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The force of water from a fosset

  1. Jun 16, 2006 #1
    I'm doing pressure calculations for a building project. I'm added all the bends and lengths of pipe up to the fosset on the top floor to find the pressure loss. In the basement there is 3,7 bars of pressure. At the top I'm left with 1,8 bars. Is this enough? What I wonder is how I can figure out what kind of force and amount the water will bring as the fosset is opened fully? Any help is appreciated.
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  3. Jun 16, 2006 #2


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    Staff: Mentor

    In the US, we design for 2.5 gallons per minute, but don't actually calculate how much water you'll get. That depends as much on the faucet as it does the piping, and they are regulated . But if you calculated 1.8 bar at whatever flow rate you need you should be fine.

    What size piping are you using, how long is it, how high does it go, and what flow rate are you looking for?
  4. Jun 16, 2006 #3


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    Are you running straight from a municipal supply, or do you have your own pumping station and reservois? If the former, then your top level has to be lower than the city water tower. Those are usually designed to feed at between 50-70 psi (sorry, I don't know from bars). You'll have to estimate your peak flow rate, as opposed to average. That will depend very much upon what kind of building it is. A large apartment complex, for example, will have a far higher peak usage in the morning (showers, toilets, etc.) than an office tower.
  5. Jun 16, 2006 #4
    The apartment needs approx 0,4 l/s, and the pipe that is to provide this measures about...12 mm inside. The water comes from a municipal supply which delivers 3,7 bars (370 000 Pa) static pressure to the basement. As mentioned, I'm left with 1,8 on top (before passing through the faucet) after taking into account all bends, height etc. The manufacturer says the faucet has a pressure loss of 1 bar.

    Say the faucet had exactly 1,8 bars of loss, what would be the outcome then? Would there come any water at all?

  6. Jun 16, 2006 #5


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    Flow through a valve is given by equations shown here:

    In metric units:

    Cv = 11.7 q (SG / dp)^.5

    q = water flow (m3/h)
    SG = specific gravity (1 for water)
    dp = pressure drop (kPa)

    If you don't know the Cv of the valve, and if the manufacturer of the valve can't give it to you, you can determine that in various ways. One way would be to take the flow rate at a given pressure drop and simply put it into the equation above. Another way is to calculate it from the geometry inside the valve. That's a bit more tricky.
  7. Jun 16, 2006 #6


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    In real life, the delta P across an object is proportional to the flow. Also, you will always have atmospheric pressure on one side. That means that if you just happen to have 1.8 bar(g) on the inlet, and you just happen to have a delta P across it of 1.8 bar, you will still have the same scenario as if you had 1.8 bar(g) inlet and no losses. Again, in reality, the losses have to be less than the driving pressure difference.
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