I have an opening that measures .3039 square inches and it is connected to a larger line at 125 psi. I need 2 separate answers, if they can even be given based on the information I have.
How many CFM (cubic foot per minute) of air are flowing through the hole, just figure 70° and dumping to atmosphere?
Now say, based on this flow rate, how long would it take to pressure up a volume of 1695 cubic feet to 1.5 psi? I know this is no longer “to atmosphere”, but a ballpark is OK on this part.
This is a 1/2" pipe, standard wall (.109" wall thickness) hooked up to an air compressor that is being used to air test a tank to 1.5 psi. We have gauges and a relief valve on the tank and of course the obvious solution would be to just pressure the thing up and time it, but the boss wants to know these 2 things and I am at a loss as to how to procede to figure it out.
if I’m reading it right, shows a volume for ¼ inch orifice at 125 psi drop to be a little over 100 scfm.
Again assuming my math is correct, a 1/4 inch orifice has a cross-sectional area of about .049 square inches.
Assuming that the ratios of the areas will be about the same as the ratios of the flow rates (which I think is fairly safe here), that gives you about 650 scfm through the ½ pipe venting directly to atmosphere.
On the second part, I can’t find the table I’m looking for that shows volume of compressed vs. uncompressed gas at various pressures. But my memory is that you aren’t going to need much extra air pumped in to raise your pressure by only 1.5 psi. Less than double, I’m pretty certain, and probably only about 10% to 15%… By that estimate, you’ll need somewhere between 1695 and 2000 cubic feet of air. At 650 scfm*, it would take between about 2.5 to 3 minutes.
*I’m using the full flow figure because I don’t think the difference between 125 psi drop and 123.5 psi drop is going to have much effect.
Just my guesses, but I think they’re pretty good ballpark range.
I missed the edit time cutoff, but I wanted to add that the 2.5 to 3 minutes in the above assumes the tank is at 0 psia, and you have to fill it up.
If it’s starting at atmospheric pressure, you only have to add the extra air required to bring the pressure up 1.5 psig. If my guesses are correct and this is only 300 cubic feet or so, you’ll get that in less than a minute, maybe as little as 30 seconds.
Nice chart. Your assumptions on flow rates look good to me, so I agree with the 650 scfm. Probably a little more, since your chart shows more like 125 scfm @ 125 psi for a 1/4 orifice. Call it 759 - 800 scfm for the OP’s orifice.
Since we’re ballparking, air is near enough a perfect gas, and atmospheric pressure is near enough 15psi, so for a 1.5psi gauge increase, you’d need about 1/10 the volume in scf – 170 scf, in other words. So time required would only be 13 seconds.
Thanks guys. I wasn’t sure what to even search for to try to Google up an answer, but that chart is something he can relate to.
And yes, the tank is just resting at atmosphere so I think your estimate will be pretty close. They normally just hook them up to a 1/4" air line that’s further restricted due to the quick connects on the line, like air tools are run on. Not sure what the reason for wanting to know this other stuff, it’s not like there is a great rush to get them to pressure or anything, he may just be curious for all I know.
And just to add, it’s not like I’m getting a raise or even any brownie points for this. He hits me up with obscure stuff all the time and never follows up on half of it…I never heard another peep about this deal all day today.
I knew if an answer could be had, my best shot would come from here, and I really do appreciate you guys taking the time to lend a hand.
I’ll let you know if I get any more info when I report back to him, and we’ll probably have a chance to test the estimates against real time here pretty quick.
Thanks zut, it’s been a long time since I did this sort of thing as part of my job, it’s good to have someone confirm I wasn’t way out in left field with my figures!
Duke of Rat, you’re certainly welcome. It was kind of fun thinking about this stuff after all these years. Time was when I had on my desk all the tables and formulas to answer the question pretty exactly in just a couple of minutes. But these days I had to think a bit and google around.