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Flow Rate and Pressure Changes when a Reducing Valve Fails

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

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

This appears, on the face of it, to be a case of the cart before the horse.

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

(OP)
LittleInch,

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

(OP)
Also,

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

Technically, no, one cannot use PRV sizing calcs (i.e. API 520) to determine flow rate through a fully open reducing valve. Use a compressible flow reducing valve equation. They mainly use Cv's. PSV's do not use Cv's. What make is the reducing valve? Use THAT manufacturer's data and equation.

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

(OP)
The reducing valve as manufactured by Grove Valve and Regulator Co., who has been rebranded and bought out by Dresser, who was then bought out by GE. I have contacted those companies as well as suppliers in hopes of finding flow charts or cut sheets, but no one has anything. The valve was manufactured in the 60's. Seems as though I'll have to make a lot of assumptions to verify calculations.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
I went through a Cv calc and am getting some insane numbers. If anyone has time to crunch this out, let me know what you get so I can compare.

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

First off your PSV set pressure (P2) is 230 psig (you didn't say why, but I'm assuming that the system was designed to operate at 87% of MAWP), not 200 psig. I don't know what GHe is so I don't know if your ideal gas assumption (Z=1.0) is valid or not. If I take a wild swing and guess that M=4 is molecular weight (which is consistent with G=0.138)species must be Helium, but what is the "G"? Helium at that temperature and pressure has a compressibility factor around 1.15 so you are giving up 15% uncertainty out of the box.

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

The effective deltaP is going to be limited by the "critical pressure ratio", i.e. choked flow.

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

That could be right. Sounds like your relief valve is something like a thermal relief valve - 220 scfm is very low. However with 150 bar one side of your valve you will get a awful lot of gas through it if fully open.

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

I got about 30,000 scfh, not scfm. There may be a time unit difference between the relief flow and reducing valve flow.

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

(OP)
So I was finally able to track down a flow chart from the manufacturer and it indicated a Qmax of 600 scfm when the pressure differential was 2200 psig (2400 u/s, 200 d/s) and the orifice is 3/16". If the regulator were to fail, it would be safe to assume that the Qmax would occur at the onset and then dip a little as the d/s pressure increased. So now my question is this: can I solve an API 520 RV sizing equation for P1 using a flow rate of 600 scfm to determine the maximum pressure in the system? I need to determine if the resulting d/s pressure would increase 20% above the MOP of 200 psig.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

What's the source of the pressure? A "full tank" with pressure going down after the relief opens?

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

(OP)
The pressure source is a tube bank of about 1200 cubic feet of compressed helium.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

That might be initially as high as 3500 psig or more. What's the max pressure rating of the bottles?

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
The bottles are set at 2400 psig.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

Quote (DMay121)

can I solve an API 520 RV sizing equation for P1 using a flow rate of 600 scfm to determine the maximum pressure in the system?

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

Isn't the overpressure calculation simply a flow in minus flow out into a fixed volume and then adding mass and pressure??

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

(OP)
The upstream volume is massive compared to the downstream. The system layout is as follows:

~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

Uploading a sketch/drawing would be a help.

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

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

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

< 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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
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?

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
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!

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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-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

(OP)
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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

(OP)
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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

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 winky smile 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.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

Another alternate solution to replacing this regulator would be to add another regulator upstream to break the pressure down to a value that will bring this existing regulator back into its controllable range, and keep within the PSV relief capacity at 1.1x LP side design pressure. Preferably pilot operated also. Agreed this option may be more troublesome than a straightforward replacement of the existing regulator.

RE: Flow Rate and Pressure Changes when a Reducing Valve Fails

Or add an orifice or flow nozzle u/s or d/s of the regulator to limit the flow less than the capacity of the PSV.

Good luck,
Latexman

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

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