Hydraulic Accumulators

Accumulators are devices, which simply store energy in the form of fluid under pressure. This energy is in the form of potential energy of an incompressible fluid, held under pressure by an external source against some dynamic force. This dynamic force can come from three different sources: gravity, mechanical springs or compressed gases. The stored potential energy in the accumulator is the quick secondary source of fluid power capable of doing work as required by the system. This ability of the accumulators to store excess energy and release it when required, makes them useful tools for improving hydraulic efficiency, whenever needed. To understand this better, let us consider the following example.

A system operates intermittently at a pressure ranging between 150 bar (2175 psi) and 200 bar (2900 psi), and needing a flow rate of 100 1pm for 10 s at a frequency of one every minute. With a simple system consisting of a pump, pressure regulator and loading valves, this requires a 200 bar (2900 psi), 100-lpm pump driven by a 50 hp (37 kW) motor, which spends around 85% of its time, unloading to the tank. When an accumulator is installed in the system as shown in Figure 7.12, it can store and release a quantity of fluid at the required system pressure.


The operation of the system with accumulator is illustrated by Figure 7.13;


At time A, the system is turned on and the pump loads, causing pressure to rise as the fluid is delivered to the accumulator via a non-return valve V3. At time B, the working pressure is reached and a pressure switch on the accumulator causes the pump to unload. This state is maintained as the non-return valve holds the system pressure.

The actuator operates between time C and D. This draws the fluid from the accumulator causing a fall in the system pressure. The pressure switch on the accumulator puts the pump on load again, to recharge the accumulator for the next cycle.

With the accumulator in the system, the pump now only needs to provide 170 1pm and also requires reduced motor hp. Thus it can be seen how an accumulator helps in reducing the power requirements in a hydraulic system.

There are three basic types of accumulators used extensively in hydraulic systems. They are:

1. Weight-loaded or gravity-type accumulator
2. Spring-loaded-type accumulator
3. Gas-loaded-type accumulator.

Hydraulic Accumulators

Like an electrical storage battery, a hydraulic accumulator stores potential power, in this case liquid under pressure for future conversion into useful work. This work can include operating cylinders and fluid motors, maintaining the required system pressure in case of pump or power failure, and compensating for pressure loss due to leakage. Accumulators can be employed as fluid dispensers and fluid barriers and can provide a shock-absorbing (cushioning) action.

On military equipment, accumulators are used mainly on the lift equipment to provide positive clamping action on the heavy loads when a pump’s flow is diverted to lifting or other operations. An accumulator acts as a safety device to prevent a load from being dropped in case of an engine or pump failure or fluid leak. On lifts and other equipment, accumulators absorb shock, which results from a load starting, stopping, or reversal.

Spring-Loaded Accumulator. This accumulator is used in some engineer equipment hydraulic systems. It uses the energy stored in springs to create a constant force on the liquid contained in an adjacent ram assembly. Figure 2-15 shows two spring-loaded accumulators.

The load characteristics of a spring are such that the energy storage depends on the force required to compress s spring. The free (uncompressed) length of a spring represents zero energy storage. As a spring is compressed to the maximum installed length, a minimum pressure value of the liquid in a ram assembly is established. As liquid under pressure enters the ram cylinder, causing a spring to compress, the pressure on the liquid will rise because of the increased loading required to compress the spring.

Bag-Type Accumulator. This accumulator (Figure 2-16) consists of a seamless high-pressure shell, cylindrical in shape, with domed ends and a synthetic rubber bag that separates the liquid and gas (usually nitrogen) within the accumulator. The bag is fully enclosed in the upper end of a shell. The gas system contains a high-pressure gas valve. The bottom end of the shell is sealed with a special plug assembly containing a liquid port and a safety feature that makes it impossible to disassemble the accumulator with pressure in the system. The bag is larger at the top and tapers to a smaller diameter at the bottom. As the pump forces liquid into the accumulator shell, the liquid presses against the bag, reduces its volume, and increases the pressure, which is then available to do work.

Piston-Type Accumulator. This accumulator consists of a cylinder assembly, a piston assembly, and two end-cap assemblies. The cylinder assembly houses a piston assembly and incorporates provisions for securing the end-cap assemblies. An accumulator contains a free-floating piston with liquid on one side of the piston and precharged air or nitrogen on the other side (Figure 2-17). An increase of liquid volume decreases the gas volume and increases gas pressure, which provides a work potential when the liquid is allowed to discharge.

Maintenance. Before removing an accumulator for repairs, relieve the internal pressure: in a spring-loaded type, relieve the spring tension; in a piston or bag type, relieve the gas or liquid pressure.