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Water content in compressed air

  1. Dec 29, 2004 #1

    A recently installed natural gas main is showing evidence of water infiltration. Water is getting into the customers regulator and meter and freezing. The gas main was cleaned and "pigged" after the installation. The system is currently operating at an MAOP of 60 PSIG natural gas.

    When the main was pressure tested (at 90 PSIG - air) it is likely that water (in the form of vapor) got into the main from the compressed air (Air compressor).

    Using a Psychometric chart I was able to calculate the amount of water at atmospheric conditions (14.7 PSIA). I assumed the Dry Bulb temp was 32 Deg, RH = 99%... etc.. Cameup with 9 lbs.

    How would I calculate the water infiltration at 90 PSIG Air? I think I need to use partial pressure?

    Any help would be greatly appreciated!
  2. jcsd
  3. Dec 29, 2004 #2


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    Staff: Mentor

    Via the ideal gas law, it would just be the ratio of the two air pressures: 90/14.7 * 9lb=55lb
  4. Dec 29, 2004 #3
    ??? I was thinking of using the steam tables - partial pressure? What you are saying makes sense. Can you please confirm?

    Thanks again!

  5. Dec 31, 2004 #4
    Thanks for the ideas...

    I think I'm on the right track. The amount of water appears low. However, the actual pipe volume is very small. Still I've read that (for example) On a 75F day with 75 percent relative humidity, a 10-hp compressor can introduce 7 gallons of water into a compressed air system.

    Instead of trying to type it all out here - I actually took the time to figure out how to use my free web space that is offered as part of my internet account. I posted the calculation there. Please forgive the very very crude looking web page. See link below.

    http://home.comcast.net/~timbickford/htdocs/Water_Vapor_In_Pipe-Rev-1.pdf [Broken]

    Last edited by a moderator: May 1, 2017
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