Dual PID Control of Temp and Pressure using Steam


Thread Starter

Marty W

I am trying to control the temperature of a Dehyration column using two PID controllers. One PID loop controls the Temperature using steam flow. The other loop controls the steam pressure. Both are using an air actuated valve.

The loops seem to fight each other and the temperature loop takes a long time for changes to take effect. As the temperature valve opens, the pressure valve has to open also, because the pressure drops.

I have tried running the pressure loop in manual, but the pressure drops out as the temperature valve opens.

Any suggestion on how to configure the temperature and pressure PID loops? Thanks


One of the schemes I've used in this same scenario is to control pressure first while monitoring your rising temperature. When the temperature reaches 1-2 degrees below setpoint, trade off to pure temperature control and let the second loop slowly rise the temp to setpoint. I've even used time based ramping of the pressure setpoint to keep the pressure from rising too rapidly.

Make certain your temperature transducers and the control valves are somewhat close or you will experience "transport lag" that can make the loop oscillate significantly in attempting to hold a setpoint.

Hope this helps,

John Kelley

R A Peterson

Off the cuff I'd say get rid of the pressure control. It seems to serve no purpose and certainly will fight the temperature control loop.

(TECO) David Bergeron

Your temperature loop should be much slower than your pressure loop. If the loops are fighting each other, this suggests that your controllers are tuned such that the loops have similar time constants. You should be able to tune the pressure loop much faster than the temperature loop.

Another solution might be to use a self actuated pressure regulator to maintaing a constant steam pressure upstream of your temperature control

Yet another solution would be to decouple the loops using decoupling methods as explained in many process control books.

David Bergeron, P.E.
Thompson Equipment Co.
You can not have two loops:
Assume that so many kgCal are needed for drying so much material [infeed], then you must implement a mass-flow loop on the steam duct. After, cascade that steam mass-flow loop from the infeed mass-flow of material. In the case you monitor the dryness of the product at the outlet, you might need little correction comming back to the steam mass-flow loop. This will have to be injected in the mass-flow algorithm. Your system may not be able digesting part of the calculations. Almost surely, don't put any derivative action.
Regards:<[email protected]>
Remember: if you don't like the temperature, then modify the steam.
You may want to take a look at your pressure supply compared to the heat requirement for the dehydration column. If there was not enough steam supply to maintain somewhat of a constant pressure when the pressure control valve was at a constant opening (assumed 50 to 75 percent open), then the heat requirement for the column was greater than the steam header was able to supply. As long as the exchanger is still within the designed pressure limits and depending on the steam header arrangement and other uses for this steam, you may be able to fully open the pressure control valve in order to prevent the steam supply pressure dropping so much. The temperature control will operate such that an increase in temperature will cause the valve nearest the exchanger to open and add more steam.

As far as modifying PID control, it may be possible to add a bias to the pressure controller output from the temperature output. This way, if the temperature increases, there will be a feed forward value added to the pressure output which then sets the opening of the pressure control valve.