Accumulators make it possible to store useable volumes of almost non-compressible hydraulic fluid under pressure. The symbols and simplified cutaway views in Figure 16-1 show several types of accumulators used in industrial applications. They are not complete representations but they illustrate general working principles.
A 5-gal container completely full of hydraulic oil at 2000 psi will only discharge a few cubic inches of fluid before the pressure drops to 0 psi. If the same container were filled half with oil and half with nitrogen gas, it could discharge more than 1 1/2 gallons of fluid while pressure only dropped 1000 psi. This is the great advantage of hydro-pneumatic accumulators.
No separator: Some original accumulators were high-pressure containers with a sight glass to show fluid level. They were filled approximately half with oil and half with nitrogen gas -- with no separation barrier between them. Before stopping the pump, a shut off valve at the accumulator discharge port was closed to prevent fluid and gas from escaping. This type of accumulator is not used on new circuits today, but there still are many in service.
Gas-charged bladder: Many accumulators now use a rubber bladder to separate the gas and liquid. A poppet valve in the discharge port keeps the bladder from extruding when the pump is off. The original design was the bottom-repair style, shown on the left in Figure 16-1. It is still offered by most manufacturers. The top-repair style on the right is now available and makes bladder replacement simple and fast.
Gas-charged piston: The gas-charged piston accumulator has a free-floating piston with seals to separate the liquid and gas. It operates and performs similarly to the bladder type, but has some advantages in certain applications. A gas-charged piston accumulator can cost twice as much as an equal-sized bladder type.
Spring-loaded piston: A spring-loaded piston accumulator is identical to a gas-charged unit, except that a spring forces the piston against the liquid. Its main advantage is that there is no gas to leak. A main disadvantage is that this design is not good for high pressure and large volume.
Weight loaded: All gas-charged accumulators lose pressure as fluid discharges. This is because the nitrogen gas was compressed by incoming fluid from the pump and the gas must expand to push fluid out. The weight-loaded accumulator in Figure 16-1 does not lose pressure until the ram bottoms out. Thus 100% of the fluid is useful at full system pressure. The major drawback to weight-loaded accumulators is their physical size. They take up a lot of space and are very heavy if much volume is required. They work well in central hydraulic systems because there usually is room for them in the power unit area. However, central hydraulic systems are falling out of favor, so only a few facilities use weight-loaded accumulators. (Rolling mills are one application where space to place large items is not a problem.) Note that there is often a long dwell time to fill these monsters.
Diaphragm accumulators: There are also diaphragm accumulators with resilient or metal diaphragms. They are used where the stored volume is small.