Drawdown Calculation....

Discussion in 'Pumps and Tanks Well Forum & Blog' started by watrax, Sep 1, 2009.

  1. watrax

    watrax New Member

    Messages:
    20
    Location:
    Philippines
    i just learned that calculating a drawdown for pressure tanks is the poduct of pump delivery rate in GPM/LPM and pump running time? is there an ideal or minimum pump delivery rate for residential uses and what is also the ideal pump running time? im clueless..pls help. thanks
  2. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas
    Size your pump for the delivery rate that you need. Size the tank to give you at least 1 minute of run time, and 2 minutes is better. Here is a draw down calculator that will help you understand.

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

    watrax New Member

    Messages:
    20
    Location:
    Philippines
    ok. is the pump delivery rate in GPM mentioned the same as the demand in GPM at the house? let say the demand at the house requires 8GPM. should i also size my pump at 8GPM?
  4. Gary Slusser

    Gary Slusser That's all folks!

    Eight gpm is not sufficient for more than a 1 bathroom house. I.E. a shower = 2.5 gpm, a toliet 1/2-1.4 gpm. a washing machine 3-4 gppm, a sink 2-2.4 gpm, a tub... large tubs with separate handles can flow over 9-10 gpm alone.

    Two person showers and 2-6 body sprays 20+ gpm.

    Pumps come in two parts, the wet end rated in gpm and the motor rated in hp. Once you know what peak demand gpm you need, then with a pump flow chart/pump curve chart, you find the horses needed to do the job.
  5. upper

    upper DIY Senior Member

    Messages:
    154
    Location:
    Fresno, CA
    First thing to know Weltrax above others,is what is your well capable of how much is avaliable.Your well does not care how many bathrooms you have,in fact it is better it does not know:D Upper
  6. watrax

    watrax New Member

    Messages:
    20
    Location:
    Philippines
    thanks for the link. DRAWDOWN (in Gals.) is the available water volums in the pressure tank regardless of its source (from well, cistern or storage tank). these drawdown will be divided by the drawdown factor to get the size of the pressure tank (in Gals). apparently drawdown depends on the selected pump delivery rate and the desired pump running time. i want know the basis of the drawdown you want to have for the pressure tank. like for instance for Storage Tanks, its size willl depend on the demand at the house. (1.) GPD per capita or (2.) Fixtures multipled by their corresponding fixture demands in gpm/gpd. THANKS
  7. Gary Slusser

    Gary Slusser That's all folks!

    You size a pressure tank so the pump is off between starts for a minimum of 60 seconds (for proper cooling of the motor) for up to and including 1.5 hp motors. Above 1.5 hp 120 seconds.

    So, run the water until the pump comes on, shut off the water and at the same time the pump comes on (switch closes) time how long it takes 'til the pump shuts off. If less than a minute, the tank is too small for the pump.

    The amount of the draw down gallons is dictated by the pressure range the pump is operated at; 30/50, 40/60 etc.. The lower the pressure, the higher number of gallons between pump off and on; the higher the pressure the fewer gallons. Of course that varies based on the size of the tank.

    Anyone selling tanks can tell you the gallons per various pressure ranges.

    This has nothing to do with your peak demand gpm in the house. That is a sum total of all the various fixtures using water in gpm at any given time but especially when the most water is being used.

    Another way to go and to be able to use a very small tank (as small as 2+ gals), is to use a CSV (Cycle Stop Valve). It allows the pump to run continuously while you are using water, which gives you constant pressure and greatly reduces the number of start/stop cycling of the pump that kills motors and, you don't need a large tank taking up a large space and their high prices.
  8. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Household fixtures are measured in SFU's (supply fixture units) the general figures can be found in any plumbing code book and most likely online also I'm sure. remember that the figures generally listed are for standard fixtures and do not include things like roman tub fillers and such. Also loads for sprinklers must be figured directly. Also remember that all of the fixtures in a home will almost never be open at the same time so you need to take that into consideration also. There is no need to figure the load as though every faucet in the house will be running at the same time.
  9. watrax

    watrax New Member

    Messages:
    20
    Location:
    Philippines
    so you're saying Gary Slusser that if i choose 20/40 pressures it will give me a bigger tank and if i choose 40/60 its will give me smaller tank
  10. Gary Slusser

    Gary Slusser That's all folks!

    No I'm not. I am saying that the tank, regardless of its size, will have more or less draw down gallons depending on the pressure range the pump is operated at. I.E. a nominal 20 gal bladder type tank with 20/40 psi pressure switch settings delivers roughly 6+ gals from pump off at 40 psi down to pump on at 20 psi. Same tank, switch settings at 30/50, less than 5 gals. Same tank, 40/60 less than 4 gals.

    It doesn't matter how many gallons, it is the length of time the pump is off between starts.

    The fewer gallons means the quicker the pump starts. You get fewer gallons because higher pressure empties the tank faster (the water moves faster; the velocity is higher) than at lower pressures.

    And you need 60 seconds off for proper motor cooling between starts.

    Pump starts kill pump motors so the fewer starts the better. All well water pumps are rated continuous duty (running constantly). And an inexpensive CSV extends pump motor life better than any other choice; especially if the choice is buying a larger tank. Large tanks are very expensive, take up a lot of space and are heavy.

    A CSV takes up no space because they are usually installed in the water line just before the pressure tank.
  11. Bob999

    Bob999 In the Trades

    Messages:
    448
    Location:
    Pennsylvania
    Gary, This doesn't seem to be correct to me. I don't think that the rate that a bladder tank empties has anything to do with how much the bladder tank will hold.
  12. Gary Slusser

    Gary Slusser That's all folks!

    Yes you're right but I meant fewer gallons in the house before the pump comes on. We are talking about how to size a tank which is based on how long the pump is off while the tank is delivering the water between pump starts.

    At higher switch settings you also are supposed to raise the captive air pre charge pressure, so there will be less space in the tank for water (fewer gallons) and then the higher pressure pushes the reduced volume of water out of the tank faster than a lower pressure range does which shortens the length of time for cooling of the pump motor.
  13. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    Bob, take a look at this chart. It will explain things a little better for you.

    http://www.goulds.com/pdf/BCPTP.pdf
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