5 replies to this topic

[KeJoSa2010]

Hello,

I was under the impression that the allowable backpressure on a pilot operated type PSV is 50% of the PSV set pressure. I recently read a report in which they said that it is 100%. I am fairly new to this so, I would appreciate anyone willing to clarify this for me.

Thanks,

[flarenuf]

hi
Pilot PSV's are not affected by backpressure as its the small pilot valve that actually sees the bp and not the main valve.
the small valve is vented to atmosphere so it really doesnt see a backpressure.

see attached sketch

Attached Files

•  PSV Pilot.JPG   83.43KB   61 downloads

[latexman]

Back pressure is not a stability (valve chatter) concern with a pilot operated type PSV because it has no effect on the main valve actuation device. However, there is an influence of back pressure on a pilot operated type PSV. It is to reduce flow capacity due to the reduction in driving force (delta P). At BP = 100% set pressure, the only driving force left is the over pressure.

Edited by latexman, 07 January 2013 - 01:42 PM.

[fallah]

_{Hi,
Allowable backpressure equal to 50% is respected to keep the nominal capacity of the pilot operated PSV, then if you have no concern about the capacity reduction it is possible having backpressure higher than 50% and even higher than 100% provided that back flow preventer has already been considered in the valve system.}

Edited by fallah, 08 January 2013 - 02:59 AM.

[KeJoSa2010]

Thank you all for the insight, it has been most helpful!

[mohagheghfard]

Mr. Fallah,
Capacity of PSV is the main concern of downstream calculations, otherwise can cause not damping the overpressure as it must. Anyhow according to API, 50% bp associated with Balance Bellows type PSV and for pilot operated the maximum allowable bp shall be liased with vendor and some time close to 100% is acceptable..