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PSV Inlet Piping Size

PSV Inlet Piping Size

PSV Inlet Piping Size

A safety valve with a socket welded inlet connection, indicates an inlet size on the valve nameplate of 0.75". How does this work? If the inlet piping is 0.75" NPS, Sched 80, the pipe ID is 0.742". But if the inlet piping is 0.75", Sched 160, the pipe ID is only 0.619". Wouldn't the Sched 160 piping create an inlet flow restriction?

RE: PSV Inlet Piping Size

Yes, you would have greater pressure drop through the sch160 line.

But what is important is not that there is less area to flow fluid. But whether if you have 3% or less pressure drop through that line if you are required to satisfy ASME VIII & API.

RE: PSV Inlet Piping Size


Thank you vladrath. But I'm confused on how the Sched 160 piping would meet the requirements of ASME Section VIII, Div. 1, Paragraph UG-135 (b)(1) where it is required the opening through the pipes must have at least the area of the safety valve inlet?

RE: PSV Inlet Piping Size


You have to weld a nominal pipe size 0.75" (or fitting) into the inlet connection of the PSV. There is no other option. The schedule, and therefore the pipe ID, is determined by you and the pipe spec for the pressure and temperature requirements it will see. If you select 0.75" Sch. 80 pipe in the inlet connection of the PSV, you have officially set "the area of the pressure relief valve inlet", right? Then, you cannot use 0.75" Sch. 160 pipe upstream of that 0.75" Sch. 80 PSV inlet pipe and downstream of the pressure vessel. You could use 0.75" Sch. 40 or 1" Sch. 160 though.

As vladrath mentioned, the 3% rule also goes into determining the pipe ID you can use.

Both criteria must be satisfied to comply.

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

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