Chemical Engineering 3P04 Process Control Tutorial # 1 Learning goals 1. Sensor Principles with the flow sensor example 2. The typical manipulated variable: flow through a conduit
Sensors: We need them to know the process conditions (for safety, product quality,.) Where are the sensors? - Located at the process equipment - Some displays near the equipment for use by people working on the equipment - Some displays transmitted to a centralized location for use by computers and people to control, monitor, and store in history
Sensors: We need them to know the process conditions (for safety, product quality,.) Central control room Sensors, local indicators, and valves in the process Valve opening determined by the signal from computer The control system does a lot! Displays of variables, calculations, commands to valves and historical data are in the centralized control center.
Sensors: What are important features for process control? Accuracy Repeatability Reproducibility Span (Range) Reliability Linearity Maintenance Consistency with process environment Dynamics Safety Cost These are explained in the pc-education site. Most engineers select sensors, do not design them.
Sensors: What are important features for process control? Accuracy = Sensors - We must see key variables to apply control Please define the following terms Reproducibility =
Sensors: What are important features for process control? Sensors - We must see key variables to apply control Please define the following terms Accuracy = Degree of conformity to a standard (or true) value when a sensor is operated under specified conditions. Reproducibility = Closeness of agreement among repeated sensor outputs for the same process variable under the same conditions, when approaching from various directions.
Sensors: What are important features for process control? Discuss the accuracy and reproducibility in these cases A B C D
Sensors: Is accuracy in flow measurement important? Petro-Canada Refinery Petroleum refinery processing 100,000 barrels/day of crude oil: A +0.50% error in flow measurement represents about 15 million $ /year extra cost to purchaser! ph 14 12 10 8 6 4 2 0 Strong Acid-Base Titration Curve 0 0.5 1 1.5 2 Flow of Base (fraction of neutralization) Add a strong base to neutralize (ph=7) a strong acid: a +0.50% error in the base flow represents A ph of about 10-11!
Titration: Do you believe in automation? Manual Automated ph control McMaster University ph Control Laboratory http://www.mpcfaculty.net/mark_bishop/titration.htm http://www.fhs.mcmaster.ca/oehl/main.html
Sensors: How do we measure fluid flow? This control system requires a flow measurement. Let s consider a situation in which the liquid is a clean fluid with turbulent flow through the pipe. FC liquid cooling
Sensors: How do we measure fluid flow? The most frequently used flow sensor is the orifice meter. What is the basic principle for this sensor? FC liquid cooling How can we use this behavior to measure flow? Velocity increases; Bernoulli says that pressure decreases
Sensors: Principles of the orifice meter P orifice Measure pressure drop pressure P orifice =P 1 P 3 Distance
Sensors: Principles of the orifice meter Nice visual display of concept. In practice, pressure difference is measured by a reliable and electronic sensor = P orifice From: Superior Products, Inc. http://www.orificeplates.com/
Relate the pressure drop to the flow rate v = velocity F = volumetric flow rate f = frictional losses = density A = cross sectional area Bernoulli s eqn. General meter eqn. Installed orifice meter (requires density measurement) 0 = aver. density C 0 = constant for specific meter Installed orifice meter (assuming constant density) F K P 1 P 3 Most common flow calculation, does not require density measurement
Sensors: Principles of the orifice meter Measured value to flow controller When an orifice meter is used, the calculations in yellow are performed. Typically, they are not shown on a process drawing. Multiply signal by meter constant K K FC Take square root of measurement Measure pressure difference P liquid cooling
Sensors: Are there limitations to orifices? Relate the pressure drop to the flow rate v = velocity F = volumetric flow rate f = frictional losses = density A = cross sectional area General meter eqn. C meter We assume that the meter coefficient is constant. The flow accuracy is acceptable only for higher values of flow, typically 25-100% of the maximum for an orifice Reynolds number
Sensors: Is there a downside to orifices? What is a key disadvantage of the orifice meter? P loss = P 1 P 2 Pressure loss! pressure P orifice =P 1 P 3 Distance Nonrecoverable pressure drop When cost of pressure increase (P 1 ) by pumping or compression is high, we want to avoid the non-recoverable pressure loss.
Sensors: Factors in selecting an orifice meter Accuracy Repeatability Reproducibility Span Typically, 2-4% inaccuracy Strongly affected by density changes from base case Much better than accuracy Much better than accuracy Accuracy limited to 25-100% of span Span achieved by selecting diameter of orifice and P orifice Reliability Linearity Maintenance Process Environment Dynamics Safety Cost Very reliable, no moving parts Must take square root to achieve linear relationship between measured signal and flow rate Very low Turbulent, Single liquid phase, no slurries (plugging) Straight run of pipe needed (D= pipe diameter), 10-20D upstream, 5-8D downstream Nearly instantaneous Very safe Low equipment (capital) cost, large number of suppliers High operating cost (non-recoverable pressure loss)
Principles of flow through a closed conduit In typical processes, we manipulate the flow to achieve desired operating conditions For liquids we typically install a pump to provide the work required for flow. Constant speed centrifugal pump liquid What is the principle for a centrifugal pump? What in adjusted to affect the flow in this system?
Flow principles: Let s look at a typical centrifugal pump For an animation and description of the basics of a centrifugal pump, follow the hyperlink below. http://www.pumpworld.com/centrif1.htm Outlet Flow = F2 (m 3 /min) Pressure = P2 (kpa) Inlet (suction) Flow = F1 (m 3 /min) Pressure = P1 (kpa) Pump Motor (work)
Flow principles: Let s look at a typical centrifugal pump What goes here? = F1 F2 Outlet Flow = F2 (m 3 /min) Pressure = P2 (kpa) > < P1 P2 Inlet (suction) Flow = F1 (m 3 /min) Pressure = P1 (kpa) Pump Motor (work)
Flow principles: Let s look at a typical centrifugal pump What goes here? = F1 = F2 Outlet Flow = F2 (m 3 /min) Pressure = P2 (kpa) > < P1 < P2 Inlet (suction) Flow = F1 (m 3 /min) Pressure = P1 (kpa) Pump Motor (work)
Principles of flow through a closed conduit Constant speed centrifugal pump P 0 = constant liquid Head at pump outlet We turn on the pump motor and let the system reach steady state. How do we calculate the flow rate that would occur? Hint: Use the plot at the left. Flow rate
Principles of flow through a closed conduit Constant speed centrifugal pump P 0 = constant liquid Head at pump outlet Flow rate Pump head curve Steady-state flow rate at given conditions system curve, pressure drop vs flow rate What if we want a different the flow in the system?
Principles of flow through a closed conduit liquid Constant speed centrifugal pump We adjust the valve opening to achieve the desired flow rate! Head at outlet of pump To achieve the desired flow, we vary the system resistance by changing the pressure drop across a valve. Flow rate
Principles of flow through a closed conduit liquid For a clear and comprehensive description of centrifugal pumps and flow in pipes, follow the hyperlink below. http://www.cheresources.com/centrifugalpumps2.shtml
Tutorial # 1 Learning goals 1. Sensor Principles with the flow sensor example 2. The typical manipulated variable: flow through a conduit Multiply signal by meter constant K Measured value to flow controller K FC Now, we understand the sensor and the flow principles! Take square root of measurement Measure pressure difference P liquid