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WE HAVE a tempered water system supplying cooling and heating to an exothermic reactor. We are using a Cascade/Split range control method. The Master controller is for the Reactor Temperature, the Slave is for the Tempered Water Temperature. The slave outputs goes to a splitter, which then controls the steam valve and water valve for the tempered water.
The diagram below is a typical split range control for example: a tempered water system. The water temperature is the PV for the PID. A typical splitter would be 0-49% for cooling, and 51-100% for heating (with steam). In some applications, another option is to shift the split point if the heating process gain is higher than that of cooling (different ‘strengths’ in the valves). For example a 0-32% for cooling, 34-100% for heating split.
My question is in regards to the difference in ‘strength’ between the heating valve and cooling valve (different process gains). Would the diagram below work? It seems that because there is a difference between the heating and cooling gains that we could have each valve have it’s own PID controller. That way we can tune each valve for it’s own gain. The Add and Subtraction of 5 degrees is to make sure each PID is not seeing the same setpoint (to give them a ‘deadband’) and are not constantly fighting each other.
Is this an acceptable method of control? What is wrong with it?
ANSWERS:THE NORMAL configuration is to use an =%, fail closed steam valve with a positioner that operates it between 50-100% of the output signal from the reverse acting slave PID set at 10-20% proportional band and a little integral. The water valve is also =%, it fails open and it operates between 0-50% of the output signal from the reverse acting slave PID. In order to prevent reset windup in the master, we also provide the master TIC with external reset from the slave transmitter output.
In your existing configuration, I do not see the need for the splitter, as the positioners fulfill that function. In the proposed new configuration, I don’t see the need for inserting dead band at the PID (particularly not such large one as +- 5 degrees F), because the dead band can be provided by setting the positioner ranges 0-49% and 51-100%.
Because the gain, time constant and deadtime of the process is different during cooling from that which exist during heating, it is reasonable to modify the tuning constants when switching from cooling to heating, but 1) most of the integral and derivative changes have to be done in the Master and 2) I would not adjust the gain in two separate slave PIDs, but in the same one. This is because if you have integral in the positional algorithm of the slave controller, the internal reference would be lost at the time of switching.
I will also ask some other colleagues about your question.
I AGREE with your assessment of the proposed system. The dead zone inserted between the two slave controllers will cause cycling in the primary loop. Additionally, the bias introduced into the set points will cause offset if external reset feedback is applied.
There is usually no conflict in a batch reactor control system between PID settings for heating and cooling, because heating only brings the primary temperature up to set point, where cooling takes over. Therefore the cascade system should be tuned for cooling, and this usually gives acceptable results for heating.
Where this is not the case, separate PID settings can be scheduled into the slave controller, depending on which valve is open. This is easy to do with digital controllers, and is a standard feature in some controllers (e.g., Foxboro).
I HAVEseen dual PID controllers used for pH control where there was an acid and base reagent. Unfortunately the difference between the set points needed to keep the controllers from fighting and insuring both valve are not open at the same time is highly dependent upon the dynamics and tuning of each PID. So I prefer a single split ranged controller with its tuning scheduled per the valve throttled as you mentioned. So far as the split range point, operators expect 50% but a different point may help compensate for the difference in process and valve gains if the controller gain cannot be scheduled. Since the process dead time and time constant is also different for heating and cooling, the integral time will also be different but if mostly proportional action is used for the slave controller, it may not need to be scheduled.
I favor the use of the splitter in the DCS configuration instead of split ranged analog positioners because it eliminates special positioner calibrations. This was particularly important in the days of pneumatic positioners. However, if the positioner is digital, smart, and has its calibration accessible from the control room, the accuracy and maintainability of split ranging in the field is no longer as much an issue. It is especially important that the valves do not have a deadband or stick-slip as the trim goes into and out of the seat that is greater than half of the split range gap. The deadband and stick-slip cited for many valves is at the ideal throttle position of 50% and does not show the effect of the extra friction from seats and seals at shutoff.
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