What happens to air in a hydrostatic test

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    Air Hydrostatic Test
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Discussion Overview

The discussion revolves around the behavior of air in a hydrostatic test of water lines, specifically addressing the implications of air presence in an enclosed pipeline during pressure testing. The scope includes practical applications in plumbing, theoretical considerations of gas solubility in liquids, and some light-hearted inquiries.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that if air is present in an enclosed pipeline during hydrostatic testing, it may dissolve into the water under increased pressure, with the implication that at 150 PSI, more air can dissolve than at atmospheric pressure.
  • Another participant questions whether all the air would be absorbed into the water if the pressure is maintained, suggesting a scenario with specific volumes of air and water.
  • A different participant provides a reference to the solubility of air in water at certain temperatures and pressures, indicating that a specific volume of air can indeed dissolve in a given volume of water at elevated pressures.
  • There is a humorous inquiry about the air speed velocity of a coconut-laden swallow, indicating a playful tone amidst the technical discussion.

Areas of Agreement / Disagreement

Participants express varying degrees of certainty regarding the behavior of air in water under pressure, with some proposing that air will dissolve while others question the extent of this dissolution. The discussion does not reach a consensus on the complete behavior of air in the system.

Contextual Notes

Participants reference specific conditions such as temperature and pressure but do not resolve the implications of these variables on air behavior in the pipeline. There are also assumptions regarding the completeness of air dissolution that remain unverified.

Who May Find This Useful

This discussion may be of interest to contractors, engineers, and individuals involved in plumbing or fluid dynamics, as well as those curious about gas solubility in liquids under pressure.

Contractor
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I am a contractor. I install water lines. Once installed we have to pressure test them. We pump the lines up to 150 PSI of water. If the PSI drops one of 2 things is wrong, either i have a leak in the line, or there is a small amount of air in the line. Generally if it is air, it stabilizes in a very short time and we get a passing test. I currently have a line that will not hold a test and I can not find the leak. My question is/are:

1. In an enclosed pipeline, supposing no leak, if there was air in the pipeline and i continue to hydrostatically pump until the pressure stops dropping, what happens to the air?

2. Is there a way to calculate how much air is in the line by how much water it takes to pump it up?
3. What is the air speed velocity of a coconut laden swallow (asking for a friend).
 
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With no expertise in the field...

1. The air dissolves into the water. At atmospheric pressure, not much dissolves. At 150 PSI, significantly more is able to dissolve. When you apply pressure, the bubbles slowly dissolve into the water and the pressure is reduced.

2. A good approximation would be that the volume of bubbles in the line is equal to the amount of water you pumped into make them go away.

3. African or European?
 
Your answer to 1 is very intriguing. So if i had 10 gallons of water and a gallon of air in the system, and then injected water up to 150 PSI and held it there, eventually all of the air would be absorbed into the water? so there would be no air pockets at all?

And I will counter you with, "Who is this man who is so learned in science?"
 
As I said, I am no expert. A quick trip to Google says that at 70 degrees Fahrenheit and 100 psig, the solubility of air in water is about 1 part in 7. That is, one gallon of air into seven gallons of water. So yes, one gallon of air would fit into ten gallons of water at 150 psig with no problem.

https://www.engineeringtoolbox.com/air-solubility-water-d_639.html
 
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