Psv inlet pressure drop 3% rule

[Justice100]

Hi all

Just looking for some wisdom on where the 3 % psv inlet dp should be calculated from. As there are differing views internally.

1. Pressure let down - a pressure control valve lets down pressure, there is no associated vessel so the psv is located on the downstream piping. Should the pressure drop be taken from the pcv or the tee off to the psv?

2. Reciprocating pumps - the pumps supply various users. The pumps have psvs at the pumps for blocked discarge case. However, one user system has a lower dp so there is a psv for the piping supplying to this system with a psv set for this system only. If supplying this user the operating pressure will be reduced accordingly. Where do you take the dp from? The view is from the pump discharge but since the length is so long then 3% is impossible to achieve.

 

[LittleInch]

Plenty of responses here: https://www.eng-tips.com/threads/psv-inlet-piping-dp-question.502295/

For me the answer is simple for the psv valve. It is the pressure from the connection of the psv to the item, vessel or pipe, being protected using the set pressure of the safety valve.

So e.g. your piping has a design pressure of 100 bar. The losses in the psv set up at full flow should not exceed 3bar. This includes tee entry losses, losses i the piping and valves to the inlet flange of the psv and losses wihin the valve itself.

Don't understanding your second case how you can operate with two different design pressures on what sounds like,e the same bit of pipe. The lowest design pressure sets the design pressure for the whole system unless there is some sort of is.ating valve or pressure control valve.

All the other things are nothing to do with the valve itself.

The 3%rule is there to stop the valves chattering and flowing then stopping then flowing because the pressure drop in the valve Inlet system is too high.

 

[Justice100]

Thanks little inch.

I am talking about where there is no vessel. For a let down station then I’ve seen it taken from the psv inlet tee normally as you say. My colleague thinks for scenarios with no vessel the dp should be from the pressure source. I can’t see that it explicitly states this in api 520 part 2 though. API only really refers to vessels and is not explicit when there is no vessel… or not that I can see.

For the 2nd one I’ll have to draw a picture…

 

[LittleInch]

Normally you leave the PSV as close as possible to the pressure source.

But the 3%is for the psv itself. If you have a large pressure drop then you might need to adjust the set pressure of the psv.