Trouble: Flow Controller is unstable!
By Jim Henry, Ph.D, P.E.
University of Tennessee at Chattanooga
2004
Our POTW (“Publicly Owned Treatment Works”) sewerage treatment plant in Chattanooga, TN, has alarge filter press to filter out the sewerage sludge solids in order to send the solids to the city landfill. The filtrate water is then processed further before returning to the Tennessee River.The filter press operates in a repeating batch mode. Between batches, the filter media and plates must be washed. The manufacturer of the press specifies that the water flow rate to the washing nozzles has to be maintained between 7 and 10 lb/min.
The flow rateof the wash water is maintained by a variable speed centrifugal pump. The motor driving the pump is a variable-speed motor. The speed of the pump is under feedback control to maintain the desired flow rate to the washing nozzles. The controller is a proportional-integral controller. A diagram of the pump, washing nozzles and control system is shown below. Also shown on the diagram are 2 other wash lines that are used by operators in the plant to wash up spills when needed.
Figure 1. Schematic diagram of the Filter Wash System
Operational Situation:
Plant personnel have been happy with the system (including the control system) for years. Ever since plant start-up, we’ve been able to maintain the wash-water flow rate within specifications. Here are graphs of the air pressure (sampled by FT-301) and Pump Speed (from FRC-301 output, expressed as %) under closed-loop feedback control for two recent periods.
Flow(lb/min) at FT-301 vs time (sec) /
Flow(lb/min) at FT-301 vs time (sec)
Pump Speed from FRC-301 Output /
Pump Speed from FRC-301 Output
Figure 2. Performance while 2 auxiliary washing stations are in use / Figure 3. Performance while 1 auxiliary washing station is in use
The controller tuning parameters were set by installation contractor and have not been adjusted. The value of Controller gain is 4.0 %/lb/min; the integral (reset) time value is 1.0 second.
Unfortunately, with a recent improvement in operating procedures and equipment upgrades, we frequently have no need for either auxiliary washing stationto be used. When neither auxiliary washing stationisin use, our operators tell us that the wash-water flow rate is very erratic. Here are graphs of the wash-water flow rate (sampled by FT-301) and Pump Speed (from FRC-301 output) under closed-loop feedback control for a recent period with neither auxiliary washing station in use.
Flow(lb/min) at FT-301 vs time (sec)
Pump Speed from FRC-301 Output
Figure 4. Performance while no auxiliary washing station is in use
Figure 4 shows that the desired flow to the wash-water nozzles goes above the desired maximum of 10 lb/min and below the desired minimum of 7 lb/min. Our maintenance chief is not happy about this.
Assignment:
Determine what the controller tuning parameters should be in order to get better performance (no oscillation, flow rate within the specifications). This can be done based on step-response testing of the system.
Fortunately, the filter press is in stand-by status today. This means that you can conduct some step-response experiments to determine the system parameters necessary to design the controller for the performance you desire.
You can conduct step-response experiments by going to this web site:
Below is what that page looks like.
Figure 5. Web page for running experiments
Fill in the form with your information in items #1 - #7, then click on the “RUN EXPERIMENT” button at #8.
The results of the experiment can be analyzed to determine the system gain, K, the system’s first order time constant, , and the system’s dead time, to.
Hints:
To do the analysis, examine the step response in detail where the step happens. See the figure 5 below. Figure 5 is a copy of what the results of the experiment will look like.
Figure 5. Sample experimental results
To expand the area of interest, you can access the data by click on the “Data as Text” icon as shown at the right.Select all the data on that page and “Paste Special” with “Unicode Text” into an Excel spreadsheet.
Plotting the data will yield a graph such as in Figure 6, below. /
Figure 6. Detail of step response
(Step input of 20% at 40 seconds)
Having the system’s parameters for appropriate operation configuration, use controller tuning formulas to determine the controller tuning parameters that you want to recommend.
Reference:
Smith and Corripio, Principles and Practice of Automatic Process Control, 2nd edition.
Demonstration of Effective solution:
Once you have improved tuning parameters, try them out on the system. You can conduct step-response experiments by going to this web site:
Possible Problems:
Instead of the “nice” response as seen in Figure 5, the experiment may give results like those shown to the right, here. These can not be analyzed. Results like this are due to the start-up effects in the system. While the system is in “stand-by” status, all the pipes drain back to the pump. So the graph shown at the right is a result of surging of water where air has been.To overcome this effect and thus get a graph that you can analyze, you can run TWO experiments consecutively. The first experiment is a “throw away,” just used to fill the pipes so the system behaves normally. /
That is, in fact, what was done to get the results in Figure 5.