Hydraulic accumulators make it possible to store useable volumes of non-compressible fluid under pressure. A 5-gal container completely full of oil at 2000 psi will only discharge a few cubic inches of fluid before pressure drops to 0 psi. The same container filled with half oil and half nitrogen gas would discharge over 1½ gal of fluid before pressure dropped to 1000 psi.
Figures 1-1 through 1-4 show symbols used for different types of accumulators. Figures 1-5 through 1-8 are simplified cutaways showing construction of different types of accumulators.
When using an accumulator, it is necessary to install a manual or automatic function to de-pressurize all fluid before working on the circuit. Several manufacturers make automatic discharge valves that work well. These automatic discharge valves are explained at the end of this section.
Most hydraulic accumulators are used in one of four applications:
1. Supplement pump flow in circuits with medium to long delays between cycles.
2. Hold pressure in a cylinder while the pump is unloading or stopped.
3. Have a ready supply of pressurized fluid in case of power failure.
4. Reduce shock in high velocity flow lines or at the outlet of pulsating piston pumps.
The following circuit images show some circuits using accumulators for the operations mentioned in 1 to 4 above. Other accumulator circuits and information follow.
Using accumulators to supplement pump flow
Some hydraulic circuits require a large volume of oil for a short time; for example to move a large cylinder rapidly to clamp a part. After clamping, the circuit needs little or no additional fluid for period of time while curing takes place. When a circuit has extended dwell time, an accumulator can be used to downsize the pump, motor, tank, and relief valve. The cost of accumulators usually offsets savings on these smaller components, but downsizing saves on operating costs.
The conventional pump, directional valve, and cylinder pictured in Figure 1-9 show horsepower and flow requirements needed for a 12.5-sec cycle time. The advance cycle requires full power, while returning the cylinder needs minimal force. Reduction of the pump and motor size is not possible if the cylinder cycles rapidly. However, if there was a 45 sec wait between cycles, the pump and motor could be almost 70% smaller with an accumulator circuit.
This reduced flow and horsepower are possible when using accumulators and the circuit shown in Figure 1-10. The extra expense of the accumulators offsets the reduced price for the power unit, but operating cost is less for the life of the machine. The directional valve and piping from the accumulators to the cylinder still has to handle the 125 gpm flow.
Using a gas charged accumulator in a pump supplementing circuit will increase maximum system pressure. The extend portion of the cycle needs at least 2000 psi working pressure, which requires filling the accumulators with fluid above 2000 psi so they can discharge oil and not drop below minimum pressure. The maximum system pressure should be as high as can be tolerated. The higher the maximum allowable system pressure, the smaller the accumulators. The drawback of high pressure is that the circuit is at this pressure when the cycle starts. If this higher pressure can cause damage or other problems, it should be lowered to a safe level.
Accumulator circuits normally have flow controls because there is a volume of oil at elevated pressure that can discharge almost instantaneously. Placing a flow control at the accumulator outlet allows free flow from pump to accumulator and adjustable flow to system.
The circuit in Figure 1-10 has a minimum pressure of 2000 psi and a maximum pressure of 3000 psi. This pressure is the limit of most hydraulic components. A 22-gpm pump driven by a 40-hp motor now meets the force and cycle time specified. All pump flow continuously goes to the circuit instead of being unloaded most of the time as in conventional circuits.
As the cylinder cycles, the accumulators supply fluid at a rate set by the flow control. Pump flow adds to accumulator flow to set the required cycle time. Cylinder cycling could be made faster than specified by increasing outlet flow from the accumulator.
The fixed-volume pump in Figure 1-10 unloads through a special accumulator relief/unload/dump valve, which sends all pump flow to the accumulators and cylinder until the system reaches set pressure. After reaching set pressure, the valve opens and unloads the pump to tank at approximately 50 psi. The pump will continue to unload until the system pressure drops about 15%. This pressure drop might be from leakage or at the start of a new cycle. Any time pressure drops, the pump will load and stay loaded until pressure tries to go above 3000 psi. With this valve, stored oil in the accumulators automatically discharges to tank when the pump stops, which makes the circuit safe to work on shortly after locking and tagging off the pump.
Figure 1-11 shows a variation of the accumulator circuit in Figure 1-10. Here a 1-gpm fixed-volume pump and a 5-gpm pressure-compensated pump supply oil until the accumulators fill. A pressure switch, set at about 2900 psi, unloads the fixed-volume pump through a solenoid-operated relief valve. After the fixed-volume pump unloads, the pressure-compensated pump finishes filling the accumulators and holds maximum pressure without fluctuations and with minimal heating.
The accumulator dump valve in this circuit will stay closed as long as the pumps are running. When the pumps stop, this valve quickly and automatically discharges the accumulators to tank.