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Effect of compressed air

  1. Jul 10, 2017 #1

    I have a question regarding the effect of compressed air. I'm working on hydrostatic pressure testing a component. The component will be filled with water and compressed to 1000 psi. I'm concerned about the small amount of air that'll be present inside and worried about the effect in case of an accident.

    Will the effect of compressed air increase with the volume of air inside? If so is there a formula to calculate the effect?

  2. jcsd
  3. Jul 10, 2017 #2
    It will be difficult for any of us to speculate of what may happen without knowing what exactly it is that you are doing. Can you elaborate more on what this is for and what you worry will go wrong?
  4. Jul 10, 2017 #3
    This experiment is to find the holding power of a seal. I'm planning to seal both sides of a component (a tube with i.d of 1.5" and 2" long) with the seal and pump water into it. To flush out the air inside, I'm tilting the component as much as i can while pumping water into it. the idea is flow of water would flush the air inside. Since the outlet nozzle from the component is in the middle of the component, there will be some air still existing inside. So if i continue to pressurize the water to a point where the seal breaks, at the moment where seal breaks, i'm concerned if the air inside would explode with the seal.

    It might not happen. but just concerned about the effects and precautionary measures to take.
  5. Jul 10, 2017 #4


    Staff: Mentor

    Yes of course the more air in the tank the more the danger. Water is nearly incompressible, so even a tiny leak relieves the pressure with no explosion. Not so with a tank full of air or steam.

    But a formula expressing what? You'll have to be more specific on your question if you want a good answer.

    I suspect the answer you seek is "How much air can I have and be safe?" The answer to that is likely to come from safety codes. Sorry, I don't know which code. My best guess is https://www.astm.org/Standards/E1003.htm
    "Standard practices for hydrostatic testing"

    Hopefully another PF member can finger exactly the code you need.
  6. Jul 10, 2017 #5
    the volume of air you have trapped when you start would be the extent of it's expansion if there were a rupture... 1000 PSI will compress 1cc of air to a VERY small volume, about 1/(1000/14.7) or 1/68th of a cc.. if there's a rupture, it'll return to being 1cc, which isn't much.

    On the other hand, if you were filling the whole thing with air and compressing it with more air up to 1000 PSI, you would get a very large volume of air... something to the tune of 1000/14.7 = 68 times the volume of the vessel.. a sudden rupture in this case would send things flying far and wide.

    I would place a barrier between you and the device under test anyhow.
  7. Jul 11, 2017 #6


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    Clearly the smaller volume of air the better; but, that is only one factor in evaluating the danger of the failure. The other factor is the specifically the manner of failure. In your initial post you state that you will pressure to 1000 psig but at the same time, in your later post you say:
    Which brings into the issue the safe pressure rating of all other components of your test fixture at the same time. So you clearly have a second maximum test pressure limit associated with those elements to consider.

    But beyond that issue, specifically related to a seal failure, depending upon the style of seal and seal containment design, you are still limited as your knowledge of exactly the manner in which the seal or seal assembly will fail; and this is critical to the level of danger the failure presents. A simple seal leakage failure can result in anything from a safe dribble of water to a very dangerous high velocity water stream or seal shrapnel discharge.

    As a result, insuring you have an adequate safety barrier and procedure during the testing process, regardless of the pressure energy, is the best way to approach dealing with the safety of your testing program.
  8. Jul 11, 2017 #7
    Thank you for the reply. That makes sense.

    And yes of course i'll put a barrier between the test equipment and me.
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