JD:
You may be a concerned EE, but I wouldn’t worry about working out this problem correctly and safely if I were you. You’ve already shown how sharp you are by coming to this forum on this type of question. If I can’t resolve it quickly enough for you, there are more than a dozen, keen and sharper engineers on this Forum that can this in their sleep.
First, I’m surprised they want you to do a pneumatic pressure test. But, that’s OK, as long as the test pressures are relatively not high and you take all the safety precautions that I’m sure you will receive here and at your site. I’m going to assume you are going to be testing at around 300 psig max. I have to know the test pressure in order to find/calculate the correct “Z” (compressibility factor for the gas you select to test with. I presume you’re thinking of using Nitrogen. Here are the steps you need to plan on taking to arrive at the identification of how much N2 you’ll need for the test:
1) Obtain an accurate calculation of the internal pipe (& any fittings) volume that you’ll be testing. You can easily add up the internal volume if you have an Excel spreadsheet with all the pipe and fittings’ volumes already figured out. If you don’t just give us the list of what you have and perhaps we can help on giving you each individual item’s internal volume.
2) You’re going to pressurize the internal volume up to test pressure and you’ll need to employ the gas equation of state:
PV = Z n R T
where,
P = the test pressure, in psia
V = the internal volume pressurized, in ft3
Z = the compressibility factor for N2 at the test pressure and temperature
n = the number of lb mols of N2
R = the universal constant, 10.7316
T = the test temperature of the gas in the internal volume, oR
Note that absolute pressure and temperature are employed – not gauge values. By re-arranging the equation, we obtain:
n = PV / Z R T = lb mols of N2
If you multiply the lb mols of N2 by the molecular weight of N2 (28.016) you’ll get the lbs of N2 required to do the job. You can also multiply the lb mols of N2 by 379.49 and obtain the “Standard” cubic feet of N2 required as measured at 14.696 psia and 60 oF.
There you have it. You should add an additional amount of estimated N2 that will be the residual remaining in the N2 cylinder source, under pressure. Here, I’m assuming that you will employ HP N2 cylinders to feed the N2 into the test pipe & fittings. Since you haven’t stated how you will obtain and feed the N2, I have to assume.
You probably have some questions and comments. With the rest of the guys writing in by now, you should have an accurate and reliable estimate in a day or two. If you give us all your basic data and scope, the answer will come faster and more accurate.
Hope this helps. Help us help you by giving all the details.
