This column is moderated by Béla Lipták (http://belaliptakpe.com/), automation and safety consultant, former chief instrument engineer at C&R and former Yale professor of process control and editor of the Instrument Engineer's Handbook. If you have automation related questions for this column, write to firstname.lastname@example.org
Q: My question is this: Can we replace a P controller with a linear function generator in controlling the level on a deaerator?
General information about the control loop(s):
- The issue is related to a deaerator (DA) level control. There are currently two loops controlling the DA level, i.e. the make-up control loop and valve and the surplus control loop and valve. The setpoints of the loops are set at 142 mm and 213 mm, respectively. The total DA level range is 0 mm to 355 mm. Make-up water is supplied indirectly to the DA, i.e. it will go first through the condenser and condenser level loop. The surplus goes back to the condenser.
- The loop design specification states that there should be a linear relation between the level control error and the corresponding valve's position. The position of the make- up valve should be between 0% to 100% for a corresponding level error range of 0 mm to 142 mm; it should start controlling when level falls below 142 mm; and fully open when level reaches 0 mm. The position of the surplus valve should be 0% to 100% for a corresponding level range of 213 mm to 302 mm; that is, it should start controlling when level increases above 213mm, and fully open when level reaches 302 mm.
- Between 142 mm and 213 mm both valves should be closed.
- Due to above requirements, P controllers have been used for both make-up and surplus loops, with gains of 2.5 for the make-up and 4.0 for the surplus. Both loops are implemented in digital control system (DCS) controllers.
The operational issues:
- The issue, according to the operator, seems to be related to the used P type controllers in both loops.
- The operator has the view that the control loops very often seem to ignore the SP, and seem sometimes to operate the valves in a direction opposite to the expected. For example when the level is <142 mm, instead of opening the make-up valve, the loop will close it.
- The above often happens when the loop has tripped to manual, and operators change it back to auto.
- As a solution, some more experienced colleagues have proposed replacing the P controllers with a linear function generator, and ensure the required logic to achieve manual-auto bumpless transfer is in place. That solution is expected to always produce the desired relationship between level control error and the valves' positions, and also ensures both valves are closed between 142 mm and 213 mm.
- I have concluded that the behavior seen by the operator is "normal" for P controllers. I am not sure if replacing the P controllers with the linear function generators is correct, but I can't figure out the reason or what consequences will appear (especially bad ones) in operating the loops if we implement this change.
Do you agree with the proposed change? What control issues could that change generate? Any suggestions for any changes in the loop would be very welcome.
A: Gap level control can be provided by using two controllers or by splitting the output of a single controller among two valves with a gap in the center of the control signal where both valves are closed.
In your deaerator level control system, the make-up valve must have an open-failure position, which means that the controller has to be reverse-acting, and therefore its gain has to be negative (Figure 1). If it's not, that explains your problems. When a proportional-only (P) controller is started up, the bias is normally adjusted to 50%, so that when the level is on the setpoint, the valve will be 50% open, and this opening will provide the "normal" flow. If the load (the required water flow) moves away from the 50% setpoint, the proportional controller is incapable of changing the flow to return the level to setpoint because it needs an error (e) in order to change its output signal (m) to the required new opening. This output is m = (controller gain)(e) + b, so that the output "m" equals the 50% bias (b), when the error is zero.
Therefore, whenever the required valve opening (the load) changes, an error must develop, and this error is called the "offset." For example, if the load "m" has changed to 60%, while the normal load (b) was 50%, the offset error is Offset Error = (60% - 50%)/(controller gain). Consequently, if the controller gain is high, (the slope of the operating curve in Figure 1 is steep), and proportional control is acceptable because the level will not change much while the valve is throttled (the offset is small).