According to Flotec, a manufacturer of water well and water pumping equipment, the water well pressure tank is a storage tank that prevents the well pump from turning on and shutting off repeatedly by using air pressure to maintain water pressure to home fixtures. As water fills the tank, the air inside is compressed, providing needed pressure to cause water to flow through pipes when a tap is opened.
Older tanks work by allowing water and air inside the same tank. Steel tanks rust over time with this setup. Newer tanks include a bladder, which is a plastic bag. These tanks store the water in the bag so it does not touch the steel. Air in the tank surrounds the bladder. A third option is the diaphragm tank that also keeps the air and water separate but uses a rubber diaphragm approximately two-thirds of the way up the tank.
Like automobile tires, tanks eventually leak air. While they still store water, they do not have the air pressure to force it into home plumbing. As a result, the well pump cycles much more often than when the tank is working properly. Tanks have a port at the top, allowing the user to add more air to the system so the tank begins working properly again. Manufacturers recommend checking the air pressure a minimum of two times yearly.