2 relief valves 4

[knowlittle]

I would like to use 2 pressure relief valves. Source is 2000 psig. First relief valve will have 25 psi set pressure. Second relief valve will be downstream of the first one and set at 3 psig. Usage pressure will be about 0.5 psig. Are there issues with multiple relief valves? Thank you.

 

[SnTMan]

I don't know jack about RV's, but... Do you want them discharging continuously? 2000 -> 25 -> 3 -> 0.5 psig?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[adammal44]

It might be more useful to explain "why?" you think you need two relief valves.

Are you trying to deal with two different overpressure cases (e.g., accidentally closed isolation valve for the 3 psig case and nearby fire for the 25 psig case)?

What service are you doing? All liquid? Two-phase?

 

[knowlittle]

My apology for asking the question without giving the full picture. The fluid is gaseous nitrogen.

Source=pressurized gas tank commonly used in laboratories, about 1' (25 cm) diameter and 5' (1.2 m) tall. 2000 psig.
Gas regulator mounted on the gas tank= Drops 2000 to 0.5 psi in 2 stages, CGA (compressed gas association) approved
Tubing=1/4" (6 mm) OD nylon tubing
My instrument=Needs only 0.5 psig

Company regulation requires pressure relief valve set @ 3 psig to protect the instrument from accidental high pressure discharge (about 90 scfm or 2500 lpm) in case of regulator failure. A 3 psig relief valve can discharge only 1/4 of it. I need an extra relief valve. Discharge rate goes up as set pressure goes up. I found a 25 psig relief valve that has 100 scfm (2800 lmp) discharge rate.

Starting from 2000 psig source, and moving downstream,

Gas tank > Gas regulator > 25 psig relief valve > 3 psig relief valve > Nylon tubing > My instrument.

Thank you.

 

[adammal44]

I may still be misunderstanding what you're trying to accomplish.

My confusion is related to your solution. Your company policy requires pressure relief to occur at 3 psig to avoid high-pressure discharge. Your current equipment can't support the flowrate/blowdown requirement. So your solution is to provide a valve that would produce high-pressure discharge @ 25 psig?
Why wouldn't you just add a larger 3 psig relief valve? Or multiple 3 psig relief valves?

 

[knowlittle]

Why not larger 3 psi relief valve?
This is a chemistry lab setup. Gas regulator has 1/4" npt port. Tubing is 1/4" OD. The 3 psi relief valve I have has 1/2" npt threads. A larger relief valve will have a larger thread. Fitting a larger relief valve will be difficult.

Why not multiple 3 psi relief valve?
The 3 psi relief valve has 1/4 of required discharge rate. If I stick to 3 psi relief valve, I will have to mount 4 relief valves.

 

[adammal44]

So, I understand the drawbacks of not doing those things, but not sure what your other options are.

Will your company accept design alternatives? Like more passive pressure regulation (orifices)? Maybe a high-pressure discharge vented directly to a safe space (into a drain)?

I also don't understand what you're doing with the 25 psig relief valve? You would just end up with that popping first and causing the same situation that your company policy prohibits (high-pressure discharge). Or is the first 3 psig relief valve required just for instrument protection and you're using this separate pressure relief valve to deal with additional flow...i.e., in the event your regulator fails, it will fail completely and (in all cases) both of these relief valves will actuate?

 

[knowlittle]

Yes, exactly as you wrote. The 3 psig relief valve is to protect my instrument. 25 psig (or an extra relief valve) is to discharge the flow load. In case of regulator failure, both will be activated.

No, we are not against passive regulation as far as it meets ASME B31.1. Any advice will be much appreciated.

 

[zdas04]

Two PSV set at different pressure is done quite often and is not at all unreasonable (e.g., a small thermal relief set at 95% of MAWP for the isolated-vessel/expanding-liquid case and a large valve set at MAWP for another overpressure scenario). You just need a clearly stated reason for doing it.

On the other hand, two PSV set at the same pressure (as was suggested a couple of times above) is a huge mistake--no valve is perfect and one of the valves will always open before the other one so each valve has to be able to handle full flow. It never works as well as you'd hope.

[bold]David Simpson, PE[/bold]
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

 

[knowlittle]

Thank you, Sir. I will proceed with the 2 relief valve approach.

 

[Compositepro]

If your issue is too much flow capacity from the first stage regulator for the relief valve to handle the usual solution is to place a restrictive orifice in the high pressure line to keep the flow under a specific value, even if the regulator fails. Most propane tank regulators contain an orifice in the inlet. An orifice is safer, cheaper, and more reliable than a relief valve. In your case you still need your one low-pressure relief valve but the orifice will keep flow down to below its capacity.

 

[curtis2004]

Hi,
I agree with Compositepro's approach. Also I think, pressure reduction from 2,000 psig to 0.5 psig seems too steep. What is allowable max pressure on low pressure side? It seems 0.5 pressure is nominal working pressure. Can your instruments accept higher than 0.5 psig?

Another alternative would be to install two consecutive pressure reducing valves. In this case, assuming only one of PRV failure you will see much lower flows. Although, in this case two pressure reducing valves need to be installed...
Regards,
Curtis

 

[Compositepro]

" pressure reduction from 2,000 psig to 0.5 psig seems too steep."

We are discussing relief valves for safety. Of course the pressure reduction would be done with a two-stage pressure regulator.

 

[mbt22]

Gas tank > Gas regulator > 25 psig relief valve > 3 psig relief valve > Nylon tubing > My instrument.

This will not work as you intend it to, the 3 psig valve will not protect your instrument. Regulator fails, pressure rises to 3 psig; 1st relief valve lifts, some flow diverted to vent but not all so accumulation continues; pressure in line rises to 25 psig; second RV lifts. The instrument will still see 25 psig.

If you actually built this, there's a decent chance your 25 psig might not even lift, as the orifice in the 3 psig will probably pass the full flow once the inlet pressure hits 12 psig or so, if it doesn't break before then (discharge pipe permitting). Don't actually try to do this!

This is not a new problem, just put an orifice upstream of the regulator to restrict the maximum flow. 0.5 psig operating could sensibly be protected by a U-tube bubbler.

Matt