Not logged inAmerican Welding Society Forum
Forum AWS Website Help Search Login
Up Topic Welding Industry / Technical Discussions / cubic ft to pressure
- - By n5uzt (**) Date 12-28-2004 16:13
any one know how much gas it will take such as nitrogen to pressurize 1200 ft. of 6 in sched 40 pipe to 850psi
i will be using 230 cf hi pressure cylinders how many cyls will it take?
each cyl starts out at around 2200 psi
will have shut off valve in place.
i had formula at one time but have lost it
Parent - By G.S.Crisi (****) Date 12-28-2004 17:44
It's simple, apply the gases laws.
G. S. Crisi
Sao Paulo - Brazil
Parent - - By billvanderhoof (****) Date 12-29-2004 05:45
Taking the 6 inch dimension as the id of the pipe you have about 236 cu ft of space in the pipe.

Most gasses follow the rules for ideal gasses well enough to use those rules. One of them is that pressure times volume is constant if the temperature is constant. Thus initial pressure times volume equals final pressure times volume or p1*v1=p2*v2. the final pressure is 850 psi final volume is 236cu ft. I believe that a 230 cu ft gas cylinder contains 230 cu ft at atmospheric pressure (I will use 15 psi as atmospheric pressure) so- 15*v1=850*236 or v1=(850*236)/15 or v1=13373. 13373/230 is 58.14 cyinders. This assumes the pipe is initially evacuated. If not about one cylinder less is required. It also assumes some sort of pump to fully empty the cylinders into the pipe. If no pump the last cylinder will have 850 psi remaining when flow stops or roughly 1/3 of its contents. Over the whole run one would expect to leave about 1/6 of the contents in the cylinders. 230*5/6=191.6 which would be the effective volume per cylinder. 13373/191=70 cylinders. I would have a little extra on hand for good measure.

I invite all to check my work, I have been known to make mistakes.

For this quantity I would expect the supplyer to bring extras and agree to buy back the unused cylinders.

It may be more economical to buy the gas as liquid.
Bill
Parent - By sparx (**) Date 12-29-2004 13:30
Well Bill, that is pretty close. The only other thing to take into consideration is temperature. Temperature affects pressure. If we use the average ambient temp of 70 degrees F, then the total cylinders required of Nitrogen in "T" size cylinders (300cf) would be 67 cylinders. At 60 deg. F the total would be 70 Cylinders, and for argument sake, at 80 deg. F it would be 64 cylinders.
I don't have the formula for K size cylinders (245cf). but this will get you in the ballpark.
hoping this helps.

sparx.
Parent - - By TimGary (****) Date 12-29-2004 21:59
Just out of curiosity...
Are you using the gas to pressure test this line?
If so, can you hydrotest with water instead to save yourself a lot of trouble, danger and expense?

Just wondering...

Tim
Parent - - By G.S.Crisi (****) Date 01-11-2005 14:19
Tim,
some pipings can not be pressure tested with water. Examples:
1. Instrument quality air. Instruments are not pneumatic any more, but instrument quality air, with dewpoint that may be as low as minus 40 degrees, is still used for many purposes.
2. Dry chlorine.
3. Carbon dioxide.
etc.
In those cases, and I ran into many of them back in my days of erector engineer, you should use a dry gas (i.e., with no moisture) as in N5UZT's case.
Giovanni S. Crisi

Parent - By TimGary (****) Date 01-11-2005 15:25
Good points Mr. Crisi.
N5UZT sent me an email and explained that the purpose of the test was to check for leaks in a natural gas line and the procedure was a customer specification.

Thanks for the advice,
Tim
Up Topic Welding Industry / Technical Discussions / cubic ft to pressure

Powered by mwForum 2.29.2 © 1999-2013 Markus Wichitill