Flow Rate and Pressure Changes when a Reducing Valve Fails

[DMay121]

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

 

[DMay121]

Unfortunately I can't upload any sketches or drawings due to the nature of the facility

 

[TD2K]

< However, whats confusing me now is the question of what happens to all of the extra SCFM >

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.

 

[DMay121]

TD2K, the 230 scfm at 200 psig was from a Capacity Chart for an incorrect relief valve. The RV im looking at does not have any capacity charts for air/gas because it was intended for liquid service. That alone means it will get replaced.

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?

 

[Latexman]

Yes, that is a proper application. It will not stay open and the pressure be exactly 243 psig. The regulator will fail. The pressure will build. The PSV will pop open. The pressure will decrease to the reseat pressure (usually about 0.93 x set pressure). The PSV reseats. The pressure will build. The PSV will pop open. The pressure will decrease to the reseat pressure (usually about 0.93 x set pressure). The PSV reseats. And on and on and on.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[TD2K]

Yes. The API equations are set up to solve for area but there's no reason you can't use them to solve for the pressure required to achieve a certain flow rate through the relief valve. Granted, with it sounding like it has a liquid trim, the discharge coefficient might be somewhat different in gas service but in looking through API 520, Part 1, it appears to be a bigger problem using a relief valve with vapor trim in liquid service which isn't your case.

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.

 

[DMay121]

I work with a safety office and am definitely aware of the possible ramifications, which is why I want to be perfectly sure that my methods and line of thinking are correct. Thanks to everyone who provided input and helped, it is greatly appreciated.

 

[DMay121]

Alright, follow up question time. Are manufacturer flow charts applicable for a state of failure for a regulator? Or are they only applicable assuming normal operating conditions? Using Cv equations from literature produce flow rates that greatly differ from manufacturer flow charts.

Thanks again!

 

[Latexman]

Regulator flow charts give normal flows. Wide open flows use the regulator Cv.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

Latexman, Your scenario is correct in most circumstances, but what the OP is describing is a situation where the incoming flow is higher than the relief valve flow at its nominal relief pressure. Hence the pressure will continue to build until the mass flow rate through the choked continuously open relief equalises with the mass flow rate entering the system from the quad banks.

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.

 

[Latexman]

Sorry, LittleInch, but in the confusion that exists in this thread, I think my assessment is correct, and you are on one of the two, or is it three now, "other" PSVs. It's not surprising that we each think we are each correct, because I know that you believe that you understand what you think the OP wrote, but I am not sure you realize that what you read is not what the OP meant. I do wholeheartedly agree though, that it would be beneficial for the OP to clarify his intentions and add some details (make, model, nozzle size, etc.) to make it easier to associate THE PSV to be used.

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.

 

[DMay121]

I can see how both of your viewpoints stem from varying things I have said. I will attempt to start from the beginning but add updated info:

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.

 

[LittleInch]

DMay

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-coefficients-d_277.html

http://www.engineeringtoolbox.com/safety-valves-gas-vapor-d_877.html

Bigger capacity relief valve needed.

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

 

[DMay121]

The Cv of the regulator is 0.6.

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.

 

[LittleInch]

Probably made an error somewhere (it's Friday afternoon), but in round numbers I get your relief valve orifice are needs to be 0.142 in^2, meaning a diameter of 0.425" vs your current size of 0.25"

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.

 

[DMay121]

Those are the exact numbers I got. This is conclusive for me that I am on the right track now. Thanks again!

 

[LittleInch]

didn't read your second line - be careful of units and figures - I got a Cv 0f 0.58 when I used your original 1800 scfm rate of Helium.

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.

 

[DMay121]

Doh! A case of the Fridays it is. Should have been 703 psi and 731 psig, not SCFM.

 

[LittleInch]

Good - what are you going to do now we sorted out that the current system is not fit to handle the potential max flow??

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.

 

[DMay121]

Well, without going into too much detail, I am writing a report that will urge the facility to install a new relief valve. The regulator in question was manufactured in the 60's, so its probably been refurbed a few times. Regardless, ~1800 scfm will take the system to over three times its MOP. This will blow some gauges, but the piping and other valves should be okay if this situation were to happen.

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.

 

[LittleInch]

The other option might be to change the trim on the regulator. If you only need 75 scfm, to have one able to do 1800 at full open implies that you're operating outside of the recommended 20% to 80% open territory.

I wouldn't put "piping should be ok" in a report if I was you ;-) Whilst in practice piping and valves will probably go to 2-3 times their design pressure before they actually break, you can't take this into account, but I appreciate you are probably aware of that. Just stick to three times over the MOP.

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.