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Thread: Pressure vs Flow

  1. #1
    Associate Engineer
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    Mar 2015
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    :bash: Pressure vs Flow

    Hello! New member... I've never wished i'd become some sort of engineer in my life... anyways. Water controls question:

    If i need to control the GPM and the Pressure off a larger (2") water feed do I want to regulate water pressure first OR flow through a ball valve first.... I'm thinking I control the pressure then the gpm...

    Thank you and pardon my lame question.

  2. #2
    Principle Engineer Cragyon's Avatar
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    Yes, you would need to regulate the pressure first then flow control. The pressure would need to be greater than the pressure needed at the desired flow rate.

  3. #3
    Associate Engineer
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    Thank you Cragyon!

  4. #4
    Associate Engineer
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    Higher pressure will cause greater flow through any given pipe size, but as the flow increases, the pressure will decrease downstream due to friction loss because water velocities increase as well.

  5. #5
    Associate Engineer
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    Flow through a pipe can be determined using the Darcy-Weisbach equation or one of the other equations for irreversible pressure drop. These equations are generally restricted to small changes in density, so if you have a gas, the density change needs to be less than about 20% for the equations to be accurate. For density changes in excess of that, we generally break up the pipe into sections and recalculate properties as the pressure changes.
    Ref: http://en.wikipedia.org/wiki/Darcy–Weisbach_equation

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