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Pressure in outdoor water tap is no good

  1. Apr 15, 2009 #1
    My company delivered an outdoor water valve (the kind you connect your garden hose to). The guy called me the other day telling me that he thought the water now had less "force" coming out the end that it used to with his old valve. The valve has a 20cm long pipe end to connect to which is Ø22 mm. Because of this the pipe leading up to this point is also done with Ø22 mm. His suggestion was to increase this supply pipe but I'm not sure this will do any good. Rather the opposite...
    Any thoughts on whether this will help and if no, what should be done?
     
  2. jcsd
  3. Apr 15, 2009 #2

    minger

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    What changes were made between the two valves? Are they of different types?
     
  4. Apr 15, 2009 #3

    FredGarvin

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    The first thing I would make sure is that both valves have the same or close to equivalent Cv (or for you European folks, an equivalent Kv).
     
  5. Apr 15, 2009 #4
    Increasing the size of the pipe supplying your valve assembly will help with the pressure at the valve, but... If the pressure was acceptable before, it may be easier to change out the valve than to change out the supply piping.

    It also occurs to me that the new valve is flowing more - and that would increase the pressure drop along the supply piping. I'm not sure if that makes sense, I have to think about it for awhile.
     
  6. Apr 15, 2009 #5
    I'm not sure, but I believe the old tap only had a 15 mm supply pipe so I suppose it probably delivered less water but with better "force" or pressure at the end. Does that make sense?
     
  7. Apr 15, 2009 #6

    russ_watters

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    Not really, no. What is on the end? If the nozzle, faucet, or whatever is on the end hasn't changed, the larger piping and valves leading to it provide lower resistance and therefore greater flow and pressure at the end.
     
  8. Apr 16, 2009 #7
    The user probably connects a 3/4" hose to the nozzle. I'm just thinking that many just pinch the end of the hose in order to get more "pressure" out the end, so won't reducing the supply pipe do sort of the same thing...?
     
  9. Apr 16, 2009 #8
    Well, if I understand this right, the new valve has a bigger diameter than the previous one. Is this correct? I also assume the water supply has not been changed?

    Now, careful not to mix force and pressure! Force is due to the water supply, while pressure is the force divided by the area of the valve (ie A = pi*radius^2 and P=F/A)

    If your customer wants greater PRESSURE from the valve, change the diameter of the valve. By mass conservation however, the water supply should not be affected by this, as the water coming in (from the water supply) must be equal to the water coming out of the valve, regardless of the diameter of the valve.
     
  10. Apr 16, 2009 #9

    russ_watters

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    Yes, at the pinch. Past the pinch, the pressure drops because the pipe gets bigger again as per Bernoulli's principle.

    It is possible I'm misunderstanding what this looks like, though - perhaps you could upload a sketch...?
     
  11. Apr 16, 2009 #10

    russ_watters

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    I can't fathom an application where someone would want a higher pressure behind a valve via changing the valve size - all you are doing is adding loss to the system.
     
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