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.