More residential well questions
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Egon
Epic Contributor
So how does the water enter the tank? How does the tank get its initial pressure?
paulsharvey
Elite Member
Yes, correct, the pump does generate the initial pressure, so your not technically wrong, but in practice, let's say you have a steel tank, with 120 psi of just water, as soon as your release any water, the pressure Rapidly drops, because fluid can't be compressed. Guess the correct way to word it is, the Tank provides pressure to the home, in that, if you didn't have a pressure tank, you wouldn't Maintain pressure to the home, in-between pump cycles.
If your pump (or foot, or riser pipe) won't build to 40 psi, the tank wouldn't magically generate pressure.
If you pump say 50 LF of 12" DI water main up to 150 psi for a pressure test, and open the value next to the gauge, and let out even a gallon of water, how much does that 150 psi drop? A lot, and that is also considering the pipe expands somewhat under the test pressure over general line pressure.
Now let's look at a typical rural home; say 100 lf of 1" down pipe/riser pipe; maybe 50-100 lf of pipe from well to home, probably 3/4"; now just a few drips from a sink would then drop the pressure at the switch, cause the pump to cycle, every few seconds, for just a second, and possibly not even maintaining pressure to the home. Now, we have surging pressure peaks when the pump kicks on, multiple times per minute, 24/7; not good for pipes; also we have an electric motor that is ment to cycle on, less often, but run longer; not good for that...
BTW, I get what you're saying; and you're right; but the simple way to explain it, is yes, the Tank provides pressure, even if that's not text book correct
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AnotherBueller
Gold Member
The bladder (or diaphragm) has air above and water below. Lets say that you have a 80 gal tank with 40 PSI air. As the pump fills the tank the pressure from the pump you bladder will compress raising the pressure in the tank until the high pressure switch turns the pump off. Now you have about 80 gal of water that comes out of the faucet. The pump will not turn back on until you use those ~80 gal.
paulsharvey
Elite Member
Correct. I dont know what % of the tank has to be used to drop below the lower trip point but, yes. When I said steel tank, I was not referring to a pressure tank, just that, theoretically, the pump is what generates the pressure, and a blank steel tank, without air or bladder or something, would not do anything. the air bladder is just what stores energy to maintain pressure in a well system, once the well pump cycles off.
Now, someone brought up a booster pump, and I've seen them for sale, but never seen a well equipped with one; I'm assuming they are used either when there is an unusually long run or elevation change from the well to the home?
Now, someone brought up a booster pump, and I've seen them for sale, but never seen a well equipped with one; I'm assuming they are used either when there is an unusually long run or elevation change from the well to the home?
Valveman
Platinum Member
I am always surprised by the misconceptions people have about how pumps work. All the water volume and pressure comes from the pump. A pressure tank is 75% air so an 80 gallon tank only holds 20 gallons of water, a 20 gallon tank holds 5 gallons of water. A tank does not add pressure or volume to the water. I have many systems without a pressure tank at all. The pressure tanks only purpose is to limit the on/off cycles of the pump. The tank can supply a few gallons before the pump comes on. But once the pump is on, water comes directly from the pump. Excess water is stored in the tank to used during times the pump is off.
Water also does not come from a water tower. That water was put up there by a pump under pressure. And again, water in the tank is just to use for small uses before the pump must come on. It takes 2.31' elevation to make 1 PSI. That is why most city water towers are 115' tall, as that is how high it must be to store and then supply 50 PSI to the system at ground level.
Water also does not come from a water tower. That water was put up there by a pump under pressure. And again, water in the tank is just to use for small uses before the pump must come on. It takes 2.31' elevation to make 1 PSI. That is why most city water towers are 115' tall, as that is how high it must be to store and then supply 50 PSI to the system at ground level.
ponytug
Super Member
In the FYI bin, an 80 gallon tank, going from 60psi to empty at 40psi will discharge about 53 gallons of water. Going from 40psi to 20psi gets you about 40 gallons, but the energy cost will be about 40% of the 60/40 pressure swing, on a cost per gallon basis.
www.engineeringtoolbox.com
All the best,
Peter
Pumping Water - Energy Cost Calculator
Calculate the energy cost of pumping water.
www.engineeringtoolbox.com
All the best,
Peter
Valveman
Platinum Member
I think what they call the maximum acceptance volume for an 80 gallon tank is about 53 gallons. But that is a stupid number to publish as it doesn't mean anything. Looking at this chart you can see the actual draw down of a WX320, 86 gallon size tank is only 23 gallons at 40/60 pressure. That is under perfect condition and laboratory testing. In real life you would do good to get 20 gallons out of a 80 gallon tank before the pump must come on.
People see these big tanks and think they have lots of water. But that is a false sense of security as water comes from the pump, not the tank. Having 20 gallons or even more in a tank is not of much use for a house that uses a minimum of 200-300 gallons a day. Again, pressure tanks don't hold much water and their only purpose is to limit the on/off cycling of the pump. In the old days using as large a pressure tank as you could get was the best way to limit the cycling. But these days when using a Cycle Stop Valve to limit the cycling, you don't need a very large tank.
People see these big tanks and think they have lots of water. But that is a false sense of security as water comes from the pump, not the tank. Having 20 gallons or even more in a tank is not of much use for a house that uses a minimum of 200-300 gallons a day. Again, pressure tanks don't hold much water and their only purpose is to limit the on/off cycling of the pump. In the old days using as large a pressure tank as you could get was the best way to limit the cycling. But these days when using a Cycle Stop Valve to limit the cycling, you don't need a very large tank.
