Air vessel

Air vessels are closed containers, in which the lower half is water & the upper half is compressed air. These air vessels are installed near the suction & delivery valve to avoid separation. An air vessel is usually fitted in the discharge pipe to dampen out the pressure variations during discharge. As the discharge pressure rises, the air in the vessel gets compressed. Similarly, air expands when the pressure falls. The peak pressure energy is thus stored in the air and returned to the system when pressure falls. Air vessels are not installed on reciprocating boiler feed pumps as they may introduce air into the de-aerated water.

Purposes of Air vessel:

1) To obtain liquid at a uniform discharge.

2) Due to air vessels frictional head and acceleration head decreases. The work for overcoming friction in suction and delivery pipe also decreases considerably, which amounts in good amount of work.

3) Reciprocating pumps can run at high speeds without flow separation.

Working:

The top half has compressed air and the lower half has the fluid which is to be pumped. Air and water are separated by a flexible diaphragm which is movable as per difference of pressure between the two fluids. Air vessel is connected close and at the level of the pump. Without air vessels, the friction head increases and reaches a maximum value at mid stroke and then drops down to zero. When air vessels are installed, the friction head remains constant throughout the stroke.