Bladder Tank Sizing for Variable Speed Pump

Discussion in 'Pumps and Tanks Well Forum & Blog' started by ecodian, Dec 4, 2009.

  1. ecodian

    ecodian New Member

    Messages:
    3
    Location:
    Indiana
    I have recently installed a new variable speed pump and I saw that the old very large bladder tank have been replaced with a small tank. My plumber said that with VSD pump I only need a small bladder pump. When I ask him how do you size the bladder tank for the VSD pump and he just said that was all I needed.

    Therefore, can anyone tell me

    1. Why a VSD pump only need a small bladder tank?

    2. how to size a bladder tank for the VSD pump?

    3. Can we run the VSD pump without the bladder tank? I have tried with the bladder tank being isolated from the system and the VSD pump run as normal.

    Thanks.
  2. dancour

    dancour New Member

    Messages:
    9
    I understand that the small bladder tank only softens the many stops and starts of the pump. If you brush your teeth, it turns on and off.

    As for the pressure setting, Franklin Monodrive recommends 70% of set pressure.

    So a system set for 50 psi, would have a pressure tank setting of 35 psi.

    See attached page from manual.

    Attached Files:

  3. valveman

    valveman Moderator Staff Member

    Messages:
    4,425
    Location:
    Lubbock, Texas
    Although that page says you can "use an existing tank with much larger capacity", it will be of absolutely no help. A pressure tank can only deliver water as the pressure decreases. When pressure remains constant, no water ever enters the tank or is drawn from the tank. It doesn’t matter if the tank is the size of a baseball or the size of your car.

    You are correct that this type system functions the same when the tank is isolated. For these VFD or VSD type systems, a tank is simply a shock absorber. It gives the system a second to see that you have turned on a faucet before the pump gets started. It also keeps the pressure from spiking extremely high when you turn off the faucet, before the pump gets shut down. The pump is still going to start every time you rinse a toothbrush, the ice maker is filled, or water is used anywhere.

    With the SubDrive/MonoDrive type systems, the pump is also going to start 45 times per minute when you are using water for longer periods of time, as when taking a shower or running a sprinkler. This type system is going to start and stop the pump 2700 times for every hour it is running. This adds up to about 2 million cycles per month, and is one of the major reasons for premature system failure. It is still going to do this, no matter the size of pressure tank.

    A CSV and a regular pressure switch will solve this problem. It allows the use of the entire amount of water stored in any size pressure tank, and also delivers constant pressure after the pump is finally started. But of course then you would not need the VFD/VSD type system at all.

    Manufacturers promote products that help keep the cash flowing. Making your pump system last a long time would be counter productive to their efforts. It is more important to find out what the manufacturer is not telling you, than to simply accept what their glossy ads are telling you. If you were convinced that the VFD system would save you energy, then you have just fallen for another "green lie" that only perpetuates "planned obsolescence". VFD systems do not save energy.
  4. masterpumpman

    masterpumpman New Member

    Messages:
    729
    Location:
    Virginia Beach, VA
    CSV, try it you'll like it!

    Valveman said it best.
  5. ecodian

    ecodian New Member

    Messages:
    3
    Location:
    Indiana
    Thank valveman for the clarification.

    Another question: do I still set the pressure tank 2 PSI below the VSD pump pressure?
  6. valveman

    valveman Moderator Staff Member

    Messages:
    4,425
    Location:
    Lubbock, Texas
    No! The chart above shows you the precharge amount. It is usually about 30% below the constant pressure. Constant set at 50 PSI, air pre-charge in the tank should be 35 PSI.
  7. ecodian

    ecodian New Member

    Messages:
    3
    Location:
    Indiana
    The VSD pump that I installed is Grunfos Sqe. Is the 30% pre-charge pressure below the pump pressure applicable to my system too?
  8. valveman

    valveman Moderator Staff Member

    Messages:
    4,425
    Location:
    Lubbock, Texas
    Last edited: Dec 8, 2009
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