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Opening/closing valves and shock types caused

  1. Sep 29, 2013 #1
    Hi,

    I`ve had past experiences with pressure switches giving "strange" (pulsed) signals due to nearby discharge valves closing or opening. Looking a little into it I run into reading about compressibility theory and "shock tubes".

    Just for info, a "shock tube" is a tube with 2 zones, "driver" with high pressure gas, and "driven" with low pressure. Separating them there is a "valve" that is suddenly opened.
    The shock tube model explains the steep pressure change evolution, and the different "zones" after the "valve" is opened.

    After the valve is opened there are two zones, start and end of the tube where the pressure is yet the initial and an "intermediate" zone where the pressure is a constant value between both.
    It appears a "shockwave" as a steep change in pressure, that is the limit between the "intermediante" and end zone, progressing towards the end of the tube, and a "rarefaction wave" limit zone progressing to the init of the tube.

    With values of (10 bar driver, and atmospheric output) in this link
    http://www.aero.iisc.ernet.in/~lhsr/web/stc.php
    can be calculated that the "shock" front (intermediate zone) is 2.85bar and the speed Mach 1.61.

    Thinking about it I see that in a simple pneumatic model (i.e. reservoir, valve and pipe to atmospheric output) the main difference with the shock tube is that the "driver" section is actually a source of pressure, only limited by the flow resistance.

    The question (sorry for the long explanation) is if the shocktube results are really related to the "shock" case in a reservoir+valve opening+tube to atmosphere system and how the "rarefaction" wave would behave upstream towards the reservoir in the real pneumatic system

    Regards and thanks for your comments
     
  2. jcsd
  3. Sep 30, 2013 #2

    Baluncore

    User Avatar
    Science Advisor

    A rarefaction wave cannot become supersonic. It adiabatically cools the gas and so must be subsonic.

    It does seem a bit extreme to invoke supersonic shock waves when the observations can be explained by normal sinusoidal pressure waves.

    To initiate a shock wave requires an almost instantaneous valve opening, and it really helps to have a vacuum on the low pressure side to sharpen the step. If at STP you open an air valve in 10ms the transition will stretch over about 11 feet. That is not a shock wave.

    Do these “strange” signals you observe cause a problem?
     
  4. Oct 1, 2013 #3
    Just used the shock tube example as I did not find any good explanation or example related to the "regular" rarefaction wave that I guess is produced also at valve opening, the usual sources always treat the "closing" water hammer wave, but not the opening case, so is not very clear for me what happens.

    I actually can not remeber the details but the behaviour was something like this
    1) a compressor is controlled by a pressure switch
    2) a valve is controlled, to exhaust enough air to assure the minimun duty cycle of the compressor (to keep it hot)
    3) the valve is near the switch, and when it is open/closed the pressure switch sometimes change state, causing the compressor to do an additional start/stop cycle quickly.

    I associated 3) with the exhaust valve opening, but thinking twice it seems now to make more sense if it was caused at closing, so the water hammer wave causes the switch to erroneously signal that the pressure was high enough stopping the compressor and then starting again when the situations goes back to normal.

    Regards and thanks for the comment
     
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