASUREMENT. INC 102 Fundamental of Instrumentation and Process Control 2/2560 FLOW MEASUREMENT INC 102 Fundamental of Instrumentation and Process Control 2/2560 TABLE OF CONTENTS A. INTRODUCTION B. LOCAL FLOW MEASUREMENT B.1 Particle Image Velocimetry (PIV) B.2 Laser doppler anemometry