What Is a Hydraulic Accumulator?

A hydraulic accumulator is a pressure storage reservoir in which hydraulic fluid is held under pressure by an external source of energy, most commonly a compressed gas such as nitrogen. It stores fluid energy while the pump has spare capacity and releases it when the system needs a burst of flow, and it also smooths pressure spikes and pulsation. Accumulators in hydraulics are therefore used for energy storage, shock damping and leakage compensation.

How a hydraulic accumulator works

A pressure accumulator has a fluid side connected to the hydraulic circuit and a gas side (or a spring, or a weight) that pushes back against the fluid. In the common gas-charged design the gas side is filled with nitrogen to a set pre-charge pressure that is lower than the minimum working pressure of the system. While system pressure is below the pre-charge, the accumulator holds no fluid. As the pump pushes system pressure above the pre-charge, fluid enters and compresses the gas; when the circuit demands flow and pressure falls, the expanding gas pushes the stored fluid back out.

Because hydraulic oil is essentially incompressible, the compressible gas is what actually stores the energy. This is why the pre-charge pressure matters: too low and the accumulator empties its usable volume quickly, too high and it stores very little fluid.

Types of hydraulic accumulators

Hydraulic accumulators are usually grouped by how they separate the gas from the oil:

Bladder accumulator: a flexible bladder holding the gas sits inside a steel shell; the oil surrounds it. Simple, quick to respond and widely used.
Piston accumulator: a sliding piston separates gas and oil. It suits larger volumes and higher pressures and can be built to a given cylinder length, at the cost of piston seal friction.
Diaphragm accumulator: a flexible diaphragm in a compact, often spherical shell separates the gas and oil. It is typically used for smaller volumes, pulsation damping and compact installations.
Weight-loaded and spring-loaded accumulators: older or specialised designs in which a mass or a spring, rather than gas, provides the pressure.

What hydraulic accumulators are used for

Energy storage: supplying a short burst of high flow faster than the pump alone could deliver, so a smaller pump can serve a peaky duty cycle.
Pulsation and shock damping: absorbing pressure spikes from pump ripple or fast valve closure.
Leakage and thermal compensation: holding pressure in a closed circuit when the fluid cools or leaks slightly.
Emergency reserve: providing a limited amount of stored fluid for a controlled operation if the pump stops.

Where accumulators appear in hydraulic test equipment

On a test bench, an accumulator keeps pressure steady while valves switch quickly and helps the rig deliver clean, repeatable data. The Neometrix dual power hydraulic test rig page lists a piston-type accumulator/dampener that smooths pulsations to give steadier pressure delivery. Accumulators are also the item under test on some rigs: the HS-748 hydraulic test rig page lists accumulators among the aircraft hydraulic assemblies it tests, and the hydraulic proof pressure test rig page includes small hydraulic tanks or accumulators among the parts it proof-tests. The aircraft tyre nitrogen charging rig page also covers nitrogen charging of hydraulic accumulators, along with struts and tyres.

How a hydraulic accumulator is tested

Testing normally covers the shell, the separating element and the gas charge. The pressure-containing shell is proof-tested with liquid to a pressure above its maximum working pressure (see proof pressure testing). Pre-charge is checked with a gauge and charging kit, with the hydraulic side fully depressurised, and topped up with nitrogen if needed. Leakage is checked on both sides of the separator, since a failed bladder, diaphragm or piston seal lets gas enter the oil or oil enter the gas side. Response and pulsation-damping behaviour are checked in the circuit itself.

Safety note: an accumulator holds stored energy even when the pump is off. Fluid pressure must be released, and the gas pre-charge handled by trained personnel, before any accumulator is disconnected or opened.

Frequently asked questions

What does an accumulator do in a hydraulic system?

It stores pressurised fluid and releases it when needed. That lets it supply short bursts of flow, absorb shocks and pulsation, keep pressure up when there is slight leakage or temperature change, and act as a limited emergency reserve.

What gas is used in a hydraulic accumulator?

Dry nitrogen is the usual gas because it is inert and non-flammable. Air or oxygen should not be used to pre-charge an accumulator because of the risk of fire or explosion with hydraulic oil.

What is the difference between bladder, piston and diaphragm accumulators?

They differ in the element that separates the gas from the oil: a flexible bladder, a sliding piston or a flexible diaphragm. Bladder and diaphragm types respond quickly and are simple; piston types handle larger volumes and higher pressures but have seal friction.

What is the pre-charge pressure of an accumulator?

It is the gas pressure set in the accumulator before the hydraulic system pressurises it. It is set below the minimum working pressure and is checked periodically, since it affects how much usable fluid the accumulator stores.

Why must an accumulator be tested?

Its shell is a pressure vessel and its separating element can wear or fail. Proof-pressure, pre-charge and leakage checks confirm it is safe and will perform as intended.

If you need a test rig with hydraulic accumulators or want to test them, contact the Neometrix team to discuss your requirement.