PRV Discharge Piping dP Question
PRV Discharge Piping dP Question
(OP)
Hi,
I'm a pretty new mechanical engineer a couple years of college. Recently I've started getting involved in PRV sizing and piping calculations at work. I've been trying to educate myself on the requirements from ASME, API, etc.
API 520 states that the Critical Pressure is the lowest pressure possible at the nozzle exit of the relief valve and provides an equation to calculate this (below).
For an example, air k = 1.4, Set Pressure = 180 psig, Fire Scenario Relieving Pressure = 232.5 psia
P(cf) = P(1)*(2/(k+1))^(k/(k-1))
For k = 1.4, P(1) = 232.5 psia, P(cf) comes out to 122.8 psia.
For a given discharge pipe, the maximum allowable dP (lets refer to this dP as P(cf) - P(2) ) is either 10% Set Pressure or 21% Set Pressure depending on the determining the case. If P(cf), which essentially defines the starting pressure of the discharge piping, is 122.8 psia and P(2)is 14.7 psia for atmospheric discharges. How can the discharge piping dP ever fall within the allowable range?
I'm a pretty new mechanical engineer a couple years of college. Recently I've started getting involved in PRV sizing and piping calculations at work. I've been trying to educate myself on the requirements from ASME, API, etc.
API 520 states that the Critical Pressure is the lowest pressure possible at the nozzle exit of the relief valve and provides an equation to calculate this (below).
For an example, air k = 1.4, Set Pressure = 180 psig, Fire Scenario Relieving Pressure = 232.5 psia
P(cf) = P(1)*(2/(k+1))^(k/(k-1))
For k = 1.4, P(1) = 232.5 psia, P(cf) comes out to 122.8 psia.
For a given discharge pipe, the maximum allowable dP (lets refer to this dP as P(cf) - P(2) ) is either 10% Set Pressure or 21% Set Pressure depending on the determining the case. If P(cf), which essentially defines the starting pressure of the discharge piping, is 122.8 psia and P(2)is 14.7 psia for atmospheric discharges. How can the discharge piping dP ever fall within the allowable range?





RE: PRV Discharge Piping dP Question
RE: PRV Discharge Piping dP Question
In a choked flow through a PSV the critical flow pressure will exist at the exit plane of the valve flow nozzle while the back pressure is the pressure at the valve outlet flange. If the PSV outlet flange size will be low enough, most critical flow pressure can be transferred to the valve outlet flange as the point of back pressure indication. Then even with a short discharge line the back pressure might be much higher than 10% of the PSV set pressure and to overcome this excessive back pressure you may have to select a larger valve outlet size or balanced bellows valve type...
Then the critical pressure in a choked flow conditions wouldn't be a starting pressure to calculate the dP along the relevant PSV tailpipe. You can back calculate using Fanno lines/curves, describing the irreversible adiabatic flow of a gas at constant mass flow, to get the actual back pressure at the PSV outlet flange...
RE: PRV Discharge Piping dP Question
So, "How can the discharge piping dP ever fall within the allowable range?" Because of the pressure discontinuity at the exit plane of the PSV flow nozzle.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: PRV Discharge Piping dP Question
If I'm understanding this correctly, due to the shock wave/discontinuity inherent in critical flow, the pressure at the PSV nozzle exit would be P(cf). However, due to the pressure discontinuity, the pressure at the valve's exit flange will be substantially less, which is found by working backwards to find the back pressure built-up?
In other terms, when API 520 states that the "nozzle exit" cannot be below P(cf), this does not mean the exit flange pressure is also P(cf)? Does anyone know of a drawing/diagram depicting these pressures inside the valve so I can better picture it? The picture attached is my rough interpretation.
RE: PRV Discharge Piping dP Question
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: PRV Discharge Piping dP Question
RE: PRV Discharge Piping dP Question
Since you are a newbie, there is a button for that.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.