An expansion tank is used either to protect piping systems and vessels from over-pressure, due to expansion of the contained fluid, and/or under-pressure or vacuum due to contraction of the contained fluid, or as a reservoir that can provide instantaneous supply of the liquid on demand, to either sustain flow in the event of system failure, or to initiate flow where system start up takes too much time.
The most common cause for fluids in a piping system to expand, or contract, is due to a change in temperature of the fluid. So whilst is is commonly known that solids will expand or contract when their temperature changes, according to their coefficient of expansion, the same is also true for fluids.
If there is a miss-match between the expansion/contraction of the piping system and that of the fluid that it contains, then an expansion vessel will be needed to prevent the piping from being over-stressed as the fluid expands more than the piping, or be subjected to vacuum like conditions as the fluid shrinks more that the piping.
Expansion tanks are normally provided with an internal bladder, or membrane to separate the process liquid from the gas charge. Bladders, or membranes are used because they separate the liquid from the gas and prevent absorption of the gas by the liquid. They also prevent escape of the gas charge from the vessel when the system is depressurised.
The tank will allow the process liquid to flow into, and out of it's volume, thereby regulating the pressure in the system. In this respect they act like a surge vessel, however the detail design requirements are quite different.
Where expansion tanks are provided to deliver instantaneous flow to support a system then it is usually necessary to regulate the flowrate out of the vessel by way of an orifice plate, otherwise the pressure in the tank may fall too quickly.