PSV Inlet Outlet lines

Intisar · science forum beginner Joined: 17 Jun 2006 Posts: 1

Dear All,

1. Can PSV outlet line reach sonic velocity???

2. And is that correct that API suggests that Inlet line pressure drop
should be 3% of set pressure and outline line pressure drop should not
create backpressure more than 10% of the set pressure??

3. A PSV orifice designation has more than one inlet outlet sizes; What
is the criteria or basis of selecting particular inlet outlet size for
a specific designation??

Best Regards,
Intisar Ali

Rob Brendel · science forum beginner Joined: 04 Feb 2005 Posts: 3

1. Sure, velocity through a PSV outlet can approach sonic. But you sure
wouldn't want it to get even close since that means the flow through
the PSV is no longer determined by the orifice you've specified; the
actual flow would be smaller than what the PSV is sized for due to the
outlet pipe dimensions (size and equivalent length).

2, The "3% Rule" is from ASME pressure vessel code (API RP 521 calls it
"non-mandatory", by the way) and is reinforced in API RP 520 part 2.
It's 3% from the protected system to the PSV pressure sensing point,
which is not necessarily the PSV inlet flanges for pilot-operated
valves.

The maximum allowable back pressure at a PSV discharge varies for
different types of valves (conventional vs. balanced bellows vs.
pilot-operated). See the API documents for what you need to know here.

3. Sounds like you need to get a copy of API documents -- this one is
covered in RP 526.

Intisar wrote:

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