pressure tank size

Discussion in 'Pumps and Tanks Well Forum & Blog. Water is life.' started by hef, Feb 6, 2011.

  1. hef

    hef New Member

    Joined:
    Feb 6, 2011
    Location:
    Rochester NY
    Hello,
    I need a new pressure tank for my cottage water system. (air pressure measured at bladder = 0)
    Symptoms are good pressure at the cottage for a brief period of about 1 min, only to drop to very low until the pump cycles back on.

    I have read about how to size a new tank but I want some other opinions as I know enough to be dangerous.;) It seems like a 20 gal total volumn would do the job, based on what I have read, but I am not sure.

    I have a 1/2 HP J5SH Goulds pump which has a switch on point of 40 psi and a switch off point of around 60-70 psi. (This is what I set it to last spring which seems to produce brief, but decent pressure)

    I have a toilet, bathroom sink, shower and kitchen sink and hot water heater.
    Rule of thumb is only one of the above is on at one time.

    The pump obtains its water from the lake - I estimate about a 5 ft lift from the water to the pump.
    The pump pushes the water UP about 75 ft.

    What is confusing to me is determing GPM for calculating tank size - if GPM varies with pressure as shown in the data below, how do you settle on a GPM for the pump?
    From the Gould J5SH manual: (See attached as well)
    AD3328 Venturi
    AN019 Nozzle
    Discharge Pressure – PSI
    20 -- --30 -- -- 40 -- --50 ---60
    Gallons per minute -------- -------- Max.Shut off
    11.5 -- 11.3 -- 11.0 -- 7.7 -- 4.8 --------- 83

    http://www.pumpsandwells.com/miva/JS+.pdf
    Thanks for the assistance!
    Regards,
    Hef
     
    Last edited: Feb 6, 2011
  2. Tom Sawyer

    Tom Sawyer In the Trades

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    You can't go wrong with big. too small will cause pump cycling. The bigger the tank, the longer the draw down and the longer your pump cycles.
     
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  4. valveman

    valveman Cary Austin Staff Member

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    Pump Controls Technician
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    Reducing the number of pump cycles is the key to sizing a tank. Pump manufacturers recommend a one minute run time and two minutes is better. The chart you supplied shows your pump produces about 11 GPM with a 30/50 pressure switch. A 20 gallon size tank only holds 5 gallons of water, which is less than 30 seconds of run time. You really need at least a 44 gallon size tank (12 gallon draw), and an 80 gallon would be better (23 gallon draw).

    As you can see the size of the tank only determines whether you have 5 gallons or 23 gallons to use before the pump must start. Even 23 gallons from a big tank is not enough for a normal shower. So the pump must always run to supply the amount of water needed. You water supply does not come from the pressure tank, it comes from the pump via the aquifer or reservoir. The main purpose of a pressure tank is only to limit the cycling on and off of the pump.

    A Cycle Stop Valve (CSV) as its name implies, will stop the pump from cycling, so the size of the pressure tank no longer determines the run time. The CSV will make the pump run for as long as you are using water. Then the CSV only lets the tank fill at 1 GPM, so a 4.4 gallon size tank that holds 1 gallon of usable water is all you need. The pump will normally cycle less with a CSV and a 4.4 gallon tank than it will with a big 80 gallon tank.

    With an 80 gallon pressure tank you will see fluctuation from 30 to 50 PSI in the shower about every 6 minutes. With a 40 gallon tank the pressure will go from high to low every 3 minutes, and with a 20 gallon tank every 90 seconds. But with a CSV and really small tank, the pressure will stay at a constant 40 PSI for as long as you are in the shower.

    I am finding 40 gallon tanks for $300+, and 80 gallon tanks for $500+, where a CSV and 4.4 gallon tank would do a better job for around $200. IMO big pressure tanks are no longer the best option.
     
  5. hef

    hef New Member

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    Feb 6, 2011
    Location:
    Rochester NY
    So the frequency of cycles (more) is bad - duration of run time is not detrimental to the motor/pump?
    You mention 11 GPM at 20psi, but its only 4 GPM at 60 psi. So which GPM is right?
    Thanks,
    Hef
     
  6. valveman

    valveman Cary Austin Staff Member

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    Pump Controls Technician
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    At 30/50 your average is 40 PSI at 11 GPM. At 50/70 your average is 60 PSI at 4.8 GPM. So it depends on where your pressure switch is set as to how much flow the pump will deliver at the average pressure. So at 50/70 you are only filling the tank at an average of 4.8 GPM, but at 50/70 pressure tanks hold less water than when at 30/50.

    Example;

    20 gallon tank at 30/50 holds 6.18 gallons.
    20 gallon tank at 50/70 holds 4.72 gallons.

    80 gallon tank at 30/50 holds 26.58 gallons.
    80 gallon tank at 50/70 holds 20.31gallons.

    So at 50/70 a 20 gallon tank will almost give you 1 minute of run time, a 40 gallon tank would give you 2 minutes, and a CSV will keep the pump running until you turn off the shower.
     
  7. hef

    hef New Member

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    Rochester NY
    Can you recommend a good manufacturer for pressure tanks - I have seen a big delta in what some tanks cost as compared to others. Also can you recommend the best material - steel over fiberglass for example.
    Thanks,
    Hef
     
  8. Texas Wellman

    Texas Wellman In the Trades

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    Owner of a Water Well and Pump Repair Business
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    SE Texas-Coastal
    Flexcon, Well-X-Trol ( by amtrol), and Well-mate seem to be the best. The Well-mates I use are fiberglass and the Flexcon and Amtrol I have seen are steel. There are many different flavors.

    Yes, the price does vary a lot. From what I have seen the Big Box stores last about 3-5 years. The Amtrol has a 5-year warranty but seems like people are getting 7-15 years out of them. The Flexcon challenger series has a good reputation among installers also.
     
  9. Tom Sawyer

    Tom Sawyer In the Trades

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    Maine
    900 bucks for a valve? I'd like to make the profit on that baby
     
  10. LLigetfa

    LLigetfa DIYer, not in the trades

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    Where are you seeing $900 quoted in this thread?
     
  11. hef

    hef New Member

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    Feb 6, 2011
    Location:
    Rochester NY
    Pressure

    Thank you for all the previous replies and informaton regarding the pressure tank.

    Regarding pressure - would you expect to see the pressue cut in and off points high (for a system like mine) in which the pump and tank are at the water suppliy (lake) and the cottage is 80-100 feet up?
    The pump is a 1/2 hp goulds jet pump J5SH

    Is 50-70 safe cut in -out points for a system as stated above?
    I assume that the air charge for the tank would have to be set to 48 psi?
    They tend to come pre-charged at around 40 - will raising it to 48 damage the bladder?
    regards,
    Hef
     
  12. valveman

    valveman Cary Austin Staff Member

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    Pump Controls Technician
    Location:
    Lubbock, Texas
    If it is 80 to 100’ up to the cottage, you are going to lose 40 PSI before it gets up there. So with 50/70 at the pump, you will see between 10 and 30 at the house. And 50/70 is your limit only because that is all your pump can do. With a bigger pump you could set it as high as you need. 100/120 setting is not uncommon for a house on top of a hill. I have done as high as 380/400 PSI when supplying water to the top of a mountain or a high-rise building.

    The adjustable CSV for that size pump is only about $200. The $900 one if for pumps up to 150 GPM. All you have to do to make the profit from either is to sell one. The installer makes the lion’s share of that profit as the manufacturer only sees a fraction of that price.
     
  13. LLigetfa

    LLigetfa DIYer, not in the trades

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    Ja, you make a good point. The OP said his cut-in pressure is 40 PSI and it sounds like he loses it all with the height. When I first read the OP, I assumed the pressure was being measured up at the house, but as you say, the pump is not capable of that.
     
  14. hef

    hef New Member

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    Feb 6, 2011
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    Rochester NY
    pressure loss bladder PSI

     
    Last edited: Feb 13, 2011
  15. valveman

    valveman Cary Austin Staff Member

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    2.31 feet equals 1 PSI. 92' equals 40 PSI. Yes that pump can do 50/70 and 48 PSI air is correct.
     
  16. LLigetfa

    LLigetfa DIYer, not in the trades

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    With a low end of 10-15 PSI up at the house, it sounds like a constant pressure CSV would be the preferred route to take. Chances are that the pump could be running a lot anyway since there could be considerable GPM losses over that distance at the top end of the pump's pressure capability. With a large tank to draw down, it would only prolong the agony.
     
  17. Texas Wellman

    Texas Wellman In the Trades

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    Owner of a Water Well and Pump Repair Business
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    SE Texas-Coastal
    I think this goes on the 5-HP thread...

     
  18. valveman

    valveman Cary Austin Staff Member

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    J5SH only pumps about 2 GPM at 70 PSI. So a CSV won't help. Best bet is to narrow the pressure switch bandwidth as much as possible. Instead of 50/70, try to make it 60/70, or 60/75. A regular FSG2 pressure switch usually won't go less than 15 PSI bandwidth ,but that would help. Just loosen all the way on the small adjustment screw to get the smallest bandwidth possible.
     
  19. hef

    hef New Member

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    Feb 6, 2011
    Location:
    Rochester NY
    I see the logic of the CSV - but I was thinking about my entire set up and had another idea. (now I know enough to be dangerous)

    My lot continues uphill from my pump at the lake to the house, and further up to the road. Would it make more sense to install a simple water tank up hill from the house, and pump the water directly into it from the lake?

    That way I would use gravity to push the water back down to the house as needed. I would need to set up a level switch in the holding tank that shuts off the pump.
    Is there a way to calculate what my pressure would be based on a height of the water tank in relation to the house?
    Thanks,
    Hef
     
  20. valveman

    valveman Cary Austin Staff Member

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    Pump Controls Technician
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    All your water is stored in the lake. The ONLY reason for any tank is to keep the pump from cycling on and off too much. At 70 PSI your pump will only produce 2 GPM, which is not enough for a good shower, much less to cause the pump to cycle on and off. You can’t store more pressure in a tank than the pump can produce. And to store the 30 PSI you have left at the house, the tank would need to be 70’ taller than your house.

    What you need is a pump that will produce more flow at a higher pressure. Average pressure switch setting for a house is 40/60. Since you lose 40 PSI going up the hill, you need a pump that will do 80/100 PSI. Then you will have enough flow at pressure that you will need to worry about the size of pressure tank, or use other ways to keep the pump from cycling itself to death. But if a 70’ tall tank is practical, you can slowly fill it with the pump you have now. The same formula applies, 2.31 feet equals 1 PSI.
     
  21. hef

    hef New Member

    Joined:
    Feb 6, 2011
    Location:
    Rochester NY
    Yea I was starting to think a pump capable of that kind of pressure is what I really need to do this right.

    Using my existing pump, to get the water up to a holding tank 70 feet above the house it would have to pump up from the lake as well which is about 90 feet.
    Therefore thats a total of 150 ft or 65 feet of loss using your formula 150/2.3 =65 PSI pressure loss.
    So I would have barely 5 psi pushing water into a holding tank 70 feet above my house.

    Even if I use the existing pump with a CSV valve, I will still lose 40 psi, with a 50-70 set up that means I will be down to 10 psi before the pump turns on right? Probably not good?

    Can you recommend a good pump that can put out that pressure?
    Thanks again,
    Hef
     
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