Pressure tank size

Discussion in 'Pumps and Tanks Well Forum & Blog' started by NathanJ, Feb 23, 2011.

  1. NathanJ

    NathanJ New Member

    Messages:
    2
    Location:
    Northern Virginia
    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. Gary Slusser

    Gary Slusser That's all folks!

    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.
  3. valveman

    valveman Moderator Staff Member

    Messages:
    4,486
    Location:
    Lubbock, Texas
    Is it a new tank? The bladder maybe busted and you have 3 gallons in the air chamber.
  4. NathanJ

    NathanJ New Member

    Messages:
    2
    Location:
    Northern Virginia
    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. valveman

    valveman Moderator Staff Member

    Messages:
    4,486
    Location:
    Lubbock, Texas
    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. Gary Slusser

    Gary Slusser That's all folks!

    " I only raised the cutoff pressure, which allows the tank to fill with more water."

    Higher pressure reduces the draw down gallons/volume.
  7. Texas Wellman

    Texas Wellman In the Trades

    Messages:
    537
    Location:
    SE Texas-Coastal
    How you figure that one Gary?

  8. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    Try 20 to 80 psi gary and report back on the drawdown. Or 40 to 90. Good lesson coming.
  9. valveman

    valveman Moderator Staff Member

    Messages:
    4,486
    Location:
    Lubbock, Texas
    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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