pump with pressure tank

Discussion in 'Pumps and Tanks Well Forum & Blog' started by Thatguy, Nov 23, 2010.

  1. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    pump with pressure tank, nothing else

    From my reading, it seems the larger the pressure tank, the more costly it is but the pump will cycle less often and so the pump life will be longer, but even under the best conditions both will eventually fail over the life of the well.

    So you'd pay a lump sum for a small tank or a bigger lump sum for a large tank and then there's a cost every few years to replace the pump.

    How long do tanks last? How long do pumps last if you don't cycle them too often?

    Is there an optimum choice for this tradeoff as to tank size?
    How would that optimum be determined?
    I need some numbers to work with.

    Thanks.
    Last edited: Nov 23, 2010
  2. Gary Slusser

    Gary Slusser That's all folks!

    Another downside of a large tank is that you draw the water level down farther in the well with each pump cycle. In some wells that can cause poor water quality and it's not a good idea with a low recovery well.

    Tanks usually last 15-20 years. Pumps 10 years or more but some fail in less than 10 years. That's pumps form old time manufacturers, not big box store brands.

    IMO you are better off with a small tank and a CSV than a large tank. You'll spend much less money, take up much less space with much less weight if that is a concern and the system is much easier to work on if needed.
  3. valveman

    valveman Moderator Staff Member

    Messages:
    4,472
    Location:
    Lubbock, Texas
    Most submersible pumps and motors are designed to last through their 5 year warranty period, while cycling the maximum number of cycles per day allowed. For a 1 HP pump this is 100 cycles per day, everyday, for 5 years. A tank will usually last 7 years under these same conditions. Everything is designed by how many cycles it can take. Even the bladder in a tank breaks like when bending a wire back and forth. Reach that critical number of cycles and you will need another pump, motor, tank, pressure switch, relay, air conditioner, refrigerator, etc., etc.

    Another thing that is critical to the life of a motor, is how many times it starts while it is still hot or warm. Motor manufacturers say a motor needs to cool for at least 1 minute before restarting, and 2 minutes is better. Sizing a tank to keep the pump off for 2 minutes can require a very large tank. Even then the motor is still warm internally after 2 minutes of being off. Everybody knows not to start a car engine after it has overheated, until it has cooled down completely. But you have no choice with your pump. If you are still using water and the tank is empty, the motor will restart even though it is still warm. This grinds off a little bit of the thrust and radial bearing during each start, and the life of the motor is shortened considerably.

    Even with a small tank, a CSV reduces the number of cycles by an average of 75%. A small tank doesn’t hold enough water to keep the pump from coming right back on when you turn on some water. However, the CSV would not have let the pump shut off, as long as any water is being used. The CSV only lets the tank fill at 1 GPM, and then only after you stop using any water. Even with a small tank that only holds 1 gallon of water, the CSV always give you at least 1 minute of run time. Even more importantly, the CSV doesn’t let the tank fill and the pump shut off, until you are completely finished using water. So the pump rarely comes on after only a minute or two of being off, like it does with a standard tank system.

    With a CSV and small tank, you would literally have to…
    1) turn off all water
    2) wait one minute for the pump to shut off
    3) turn on a faucet and use at least 1 gallon of water to restart the pump
    4) then turn off all water again
    5) wait another minute for the pump to shut off
    6) and repeat over and over if you want to cycle the pump more than once.

    The CSV makes it very hard to cycle a pump even if you were trying manually and timing everything you did. It is almost impossible to rapid or multiple cycle a pump during a day of normal house water use.

    I still have some accelerated test going that were started 18 years ago. So I really don’t know exactly how much longer a CSV will make pumps, motors, tanks, etc., last. I have a few accelerated tests that have been completed. In these test the pump had previously been in a condition that cycled it to death every two years. The CSV made one test pump last 12 years, and another lasted 14 years. The CSV made these pumps last 6 to 7 times their normal life span. I am guessing the CSV will increase the average life of a pump by 4 times. The oldest CSV tests have been running for 18 years now. If they last to 28 years, that will be 4 times the average. Only time will tell.

    You can also bet that many pump, motor, and tank manufacturers have already run these numbers. That is why you won’t find any of them promoting a CSV. As a matter of fact they do just the opposite. Call one of these manufacturers and their technical assistance will inform you that a CSV will destroy a pump/motor. After 18 years of this not happening, it has become comical to hear them try to talk someone out of a CSV.
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    OK, this will take me a while to digest this info. I forgot about well considerations. Thanks, guys.

    I hope I can come up with some recipe or program or algorithm for selecting a tank in at least some cases. The CSV is another element that I will consider later, depending on how I do with just a well, pump and tank.
  5. masterpumpman

    masterpumpman New Member

    Messages:
    729
    Location:
    Virginia Beach, VA
    If you don't have a limited well gpm install the CSV with a small tank or a Pside-Kick. It will give you constant pressure while taking showers and you'll love it.
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Sizing a pressure tank, version 1

    Comments, questions, test cases and revisions welcome.

    The input to this algorithm is the maximum well drawn down WDD in gallons. The output is max and min pressure tank capacity PTC in gallons. The pressure tank can be one tank or several plumbed in parallel.

    Rule 1: PTC < WDD
    Rule 2: tank/pump cycles per day CPD is less than 100. It's probably not worth the money to have a tank large enough to get down to 10 CPD
    Formula 1 from rule 2: PTC selected size < PTC/CPD, where CPD varies from 100 down to 10

    Ex 1: WDD = 15 gallons. By rule 1 the PTC is less than 15 gallons. By formula 1 the PTC selected size is between 15/100 and 1.5 gallons.

    Ex 2: WDD = 150 gallons. By rule 1 the PTC is less than 150 gallons. By formula 1 the PTC selected size is between 1.5 and 15 gallons.

    BTW, I'm on city water. I like looking for, or deriving, formulas.
    Last edited: Nov 24, 2010
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