Question - Pressure relief valve 2

[knowlittle]

I work in a gas analysis lab.
3000 psi gas cylinder > pressure regulator (Cv 0.05 to 0.1) to 70 psi depending on gas > manual valve > 1/8" (3 mm) OD metal tubing, 1 m long > my gas analysis instrument.
Per company safety code, I have to install a pressure relief valve in case of pressure regulator failure.

I checked Circle Seal catalog. 1/4" thread, deflector cap, 80 psi crack open, 36 scfm @88 psi, 50 scfm @100 psi. Is this flow sufficient?

Thanks.

 

[LittleInch]

I don't think so. If your regulator cv is 0.1 fully open, I used an online calculator for choked flow for nitrogen and got approx 150 scfm. Check with a simple CV calculation for your particular gas sg as the mass flow varies with sg.

Just search flow calculation choked flow or valve flow and this will give you proper value. Don't skimp on this and add a bit for error. Pressure in this system will build in seconds if the regulator fails open and you need a bit of spare.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[knowlittle]

Thank you for not only answering my question but teaching me the guideline. Problem solved.

 

[Tuckabag]

I'd back up LittleInch's advise by saying you can also help yourself out by selecting a different regulator with a smaller Cv.
Something with a wide open Cv of around 0.05/0.06 is usually more than sufficient for a lab analysis type situation (I assume this is a carrier/cal gas application?) and will help reduce the size of relief valve substatially.
We use some instrument regulators that due to their wide open Cv rating (0.138) will usually require a relief valve somewhere around 1" inlet connection when used on natural gas applications (size dependant on process conditions of course).

 

[knowlittle]

True. Some of our regulators are the welding type. Cv greater than 0.2. Whoever bought them apparently went by CGA fitting. I am replacing them with Cv 0.05.

Yes, the gases are for cal, carrier, fuel for flame ionization detector. Cv 0.02 can give sufficient flow.