We are using Azbil make smart positioners. Recently we are observing that quite a few valves inlet pressure to positioners is hunting (that is lot of variation is seen in the inlet pressure, although output is stable). My question is what is the reason for this variation and what should be possible solution.

 

The inlet air is has a mechanical pressure regulator somewhere upstream.

The changes in pressure could be
a) a regulator not sized properly for the load

b) a properly functioning proportional-only regulator and the changes in output pressure reflect the 'droop' inherent in varying flow rates. The 'droop' is change in pressure with change in flow. It's inherent in how proportional-only regulators operate.

c) malfunctioning regulator

Go check out the pressure regulator upstream and see what it's doing.

 

Sir,

The regulator is giving a constant pressure. there is variation only in positioner inlet gauge.

 

The Azbil SVP3000 'smart' positioner is a HART 4-20mA current pneumatic valve positioner.

The only air 'inlet' on this (or any electropneumatic) positioner is supply air.

The control signal is an input to the positioner.

The positioner's pneumatic OUTPUT drives the actuator, not the input.

A positioner's pneumatic OUTPUT is supposed to vary and change as needed to achieve the correct valve position determined by electrical control signal and the positioner's configured response characteristic. Doing so is its purpose and function.

Use a HART communicator or a milliameter to read the current input control signal and see the correlation between the control signal and positioner's output pressure.

 

Sir,

Thanks for your reply...My issue is not this..

I think i am not able to explain the issue...i will try to explain again..

We have a positioner.. Inlet pressure to this positioner is through a AFR.

AFR is giving a constant pressure. Positioner has this pressure as inlet pressure...

My problem is that even though the pressure from AFR is constant, the Positioner inlet pressure is continuously hunting as seen on the inlet gauge (in my case its varying between 4 o 4.5 kg/cm2)

During this time, there is no hunting in the outlet. i.e if valve is at 75 % pos., it remains at this position itself.

We have around 68 control valves in our plant. Out of this 8 are exhibiting this variation but the remaining don't have this issue.

Kindly suggest the possible cause for this behaviour.