How Your Well Works
Part III - The Pressure Tank
Probably the most visible part of your water system is your pressure tank. Whether you have a submersible pump or a jet pump, you will have some kind of pressure tank. The function of the pressure tank is twofold. First, it provides water pressure to run most any household fixture. Second and perhaps most important, it prevents the pump from cycling too rapidly. This rapid cycling of the pump is the primary cause of premature pump failure as it causes strain on the motor and bearings.
A pump cycle begins when the tank is nearly empty. The pressure tank is filled with air up to about 2psi less than the cut-in pressure of your pressure switch. (Typically this cut-in pressure is 38psi.) Because the tank is at or below the cut-in pressure, the pressure switch contacts close, turning the pump on and beginning the cycle. As water enters the tank, the air above the bladder is compressed acting much like a spring to push against the walls of the bladder. Once enough water has entered the tank to compress the air to the cut-out pressure of your pressure switch (typically about 60psi), the contacts on the switch open and the pump turns off, ending the cycle.
If the tank were not pressurized with air, the water entering the tank would need to compress in order to create the pressure needed to push the water out of the tank. Because water is much more dense than air, you can't fit as much of it in the same amount of space. As a result, a fraction of an inch of water drawn from the tank could cause the pressure to drop from the cut-out pressure to the cut-in pressure almost every time a faucet is turned on or a toilet is flushed. This in turn would cause rapid failure of the pump or motor in a matter of days or weeks.
This situation known as waterlogging of the pressure tank was caused frequently in older tanks where there was no rubber bladder to separate the water from the air. Because air can be dissolved in water, over time the volume of air in the upper portion of the tank would gradually diminish causing a loss in pressure and eventually a cycling problem. In modern tanks with rubber bladders, the main cause is rupture of the bladder itself which allows water to mix with and eventually dissolve the air in the tank. This situation can be remedied easily by simply adding more air to the tank, however the situation would continue until the tank is replaced.