A hydraulic accumulator mainly consists of a chamber in which a fluid is held under pressure by a spring or a raised weight or a volume of compressed gas (nitrogen). It is, thus, possible to store potential energy in the accumulator, when the associated system pressure is greater than that of the accumulator. A hydro-pneumatic accumulator is pre-charged to a certain % of the minimum or median working pressure. The accumulator can release the stored energy back, when the system pressure falls below that of the accumulator. The most important advantageous of accumulators are highlighted below:
Energy stored in a fully charged and appropriately-sized hydraulic accumulator can be used to meet the sudden demand for a high level of power for a comparatively short time to complete a cycle or as a source of power in an emergency during power failures or for leakage compensation. With the addition of an accumulator in a hydraulic system, its pump unit may be re-sized for the discharge of the high-pressure fluids to match the average power requirement. An accumulator can be sized by using certain empirical formulas or software sizing tools.
When the flow through a hydraulic system is blocked abruptly, for example, with the rapid closure of a valve in the system, a pressure wave is developed that travels back and forth through the system. The shock pressures may rip tubing, blow seals, produce fatigue failures, jar parts, and split pump housing. The accumulator can absorb the kinetic energy of the moving column of the fluid and can suppress the shock. Other system parts such as pumps, valves, hoses, and fittings do get away from the pressure spikes and thus avoid damages to the components. For the purpose of shock suppression, accumulators are used in presses, vehicles, and equipment used in the construction, offshore, and mining fields.
Pressure pulsations are caused by delivery fluctuations from the pump, irregularities in the fluid flow, thermal variations, or excessive loads. The pressure fluctuations can cause variations in the actuator’s speed and inferior system performance. Adding an accumulator cushions the pressure pulsations and keeps the pressure relatively constant. The cushioning leads to reduced noise levels in the system.
A hydraulic accumulator is a pressure vessel that stores an enormous amount of potential energy. Accumulators can be dangerous to personnel and property if they discharge the stored pressure inadvertently. It is necessary to isolate the accumulators from the associated systems and discharge pressures from the accumulators, during periods of maintenance or emergency. Typically, safety devices must be incorporated to provide a shut-off facility and pressure-limiting and pressure relief features.
Accumulators are definitely a hazard in hydraulic systems and they add cost to the systems, so they should not be used except in systems where the use of accumulators offers an overwhelming advantage over systems with conventional circuits (without accumulators).
References: (1) Hydraulic Accumulators and Circuits (In the SI Units) and (2) Accumulators in Hydraulic Systems (In the English Units), by Joji Parambath
Hydraulic Accumulators and Circuits In the SI Units
Accumulators in Hydraulic Systems (In the English Units)
Authored by JOJI Parambath, Founder/Director, Fluidsys Training Centre, Bangalore