Calculate Nitrogen Gas Flow From PCV
Calculate Nitrogen Gas Flow From PCV
(OP)
I have a pressure reducing valve in a nitrogen line. Currently, the valve is set for an outlet pressure of 3 psig with an inlet pressure of 120 psig (estimated). I want to calculate the current flowrate and proposed flowrate if we decreased the outlet pressure to 2 psig.
Do I used the Fisher universal gas sizing equation (the one with the Sin function in it) for this calculation? It is a Fisher regulator, so the Fisher handbook should be applicable to this valve.
How would one find the maximum flow through this valve at 100% open?
Thanks.
Do I used the Fisher universal gas sizing equation (the one with the Sin function in it) for this calculation? It is a Fisher regulator, so the Fisher handbook should be applicable to this valve.
How would one find the maximum flow through this valve at 100% open?
Thanks.





RE: Calculate Nitrogen Gas Flow From PCV
In either case, the flow of nitrogen through the regulator is determined by other parts of your system. For example, if feeding into a low pressure nitrogen header, it will be the consumption of low pressure nitrogen that will determine the flowrate. To get the maximum flowrate, I think the universal gas sizing equation would work fine. You need to know or be able to look up the Cv of the currently installed valve. The downstream change from 3 to 2 psig is insignificant. It merely increased your differential pressure from 117 to 118 psi.
HTH,
Doug
RE: Calculate Nitrogen Gas Flow From PCV
The nitrogen is fed to puff a bulk bag to inert the inside and to expand it. The feed rate of the design is 450 cfm. I'm trying to figure out if we are adding too much nitrogen (over 450 cfm). The valve is set at an exit pressure of 3 psig and the pressure before the valve is about 120 psig. I was going to calculate the flow through the valve now and what it would be if we decreased the outlet pressure (to 2 or 1 psig). So could I calculate this using the Universal Gas sizing equation (flow before and after) or is there another method?
Thanks
RE: Calculate Nitrogen Gas Flow From PCV
I'll carry this one step further and say that the pressure in the bag remains fairly constant during this operation at just above atmospheric. Assuming I am correct, I would suggest that you use a restriction orifice/solenoid valve combination. With near constant dP, a restriction valve is all that's needed to "control" the gas flowrate. The solenoid would control the time period for "the blow" to occur.
Sorry, I've gotten off on a tangent. The Fisher Universal Gas sizing equation applies as long as you do not have sonic flow. With your pressure drop, however, you WILL have sonic flow. Downstream pressure changes in such a situation do NOT impact the gas flowrate. The equation to use is:
Qcritical = Cg * P1 * sqrt(520 / (G * T))
You'll note that the downstream pressure does not appear in this equation because it does not impact the flowrate.
Doug
RE: Calculate Nitrogen Gas Flow From PCV
Thanks.
RE: Calculate Nitrogen Gas Flow From PCV
RE: Calculate Nitrogen Gas Flow From PCV
High delta P's with gases are noisy. They can be dangerously noisy, the amount of noise dependent upon the valve design, pressure drop and flow rate.
Paul Ostand
www.ostand.com
RE: Calculate Nitrogen Gas Flow From PCV
A speicalized pressure drop valve (like drag valves, lincoln logs, and other tourtuous path valves) is basically a valve that "stretches" out the pressure drop.
A cheap alternate way to do it is multiple valves in series. It takes more room, but is cheaper.
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RE: Calculate Nitrogen Gas Flow From PCV
Thanks.
RE: Calculate Nitrogen Gas Flow From PCV
With a vent pressure of 1-3 psig, changing the pressure is not going to help.
If you have access to the equations for noise prediction, you can visualize this. ISA publishes the standard. Fisher tech people might also assist you.
Paul Ostand
www.ostand.com
RE: Calculate Nitrogen Gas Flow From PCV
So the conttrol valve brings the pressure down from 120 psig to 3 psig. For this valve this comes out to about 917 SCFM. Wouldn't adding a correctly sized orifice downstream of the control valve help decrease the pressure even more (I sized it for a delta P of 2 psig) with a decrease in flowrate (sized for 408 SCFM) help decrease the noise? Fisher has calculated that the noise out of the valve should only be around 80 dBA and we are getting about 110 dBA coming out of the vent header.
Thanks
RE: Calculate Nitrogen Gas Flow From PCV
In their defense, they cannot test everything. And also, their testing data is derived from controlled testing and placement of instruments. Field testing may not be able to match those conditions.
I believe that you would decrease the noise level. Do the calcs and see what they say, even if your numbers don't agree, the trend should be indicative.
Paul Ostand
www.ostand.com
RE: Calculate Nitrogen Gas Flow From PCV
Whats your intended downstream enduser pressure?