Why are accumulators used?
To supplement pump flow: The most common use for accumulators is to supplement pump flow. Some circuits require high-volume flow for a short time and then use little or no fluid for an extended period. Generally speaking, when half or more of the machine cycle is not using pump flow, the application is a likely candidate for an accumulator circuit.
The circuit in Figure 16-2 uses several accumulators to supplement pump flow because the dwell time is 45 seconds out of the 57.5-second cycle time. This circuit’s 22-gpm fixed-volume pump operates on pressure during most of the cycle to fill the cylinder and the accumulators. Without the accumulators, this circuit would require a 100-gpm pump driven by a 125-hp motor. The first cost of the smaller pump and motor plus the accumulators is very close to that of the larger pump and motor. However, energy savings over the life of the machine make the pictured circuit much more economical.
One drawback of using accumulators to supplement pump flow is that the circuit must operate at a pressure higher than needed to perform the work. In the circuit in Figure 16-2, a minimum of 2000 psi is necessary to perform the work. This means the accumulators must be filled to a higher pressure so they can supply extra fluid without dropping below the minimum pressure. This circuit uses 3000-psi maximum pressure to store enough fluid to cycle the cylinder in the allotted time and still have ample force to do the work. The flow control in the circuit is necessary to keep the cylinder from cycling too rapidly. An accumulator discharges fluid at any velocity the lines can handle at whatever the pressure drop is when a flow path is opened.
The circuit in Figure 16-2 uses a fixed-volume pump and an accumulator unloading-and-dump valve. The valve forces pump flow to the accumulators when pressure drops approximately 15% below its maximum set pressure. At set pressure, the unloading valve opens and all pump flow bypasses to tank at 25- to 50-psi pressure drop. While the pump is bypassing, a check valve keeps the accumulators from unloading to tank. The dump valve (which is a high-ratio, pilot-to-close check valve) is held closed by pump idle pressure until the pump shuts down.
To maintain pressure: Another common application for accumulators is to maintain pressure in a circuit while the pump is unloaded. This is especially useful when using fixed-volume pumps on long holding cycles. The laminating-press circuit in Figure 16-3 clamps material and holds it at force for one to five minutes. If the pump were flowing across the relief valve at high pressure for this length of time, a lot of heat would be generated, wasting energy. With a pressure-compensated pump, energy loss would be less, but the system might still overheat in a short time.
Adding an accumulator, flow control, and pressure switch to the fixed-volume pump circuit allows the pump to unload when pressure is at or above the pressure switch’s minimum setting. If leakage at the valve or cylinder seals allows pressure to drop about 5%, the pressure switch shifts the directional control valve to pressurize the cylinder cap end and build pressure back to maximum. The only time the pump is loaded is when fluid is required. This circuit will laminate parts continuously and does not need a heat exchanger. The flow control should be set at a reduced rate so the accumulator does not dump too rapidly when the directional control valve shifts to retract the platen. Flow to make up for leakage is minor and does not need a high rate.
The accumulator dump valve in Figure 16-3 is a high-ratio pilot-to-close check valve that is held closed by the low pressure when the pump is unloaded. It opens to discharge any stored energy when the pump shuts down.
To absorb shock: Fast-moving hydraulic circuits can produce pressure spikes that cause shock when flow is stopped abruptly. Accumulators can be installed in such shock-prone circuits to reduce damaging pressure and flow spikes to an acceptable rate -- or eliminate them completely. (Accumulators can handle other pressure-spike concerns with some valve additions for special instances.)
Figure 16-4 depicts an accumulator installed to eliminate the pressure spike caused by sudden flow blockage. The nitrogen charge in this installation should be 5 to 10% above the working pressure. This keeps the accumulator out of the circuit except during pressure spike situations. A bladder-type accumulator works best here because of its fast response to pressure changes. (Use caution when applying accumulators to shock situations. It is possible to actually increase shock instead of reducing or eliminating it.)
As an emergency power supply: Some hydraulically operated machines may always need to stop in the open position to keep from damaging product or equipment. When a power failure shuts the hydraulic pump off and the machine happens to be some position other than open, there needs to be some way to get it open. An engine-driven standby pump could fill the bill and in some instances might be the best remedy. Another option is to use accumulators that are charged before the first cycle and held that way until the machine shuts down. The stored energy is ready to cycle the machine to the open position in case of a power failure.
The circuit in Figure 16-5 operates a slide gate on a waste material bin that opens hydraulically to fill a transfer truck. The circuit is located in a remote area that is prone to power failure, so it was designed to automatically close the gate in case power went off.
The schematic diagram shows the cylinder at rest with the pump running. When the unit starts, solenoids C and C2 on the normally open 2-way directional valves are energized. They stay energized while the pump is on. The first pump flow goes through the check valve and fills the accumulator with enough fluid to extend the cylinder from any open position. When electrical power is available, the gate can be opened and closed to dump waste material into the waiting truck. If a truck is filling and a power failure occurs, the pump stops and all solenoids de-energize. At this point the accumulator is ported to the cylinder cap end and fluid in the cylinder rod end has a free path to tank.
Notice the manual drain connected to the line between the check valve and the accumulator. This drain must be opened before working on the circuit. A placard on the machine warns maintenance personnel of the potential danger if the accumulator is not drained. Emergency power supplies are the only accumulator circuit that cannot be drained automatically in most cases.