CHEMICAL ENGINEERING LABORATORY CHEG 239W. Control of a Steam-Heated Mixing Tank with a Pneumatic Process Controller

Transcription

1 CHEMICAL ENGINEERING LABORATORY CHEG 239W Control of a Steam-Heated Mixing Tank with a Pneumatic Process Controller Objective The experiment involves tuning a commercial process controller for temperature control of a steam-heated mixing tank. The objective is to gain an understanding of the operation of an analog controller by generating process reaction curves to determine tuning settings, optimizing these settings through setpoint changes, and investigating the controller s ability to reject disturbances. These tests will be done for both P-only and PI-control. Equipment The apparatus is a steam-heated tank wherein steam is supplied through a circular coil in the tank. A pneumatic control valve in the steam line controls the flow of steam through the coils and thus the heat input to the tank. Water flows continuously into and out of the tank. The flow into the tank is regulated manually by a globe valve through a range of 0 to 6 GPM and can be quantified using a inline rotameter. A constant water volume in the tank is maintained by an overflow conduit located in the middle of the tank. The tank is equipped with a mixer to give a reasonably uniform temperature in the water in the tank. Figure 1 is a diagram of the system. A Foxboro 760 controller regulates the tank temperature by manipulating a pneumatic control valve on the steam line. A thermocouple in the tank sends the temperature signal to a Foxboro Electronic Temperature Transmitter, which produces a signal that is sent to the controller. The controller in turn produces a 4-20 ma output signal which is sent to a converter that changes the 4-20 ma current signal to a 3-15 psig air signal. The air signal is then sent to the pneumatic control valve on the steam line. The air line to the control valve is equipped with a pressure gauge. At a reading of 3 psig the valve is closed, and at 15 psig the valve is fully open. The tank temperature (controller input), valve position (controller output) and set point are sent to and recorded on a Foxboro strip chart recorder. The pressure of the air supply to the pneumatic converter is held constant by a pressure regulator. This air supply line also contains a water trap to remove any condensate that may build up in the air line. Water collected in the trap can be discharged through the connected plastic tubing by pressing the white button on the back of the trap. Foxboro 760 Controller The controller can be operated as P-only, PI, PD, or PID. You will use only P and PI modes. The controller panel is described on pages 9 and 10. You will not be concerned with the alarm indicators. As can be seen, the display panel shows three bar graphs with a digital reading above the graphs. The bar graphs show, from left to right, the values of set point, controller input (temperature) and controller output (valve position) from 0 to 100% of the range of operation. A digital reading of one of the three variables appears Checked: 1/17/2006

2 2 above the bar graphs. A dot is illuminated over the bar graph value that is being displayed. The desired digital display can be chosen by using the SEL button, which moves the illuminated dot over the bar graph that is being displayed. The display gives temperature readings, in degrees F, for tank temperature and set point, and valve position as 0 to 100% open. The buttons at the bottom of the controller are used to change settings and controller mode. The A/M button changes between automatic and manual control. You will not be concerned with the W/P button or R/L button. The controller has three settings: normal, read and set. Normal corresponds to normal controller operation, whereas read and set correspond to reading or changing the controller mode or controller parameters. In normal, the SEL button is used to change which parameter is being displayed above the bar graphs, while the up and down arrow buttons adjust the valve position for manual operation and the set point for automatic operation. The read and set settings contain the controller logic. The controller logic is set up in a flowsheet format that is embedded in the memory of the controller. Adjustments to the controller are made by following these flowsheets in a step-wise fashion, where each step is shown in the digital display. These steps must be followed to read current parameters or to change them. The flowsheets that you will need are attached. From normal, pressing the TAG key changes the controller into read (though the controller continues to operate with the same parameters). If the message VERSION/K0143RC is displayed, pressing the TAG key a second time will put the controller in read state. This occurs if read state is being activated for the first time since the controller was energized. Read allows for you to read, among other things, the present controller mode (P, PI, PID, etc.). However, you cannot make any parameter changes in the read state. Use the flowsheets on pages 11 and 12 of the manual to determine the present controller state. When in read or set state, you move through the flowsheet by pressing the ACK key to move along the solid lines in the flowsheet and the up and down arrow keys to move along the dotted lines. The current flowsheet box is displayed on the controller. Pressing the TAG key at any time will return you to normal operating state. While in read or set, repeatedly pressing the SEL key will step you back through the flowsheet along the solid lines. The controller mode can be seen in set or in read states. Set allows you to change both the controller mode and current parameters. Follow the flowsheets on page 11 and page 13 to make changes. Follow the flowsheet on page 11 to the box labeled SET? From this point, follow the flowsheet on page 13 to the box labeled OPTUNE/MODES? You are now able to change the parameters. If you make a mistake, pressing the SEL key will move you back through the diagram. Remember that the controller is still operating with the same preset parameters while it is in the set state. You will be concerned with two controller parameters, PF and IF. PF is the proportional band, and IF is the reset time, τ I. To change these parameters, follow the flowsheet on page 13 to the block with PF on the upper line. The present value of PF will be displayed on the lower line. The arrow keys are used to change the value. Once you have set the parameter to the desired value, press the ACK key to enter the value and move on to the next block. From this point, you can either move on to the IF block; or, if 2

3 3 you want P-only control, press the TAG key to return the controller to normal operation. Make sure to press ACK after entering the desired value, otherwise the new value will not be saved by the controller. Although you will only be concerned with P-only and PI control, there is no direct way to change the controller mode. The controller, as it is presently configured, is set up for PI control. In spite of this, you can closely approximate P-only control by setting the value of IF to its maximum value: 200. If you look at the equation for PI control (Stephanopoulos, p. 246), notice that the reset time, τ I, is in the denominator of the integral term. As a result, a large value of τ I will effectively eliminate this term and cause operation to approximate closely a P-only controller. Thus, for PI control, set IF to any desired smaller value of reset time, and, for P-only control, set IF equal to 200 (maximum). Once you have changed the parameters, make sure the controller is configured for automatic operation. Foxboro Chart Recorder The chart recorder has three pens. The green pen records the valve position, the red pen the tank temperature and the blue pen the set point. The chart paper is calibrated from 0 to 250, so you will have to convert the values on the chart paper to the true values based on the appropriate range. The temperature range, for both the tank temperature and set point, is 50 to 200 F, while the valve position ranges from 0 to 100% open (3-15 psig). These limits correspond to the bottom and top of both the bar graphs on the controller and the limits of the recorder chart paper. There is a linear relationship between the ranges and the chart paper scale, For example, a reading of 100 on the chart paper (40%) corresponds to a temperature of 110 o F (red or blue pen) or a valve position of 40% open (green pen). Note that it is good practice to check the recorder output against reliable instruments such as the the tank thermometer. The recorder has two operating speeds, slow and fast. The lever to change the speed is inside the glass door, underneath the paper tray. It is convenient for this experiment to operate in fast mode. In fast mode, the paper moves at a speed of 0.75 in./min. Also notice that the pens are not at the same horizontal position on the chart paper. The blue pen trails the red pen by about 2 mm, and the red pen trails the green pen by the same amount. Thus, at any time, the current value of the green pen (controller output) corresponds to the value of the red pen (controller input) that is 2 mm behind. This is an important consideration when evaluating the process reaction curve and especially when estimating dead time. This discrepancy can be removed automatically by scanning the charts, digitizing the traces, and applying the correction to reduce all "traces" to the same time scale. Note that the swing of the pens will need to be also corrected, although both of these corrections are straightforward. Occasionally the ink may stop flowing out of a pen, especially the green pen. Rapid movement of a pen also may cause it to temporarily stop writing. If it does not start writing once the pen has stabilized, carefully slide the recorder out of its housing and locate the ink reservoirs on the right hand side of the recorder. Insert the white plastic syringe into the vent hole for the particular color, which is in the upper left-hand corner of the reservoir, and push the syringe plunger to start the ink flowing again. See page 14, part 4 of the controller manual. 3

4 4 Calculating Parameters From Process Reaction Curves In the Foxboro controller, proportional band (PF) is defined as 100/K c and is a dimensionless controller gain. The dimensionless controller gain will be calculated from process reaction curves using Cohen-Coon equations with a dimensionless static gain. The dimensionless static gain is calculated as the % change in process output (tank temperature) divided by the % change in process input (valve position) for an open loop step. The percent changes are calculated as the magnitude of the change divided by the total range (150 F or 100%). For example, say a 10% change in valve position resulted in a temperature change of 30 F. The percent change in temperature is (30/150) x 100 or 20%. The dimensionless static gain is then 20/10 or 2. This is the value of K used in the Cohen- Coon equations. The reset time, IF, has the usual meaning. It is also calculated using the Cohen- Coon equations. Start-up Power-up the controller. You may want to learn how to change the controller parameters before you actually start the experiment. Once the controller is operating, learn how to follow through the flowsheets. This can be done without any of the equipment turned on. Also make sure you can change the set point or valve position, depending on whether the controller is operating automatically or manually. Once you are confident that you can change the controller settings, start the experiment. Before doing anything else you should purge the air line of any condensate that may have accumulated. This is especially necessary if it is humid. If the air going to the converter is not dry, moisture will damage the converter. There is an open-ended air line just inside the door in the room next door, 114-H. Open the valve on the line for about ten seconds and then close it. Next, power-up the recorder, making sure it is in fast mode. Turn on water flow to the tank and set the flow rate to about 50% on the rotameter scale, which corresponds to about 3 GPM. Adjust the mixer to a dial setting of 2. Put the controller in the manual state by pressing the A/M button so that the M on the right side of the controller illuminates (it may already be in manual state). Use the SEL button to select the bar graph corresponding to valve position (the rightmost bar graph). Use the down arrow to set the valve position at 0%. Be sure to mark on the chart paper where you started the experiment, and record all information in your notebook. Open the valve on the system air line, but do not adjust the regulator yet. Press the white button on the back of the water trap and hold it down for at least ten seconds to further purge water from the air line. Then turn the pressure regulator clockwise until the gauge on the air line reads 22 psig. After the water in the tank has reached the overflow outlet, open the steam valve. BE VERY CAREFUL, the uninsulated steam lines are VERY HOT! Adjust the valve position to 25% and let the system come to steady state. This should take about ten minutes. The pressure gauge on the air line should read about 6 psig. 4

5 5 Experimental With the control set to manual, make a step increase in the valve position and generate a process reaction curve. Then return the valve to its original position and generate a second reaction curve. Compute the Cohen-Coon settings for both P-only and PI-control from these curves. Average the results. Repeat at different valve position. How reliable are these values? Remember that you want to calculate a dimensionless process gain based on the percent change of the input and output. Also be careful to properly convert the values read from the chart paper to the true values based on the operating ranges. Put the controller in automatic by pressing the A/M button. Using the attached flow sheets, implement P-only control with the computed Cohen-Coon setting for PF. Demonstrate P-only control in tracking set point changes. Adjust PF in a systematic fashion and record dead time, rise time, settling time, offset, oscillation amplitude and period, where feasible. Examine all these variables for their dependence, if any, on PF. Now, with a chosen value for PF, test the controller s ability to reject disturbances. Do this by making a sharp step increase in the inlet water flow rate. Once a new steady state has been reached, return the flow rate to 50% (3 GPM). Then repeat this procedure with a step decrease. Implement PI control and repeat the above procedures and operations. Do not forget to note the values of the parameters in your notebook as well as on the chart paper. Do experiments to clearly demonstrate reset windup behavior. Compare and discuss the results. Be sure you understand reset windup and explain it clearly in your report. Describe the strategies that can be used to eliminate or reduce reset windup. Shutdown Once you have completed the experiment, turn the regulator counterclockwise to shut off the air flow then close the valve on the air line. Close the steam and water supply valves, turn off the mixer and power-down the controller and recorder. Remove the portion of the chart paper with your data from the recorder. Lab Reports The following items should be considered in preparing your report: 1. Include the controller output equations for proportional-only #(P-only) and proportional-integral #(PI) control. 2. In determining the controller constants from process reaction curves, use the fraction-incomplete method to determine the dead time and reset time. If you are able to digitize the curves, try describing the entire response with FOPDT model. 3. Explain the significance of dead time, reset time, and controller gain for both P- only and PI control. 4. Explain the advantages and disadvantages of P-only and PI control. Which mode provide more advantageous control of the water temperature during transients and steady operation? 5. Work out an empirical model that correlates an important characteristic of the system (e.g., overshoot, settling time) with controller variables (i.e., proportional band and reset time). Use this model to predict the optimum setting for the chosen characteristic. 5

6 6 Be sure to use graphs, models and tables to present and discuss your experimental results. The text must refer to and discuss the individual graphs, models and tables. Remember that this experiment is not exempt from random errors; handle appropriately. Apparatus Diagram. Your report should contain a schematic diagram that shows all the important features of the apparatus. On this diagram indicate the path of signal flow for the control loop using dashed or dotted lines in the conventional fashion. Clearly label the components shown on the diagram. Control-Loop Block Diagram. Provide a block diagram of the control loop. Be sure to identify the components and blocks to demonstrate clearly the relationship between the apparatus diagram and the control-loop block diagram. References: Bequette, B. W. Process Control, Modeling, Design and Simulation, Prentice Hall, Upper Saddle River, NJ Chapters 4 and 6. Stephanopoulos,?? Figure 1. Schematic of apparatus for the experiment 6