Over sized relief valve with reduced discharge piping
Over sized relief valve with reduced discharge piping
(OP)
I am new to the pressure relief valve world and having an issue on a relief valve that has already been installed. The valve is a conventional relief valve for pipe thermal relief of vapor and the outlet is piped to a vent. The capacity of installed valve is about 4 times of the flow rate needed. The valve inlet/outlet sizes are 1/2" and 3/4". However, the discharge line is reduced to 3/8" due to limited space. I am concerned about if the valve will still function properly. I understand the valve capacity will be reduced due to the smaller discharge line and the backpressure probably way beyond 10% of the set pressure. Since the valve is oversized, is it possible to use the backpressure correction factor to calculate the reduction on the capacity? Are there other concerns, chattering?
Any education and advice are greatly appreciated.
Arbor
Any education and advice are greatly appreciated.
Arbor





RE: Over sized relief valve with reduced discharge piping
Does the valve vent (discharge) to atmosphere?
Sometimes its possible to do all the right things and still get bad results
RE: Over sized relief valve with reduced discharge piping
RE: Over sized relief valve with reduced discharge piping
Have you found the outlet pressure drop on the PSV record, or calculated it yourself? Is it a problem or not?
Chattering and fluttering are the concerns. Unstable operation which can lead to failure. Depending on size, pressures, and the fluid handled, sometimes the failure can be catastrophic!
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
P1^2-P2^2=25.2*(S*Q^2*Z*T*f*L)/D^5.
p1: upstream pressure psia
P2: downstream pressure psia
S: Specific gravity
Q: gas flow MMSCFD
Z: compressibility factor
T: gas flow pressure
f: friction
L: pipe equivalent length
D: pipe ID
The pressure drop is quite low, below the 10%. However, I am not really confident if my approach is correct,i.e., are there any other items contributing to the backpressure? Is the capacity really OK even the line size is half of the outlet size? Am I missing something?
Thanks
RE: Over sized relief valve with reduced discharge piping
For further help we would need a LOT more detail. In fact, we'd need all the detail you needed to do the calc. Why don't you attach that?
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
If the fluid disposed poses some HSE issues, then all related design mitigations should be in place. The fluid released should also not condense in the exit line at the lowest ambient temp ( and create a liquid static head backpressure) , else you'll need some kind of low point drip leg in the exit line.
RE: Over sized relief valve with reduced discharge piping
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
Thank you,
RE: Over sized relief valve with reduced discharge piping
The 0.7 psi vs. 150 psi dP indicates problems in the methodology to me.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
As Latexman says, the receiver (landing) pressure is known. Assume flow, calculate pressure drop (= get pressure downstream of the PSV), then calculate flow for given pressure differential upstream/downstream PSV. The calculated flow must be equal to the assumed flow.
In this way you can confirm what the valve (and the piping) will actually pass through, and then it will be easier to see if other phenomena will occur as well.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Over sized relief valve with reduced discharge piping
Take that flow at atmospheric pressure and at whatever temperature assumption you are going to make (adiabatic to isothermal), and see if the exit plane velocity calculates to Mach << 1, Mach < 1, Mach 1 , or Mach > 1. That screening will give you a hint how the problem should be handled.
If Mach << 1, probably incompressible. This method may have to be adjusted later depending on the results.
If Mach < 1, Mach 1 , or Mach > 1; definitely compressible flow.
In all cases though, it's easier IMO, to work backwards, from the atmospheric exhaust to the PSV exit nozzle connection.
If it's not possibly to accommodate a conventional PSV and you have to use a bellows PSV, then flow is an unknown since Kb can change.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
Dejan IVANOVIC
Process Engineer, MSChE
RE: Over sized relief valve with reduced discharge piping
Thank you again,
RE: Over sized relief valve with reduced discharge piping
You ask a lot of good questions, some of which are confusing to me, like "What would be my upstream pressure, 235 or 26 psi?". This makes me think I do not fully understand your installation. And, after about 14 posts we still do not know all that is needed to answer on this specific installation. What is the PSV set pressure? I don't think you ever said. What is the normal operating pressure? What is the gas/vapor, or at least the MW? How else will we estimate and double check the density? A dimensioned sketch would be nice. What about that vent mentioned in post # 1? Diameter, length, etc? For us to be the most help to you, we really do need about the same data as you needed to run the calcs. If not, I'd have to be the Amazing Kreskin to get it right; and I'm not. So, if you want some answers to your questions, provide the information, up front.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
The isothermal compressible flow calc should be used with known vent tip exit press to derive the pressure at PSV exit. Max line velocity should typically be less than 0.5Mach. Pressure upstream of the RV would be set press plus accumulation permitted by the Code applicable at your location.
RE: Over sized relief valve with reduced discharge piping
Just for reference, from my Crane TP410, for 10 cfm of "free air" (60o F and 14.7 psia, which is close to a lot of "standard conditions") the pressure drop for 100 feet of Schedule 40 3/8" pipe is 1.26 psi. 20 feet would be 0.25 psi.
When I don't know much about the fluid being handled, I look at what seems reasonable with air, water, or steam to try to "rough in" the answer. In this case, air seemed most reasonable to me.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.
RE: Over sized relief valve with reduced discharge piping
RE: Over sized relief valve with reduced discharge piping