Accumulator Tank

Shurflo Accumulator Tank 

The Accumulator Tank smooths water flow and reduces on/off cycling of the pump by lessening the variation in pressure and flow between the pump and the outlets in the system. The even flow of water gives better control of water and regulating temperature.

Example in practice
When you turn on your tap the water accumulated in the pressure tank is first released from the tank before the pump has to turn on. If there is enough water in the pressure tank for the application ( say washing your hands ) then the pressure pump will not have to start. When the tap is turned off or if there is excess pressure in the line than what is being used at a given time, the back pressure is absorbed back into the pressure tank thus protecting the pump, making less noise and reducing water pulsing. 

Needs to be installed on the fixture side of your pump between the pump & the fixture. "pressure side"