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Pipe deformation under pressure

  1. Jul 5, 2009 #1
    If a soft malleable metal pipe has a bend in it and is then pumped up with high internal static pressure, would this soft metal pipe seek to straighten out or would it simply remain in its bent shape due to pressure acting in many directions.

    I wonder if the most natural position of a pipe is straightened out, or whether there is no such natural position since the pipe was already bent and that IS its natural position as of then. I would guess that possibly the metal atoms would seek to find the most natural position under pressure. Maybe the most equal atomic surface contact and spacing (as much lack of deformity as possible). I am not sure what the most natural position for a pipe is if it is currently bent, if it has the ability to "reform" due to malleability.

    Maybe a factor that might affect this, is how recently the pipe was bent. If it had been bent into that shape 5 years ago, or 5 minutes ago. The atoms may have settled over time.

    (Assume that the pressure is indeed high enough to have any effect, if it could.)

    For visualization, we could dream of a pipe that was as soft as a garbage bag twist tie.. it would be very very very easy to reform this item. So if a pipe was like that, and subjected to internal pressure, but not enough pressure to burst it, would it straighten out first before trying to burst or expand.
    Last edited: Jul 5, 2009
  2. jcsd
  3. Jul 6, 2009 #2
    Oh boy, I have just reinvented the Bourdon-tube pressure gauge.

    Still questions though. Would like to know whether or not the tube in his gauge expands so the internal hollow volume increases, or does it just deform to a new elongated shape (volume the same - just reshaped)? Example: when an a hollow elastic tube is stretched... internal volume could change but it could also stay constant and just the gets narrower as you stretch it out.
    Last edited: Jul 6, 2009
  4. Jul 6, 2009 #3


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    You're thinking along the right lines: a pressure increase makes it energetically favorable for the pipe to deform in a way that the interior volume is increased. So would straightening a bent pipe be energetically favorable? You have to compare the change in interior volume to the energy required to permanently deform the pipe material. What occurs in a fabric hose could be very different that what occurs in a metal pipe (and this should match your intuition).
  5. Jul 6, 2009 #4
    Well the pipe doesn't have to be "permanently" deformed if one uses the right material - but in my original post I implied permanent deformity for the example I guess.

    p.s. with Schrader gauges I believe they have a bleeding function to reset the gauge whereas a Bourden measures the real time pressure and no bleeding? I don't have a Bourden gauge to analyze in person, I have to buy some and closely analyze :-)

    I think I need some formulas to calculate the volume differences between a bent pipe and a straight pipe. Would probably be best mathematically setting up a round pipe like a hoola hoop, and comparing to a straight pipe of the same. If I took a 1 meter straight pipe and bent it into a perfect circle without crimping the pipe.. the volume would be calculated via.. hrmmm.. The outside perimeter would expand while the inside would contract.
    Last edited: Jul 6, 2009
  6. Jul 7, 2009 #5
    Apparently the reason pipes or Bourdon gauge tubes straighten is because of P=F/A where the inside of the circular tube is a smaller area and the outside is larger. F=P*A, and with a larger area we have more force winning the battle.

    Vacuum (well actually the positive air pressure outside the tube) would do the opposite on the curve. If the pipe was near perfectly straight something interesting could happen - well the pipe is never perfectly straight so there is always an imbalance leading for it to deform one way more than the other.

    Liquid filled guitar strings, in addition to springs, were something funny that came to my head too.
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