We use Lowara Pressur Vessels, they have a steel shell (Mild and Stainless Versions available), with a Butyl rubber bag inside. Pressurized air is held between the bag and shell, (air pressure is generally 3psi below the pump cut in pressure). Water is pumped into the bag, the air compresses allowing the water to be stored under pressure, a sensor of some kind will turn off the pump at the desired pressure. When a tap is opened the stored water will leave the vessel, once sufficient water is expelled the pressure inside will drop and cut the pump in, filling the vessel again whilst meeting the systems demand. The size of the vessel should be calculated based on the pump size and systems peek demand, bigger is not always better and conversely too small a vessel can be detrimental to the pumps and the vessels life. Regular maintenance is highly recommended, water pressure in the vessel would be determined by the pumps capability and the system requirements, air pressure should be set to 3 PSI below the cut in of the pump.