While validating the fire relief case for a propane drier, I came across a discharge pipe sizing issue that I'd like some advice on.
The drier has a balanced bellows style PSV with a set point of 445 psig. Using the API 521 guideline for vessel relief when exposed to external fire, I calculated the required relief capacity to be 26,412 pph. Allowing 121% overpressure in the fire case, I found that the Pcritical = 328 psia, the flow will be choked, and the PSV required an orifice of 0.595" to meet the required relief capacity assuming the backpressure is low enough that Kb = 1.0 (which I later verified). The current PSV has an H size orifice of 0.785" (which satisfies the >0.595" required).
The question I have centers on the discharge pipe backpressure. The discharge line from the PSV is ~100 ft effective pipe length that is 3" nominal diameter schedule 40. This 3" line connects to an 18" flare header that runs ~2 psig.
Using the required 26,412 pph mass flow along with the 2 psig downstream flare header pressure, I determined the outlet pipe pressure drop is large enough to cause the gas to choke a second time in the 3" PSV discharge piping. Can double choking like this occur?
Assuming that double choking can occur, I assumed adiabatic compressible choked flow through the pipe (since it is a short run and flowing quickly through the pipe) and iterated the adiabatic compressible pipe flow equation to determine what inlet pipe pressure would be needed in order to satisfy mass balance continuity. From this iteration, I found that once the discharge pipe inlet pressure reaches ~63 psig, mass balance continuity is achieved across the PSV choke flow and the pipe choke flow. Since the Pcritical for the PSV outlet is 328 psia (313 psig), the PSV is still choked so the increase in downstream pressure shouldn't affect the relief case, right? Furthermore, from the API 520 Kb chart for bellows backpressure the backpressure/set pressure isn't high enough to cause the Kb to drop below 1.0. It also should be noted, the 63 psig in the flare discharge piping is within the flare pipe design MAWP.
Anyway, I'm just curious if anyone else has ever come across such a second choke in flare outlet piping - and if you agree with the methodology I used (adiabatic compressible flow in discharge pipe) to determine backpressure and verify relief valve capacity. Personally, I don't think choking in the PSV discharge piping is a good idea and should be avoided in new designs - but this is a case where it is an existing design.
I am a young process engineer and somewhat new to this, so any advice from the experienced process engineers is much appreciated
The drier has a balanced bellows style PSV with a set point of 445 psig. Using the API 521 guideline for vessel relief when exposed to external fire, I calculated the required relief capacity to be 26,412 pph. Allowing 121% overpressure in the fire case, I found that the Pcritical = 328 psia, the flow will be choked, and the PSV required an orifice of 0.595" to meet the required relief capacity assuming the backpressure is low enough that Kb = 1.0 (which I later verified). The current PSV has an H size orifice of 0.785" (which satisfies the >0.595" required).
The question I have centers on the discharge pipe backpressure. The discharge line from the PSV is ~100 ft effective pipe length that is 3" nominal diameter schedule 40. This 3" line connects to an 18" flare header that runs ~2 psig.
Using the required 26,412 pph mass flow along with the 2 psig downstream flare header pressure, I determined the outlet pipe pressure drop is large enough to cause the gas to choke a second time in the 3" PSV discharge piping. Can double choking like this occur?
Assuming that double choking can occur, I assumed adiabatic compressible choked flow through the pipe (since it is a short run and flowing quickly through the pipe) and iterated the adiabatic compressible pipe flow equation to determine what inlet pipe pressure would be needed in order to satisfy mass balance continuity. From this iteration, I found that once the discharge pipe inlet pressure reaches ~63 psig, mass balance continuity is achieved across the PSV choke flow and the pipe choke flow. Since the Pcritical for the PSV outlet is 328 psia (313 psig), the PSV is still choked so the increase in downstream pressure shouldn't affect the relief case, right? Furthermore, from the API 520 Kb chart for bellows backpressure the backpressure/set pressure isn't high enough to cause the Kb to drop below 1.0. It also should be noted, the 63 psig in the flare discharge piping is within the flare pipe design MAWP.
Anyway, I'm just curious if anyone else has ever come across such a second choke in flare outlet piping - and if you agree with the methodology I used (adiabatic compressible flow in discharge pipe) to determine backpressure and verify relief valve capacity. Personally, I don't think choking in the PSV discharge piping is a good idea and should be avoided in new designs - but this is a case where it is an existing design.
I am a young process engineer and somewhat new to this, so any advice from the experienced process engineers is much appreciated
