Expansion / Diaphragm tanks are pressurized water vessels where the water and air are separated by a flexible diaphragm allowing the water to expand and absorb pressure surges.

In booster applications, diaphragm tanks can be used on both the discharge and the suction manifold. Connected to the suction manifold, a tank serves to counteract fluctuating inlet pressures.
 
Pressure in low flow periods
Diaphragm tanks are commonly used on the discharge manifold to provide water storage for the system in very low flow situations. It decreases the running time of the booster pumps, thereby making the system more energy effective.
 
Eliminates water hammer
Since the diaphragm is flexible, it is able to adjust to sudden pressure changes, thus counteracting water hammering.
 
When to use diaphragm tanks…
If low or no-flow conditions are expected, a diaphragm tank should be used to work in conjunction with the Stop Function of the booster system.
 
In areas where inlet pressure fluctuates, diaphragm tanks are often installed on the suction manifold to reduce damaging peak pressures.