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Thread: fluid dynamics - constrictions in gate valves

  1. #1
    DIY Junior Member tbb2's Avatar
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    Question fluid dynamics - constrictions in gate valves

    This is something that nags at me whenever I install a gate valve.

    If I have a 3/4" copper supply that goes into a gate valve the valve body cuts the pipe ID in half (or more).

    Why isn't there a drop in supply pressure on the outgoing side of the valve?

    Or is it that the difference isn't obvious?

    If it is that the difference is not obvious then why do I need 3/4" when the valve is only allowing 1/2" at best to pass thru it (don't add a 1/2" gate valve into the discussion) ?

    Or is this just why I like ball valves?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Venturi effect...personally, I do like ball valves since it's rare the seals go bad and the connection to the gate doesn't break.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the Trades Gary Swart's Avatar
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    Pressure is not affected by reducing the size of the pipe (valve). Flow can be affected, but when it's just a short distance like the valve, it would be hard to measure. As Jim notes above, ball valves are a superior valve.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, a ball valve doesn't work all that well as a throttle, works better as an on/off valve. Normally, though, that's what you want with a supply valve.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Member BillTheEngineer's Avatar
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    You are probably not seeing a drop in flow or pressure through the gate valve because you are not at the limits of the valve. There is a drop in pressure through it but it is small, maybe a 1 to 3 psi at the highest flows you are putting through the valve. A gate valve has a pretty unrestricted path only sightly less than a ball valve.

  6. #6
    DIY Senior Member bluebinky's Avatar
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    Another way to look at it (a pretty good approximation at reasonable flow rates) is that a restriction like a valve is equivalent to some length of the pipe you are using. The same goes for fittings...

  7. #7
    Moderator & Master Plumber hj's Avatar
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    You are buying "cheap" valves, '(but who uses gate valves any more in the first place)? ALL the good ones are "full port" which do NOT have the restriction. IF you were to use a Jet-Swet, or similar stopper, to add or change a valve, you would need a full port valve or you would not be able to extract the stopper through the valve.
    Licensed residential and commercial plumber

  8. #8
    DIY Senior Member Hairyhosebib's Avatar
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    I agree with HJ. use FULL PORT ball valves. If you are using them in a sweat copper system use threaded valves and use male pipe thread adapters. If you plan to insulate around the ball valve find a brand that has the handle extended up from the top of the valve. The extended shaft is enclosed in a plastic tube so that you will only need to cut a small hole in the insulation. Otherwise ball valves are difficult to insulate. If you were installing a chilled water piping system in an area of the country where humidity is high they will sweat quite a bit.

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