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O-rings and altitude

  1. Aug 30, 2006 #1
    I am wondering if anyone knows how o-rings (such as nitrile, etc.) react when exposed to high altitudes -- especially in the case when under pressure. I guess I am thinking the thin air might harden them and cause a breakdown of the material -- but I'd like to hear some opinions or sources on this.

    Thanks!
     
  2. jcsd
  3. Aug 30, 2006 #2

    brewnog

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    Nope, not heard anything about this at all.

    Low temperatures, however, can cause embrittlement of most polymers, as the Challenger crew found to their demise in 1986.

    Naturally, in a high altitude environment you might expect to witness low temperatures, depending on your application.
     
  4. Aug 30, 2006 #3

    Gokul43201

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    Low temperatures cause loss of resilience in most rubber o-rings (recall the Challenger disaster), but low pressures have no effect - I've built vacuum equipment with o-ring seals.
     
  5. Aug 31, 2006 #4

    Astronuc

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    Why would one think that thin air would harden O-ring material? As Gokul mentioned such material is used in vacuum, and the only differing is a decrease of less than 1 atm.

    When the Solid Rocket Boosters are operating, they get quite hot! Then the issue is resilience at temperature.
     
  6. Aug 31, 2006 #5
    I was thinking about the thin air and how it affects temperature. See, in a vacuum, the objects are still in contact (at some point) with regular atmosphere and thus temperature is easily measurable. However, when you get closer to space -- the concept of temperature is more tricky. An object in space (or VERY high in the atmosphere) doesn't experience temperature in the same sense it does in a normal atmosphere. An object the size of an o-ring may not experience very many collisions with particles, so the o-ring may "experience" temperature differently than say taking a large object which would get a better estimate of the average KE of the particles in the area.

    I don't think I am cracking up -- but of course I am only on cup #1 of my Caribou Coffee!
     
  7. Aug 31, 2006 #6

    LURCH

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    I don't know if the effects of vacuum on an O-ring by itself have ever been tested. An o-ring on the Shuttle is direct contact with a very hot piece of metal, and heat gets transferred redily. An o-ring in a vacuum and not in contact with any other object might be very different.
     
  8. Aug 31, 2006 #7

    Gokul43201

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    An o-ring in a high altitude vehicle also sees low pressures only on one side of it - this time, it's the outside.
    This only changes the thermal time constant for equilibrating with the ambient temperature. If the o-ring started off warm, it will take longer to lose its thermal energy because of less frequent collisions.
     
  9. Sep 5, 2006 #8
    Right Gokul, but would the process be a little different in a vacuum sense? Meaning, wouldn't the o-ring lose more "heat" through evaporation of the material itself and blackbody radiation. This is my concern since it seems there is erosion of Nitrile o-rings on both sides of the o-rings in a CO2 pressurized system. I know CO2 is corrosive under pressure, but I want to eliminate any other sources of this corrosion in my system.

    This may be a shot in the dark, but could there be a radiation component to this problem? Perhaps repeated use in planes, etc. is exposing it to high energy radiation -- thus causing a breakdown? I know there is thought of making pilots and crew wear radiation badges because they can get pretty high exposures depending on what's hitting the Earth that day.
     
  10. Sep 5, 2006 #9

    Q_Goest

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    Hi wxrocks.

    There is no evaporation that might result in a heat flux, nor any significant evaporation at all. Nitrile O-rings are used for vacuum systems all the time, and long before you'd see any significant heat transfer due to evaporation, you'd see a sharp rise in the vacuum pressure but that doesn't happen. Nitrile is perfectly acceptable to use in a hard vacuum.

    Regarding blackbody radiation, an O-ring should be completely enclosed by some metal gland, so the O-ring itself doesn't see significant radiation heat transfer, it's the metal parts around it that do. That said, your other parts shouldn't be any colder than the environment. If in a vacuum, the temperature will still come to some thermal equilibrium through radiation heat transfer.

    Looking in a chemical resistance guide for elastomers, I see nitrile is the proper material for sealing CO2, it won't be attacked, even with wet CO2. CO2 shouldn't be corrosive under pressure, but if CO2 comes into contact with water it can create an acid. Nevertheless, nitrile is still compatible and won't be attacked by that.

    - What metal is the nitrile and CO2 in contact with?

    Checking material compatibility again for nitrile versus radiation (ie: nuclear radiation) I see nitrile is rated BC which is not great, but not catastrophic either, and that's for very heavy nuclear radiation as might be exposed in a nuclear power plant. So to answer the question, nitrile shouldn't be significantly affected by high altitude (ie: aircraft) radiation.

    - What are the symptoms you're seeing in the O-ring? What conditions is it exposed to (ie: altitude, temperature, CO2 pressure, etc...)? What is the extrusion gap (ie: the gap the O-ring would extrude through if it were a fluid)?
     
  11. Oct 12, 2006 #10
    I use o-rings all the time in my vacuum designs and usually they work great given the surface finish is 32um or better. For more info read the Parker O-ring book. there are several materials that fit different needs.
     
  12. Oct 13, 2006 #11

    FredGarvin

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    We have no problems with our o-ring applications at altitude. The only real issues to deal with are the decrease in temperature and fluid compatability. Gland design takes into account pressure direction. Your medium you are sealing is going to have more effect on the o-ring than the evaporative or radiation effects (if they even exist).
     
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