Description

Hydraulic accumulators use an incompressible fluid and a compressed gas, spring or raised mass to

• Store energy
• Absorb shock
• Eliminate noise

• Counter-balance loads

Types

• Compressed Gas Accululators use a gas such as nitrogen usually include separate fluid and gas compartments. The fluid section connects to the hydraulic circuit so that as pressure rises, fluid enters the accumulator and the gas is compressed. As pressure falls, the compressed gas expands and forces the stored fluid back into the system.
• Spring-loaded hydraulic accumulators are small, lightweight devices that are suitable for mobile applications with low volumes and pressures below 500 psi.
• Hydraulic accumulators use a bellows as a spring cushion.
• Raised mass or weight-loaded devices often use concrete discs loaded onto an oversized piston. Typically, these units are found in high-demand applications such as steel mills.

Different Kinds of Compressed Gas Accumulators

   
Most modern, fluid power systems include hydraulic accumulators that use compressed nitrogen gas and a piston, bladder, or diaphragm that separates the compressed gas from the hydraulic fluid.

• Piston accumulators have an outer cylinder tube, end caps, a piston element, and sealing system. The cylinder maintains fluid pressure and guides the piston, which forms the separating element between the gas and fluid.
• Bladder accumulators consist of a pressure vessel and an internal elastomeric bladder than contains the gas. The bladder is charged through a gas valve at the top of the accumulator, while a poppet valve at the bottom prevents the bladder from being ejected with the fluid outflow.
• Diaphragm accumulators include a spherical or cylindrical pressure vessel and use an elastomeric diaphragm as the separating element. There are two basic designs: welded and threaded. Welded devices are more expensive, but can be repaired from the top, the bottom, or from either the top or the bottom.  

Specifications

Specifications for hydraulic accumulators include

• Capacity
• Operating pressure
• Maximum flow rate
• Nominal bore size
• Housing material
• Mounting style

Typically, devices are sized according to their effective or actual gas volume when all of the hydraulic fluid is discharged. The available volume of fluid depends upon the available volume of compressed gas, an amount known as the working volume. Because working volume varies according to changes in pressure and temperature, the rates of charge and discharge are important considerations when sizing hydraulic accumulators.

Materials and Installation

In terms of housing materials, devices are usually made from steel, stainless steel, aluminum, copper, or plastic. Normally, hydraulic accumulators are installed vertically, with the hydraulic port down. Mounting a bladder-style device horizontally can result in accelerated bladder wear if the bladder rubs against the shell while floating on the hydraulic fluid.

Related Standards

AD 87-26-06 - Hydraulic brake accumulators

BS 7201 P2 - Hydraulic fluid power

GOST 16769 - Hydraulic accumulators - general technical requirements

References

Image Credit:

Haskel International, Inc. | Royal Systems Group, Inc. | Senior Aerospace Metal Bellows