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Reducing Water Pressure (Pipe sizing)

  1. Dec 13, 2013 #1
    I have a problem at work and unfortunately it has been too long since I have been in a class room for me to reason this one out, never did like fulid mechanics much :).

    We have filter that is pressurized to approximately 55 psi. Once ever 12min or so this filter purges a volume of ~2L that for the sake of this problem we can call water.

    The facilites side of our building says that they can not accept pressurized liquids to this system. As far as I know it is an open drain line that is usually gravity drained to a waste collection barrel on the floor beneath the tool.

    The thought from some of my coworkers was first to add a collection tank and then gravity drain from there. Then the though was what if we just expanded the drain line to ~4" line and then reduced to tie into the facilities line (approximately 2" line).

    The question came up, How long of a section of 4" pipe do we need to get this 2L (55psi) volume of water down to around 10psi.

    I thought oh this will be easy but then I remember... well this isnt a gas so i can't use the equation I thought I was going to. I don't have constant flow in the pipe either so Bournoulli's gets a little weird as well.

    Is this event the right way to think about fixing our pressure problem? If so, any help on how I can calcuate how long of a section of 4" pipe we need to install to reduce our water pressure to the desired 10psi?
  2. jcsd
  3. Dec 13, 2013 #2
    Pressure is independant of volume. Listen to your coworkers and use an intermediate receptacle in the drain line. The receptacle should be open to the atmosphere, so that receptacle simply acts as a funnel.
  4. Dec 13, 2013 #3
    What if we just used an orifice plate? I still don't think any of this is necessary as there is no way that our 2L of 55psi is going to pressurize the open drain line but some people require extra assurance.
  5. Dec 13, 2013 #4
    The drain line must remain at atmospheric pressure, by plumbing code(law), because otherwise you'd be making a pressure tight connection between a building drain and a potable(?) water source, and the drain system is not designed for pressure applications at all. If your drain gets plugged, your orifice plate installation will provide for a ruptured drain.
  6. Dec 13, 2013 #5


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    You really haven't described your building's drainage system in too much detail. I'm having a hard time visualizing how 2L of water discharged every 12 minutes or so will pressurize any piping system that isn't already full.
  7. Dec 13, 2013 #6
    I agree and this is exactly why I think it is unnecessary, but I need a good concise way to explain it to the team so that they also agree. I think it should be fine for us to drain staright from the filter to the facilities line without making any changes. It is simply a gravity fed drain line open to a waste collection tote on the floor below. It just that the 2L purge is pressurized liquid which seems to scare people. The filter is simply filtering out solids from the tool chemistry so no this is not connected to a potable water source. The line is dedicated for this purpose so theoretically it should be empty every time we dump to it.

    Edited for clarity I hope

    I think my orifice plate idea is a bust anyways. Quick calcuation puts me at needing a plate with a 1.35mm hole in it to reduce the pressure to 10psi (just assumed it was continuos flow) and there's no way that would hold up.
    Last edited: Dec 13, 2013
  8. Dec 13, 2013 #7


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    If you try reducing the pressure by restricting flow, it can generate a lot of noise. Best to have a large open tank and a large bore drain pipe from it. Simple drainage systems do not expect to cope with more than a very few metres of head. You may need some form of silencing for this vent or the periodic sound of the purging could disturb people.
  9. Dec 14, 2013 #8
    What happens when the final outlet blocks and the contents of any traps followed by your contaminated waste shoot out of any properly plumbed outlets at 55psi every 12 minutes? (At least you'll know it's blocked...)
  10. Dec 14, 2013 #9


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    Could be fun!
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