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Expansion Vessels for Pumps

Expansion vessels or pressure vessels as they are sometimes called are used to store water under pressure to reduce the on/off cycling of electric powered pumps.


How it works

Inside the tank is a rubber membrane. On one side of the membrane is compressed gas (air or nitrogen) and on the other side is water. When the pump is started, water is forced into the pressure vessel which causes the membrane to expand, compressing the air on the other side of the membrane. When the set pressure is reached, the pressure switch stops the pump and a non-return valve closes, holding the pressure in the system. When water is drawn from the system, the gas pressure forces water out until the pressure falls to the lower set level of the pressure switch. At which point, the pump re-starts and the cycle is repeated.


How to calculate the correct size Pressure Vessel for your pump.

Graph to show which size pressure vessel you'll need for your pump


How to set the air pressure in your expansion vessel.

The pre-charge pressure for operation with a pressure switch should set at 0.1 bar (1.5psi) below the cut in pressure of your pump. e.g. The pump turns off at 5 bar (75psi) and cuts back in at 3 bar(45psi). The air pressure in the expansion vessel should be set at 2.9 bar (43.5psi).

For pumps controlled by a variable speed (inverter) drive, the pre-charge pressure should be set at 65% of the set pressure. e.g. The pump running pressure is set to 3 bar, the pre-charge pressure should be set to 1.95 bar. (3X0.65).

The pre-charge pressure can be increased using an air compressor or foot pump with a standard Schrader (tyre) adaptor and should be checked periodically with a tyre pressure gauge.


Please Note: When checking the pressure in the expansion vessel, there must be no water pressure in the system as this would give a false reading.



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Whisper Pumps Ltd, Leen Court, Boughton, Newark, Nottinghamshire, NG22 9ZD