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

 

[LittleInch]

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.

 

[DMay121]

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?

 

[DMay121]

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?

 

[Latexman]

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.

 

[DMay121]

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.

 

[DMay121]

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.

 

[zdas04]

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

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

 

[Latexman]

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.

 

[LittleInch]

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.

 

[Latexman]

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.

 

[DMay121]

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.

 

[racookpe1978]

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?

 

[DMay121]

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

 

[BigInch]

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

 

[DMay121]

The bottles are set at 2400 psig.

 

[Latexman]

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.

 

[LittleInch]

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.

 

[DMay121]

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?

 

[Latexman]

Uploading a sketch/drawing would be a help.

Good luck,
Latexman

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