Hi,
For a pressure control valve in the flare line, PID action is set to direct, and when the PV (pressure) is less than SP, the valve is fully closed (MV = 0). When PV (pressure) started to increase to match the SP, the PID controller demanded OUT (MV) be opened and closed.
When the PV < SP, why does the valve open and close when it has to be closed until the PV goes above SP?
Gain: 2
Reset: 150s
Rate: 0s
PID Structure: PI action on error, D action on PV.
See next reply...
Hello Rishi,
First of all: Your graph has only a few datapoints. This obscures the details of what is happening.
DeltaV uses a velocity form algorithm of the PID equation. Essentially, this means that the loop does not work with the error, but with the change in error. If that would not be the case (i.e. using a positional form algorithm) the valve would not start to open until the PV reaches SP, like you mention. But then you are going to encounter a large overshoot as the valve starts to open too late.
To prevent the valve from opening too early, I think DeltaV uses the concept of one-half proportional bands like Provox did. The valve does not start moving until thePV is within one-half of the proportional band from the SP. Or, said differently: The valve does already start moving as soon as the PV is within one-half of the proportional band from the SP.
I'm mentioning Provox, as I cannot find it in the DeltaV Books Online. Here is an excerpt from the Provox manual "Configuring the SRx Controller Family Volume 1":
Not a complete answer, but I hope it helps.