Accumulators are simple devices that store energy in the form of fluid under pressure. Because of their ability to store excess energy and release it when needed, accumulators are useful tools in developing efficient hydraulic systems. They are generally classified by the means of stored energy: gas loaded, spring loaded, or weight loaded. The latter are typically very large devices, with concrete disks loaded onto an oversized piston. Often found in older, high-demand applications such as steel mills, weight-loaded accumulators have a large capacity and uniform output pressure but present problems with installation and servicing.

Spring-loaded accumulators are at the other extreme. Generally small and lightweight, making them useful in mobile applications, a spring gives a repeatable output force. However, spring-loaded accumulators are limited to small volumes and pressure below 500 psi.

The bulk of hydraulic accumulators are gas loaded. They use the compressibility of a gas -- usually nitrogen -- for storing energy. Basically, a hydropneumatic accumulator has a fluid compartment and a gas compartment, with a gas-tight element separating the two. (Some types, however, such as air receivers, have no separating element.) The fluid section connects to the hydraulic circuit so that as pressure rises fluid enters the accumulator and the gas compresses. Then, as pressure in the system falls, the compressed gas expands and forces the stored fluid back into the system.

Hydraulic accumulators with separating elements are further divided into bladder, diaphragm, and piston designs.

Bladder accumulators consist of a pressure vessel and an internal elastomeric bladder that 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 outflowing fluid.

The poppet valve is sized so that maximum volumetric flow (typically to 15 liter/sec, but up to 140 liter/sec for high-flow designs) cannot be exceeded. The bladder can be replaced, usually through the fluid end of the vessel.

To operate, the bladder is charged with nitrogen to a pressure specified by the manufacturer according to the operating conditions. When system pressure exceeds gas-precharge pressure of the accumulator, the poppet valve opens and hydraulic fluid enters the accumulator. The change in gas volume in the bladder between minimum and maximum operating pressure determines the useful fluid capacity.

Diaphragm accumulators are usually comprised of a spherical or cylindrical pressure vessel, containing the separating element -- an elastomeric diaphragm. There are two different designs: welded and threaded. In the former, the diaphragm is pressed into the bottom half of the vessel before the seam is electron-beam welded; in threaded models, the diaphragm is held between top and bottom halves of the vessel by a threaded ring. As in the bladder type, a poppet prevents the diaphragm from being ejected through the fluid connection.

Threaded models can be disassembled and the diaphragm replaced. Welded diaphragm accumulators are nonrepairable. However, they are lighter and cost substantially less than threaded or bladder designs.

Another difference is the ratio of maximum operating pressure to gas-precharge pressure. Pressure ratios are generally 10:1 for threaded, 8:1 for welded, and 4:1 for bladder accumulators. In other words, for an accumulator with a maximum operating pressure of 3,000 psi, minimum gas-precharge pressures would be 300 psi for threaded, 375 psi for welded, and 750 psi for bladder types. Accumulators with a higher pressure ratio are more efficient because they have a greater volume of usable fluid.

Piston accumulators have an outer cylinder tube, end caps, a piston element, and sealing system. The cylinder holds fluid pressure and guides the piston, which forms the separating element between gas and fluid. Charging the gas side forces the piston against the end cover at the fluid end. As system pressure exceeds the minimum operating level for the accumulator, the piston moves and compresses gas in the cylinder.

Each type of separated, hydropneumatic accumulator has advantages, but bladder designs are generally considered the most versatile of the three. For shock and pulsation, for example, bladder and diaphragm models are ideal. Piston units are not recommended because they are too slow to react to shock waves. For emergency service, though, many users prefer piston accumulators. However, the seals in piston types are not designed to hold pressure indefinitely without being cycled. The system can leak and fail with no obvious indication to the outside. An advantage diaphragm accumulators hold is that they can be mounted in any position. For piston and bladder accumulators vertical mounting is the preferred orientation, with the gas side up.

Diaphragm accumulators are the least expensive, but maximum capacity is only about one gallon. Bladder designs predominate between one and 15 gallons, but are available to about 125 gallon capacity. The largest piston designs are typically about 100 gallon capacity. For similar quality and performance, bladder and piston models are competitively priced.