Pressure vessels are connected to a pumped irrigation system to store a reserve of pressurised water. If the water need is small, the water is supplied from the vessel rather than operating the pump. Electric pumps should not start and stop more than 20 times per hour, so using a pressure vessel reduces the number of times the pump needs to operate.
Pressure vessels consist of an outer metal cylinder and an inner flexible membrane. The area between the metal body of the pressure vessel and the membrane is pressurised with air. The air is compressed when the pressurised water is pumped into the pressure vessel and then expands when there is a water demand. A pressure switch brings on the pump to re-pressurise the vessel once the water pressure drops below a preset point.
Mist propagation systems often require a pressure vessel as the propagation nozzles operate for short periods of time very frequently. Hand watering systems in garden centres also use pressure vessels, as the watering lances are turned on and off frequently.