Private Water supply query


I have a private water supply which is 150 ft from the house and 45 ft below.I have a pump. a pressure switch and an aquabox The pressure switch is set to start pumping at 22 psi and stop at 30 psi The aquabox is pumped up to a pressure of 19psi ( 3psi less than the starting pressure) I want to understand how the system works i.e.; with the bladder in the aquabox filled and at 30psi I can see that opening a tap in the system will reduce the pressure and allow the water to flow but what drives it? The 19psi in the aquabox.? I am still confused what the 19psi actually does. Can someone explain in clear terms please. The main reason I ask is I require to check and correct the pressure in the Aquabox which must be done with no pressure in the water system and instead of purging the whole line to reduce pressure I wondered if having switched off the pump I could simply open a tap in the system and thus empty the bladder and thus reduce the pressure. Hope I have made sense Blair
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bm wrote:

If you have a tap below the box, switch the pump off and allow that tap to run until water stops running - fine. Do that and set the box pressure. You can skip the rest of this!
How it works is fairly simple. The bladder is like a balloon - it takes a few psi to force water into it, against the elastic forces that are trying to shrink it back to its empty state.
By pressurizing the tank surrounding the "balloon", the water inlet pressure has to overcome that pressure, plus the balloon elastic forces pressure, in order to force water into it.
Under normal operation, the pump starts running when there is still a little water in the bladder. ie the bladder is still exerting a pressure of a couple of psi, to add to the tank pressure. It stops running when the bladder is full. At that point, the water pressure is 30 psi and the tank pressure will be a couple of psi below that (the elastic forces of the bladder adding the other couple of psi).
If the tank pressure, when empty, is pumped up too high, the bladder will empty in use and no water will come out of the taps - but the pump won't run.
If the tank pressure is set too low, when empty, there is a risk that the bladder will fail through over-stretching - the pump keeps pumping although the bladder is full. Plus the pump will short-cycle and run far more often than necessary, even if the bladder doesn't rupture.
Setting the tank pressure is easy - there must be no pressure in the water system though.
If the tap is above the tank - even when it stops running, there will still be a head of water left in the system. That head being the height difference between tank and tap. The bladder will still be extended.
It would be possible to either measure or calculate the pressure and compensate for it - but the most accurate way is, I'm afraid, to remove the head of water by draining down or at least loosening a union at the tank.
-- Sue
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I am extremely grateful to you for you very comprehensive reply I will print it and study the second part so that I really understand it. If you don't mind I would like to come back after taking it all in I did switch of the pump and opened a tap which was lower in the system and was surprised at the pressure the water came out at but as you say that it would be near 30psi. The system pressure then went to 0 Am I right in assuming that tall he water came form the Aquabox and not from the supply line as well My concern was drying out the pump Thanks again Blair
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bm wrote: <snip>

Any time..

The pump will be fine. The water would only have come from the system above the tap (water doesn't run uphill) - This will include the box (as it is above the tap) and the /demand/ pipework above the tap.
I very much doubt that the pump is above the tap. Even if it were, the system will have non return valve(s). Otherwise, every time the pump stopped, the box pressure (and gravity) would start emptying the system back into the bore hole, via the pump. When the pressure fell low enough, the pump would start and the cycle start again. The electric bill would be quite something..
You haven't mentioned why you think the pressure could be wrong..
-- Sue
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