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Thread: Pressure tank size

  1. #1
    DIY Junior Member NathanJ's Avatar
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    Default Pressure tank size

    I have an Amtrol PF32 pressure tank that is supposed to have a 9.4 gallon drawdown at 40/60. My problem is that I am only getting 6-7 gallons out of it before the pump kicks on.

    My pump flows 10 GPM and I have even temporarily upped the pressure so it is 40-65psi. My pump is able to fill the tank in 39 seconds with these settings.

    I have drained my tank and increased the pressure to 38PSI from 28 PSI ( the switch was set to 30-50). When drained, the tank feels light like all the water has been removed.

    How come I am getting 3 gallons too little out of this tank?

    Thanks,
    Nathan Jones

  2. #2
    That's all folks! Gary Slusser's Avatar
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    The tank is rated at 40/60 and you're operating at 5 lbs higher, which reduces the drawdown gallons but I'm not sure it would reduce it by 6-7 gals.

    Your pump may be delivering more than 10 gpm due to the water level in the well being higher than the level used to rate the pump at.
    Gary Slusser Retired (= out of business)
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  3. #3
    Moderator valveman's Avatar
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    Is it a new tank? The bladder maybe busted and you have 3 gallons in the air chamber.

  4. #4
    DIY Junior Member NathanJ's Avatar
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    Thanks for the replies guys. I only raised the cutoff pressure, which allows the tank to fill with more water. The 10 GPM was actually calculated by taking the volume of the drawdown and the time it took to repressurize the tank. It was nearly exactly 10GPM.

    The pressure tank was installed in Nov 2004, so it isn't new, however it didn't feel like there was any water in it after I drained it. I was able to rock it side to side easily.

    I'm thinking about getting a larger pressure tank, but before I do that I would like to understand what is happening to mine. The only thing I know to check is the bladder pressure with another gauge. But I believe mine to be pretty accurate.

  5. #5
    Moderator valveman's Avatar
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    You could still have three gallons on top of the bladder, and it would still feel light. Bladders are busted from the pump cycling on and off. The diaphragm breaks like bending a wire back and forth. A tank sized for a one minute cycle is required, two minutes is better, and no cycling is even better. A CSV to stop the cycling will work with as small as a 4.4 gallon tank like pictured on the left. You can see how it works at this link. www.cyclestopvalves.com

  6. #6
    That's all folks! Gary Slusser's Avatar
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    " I only raised the cutoff pressure, which allows the tank to fill with more water."

    Higher pressure reduces the draw down gallons/volume.
    Gary Slusser Retired (= out of business)
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    CAUTION, as of Nov 12 2013 all YouTube videos showing how to rebuild a Clack valve have an error in them that can cause damage.

  7. #7
    In the Trades Texas Wellman's Avatar
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    How you figure that one Gary?

    Quote Originally Posted by Gary Slusser View Post
    " I only raised the cutoff pressure, which allows the tank to fill with more water."

    Higher pressure reduces the draw down gallons/volume.

  8. #8
    General Engineering Contractor ballvalve's Avatar
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    Try 20 to 80 psi gary and report back on the drawdown. Or 40 to 90. Good lesson coming.

  9. #9
    Moderator valveman's Avatar
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    A 32 gallon tank has 20.27 gallons of draw down at 20/80. That is stretching the heck out of the bladder though. Many of these tanks have a metal dome above the diaphragm, so they only stretch so much, no matter how high the pressure goes. You can play with different pressure settings and see the draw down using the calculator at this link.

    http://www.cyclestopvalves.com/runtime_app.php

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