Water content in compressed air

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Discussion Overview

The discussion revolves around the issue of water content in compressed air within a natural gas main, specifically addressing the infiltration of water vapor into the system during pressure testing. Participants explore methods for calculating the amount of water present at different pressures and conditions.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Exploratory

Main Points Raised

  • One participant describes a scenario where water infiltration is causing freezing issues in a gas regulator and meter, suggesting that water vapor from compressed air may be the source.
  • Another participant proposes using the ideal gas law to calculate the water content at 90 PSIG, suggesting a direct ratio based on pressure differences.
  • A different participant considers using steam tables and partial pressure for the calculations, seeking confirmation on this approach.
  • One participant notes that the calculated amount of water seems low, referencing a case where a compressor can introduce significant amounts of water into a system under certain conditions.
  • A participant shares a link to a personal webpage where they have posted their calculations related to water vapor in pipes.

Areas of Agreement / Disagreement

Participants express various methods for calculating water content, but there is no consensus on the best approach or the accuracy of the calculations. Multiple competing views remain regarding the methods to be used.

Contextual Notes

Participants mention assumptions regarding temperature and relative humidity, but these are not fully detailed. The discussion does not resolve the mathematical steps or the implications of different calculation methods.

Who May Find This Useful

Individuals interested in the effects of humidity in compressed air systems, gas distribution systems, or those involved in engineering applications related to gas and air pressure management may find this discussion relevant.

JackRuby
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Hi-

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

Thanks again!

JR
 
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

Thanks..
 
Last edited by a moderator:

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