PRVs downstream of regulators

[cliffy]

Hi all. I found some info on this topic but that thread is closed so I will ask away.
I am required to spec, install and maintain the history record for PRV/PSVs on compressed gas regulators on 250 co.ft cylinders.
We use mostly nitrogen, air, helium and 5% hydrogen/ 95%nitrogen mixtures. I can't seem to be able to find any regulation or requirement or rule that specifies the pressure setting of the PRV as a percentage of the MAWP of the system.
I suppose that 80 or 90% of MAWP would be acceptable from a safety standpoint but I can't cite a source other that the seat of my britches, which the poo-baas don't appreciate.
I've looked all through the CGA guidelines but they don't mention the pressure rating other than for testing.
Any ideas?
Thanks


My hovercraft is full of eels.

 

[iainuts]

If you're designing to a piping code (in the US) the set pressure is defined there. For example, ASME B31.3, para. 322.6.3 (1990 edition) states:

(a) Pressure relieving devices required by para. 301.2.2(a) shall be in accordance with the BPV Code, Section VIII, Div. 1, UG-125(c), UG-126 through UG-128, and UG-132 through UG-136, excluding UG-135(e) and UG-136(c). The terms "design pressure" and "piping system" shall be substituted for "maximum allowable working pressure" and "vessel," respectively ...

Basically, you protect the piping as you would a vessel. Relief valves are set at the design pressure of the piping.

Note: I have an old copy of the piping code which I've quoted, I simply haven't used it for a while.
I'll leave it to you to verify the requirements in the newest version of the code that's applicable to your system.

 

[cliffy]

Thanks iainuts for the reply. I was hoping to get a quote from B31.3 so I didn't have to buy it but it might be worth the investment after all.

My hovercraft is full of eels.

 

[JimCasey]

iainuts has supplied good information. Also, do not set your regulators to more than 90% of the SRV set pressure because if the SRV ever lifts, it won't reclose and you'll lose a lot of your expensive compressed gas. Also, calculate the flow through the regulator at P1 = upstream system oressure, Cv=published Cv, and P2=SRV set pressure +10%. The SRV must be able to handle this capacity or more. If the SRV capacity is much more than double, it might go into chatter and beat itself into nonfunctionality. THat would be bad.

 

[MortenA]

Jim - why wont the PRV reclose - do you mean ever or just assuming that since the condition that actually causes the pressure to go up dont go away be relieving then relief may continue for a while untill somebody notice?

Best regards

Morten

 

[zdas04]

Morten,
If I can speak for Jim, the tendency of PSV is that they will be bubble tight until the first time they lift. Then the risk of trash from the dead leg fouling the seating surface is real. I always assume that someone needs to check a PSV in the field for leakage every time one lifts, and have generally had to replace a significant portion of them. Inside plant fences, the PSV's seem to lift more often and have a better track record for resealing, but piping and vessels outside the plants really can't be counted on to reseal.

David

 

[iainuts]

Also, do not set your regulators to more than 90% of the SRV set pressure because if the SRV ever lifts, it won't reclose and you'll lose a lot of your expensive compressed gas.

I'd agree with that. If a reg is set at more than 90% of PSV set pressure, and the valve lifts, then theoretically, the blowdown (often as much as 10%) will not allow the valve to reseat as the reg tries to maintain some pressure greater than the reseat pressure.

Personally, I don't like using any kind of pop acting (ie: ASME coded) relief device on a piece of pipe. They're really intended for large volumes such as tanks, not very small ones which can change pressure within a few milliseconds. The problem is exactly as Jim describes, the valves chatter and beat themselves into non-functionality.

It really doesn't matter though how you size them in relation to the source flow. If you do the math on a typical pipe, you'll find there is a volume of gas which is very small that can be evacuated by the RV regardless of the length of the pipe because of dynamic pressure drops in the pipe. The pipe diameter has to be very large in comparison to the relief valve inlet size in order to prevent dynamic pressure losses in the pipe from limiting the volume that's blown down. The resulting noise from the RV is what can be described as a "jackhammer" or "machine gun".

The chances you can find a relief valve that flows just a tiny bit more than a reg is slim. They will typically be 50% or more larger in flow capacity. Even if you find one that's exactly sized, the chances the reg will fail full open are slim. More likely, they will leak or have some other issue that will leave it partially open and the result will be a chattering PSV that hammers itself into non-functionality. So I'm not in favor of using any pop acting PSV.

A better alternative is to use a modulating valve. These valves are often not built to ASME standards though and many folks don't like using them because of that. I'm not refering to pilot operated valves here, many valves are simple spring loaded ones that are fairly inexpensive.

 

[JimCasey]

Thanks for jumping in for me. My rationale was that the reseat pressure is normally 10% below set for a Section VIII valve, thus if the reg is set higher than 90% of the SRV setting, the SRV will never reclose, even if it only pops on a transient.

It would be possible to protect the system with a rupture disc: If it blows, you shut down and fix it. Even if you protect the line with a rupture disc, you have to get pretty advanced reverse-buckling discs to get a 90% operating to burst ratio. Tension-type discs will pop at less than rated pressure if the operating pressure ever exceeds 70% of rated burst.