In my refinery, I encounter several situations where the flow rate (PV variable) varies significantly even when the controller (OP variable) is set in manual. Transmitters are Vortex flow. Can anyone of you tell me the reason for this and the approach for troubleshooting it?

 

Inlet pressure to the valve changes.

With the controller in manual mode holding the valve at a fixed, given position, the changing inlet pressure results in a changing DP across the valve which results in a changing flow rate, as reflected in the vortex readings.