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Accumulators are pressure vessels where hydraulic energy is stored and when needed, delivered back to the system. In hydro-pneumatic accumulators, the compressibility of gas (nitrogen) is used to store pressurized fluid.

The accumulator starts with a pre-charged gas and no fluid. By increasing the fluid pressure, the accumulator fills with oil and the gas is compressed. When the pressure in the fluid side is reduced, the gas expands to equalize and pushes the fluid out of the accumulator and into the fluid system.

There are three main types of accumulators, gas (compressibility of gas), dead weight (gravity) and spring (mechanical). The three main construction types of gas-charged accumulators are diaphragm, bladder, and piston.

Diaphragm Accumulators

Bladder Accumulators

Piston Accumulators


There are many advantages for using an accumulator including the following:


Accumulators are pressure vessels and as such are manufactured, tested and certified according to statutory standards. In the United States, for example, the relevant standard is the ASME Boiler & Pressure Vessel Code VIII, Division 1.

All pressure vessels manufactured to these standards are considered to have a finite service life depending on the number of pressure cycles experienced during normal operation. The typical design life for a hydraulic accumulator is 12 years.

In many jurisdictions, periodic inspection and recertification is required. This particularly applies to hydraulic accumulators which have relatively large volumes and operate at high working pressures. Inspection may be required at predetermined intervals (i.e. every two, five or 10 years) or when a certain percentage of usable design life is deemed to have been reached.