Calculate Nitrogen Gas Flow From PCV 1

[RJB32482]

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.

 

[djack77494]

I suspect the valve is acting as either a pressure control valve or a backpressure control valve. In the former case, the pressure downstream of the valve would determine its position; in the latter case, pressure on the upstream side would enable the valve. So what are you trying to control? Are you venting nitrogen to maintain the header pressure at 120 psig or are you bleeding nitrogen into a low pressure system operating at 3 psig?

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

 

[RJB32482]

Thanks for the reply.

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

 

[djack77494]

Your last post changes the nature of your situation significantly from what I had assumed. If i understand your problem correctly, you don't much care about the upstream or the downstream pressure. You're just trying to get a blast of nitrogen into the bag to expand it. So it's more a quantity of N2 that you need, rather than a pressure. There's probably some danger that you could be overly vigorous in the expansion and either break or blowoff the bag.

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

 

[RJB32482]

OK so I have sonic flow (120 psig at the beginnig of the piping with an discharge at the end of the pipe after the PCV at 0 psig). Is there any way that I can change the setting on the valve to not have a high noise attenuation (sonic velocity), or because the inlet pressure is so high, I cannot change the sound even if the valve is turned down so the outlet of the PCV is 1 psig?

Thanks.

 

[pleckner]

Drop the pressure in stages. Nothing says you have to drop the pressure all in one valve.

 

[proinwv2]

You might also contact different manufacturers to discuss their trim and the noise inherent in the various valves. Trim design influences noise.

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

http://www.ostand.com

 

[Ashereng]

If noise is a problem, then drop the pressure in stages.

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.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[RJB32482]

Now with the pressures I have, I can still decrease the opening of the valve to decrease the flow through the valve correct (changes the Cg of the valve)? I'm not worried about the noise caused by the valve (predicted by Fisher to be below our max noise exposure), but concerned on decreasing the noise cause by the velocity of the gas through the vent.

Thanks.

 

[proinwv2]

If I understand then the only way to decrease the noise level of the vent valve is to decrease the flowrate or increase the vent size.

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

http://www.ostand.com

 

[RJB32482]

Here's another question I have on this subject (bringing it back from the past).

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

 

[proinwv2]

I strongly suspect that noise information offered by valve manufacturers, can be incorrect because the information is derived from a limited amount of testing and extrapolated.

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

http://www.ostand.com

 

[frogcurry]

Please clarify - an orifice for going from 3 psig to 1 psig or an orifice to go from 5 psig to 3 psig?

Whats your intended downstream enduser pressure?