It manufactures a Vessel Pumps which is generally works on the following: -
It is a Variable pressure system which means that when a tap is twisted on water is drained and when the pressure is down up-to 20psi/140kpa. It's Pressure Vessel Pumps the pump will start up and when the pressure accomplishes in the pump will turnoff.
The quantity of water that can be exhausted off before the pump starts up is based on the size of the pressure vessel. The larger the pressure vessel has the bigger volume of water that can be drained off earlier the pump start again.