Refer to figure.
Hydraulic Accumulators are usually installed in hydraulic systems to store energy, in the form of hydraulic fluid under pressure (which will allow limited operation of hydraulic services when the pump is not operating or in case of an emergency) and to dampen out pressure fluctuations within the system. They are often used after a pump in a hydraulic system to do both these jobs, and by doing so increase efficiency and reduce the stresses on the hydraulic system. They can use a diaphragm (annex above) or a movable piston, with one side connected to the hydraulic lines and containing hydraulic fluid, but with the other side filled with gas (usually nitrogen), which can compress to the same pressure as the system.

They can also help with absorbing the pressure of thermal expansion within closed hydraulic systems, when the fluid expands or contract due to temperature changes, but cannot flow back to the resevoir. When the system is open, any excess fluid will flow into the reservoir.

The are not used for aiding cooling.