Courseware Sample F0

Transcription

1 Instrumentation and Process Control Courseware Sample F0

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4 INSTRUMENTATION AND PROCESS CONTROL COURSEWARE SAMPLE by the Staff of Lab-Volt (Quebec) Ltd Copyright 2004 Lab-Volt Ltd All rights reserved. No part of this publication may be reproduced, in any form or by any means, without the prior written permission of Lab-Volt Quebec Ltd. Printed in Canada June 2004

5 Table of Contents Introduction... Courseware Outline Multi-Process Station Sample Exercises Extracted from Multi-Process Station Ex. 2 Level Measurement II Calibration of a Level Transmitter... 3 Ex. 6 Level Process Characteristics with Variable Speed Pump Ex. 15 Ultimate Period Tuning of a Level Process III

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7 Introduction The Lab-Volt Mobile Instrumentation and Process Control Training System, Series 3500, consists of self-contained workstations designed for hands-on training in the measurement, control, and troubleshooting of pressure, flow, level, temperature, heat exchange, and analytic processes. The stations can operate independently, or be interconnected into several configurations to simulate complex processes. All processes are sized to be "real-world" in time lag and process response. Process dynamics can be altered by various means to provide several degrees of stability and damping. The Flow, Level, Heat Exchanger, Multi-Process, and Analytic Process Stations utilize water as the process media, while the Pressure and Temperature Process Stations utilize air. All Process Stations include a microprocessor-based controller, a dual-speed strip chart recorder with single or dual pen, and alarm lamps, with all connections terminated by banana jacks on the station main control panel. V

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9 Courseware Outline MULTI-PROCESS STATION Exercise 1 Exercise 2 Exercise 3 Exercise 4 Exercise 5 Exercise 6 Exercise 7 Exercise 8 Exercise 9 Level Measurement I Dry Method using a Bubble Pipe Level Measurement II Calibration of a Level Transmitter Pressure Measurement Flow Measurement: Differential Pressure vs Flow Using a Venturi or Orifice Plate Level Process Characteristics with Control Valve Level Process Characteristics with Variable Speed Pump Flow Process Characteristic with Control Valve Pressure Process Characteristic Proportional Control Level Process with Control Valve Exercise 10 Proportional Control Flow Process with Variable Speed Pump Exercise 11 Proportional Plus Integral Control Level Process with Control Valve Exercise 12 Proportional Plus Integral Control Pressure Process Exercise 13 Proportional Plus Integral Plus Derivative Control Level Process with Control Valve Exercise 14 Proportional Plus Integral Plus Derivative Control Flow Process with Variable Speed Pump Exercise 15 Ultimate Period Tuning of a Level Process Exercise 16 Ultimate Period Tuning of a Flow Process Approximation Method Exercise 17 Open Loop Tuning of a Level Process using the Reaction Rate Method Exercise 18 Open Loop Tuning of a Pressure Process Exercise 19 Troubleshooting a Level Control Process Exercise 20 Operation of a Two Element Control Process VII

10 Courseware Outline MULTI-PROCESS STATION Exercise 21 Three-Element Control Process Exercise 22 Auto-Tune Controller Appendix A Symbols Used in Diagrams B Venturi Tube Flow Curve VIII

11 Sample Exercise Extracted from Multi-Process Station

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13 Exercise 2 Level Measurement II Calibration of a Level Transmitter OBJECTIVES At the completion of this exercise, you will be able to calibrate a differential pressure transmitter, using the process, to measure level. DISCUSSION A Differential Pressure (D/P) Transmitter may be used for the measurement of liquid level or flow of a fluid in a pipe. In this exercise you will calibrate the Differential Pressure Transmitter by varying the height of the water column in the level tank. A Differential Pressure Transmitter measures the difference of pressure applied across its measuring element. The differential pressure detected by the Differential Pressure Transmitter is related to a column of fluid by the following relationship: Pressure = Density of fluid x Height of fluid Differential pressure transmitters produce an output proportional to the difference in pressure across its high pressure, and low pressure ports. The height of fluid is normally expressed in inches/centimeters of water. If the density of the fluid remains constant, which is normally the case, then the pressure is directly related to the height of the fluid. Therefore, accurately determined, reproducible pressures can be applied to a Differential Pressure Transmitter by varying the height of a column of fluid of a known density. Calibration of a Differential Pressure Transmitter is the process of matching the zero and full scale outputs of the transmitter to the minimum and maximum differential pressures applied. The actual differential pressures that are to be applied to the Differential Pressure Transmitter are derived from the specific application. As for most transmitters, the two adjustments available for the calibration are the zero and span of range. It is necessary to determine the upper and lower range values of differential pressures which will be applied to the transmitter. The level process tank is graduated in centimeters and inches. The bottom of the tank has two pressure taps, and mini valves labelled V6 and V7. If the tank overflow valve V13 is opened, then the tank will be vented to atmosphere, and we need only to connect the high pressure part of the D/P transmitter to V6 and V7. The tank level will provide a pressure on the D/P cell proportional to its height, and the D/P electronics will give a current output of equivalent to the range the D/P cell is calibrated to. 3

14 Level Measurement II Calibration of a Level Transmitter When we connect the D/P cell to the bottom of the tank, two problems occur: 1) The air trapped in the tubing will compress as the water column height increases. This requires that the D/P cell be opened to release the trapped air, which is a technique called bleeding the sensing lines and the D/P cell. 2) The bottom of the tank is not necessarily the real bottom of the water column. The actual bottom is the lowest point of the tubing in relationship to the height of the D/P cell. To solve 1) we must bleed the tubing and the D/P cell to ensure no air is trapped. All D/P cells have small vents to permit this. To solve 2) we must adjust the electronics to elevate or suppress the zero output of the D/P cell (4.0 ma) to be equal to the real level zero in the tank. Again this is not always the bottom of the tank. In this exercise we calibrate the transmitter for a zero = 4" of water and a span of 20 inches of water. This means our range will be 4-24 inches of water. Range span = zero. EQUIPMENT REQUIRED DESCRIPTION MODEL Multi-Process Station 3505-M0 D/P Transmitter (LT) Digital Multimeter INSTRUMENT DATA DEVICE MODEL SERIAL NO. CALIBRATED LT 0-30" WC/ PROCEDURE CAUTION! Water and electric power are present in this laboratory exercise. Be careful of possible electrical shock hazard. * 1. Connect the equipment as shown in Figure 2-2. Open or close the valves as shown. * 2. Program the variable speed drive for manual operation. Close valve V8. 4

15 Level Measurement II Calibration of a Level Transmitter * 3. Start the pump and fill the level tank to 26 inches (65 cm) and close valve V2. Stop the pump. * 4. In this step you will bleed the air from the tubing between V7 and the D/P cell. Using a small wrench, open the D/P cell high side vent, and bleed the cell into a small cup. You need to bleed 2 or 3 inches of water into the cup to ensure all air is out. Close the D/P cell vent. * 5. Check again that the water level in the tank is exactly 24 inches (60 cm). If not, add or release water until correct. * 6. Following the procedure in the manufacturers manual for the specific D/P Transmitter, set the span adjust so that the transmitter output, as indicated on the DMM, reads 20.0 ma. * 7. Open V8 and drain the tank level down to 4 inches or 10 cm and close V8. As for step 6, follow the manufacturers instruction for setting zero, and set the zero adjust so that 4.0 ma is indicated on the DMM. * 8. Refill the tank to 24 inches (60 cm) and reset the span adjust for 20 ma. Drain the tank to 4 inches (10 cm) and reset the zero to 4 ma. Some transmitters require that you repeat this several times because the zero and span adjustments are often interactive. New microprocessor based instruments have virtually no interaction, and the zero/span need only to be set once. * 9. Note that we have set the zero at 4 inches (10 cm) and upper range to 24 inches (60 cm) for a 20 inches (56 cm) span. If time permits recalibrate the D/P transmitter to a zero of 10" (25 cm) and an upper range value of 20" (50 cm). * 10. Complete the calibration data sheet and plot a graph of the results. Check to see if there is any non-linearity on hysteresis visible. CONCLUSION In this exercise you learned to calibrate a Differential Pressure Transmitter. You observed the interaction of the zero and span adjustments for a specified range of operation. The zero adjustment does not normally affect the span/range adjustment. However, the span/range adjustment does affect the zero adjustment. You also learned that a Differential Pressure Transmitter needs to be vented to produce correct readings. 5

16 Level Measurement II Calibration of a Level Transmitter CALIBRATION DATA SHEET APPLICATION DATA INSTRUMENT NUMBER: FUNCTION: LOCATION: INPUT RANGE: INSTRUMENT NAMEPLATE DATA MANUFACTURERS NAME: MODEL NUMBER: SERIAL NUMBER: OUTPUT RANGE: REQUIRED ACCURACY: DATE OF CALIBRATION: INPUT % SPAN DESIRED OUTPUT ACTUAL OUTPUT REMARKS ALARMS ALARM FUNCTION: ALARM SETTINGS: LOW SETPOINT ACTUAL TRIP POINT HIGH SETPOINT ACTUAL TRIP POINT 6

17 Level Measurement II Calibration of a Level Transmitter Figure

18 Level Measurement II Calibration of a Level Transmitter SUPPLY + + FREQUENCY INPUT F1 F MULTI-PROCESS STATION 24 V DC COMMUNICATIONS A ALARM INDICATORS 24 V DC CAUTION: DO NOT SET F max. ABOVE 70 Hz OR PUMP MAY BE DAMAGED B 25 COM. 22 VSD RS 485 ALM 1 ALM 2 ANALOG INPUTS OUTPUTS TO PUMP COM DRY CONTACT INPUTS V OPEN CIRCUIT CONTROL OUT AUXILIARY OUT CONTACT OUT RATED 24 V 1 A DC RELAY 1 ISOLATED CO 1 CO 2 (32) (31) INPUT SIGNAL + PEN 1 SPAN INPUT PEN 2 SPAN INPUT D / P 1 TO +24 V DC RL D / P 2 RL SUPPLY 20 psi 140 kpa I / P 3-15 psi kpa SUPPLY 24 V DC SUPPLY + SOLENOID VALVE SV-1 SUPPLY PUMP MAINS INTERLOCK MAINTAINED DRY CONTACT MAINTAINED DRY CONTACT REMOTE 24 V DC OPEN CIRCUIT 100 ma 20 psi / 140 kpa REMOTE 24 V DC OPEN CIRCUIT 100 ma PUMP-HEATER Figure 2-2A. 8

19 Level Measurement II Calibration of a Level Transmitter LEVEL COLUMN V 3 U 8 U 9 SUPPLY 100 psi 700 kpa PRR 1 V psi 0-20 kpa V 4 TO +24 V DC V 13 OVERFLOW V 8 U 3 HA 2 V 6 V 5 TE SV-1 V 7 V 14 U 7 U 6 FI U 5 V 2 D / P 1 H L U 4 V 11 V 10 PUMP CV 1 V 1 V 12 HA 1 U 2 HOLDING TANK 20 GALLONS 75 LITRES U 1 Figure 2-2B. REVIEW QUESTIONS 1. What is the function of a Differential Pressure Transmitter in a level measurement channel? 2. Why is it necessary to purge all air from the transmitter before using water as the calibration medium? 9

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21 Exercise 6 Level Process Characteristics with Variable Speed Pump OBJECTIVES At the completion of this project, you will be able to use standard process instrumentation to determine the characteristics of a level process, controlled by a variable speed pump. DISCUSSION Understanding how a process responds to changes is important for the individual who must calibrate the instruments used for controlling the process. It is also important to know how quickly the output of the measuring instrument responds to changing conditions. In the previous exercise, we used a pneumatic control valve, which is a relatively slow control element. In this exercise, we will do exactly the same experiment, but use a much faster control element. Procedural Notes 1. The procedure involves a step change to the calibrator (Figure 6-1), performed by quickly increasing the output from 8 to 12 ma. Increasing the Variable Speed Drive input signal will increase the flow. 12 ma STEP CHANGE 8 ma Figure The process will take some time to react to the instantaneous step change. This is due to several factors, including the time it takes for the water to travel through the system and the water level to increase. The actual step change will be considered instantaneous to simplify calculations. The step change and resulting process reaction are shown in Figure

22 Level Process Characteristics with Variable Speed Pump 12 ma (A) STEP CHANGE VARIABLE SPEED DRIVE INPUT 8 ma T = 63.2% (B) PROCESS REACTION t o t d TIME Figure 6-2. where t o = Initial Time t d = Dead Time (time taken by the process to start to react to the step change) T = Process Time Constant (time taken by the process to reach 63.2% of final steady state value) EQUIPMENT REQUIRED DESCRIPTION MODEL Multi-Process Station 3505-M0 Differential Pressure Transmitter (LT) Strip Chart Recorder (LR) Current to Pressure Converter (I/P) Electronic Calibrator 3550-M0 INSTRUMENT DATA DEVICE MODEL SERIAL NO. CALIBRATED LT LR I/P 0-30" WC/ /0-100% /3-15 psi PROCEDURE CAUTION! Do not run pump for prolonged periods with a shut off head! * 1. Set up and connect equipment as per the loop diagram. Close or open valves as shown in Figure 6-4. Program the variable speed drive for manual operation and 40 Hz. 12

23 Level Process Characteristics with Variable Speed Pump * 2. Fill the tank to approximately 10 in./25 cm water and bleed the HP side of the transmitter. * 3. The low pressure side of the transmitter is left open and will therefore measure atmospheric pressure. The high pressure side is connected to valve V7 and measures the pressure of the column of water plus atmospheric pressure. The difference between the two is the pressure, and therefore the height, of the column of water. Because we are calibrating this transmitter using water, it is necessary to purge the air from the high pressure side. Hold a cup under the HP vent and loosen the vent plug until a constant stream of water is flowing through. Tighten the vent plug. * 4. Temporarily connect the I/P Converter input to the PID Controller output. Place the controller in manual and adjust the output to 100% (20 ma). Adjust the variable speed pump (and V2 if necessary) to a flow rate of 5 GPM (18 lpm) with inflow to tank at maximum. This step will permit the process to stabilize at approximately 24 to 30" of water in the column. When we switch between 8 and 12 ma the process will stabilize at two lower levels. * 5. Set the calibrator output to 8 ma. * 6. Allow the process to stabilize. Adjust V8 (tank inflow) to balance the inflow and outflow. * 7. Start the recorder at 10 in./min (1500 mm/s) and rapidly change the calibrator from 8 to 12 ma. When the process has stabilized, stop the recorder. * 8. Record the change in process level as read on the tank scale. L start L finish * 9. Perform calculations (see NOTES/CALCULATIONS). NOTES/CALCULATIONS Level start (Ls) Level finish (Lf) Level change Lc = Lf Ls = 13

24 Level Process Characteristics with Variable Speed Pump Process Gain 1. Convert the Level Change (Lc) to a percent of transmitter span: Lc/30" x 100% = 2 Express the variation of the VSD input signal as a percent. P$ P$ P$ î 3 3URFHVV *DLQ 2XWSXW FKDQJH LQ RI VSDQ,QSXW FKDQJH LQ RI VSDQ Process Dead Time 4 Process Dead Time (t d ) = time difference between point when the VSD input signal was changed from 8 to 12 ma and when process level started to rise Process Time Constant 5 Process Time Constant (-) = time taken to reach 63.2% of final steady state value 14

25 Level Process Characteristics with Variable Speed Pump Figure

26 Level Process Characteristics with Variable Speed Pump SUPPLY + + FREQUENCY INPUT F1 F MULTI-PROCESS STATION 24 V DC COMMUNICATIONS A ALARM INDICATORS 24 V DC CAUTION: DO NOT SET F max. ABOVE 70 Hz OR PUMP MAY BE DAMAGED B 25 COM. 22 VSD RS 485 ALM 1 ALM 2 ANALOG INPUTS OUTPUTS TO PUMP COM DRY CONTACT INPUTS V OPEN CIRCUIT CONTROL OUT AUXILIARY OUT CONTACT OUT RATED 24 V 1 A DC RELAY 1 ISOLATED CO 1 CO 2 (32) (31) INPUT SIGNAL + + CAL ma PEN 1 SPAN INPUT PEN 2 SPAN INPUT D / P 1 TO +24 V DC RL D / P 2 RL SUPPLY 20 psi 140 kpa I / P 3-15 psi kpa 3-15 psi kpa SUPPLY 24 V DC SUPPLY + SOLENOID VALVE SV-1 SUPPLY PUMP MAINS INTERLOCK MAINTAINED DRY CONTACT MAINTAINED DRY CONTACT REMOTE 24 V DC OPEN CIRCUIT 100 ma 20 psi / 140 kpa REMOTE 24 V DC OPEN CIRCUIT 100 ma PUMP-HEATER Figure 6-4A. 16

27 Level Process Characteristics with Variable Speed Pump LEVEL COLUMN V 3 U 8 U 9 SUPPLY 100 psi 700 kpa PRR 1 V psi 0-20 kpa TO +24 V DC V 13 V 4 FI D / P 1 OVERFLOW V 8 U 3 HA 2 V 6 V 5 TE SV-1 V 7 V 14 U 7 U 6 U 5 V 2 H PT L U 4 V 11 VM5 VM7 V 10 PUMP CV 1 V 1 V 12 VM6 HA 1 U 2 HOLDING TANK 20 GALLONS 75 LITRES U 1 Figure 6-4B. REVIEW QUESTIONS 1. Why did the process level reach a steady state value rather than completely filling the tank to the overflow line? 17

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29 Exercise 15 Ultimate Period Tuning of a Level Process OBJECTIVES At the completion of this laboratory exercise you will be able to use standard process instrumentation to observe and analyze the effects of setpoint and gain changes on a controller and, using the observed information, determine the optimum settings required to tune the controller. DISCUSSION The basic purpose of tuning is to match the P + I + D settings within the controller, to the dynamics of the process. There are two basic approaches to loop tuning: a) Open loop, which we will examine later, and b) closed loop, which places the process in oscillation. The desirable goal is to upset or disturb the process just enough to determine the PID values without upsetting the plant. There are many theoretical tuning methods. In this exercise we will examine the ultimate period or Ziegler-Nichols method. Because overall plant efficiency relies heavily on optimum tuning of all processes in the plant, it is important to understand this method of tuning. In Exercises 9 and 12 we have observed that increasing the controller gain may lead to increased instability. Any control loop will oscillate in the controller gain (K P ) is high enough. The period of the oscillation is called the natural or ultimate period (P U ). The ultimate period method requires placing the process in continuous amplitude oscillation and then using the controller setting and measurements from the strip chart to determine the optimum settings of gain, Integral action and derivative action for the controller and the process. 19

30 Ultimate Period Tuning of a Level Process MEASURED VARIABLE Pu TIME Figure EQUIPMENT REQUIRED DESCRIPTION MODEL Multi-Process Station including: 3505-M0 Microprocessor PID Controller (LIC) Differential Pressure Transmitter (LT) Variable Speed Pump (VSP) Strip Chart Recorder (LR) INSTRUMENT DATA DEVICE MODEL SERIAL NO. CALIBRATED LT I/P LR 6-26" WC/ /3-15 psi /0-100% Controller Configuration (See note in Exercise 9) 1. Setpoint = 50 % 2. Gain = 1 (PB = 100 %) 3. Reset = minimum rep/min (max. integral time min/rep) 4. Derivative = 0.05 min. 5. Auto/Manual = Auto 6. Action = Reverse 20

31 Ultimate Period Tuning of a Level Process PROCEDURE CAUTION! Do not run pump for prolonged periods with a shut off head! * 1. Set up and connect equipment as per the loop diagram. Valve settings as per diagram Figure Configure the VSP to provide 0-10 GPM (36 lpm) for an input signal of. * 2. Calibrate the level transmitter for 6-26" WC. * 3. Set the controller as per the Controller Configuration. * 4. Manually adjust the controller output until the measured variable equals the setpoint. Start the recorder and place the controller in automatic. The process will stabilize close to the setpoint. * 5. Disturb the process by increasing the setpoint for 5 seconds then reduce it back to 50 %. If the chart recorder displays the process as being in continuous amplitude oscillations proceed with step 9. Otherwise proceed with step 6. * 6. Allow the process to stabilize until the process stabilizes. * 7. On the controller, increase the gain (decrease the proportional band) to give more proportional action. The normal practice is to make steps in factors of 2 (i.e. PB = 100 % Ú50 % Ú 25 % Ú 12 % Ú 6 % etc.) * 8. Repeat steps 5 to 7 until the process responds with constant amplitude oscillations. * 9. Use the proportional setting and the period of oscillation in the Ziegler- Nichols equations to determine optimum controller settings. Note: Some texts show slightly different coefficients on the equations. * 10. Using the three calculated settings, evaluate the controller response to supply and demand disturbances. Fine tuning may be necessary. Changes in process gain due to transmitter and VSP calibration variations will result in values differing as much as 20 % or more. 21

32 Ultimate Period Tuning of a Level Process NOTES/CALCULATIONS K p = Calculated controller gain setting PB = Calculated proportional band setting T i = Integral time (min/repeat) RPM = Reset (repeats/min) t d = Derivative time (min) K u = Controller gain setting which resulted in constant amplitude oscillations P u = Period of oscillation (minutes) Proportional K p = 0.5 K u = PB = 2 Pbu = Proportional and Reset K p = 0.45 K u = PB = 2.2 PBu = T i = Pu/1.2 = RPM = 1.2/Pu = Proportional and Reset and Rate K p = 0.6 K u = PB = 1.66 PBu = T i = Pu/2 = RPM = 2/Pu = t d = Pu/8 = 22

33 Ultimate Period Tuning of a Level Process Figure

34 Ultimate Period Tuning of a Level Process 3505 MULTI-PROCESS STATION SUPPLY + + FREQUENCY INPUT F1 F2 24 V DC COMMUNICATIONS A ALARM INDICATORS 24 V DC CAUTION: DO NOT SET F max. ABOVE 70 Hz OR PUMP MAY BE DAMAGED B 25 COM. 22 VSD RS 485 ALM 1 ALM 2 ANALOG INPUTS OUTPUTS TO PUMP COM DRY CONTACT INPUTS V OPEN CIRCUIT CONTROL OUT AUXILIARY OUT CONTACT OUT RATED 24 V 1 A DC RELAY 1 ISOLATED CO 1 CO 2 (32) (31) INPUT SIGNAL + PEN 1 SPAN INPUT PEN 2 SPAN INPUT D / P 1 TO +24 V DC RL D / P 2 RL SUPPLY 20 psi 140 kpa I / P 3-15 psi kpa 3-15 psi kpa SUPPLY 24 V DC SUPPLY + SOLENOID VALVE SV-1 SUPPLY PUMP MAINS INTERLOCK MAINTAINED DRY CONTACT MAINTAINED DRY CONTACT REMOTE 24 V DC OPEN CIRCUIT 100 ma 20 psi / 140 kpa REMOTE 24 V DC OPEN CIRCUIT 100 ma PUMP-HEATER Figure 15-3A. 24

35 Ultimate Period Tuning of a Level Process LEVEL COLUMN V 3 U 8 U 9 SUPPLY 100 psi 700 kpa PRR 1 V psi 0-20 kpa TO +24 V DC V 13 V 4 FI D / P 1 OVERFLOW V 8 U 3 HA 2 V 6 V 5 TE SV-1 V 7 V 14 U 7 U 6 U 5 V 2 H L U 4 V 11 VM5 VM7 V 10 PUMP CV 1 V 1 V 12 VM6 HA 1 U 2 HOLDING TANK 20 GALLONS 75 LITRES U 1 Figure 15-3B. REVIEW QUESTIONS 1. Is the ultimate period method an open-loop or closed-loop method of controller tuning? Explain. 2. For the ultimate period method, why is the calculated gain value different for PI control and straight proportional control? 25

36 Ultimate Period Tuning of a Level Process 3. What information must be obtained to tune a controller using the ultimate period method and what is it used to determine? 26