When sized and applied correctly, accumulators produce a win-win situation. First, accumulators can reduce the size of the pump needed to power a hydraulic system. And because a smaller pump carries a lower price tag, OEMs can save on initial cost of the power unit. But users can also benefit from higher system efficiency and less electrical use in the long run.

Strategic application of accumulators allows for specifying smaller pumps because accumulators store hydraulic energy. During periods when equipment requires little or no hydraulic power, the pump charges the accumulators with pressurized fluid. Then, when the system demands full flow — which the pump alone cannot accommodate — the accumulators release their stored energy to assist the pump in meeting flow demand. When the system again demands less power, the pump recharges the accumulators to prepare them for the next period of high flow demand.

Higher energy efficiency stems from a smaller motor needed to drive the smaller pump. Furthermore, when the system runs in standby mode, the smaller motor-pump combination consumes less power than a larger unit running in standby. The smaller motor consumes less power during peak flow demands as well, because the accumulators reduce the peak power requirement of the motor.

Learning by example

To demonstrate, assume a system is designed for a maximum operating pressure of 3000 psi. This is the highest fluid pressure with which the accumulator can be charged. Figure 1 shows the minimum system operating pressure to be 2000 psi, which normally is determined by the minimum system pressure requirement of the actuators. Based on these parameters, the first calculation to be made is precharge pressure for the accumulator.

Gas precharge pressure for piston-type accumulators generally is 100 psi less than minimum operating pressure. So for our example, precharge pressure is 1900 psi. This provides the maximum usable fluid discharge from the accumulator, without bottoming out the piston.

Still referring to Figure 1, the machine operates in 30- sec cycles, consisting of 19 total sec of fluid demand by system actuators and 11 total sec of dwell time. This is typical of automated machinery: certain functions of a process require more hydraulic power than others. The variation in hydraulic power demand provides an opportunity to use an accumulator so that the pump can be sized more on the basis of average power demand rather than peak demand. The accumulator recharges whenever pump discharge flow exceeds system flow requirements.

Sequence of events
At zero seconds, system pressure is maximum (3000 psi), and the accumulator has been fully charged. According to Figure 2, it would contain 9.12 gal of fluid. The accumulator actually has a total volume of 25 gal, but nearly 16 gal is occupied by precharge gas. As the actuators demand flow, system pressure gradually drops, and fluid flows from the accumulators.

When system pressure drops to its lowest point in the cycle, 2026 psi, the accumulator contains 1.54 gal of fluid. This means it has provided 7.58 gal in the first 10 sec of the operating cycle to supplement pump flow. After 10 sec, the flow requirement drops to well under 40 gpm (Figure 1), so excess flow from the pump begins to recharge the accumulator. This is evident in Figure 2; fluid volume in the accumulator steadily decreases, but shows a temporary increase from 10 to 12 sec. The peak system flow demand is 100 gpm, but because the accumulator supplements pump flow, a pump rated for 40 gpm can be used. Its flow is represented by the green areas in Figure 1.