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Thread: Calculating Flow Rate From PSI Reading?

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    flyboy320's Avatar
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    Calculating Flow Rate From PSI Reading?

    We are installing a UV system for our pool, and the manufacturer recommends a max flow rate of around 50gpm. There is not enough room to install a flow meter, but there is a psi gauge at the inlet of the UV unit. Is there a way to estimate the flow rate using the psi gauge, and the inlet pipe diameter?

    There is a by-pass installed at the UV system so some of the water is by-passed around the UV and onto the heater, so it's not enough to just get the flow rate of the pump, as that wouldn't accurately give the water flowing into the UV, so all I have to go on is the PSI gauge.
    10K gal IG, vinyl, 1 HP Jandy Stealth pump, Jandy CS cartridge filter, Delta UV system, Hasa Liquidator

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    mas985's Avatar
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    Re: Calculating Flow Rate From PSI Reading?

    It depends on your plumbing somewhat but I have written a proceedure in this post to estimate flow from PSI readings. The first step is to determine the head loss of the plumbing and the second step is look up the flow rate on the pump's head curve. Here is the excerpt:

    Plumbing Head Loss

    There are several rules of thumb regarding head loss and most pools are usually between 45-65 feet of head with a clean filter, no solar and no water features. Generally, larger pumps are at the high end of the range and smaller pumps are at the lower end of the range. However, there are other methods to get a better estimate of head loss.

    Return dynamic head can be estimated from a filter’s PSI reading with this formula:

    Return Dynamic Head = 2.31 * Filter PSI + 3

    The 3 is because the filter guage is usually 3' above the pump so that needs to be taken into account.

    Suction dynamic head can be estimated from a vacuum gauge at the input to the pump (i.e. drain plug). Vacuum gauges can be found at automotive stores or Sears for about $20. If you have one, you can determine suction head loss with this formula:

    Suction Dynamic Head = 1.13 * Inches Mercury (vacuum gauge reading)

    If a vacuum guage is not available, there are ways to "guess" at the suction head loss. For a standard plumbing system with a filter, heater and without solar, water features or cleaners engaged, the suction head loss can be approximated by the following:

    Suction Dynamic Head = 75% of Return Dynamic Head for 1.5" suction and 1.5" return pipe
    Suction Dynamic Head = 50% of Return Dynamic Head for 2" suction and 2" return pipe
    Suction Dynamic Head = 30% of Return Dynamic Head for 2.5" suction and 2.5" return pipe
    Suction Dynamic Head = 17% of Return Dynamic Head for 3" suction and 3" return pipe

    Suction Dynamic Head = 22% of Return Dynamic Head for 2" suction and 1.5" return pipe
    Suction Dynamic Head = 22% of Return Dynamic Head for 2.5" suction and 2" return pipe
    Suction Dynamic Head = 14% of Return Dynamic Head for 3" suction and 2.5" return pipe
    After you determine the head loss, you can then look up the flow rate on the pump's head curve.
    Mark
    Hydraulics 101; Pump Ed 101; Pump and Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater, ThePoolCleaner

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    flyboy320's Avatar
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    Re: Calculating Flow Rate From PSI Reading?

    Thanks mas985, much appreciated...
    10K gal IG, vinyl, 1 HP Jandy Stealth pump, Jandy CS cartridge filter, Delta UV system, Hasa Liquidator

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    Re: Calculating Flow Rate From PSI Reading?

    Whoa, deja vu dude...
    Factory Warranty Station for:
    Jandy, Pentair, Sta-Rite, Raypak, Polaris,
    and Paramount pool cleaning systems.

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    flyboy320's Avatar
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    Re: Calculating Flow Rate From PSI Reading?

    Quote Originally Posted by Pool Clown
    Whoa, deja vu dude...
    LOL.....more opinions the better though... :P
    10K gal IG, vinyl, 1 HP Jandy Stealth pump, Jandy CS cartridge filter, Delta UV system, Hasa Liquidator

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