Flow Rate Restriction 1

[chris1982]

I have a low pressure nitrogen gas system supplied from a 3000 psig source through a regulator. The regulator output flows through 6 feet of 1/4" diameter hose to the low pressure components. To limit the low pressure system pressure to 80 psig, the designer used a relief valve.

Problem. The full flow rate of the relief valve is less than that of a failed regulator. I need a method to limit flow to about 5-6 scfm. For a 3000 psig inlet pressure and 2950 psig pressure drop, the Cv is on the order of 0.001 but the Cv for a 0.03" orifice is only 0.016. Too many orifices. Needle valve maybe?

Any suggestions are welcome.

 

[TD2K]

Is a smaller regulator an option?

Different PSV maybe? 80 psig should be enough pressure to get 5 to 6 scfm even at 1/4" (though I will admit I haven't crunched any numbers). I've seen Swagelock spring loaded check valves used as PSVs in some service, have you looked at something like that?

 

[chris1982]

Thanks TD2K. A smaller regulator might work, but considering complete failure of the regulator as an accident condition results in large Cv. The swagelok check valves are good - I used them in a lab environment before - but the current PSV is mounted in a package and I'm trying to minimize field changes. The regulator is mounted directly to the bottle with an inlet gage to monitor bottle pressure. The bottle and regulator are not packaged and are therefore easier to modify or replace.

 

[DiegoMartinez]

Chris 1982,

Is your application precharging pulsation damper bladders on suction of a pump?

 

[chris1982]

No - it's pressurizing a water filled tank to transmit the test pressure to a connected detector. Low pressures and low flow rates.

 

[DiegoMartinez]

For your application, use Swagelock spring loaded check valves as psvs. Maybe a couple or three of them!

 

[chris1982]

I've looked at the orifice plate flow restriction with some different reference material. Used m=.53*Cd*P1*A2/SQRT(T). It appears that the 0.03" orifice will result in critical flow rates on the order of 0.045 lb/min (about 3-4 SCFM) which is within the capacity of the relief valve.

I'll keep the check valves in mind should I need to increase the relief flow rates rather than restrict the inlet flow.

 

[JLSeagull]

Fortunately your nitrogen stream is likely a clean fluid. I hate holes smaller than 1/8-inch if the line could be dirty.

 

[mbeychok]

Chris1982:

You might want to look at faq798-1196 on how to calculate the choked flow of a gas through a restriction orifice.

Milton Beychok
(Visit me at www.air-dispersion.com)
.