Flow Rate and Pressure Changes when a Reducing Valve Fails
Flow Rate and Pressure Changes when a Reducing Valve Fails
(OP)
I am verifying the integrity of a pressurized piping system that is utilizing GHe. The inlet pressure is 2400 psig, and the outlet pressure is 200 psig. I am trying to analyze how the pressure on the outlet side will change if the pressure reducing valve fails. The relief valve on the outlet side has a set pressure of 230 psig, and at that pressure it can offload 220 scfm.
Can I use relief valve sizing calcs to determine the scfm through the regulator if it fails? And if so, how can I calculate the resulting pressure change that would result from the excess scfm assuming the relief valve can't offload the entire flow rate through the failed regulator?
Thanks,
David
Can I use relief valve sizing calcs to determine the scfm through the regulator if it fails? And if so, how can I calculate the resulting pressure change that would result from the excess scfm assuming the relief valve can't offload the entire flow rate through the failed regulator?
Thanks,
David





RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
first action should be for your downstream system to identify your worst case relief scenario and flow - in this case working out what the highest flow is through your control valve is fully open. You will probably have choked flow, but work out from the valve max CV what your flow is with 2400 u/s and 200 psi d/s. Then you fit relief valves able to handle that flow.
you appear to be doing this the wrong way around. Depending on your flow in, flow out and volume available, the pressure rise up to maximum inlet pressure could be < 1 second or could be > 1 hour/day. That also has some impact, but normally the pressure rise is sufficiently fast and at such a level that your d/s system will become over pressured very quickly and then potentially rupture. you don't want that to happen.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
It is most definitely cart before the horse. The system has already been installed and used for decades, but no one has ever verified the system in this manner. I am trying to reverse engineer the system to see if catastrophic failure is likely if the reducing valve fails.
In your experience, can PRV sizing calcs (i.e. API 520) be used to determine flow rate through a fully open reducing valve? Or are those calcs only applicable to relief valves?
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
If the reducing valve fails, the initial conditions would be 2400 u/s and 200 d/s. If the relief valve is sized correctly, shouldn't these pressure remain the same? If the relief valve is undersized, then the d/s pressure would increase until equilibrium was reached, correct?
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
P1 = 2414.7 psia
P2 = 214.7 psia
DeltaP = 2200 psi
x = 0.9167
T1 = 535 R
M = 4.0
G = 0.138
k = 1.66
xT(assumed) = 0.6
Z = 1.0
Fp = 0.9148 (since reducer is present in valve)
For the given regulator, I have a spec sheet for the rebranded valve in question indicating the orifice is 3/32" and the Cv is 0.1. I solved the Cv equation for Q. The relief valve, at a set pressure of 230 psig, can let off 220 scfm. When I solved for Q I got around 30000 scfm, which is the flow rate from the reducing valve assuming choked flow. If this is true, this relief valve is grossly undersized.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
The equation in API 520 has been tweaked (with the various "K" terms) to be specific to a PSV. I would use the compressible flow equation in FAQ378-1864: What does the flow profile look like for a Pipeline Blowdown?
Finally, the techniques used in the 1960's pre-dated the concept of "credible scenarios" and tended to be way off by today's calculations. One common technique was to size the PSV based on the biggest single-component volume (the assumption was that you would build a bigger vessel for bigger flow). Using credible scenarios almost always indicates that the PSV is not optimum (if it is way too big most people just leave it, if it is undersized, you really can't get away with just leaving it).
David Simpson, PE
MuleShoe Engineering
In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
However the system appears to have survived for 50 something years and although it could fail tomorrow in a way it hasn't so far, something must be working well.
One technique which is sometimes used is to physically stop the control valve exceeding a certain percent open. maybe that is applied here?
what is the normal percent open / flowrate through this thing? Can this section actually be isolated?
You probably know all that, but it would seem on the face of it that the relief valve is not able to protect this system in the event of a wide open valve
What is the actual pressure rating on the d/s side? There's a lot of questions.....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
The PSV is still too small, just not as much.
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
A compressor that will be turned off if the pressure goes too high?
or a massive tank or very long pipeline that will essentially stay at the initial "too high" source pressure (at the relief valve setpoint) "forever" even if the relief valve opens?
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Sure. Why not?
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
what is the volume of the downstream section compared to the volume of the tube bank?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
~1200 cubic feet of compressed helium in the form of four 300 cubic foot tanks runs through a manifold that has its own relief valves, check valves, etc. It then exits the manifold and penetrates a building with 1/2" tubing where it connects to a user-operated panel that has an upstream gauge (2400 psig), a downstream gauge (~200 psig), and two PRV's set up in a pilot-valve combination. There is a relief valve downstream of the PRV's as well as a hand valve that is only turned on when the user needs the helium. The volume of tubing downstream of the PRV is orders of magnitude smaller than the tube bank. We are talking 1/2" and 1/4" tubing that may be 5 foot in length at the most. My initial plan of attack was trying to find an equation that involves flow in minus flow out, but then I figured the pressure differential was all I needed to find the flow through the primary PRV since the flow was restricted by the orifice in the PRV anyways.
Using manufacturer flow charts, I computed the maximum flow rate through the PRV using an upstream pressure of 2400 psig and a downstream pressure of 200 psig. This resulted in a flow rate of 600 SCFM. If the PRV and its pilot-PRV fails, this is how much air would be entering the downstream side based solely on the pressure differential. If I understand this correctly, API 520 calcs show that at that flow rate, the relief valve, which has a set orifice size of 0.25", would maintain the downstream pressure at ~243 psig.
However, whats confusing me now is the question of what happens to all of the extra SCFM. If the PRV fails and is constantly introducing 600 SCFM to the system and the RV can only offload so much, then the pressure will constantly build on the downstream side. When the system is functioning properly, then when the PRV introduces too much flow and the pressure builds, the PRV eventually stops the flow and the relief valve will let out some of the helium to maintain the system pressure. What happens if the PRV never stops the flow? How can I determine the resulting pressure or pressurization rate that would occur?
*EDIT: Do the API 520 calcs show that the RV is offloading all 600 SCFM and that it is resulting in a pressure of 243.6 psig?
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
You have gas coming into the system through the failed valve, gas leaving the system through the relief valve and potentially, some gas still flowing into the process/users (or you can assume that is zero which is the conservative approach and likely recommended unless you are sure that the users are ALWAYS taking gas).
Basically when the capacity of the relief valve is less than what is flowing through the failed regulator, the pressure in the downstream system increases because you are adding more mass than you are removing. As you increase the pressure in the downstream system, the capacity of the relief valve increases. Given the 2400 psig inlet pressure to the failed regulator, you won't see any reduction of gas flowing into the low pressure system as its pressure increases because flow through the failed regulator will be choked. Just keep increasing the low pressure system and solve for the capacity of the relief valve until the capacity of the relief valve equals 600 scfm. That should be about 600 psig (230 scfm at 200 psig set pressure per your first post if I've understood what you posted correctly).
Then you can look at the low pressure system and determine what happens when it's pressurized to this system. That will tell you if you have to replace the relief valve, add another relief valve, do nothing, etc.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
However, due to the lack of a proper capacity chart for this given relief valve, I did an API 520 calc where I solved the required area equation for P and then substituted in the 600 SCFM flow rate and the other parameters and it resulted in a pressure of 243 psig. Is that a correct application of the sizing equation?
Do the API 520 calcs show that the RV is offloading all 600 SCFM and that it is resulting in a pressure of 243 psig? Is it plausible to think that with 600 SCFM coming into the low pressure section through the failed regulator and 600 SCFM leaving through the RV that the resulting pressure is 243 psig?
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
243 psig versus 230 psig set pressure is less than 6% overpressure so I wouldn't take the 243 psig as gospel but it's in the ballpark. Maybe it's 275 psig but it won't be 600 psig.
What happens if you overpressure the system? Do you have a loss of containment? What's the effect of that? Are people going to be injured? Is equipment going to be damaged? That gives you some idea of the urgency of the issue.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Thanks again!
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
I must admit I've now got completely lost with all the different data and it's not this valve it's that one and hence it may be best for the OP to start again and list what he now knows in terms of
Upstream pressure in the quad (2,400 psig?
Pressure downstream the regulator 200 psig
size or CV or max flowrate of the regulator based on the pressures and gases going through it - note mass flowrate might be better to use
size or orifice size or flowrate of the relief valve at 230 psig (relief pressure)
design pressure / MAWP of your downstream system
what pressure is needed through that valve to equal the incoming flowrate. I find it difficult to see how a small increase in pressure to 243 psig makes such a big difference in relief flow rates, but as I said I've got lost in the changing data being listed here. There will be a point at which increased upstream pressure increase mass flow for a choked relief, but whether it is acceptable or not I don't know.
If the relief valve needs replacing then simply work out max incoming flow and make sure your relief valve can handle that plus 10% at your relieving pressure.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
It also wouldn't surprise me if I'm the one that is confused!
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
I am trying to determine if the relief valve d/s of a regulator will be able to maintain 120% system MOP if the regulator were to fail.
u/s of the regulator is 2400 psig of pressure, and it needs to maintain a MOP of 200 psig d/s. The set pressure of the relief valve is 230 psig. The orifice size of the regulator is 3/16" and 1/4" for the relief valve.
Initially, I had no flow/capacity charts for the regulator or the relief valve. I was able to track down a capacity chart for the regulator, but will not be able to obtain one for the relief valve because it was intended for liquid service despite its current use with helium. However, after talking with some regulator manufacturers it was determined that flow charts shouldn't be used for a failed regulator since the Cv's are different between normal operation and wide-open. These manufacturer's also stated that most Cv's that are found in spec sheets are for the "wide-open" scenario since those values are almost always used for sizing relief valves.
As such, this is the process I landed on:
1) Use Cv equation (Q=0.471*N2*Cv*P1*SQRT(1/(Gv*T))) to determine flow rate through wide-open regulator
2) Solve API 520 equation for P1 and substitute in the flow rate
3) If P1 is less than 120% of the d/s system MOP, then the system is compliant from my organizations POV.
Using this method, the flow rate through my failed regulator is ~1800 SCFM vice the 600 SCFM stated earlier. I also want to say that I incorrectly used the capacity chart for the regulator because I failed to take into account the dome-loading capability. The operational flow rate of the regulator is around 75 SCFM, which is not excessive for the relief valve.
I apologize for all of the confusion, I have been learning this from scratch with little to no information available.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
You haven't given us the valve CV you used, but look at the two links below which may help you check your answer.
Even with the different orifice sizes, the much larger u/s pressure on the regulator will lead to a pressure much higher than 230 psig once steady state is reached. You are always better off working in mass units for something like this.
http://www.engineeringtoolbox.com/flow-coefficient...
http://www.engineeringtoolbox.com/safety-valves-ga...
Bigger capacity relief valve needed.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
EDIT: The Cv calculator you provided, when solving the air/gas equation for Q using the Cv for the regulator results in a flow of 703 CFM. Using another Cv equation that I have found from Swagelok and Sandia National Laboratory results in a Q of 731 SCFM. Relatively close, I guess, but I am more apt to choose the higher of the two to be safe.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
With your CV of 0.6 and the relief valve orifice size of 1/4" to relieve the incoming mass flow rate, which is not affected by the rise in pressure as it is still choked flow, you would need approx. 700 psig u/s the relief valve before the pressure stopped rising
given the raw data that has the right "feel" to it for me.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
I get approx. 700 scfm only when I have air (SG of 1). SG of helium which that equation uses is 0.138
It probably works out to be the same mass flow hence similar answers, but it's much better to be consistent and work it all out as helium.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
We always like a happy ending on these threads.....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
What blows my mind is that multiple PE's have supposedly "verified" this system. I finished my undergrad last year and I wish I could say this was the first major problem I have found.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
I wouldn't put "piping should be ok" in a report if I was you
once something is in place and woring, a lot of people will just assume that it must have been designed Ok and as it's still working there's no need to actually verify anything.... On a 50 year old plant you're going to uncover a lot more of these I think. good luck and let un know what finally happens, even in a few months time.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
RE: Flow Rate and Pressure Changes when a Reducing Valve Fails
Good luck,
Latexman
Technically, the glass is always full - 1/2 air and 1/2 water.