Hydraulic Accumulator Sizing Calculations

Most accumulators used within industry are limited to an operating pressure of 3000 psi. Accumulators are available which operate at higher pressures. In general, hydraulic accumulators are pre-charged one half of the maximum operating fluid pressure, this is adequate for most applications. For a system operating at 3000 psi, a properly rated accumulator should be pre-charged (nitrogen is typically used) to 1500 psi.

Accumulators are typically rated by their manufacturer at gas volume when all fluid has been expelled. The amount of fluid which can be stored within the accumulator is normally one half of the gas volume.

Accumulators are selected based on the fluid pressure and volume requirements of the system which they are to be installed into. The accumulator is sized such that the system fluid pressure will not fall below a value resulting in degraded system performance.

A general formula for most accumulators:

D = (e x P1 x V1)/P2 - (e x P1 x V1)/P3

Where:

D = Volume of fluid discharge (in3 )

P1 = Pre-charge pressure (psi)

P2 = System pressure after volume D has been discharged

P3 = Maximum system pressure at full accumulator pressure

V1 = Rated accumulator gas volume (in3)

e = System efficiency, typically 0.95

Allowing for Extra Capacity

As fluid enters the accumulator, the gas charge (normally nitrogen) is compressed. As the fluid gas is compressed, the temperature will rise (Charles Law). Unless the gas is allowed to cool to ambient temperature, elevated gas temperature will result in the volume of the fluid entering the accumulator will be less than the calculated amount, resulting in the fluid discharge to be less as well. Compensating for extra capacity may be facilitated by increasing the require accumulator volume, 5% is a common value.