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Hydraulic and Pneumatic Knowledge
Fluid Power Equipment
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.
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