• Not Answered

What causes the direct acting PID controller OUT to open and then closes, when the PV is less than SP?

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.