30
Forbes Marshall Pilot Operated Pressure Reducing Valve
3)
Hunting:
Hunting or Pressure Fluctuations may coincide with variations in steam load. In such case,
check following before dismantling the Forbes Marshall Pilot Operated Pressure Reducing
Valve.
1.
WET Steam- Ensure steam is not wet and Moisture separator is installed before the
pressure reducing valve & the steam trap below it, is operational.
2.
Up Stream Pressure – It is recommended to have stable upstream pressure however the
Forbes Marshall Pilot Operated Pressure Reducing Valve will give constant downstream
pressure with + or - 20% variation in designed Upstream pressure.
3.
Partial Blockage in Upstream - If the pressure drops during full-load conditions, it is
possible that there is a partial blockage in the upstream line or that the upstream pipe
work is undersized. Steam Line should be sized properly to carry the required steam
flow rate at given pressure considering steam velocity of 25 M/S. Please refer steam line
sizing chart for correct steam line size.
4.
Isolation valves - Ensure Upstream & Downstream Isolation valves are in full open
condition. By pass valve should be in full closed condition and it should not be leaking.
5.
Upstream Strainer Clogged – Ensure upstream Strainer is in clean condition: Clean it if it
is found as clogged.
6.
Bypass valve – Check whether the bypass valve is leaking. Replace if found so.
Failure Mode Possible Cause
Remedy
Hunting
Sticking of push rod
in the Main Valve
Chamber
Check that the Main valve pushrod is not sticking. Open & clean
it. Also check whether the push rod outer surface is not
deformed. Replace the push rod if it is found deformed.
Diaphragms Over
Stretched /
permanently
deformed
If Pilot diaphragms or main diaphragms overstretched or
deformed -replace the same.
pressure reducing
valve sizing.
Set the pressure reducing valve in no flow condition i.e. all
process valves are closed. Now Apply full- load to the pressure
reducing valve by opening all the process valves. If the
downstream pressure drops excessively during full-load
condition but it is maintaining by opening of by-pass valve, it is
likely that the valve is undersized; in which case it should be
replaced. Please refer the pressure reducing valve sizing chart
given in Technical information sheet to know the correct
pressure reducing valve size. Consult Forbes Marshall for correct
sizing of the valve.
Check Split pin port
Check the port is clean and the split pin is free to move
