outlet piping pressure drop and built-up backpressure 2

[Mous1747]

the PSV outlet relieves to the atmosphere, so immediately i know the backpressure is 0 psig

however the outlet pipe + fitting will develop a pressure drop of 120 psig

my question is, what is my backpressure ? is it 0 or 120 ?

API 520 1.2.3.3 (J) built-up back pressure is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device opens

so my actual question is whether the outlet pipe pressure drop is considered backpressure?

the turbulent flow generated at the outlet fitting is counted as backpressure ?

 

[georgeverghese]

And if you are switching out to another type of PSV to handle the higher backpressure with no influence on PSV set pressure, redo the mandatory PSV inlet side pressure drop calc for the RATED capacity of the new PSV also.

 

[LittleInch]

Mous 1747

You have gone quiet on this and I' still unclear if you actually accept that regardless of the type of PSV your backpressure of 120 psi on top of your set pressure (which in your case = MAWP) results in a pressure 19% greater than the MAWP.

15th Oct post "@don1980 : yes the MAWP of the vessel equals the set pressure of the PSV at 615 psi , so yeah it's only 10% overpressure."
OP - "however the outlet pipe + fitting will develop a pressure drop of 120 psig "

615 + 120 = 735 psig
615 X 110% = 676.5
Exceedence of max overpressure - 58.5 psi

This exceeds the allowable 10%.

You don't appear to be addressing this issue.
I trust that someone is reviewing and certifying this design?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

LittleInch,

The 120 psi is not superimposed backpressure. It is built-up backpressure. If it was a conventional PSV, the back-pressure of 120 psig is outside the flow nozzle. The flow nozzle will be choked and the critical pressure ratio implies the pressure just inside the nozzle exit is roughly 332 psi (k = 1.27). So, there is a pressure discontinuity across the shock wave from about 332 psi to 120 psi at the nozzle exit.

But, he will need a balanced bellows PSV for the backpressure of ~20% overpressure.

Good luck,
Latexman

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

 

[LittleInch]

Ok, my view was fairly simplistic, but what you're saying is that this developed or built up back pressure for a BB valve makes no difference to the flow rate or max pressure upstream of the PSV??

It still doesn't sound a good idea to me, but I'll bow to greater knowledge.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

Yeah, here's a typical capacity vs % built-up back pressure curve:

Good luck,
Latexman

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

 

[e43u8]

The main point following to built-up back pressure about the 20% of the set pressure of conventional PSV has been described here; is reduction the PSV capacity at PSV relieving pressure due to higher back pressure than 10% (simultaneous lower disc lifting and lower differential pressure across the PSV) will force having the PSV overpressure equal to 20%, which isn't compliance with code and API requirement (limitation of 10% overpressure); hence a BB PSV should be applied.