Accumulator dump valves
In all the foregoing accumulator applications (except the one for emergency power supply), the accumulator fluid was drained automatically at shut down. This is very important because accumulators store energy that can be a safety hazard and can cause damage to the machine. Here are examples of different types of accumulator dump valves and circuits.
Figure 16-8 shows one frequently used circuit. A normally open, solenoid-operated, 2-way directional control valve is teed into the pump line between the isolation check valve and the accumulator. The solenoid is wired so that it is energized when the pump starts and de-energized when the pump stops. An orifice in front of the 2-way valve controls flow when the accumulator is discharging to prevent damage to the valve. This arrangement works equally well with fixed-displacement or pressure-compensated pumps.
A note of caution: Some solenoid valves, even though they are designed for continuous duty, get very hot when energized for long periods. Such overheating can cause varnish deposits to form and lock the valve’s internal parts in the closed condition after the pump shuts down. This means the trapped energy does not get discharged and the accumulator can cause harm to anyone working on the circuit.
The dump circuit in Figure 16-9 is only for pressure-compensated pumps. A packaged set of valves isolates the accumulator while the pump is running and automatically dump it at shut down. The package consists of an isolation check valve, a pilot-to-close check valve, and a flow-control orifice.
At pump startup, flow goes to the circuit and the accumulator. Pressure from the pump outlet shifts the pilot-to-close check valve, blocking flow to tank. When the accumulator is full, the pump compensates to no flow and the circuit waits for a new cycle. When pressure drops, the pump comes back on stroke and makes up for flow going to the circuit. At pump shut down, pilot pressure to the pilot-to-close check valve drops and the valve shifts to open. Now, stored energy in the accumulator is ported to tank through the orifice. This circuit is very reliable because it depends on system or pump pressure to close and/or open valves.
A fixed-volume pump must be ported to tank at very low pressure when its flow is not doing work. A common circuit for unloading a fixed-volume pump and dumping an accumulator is shown in Figure 16-10. An internally piloted unloading relief valve with integral check valve forces all pump flow to the circuit and the accumulator until the system reaches the set pressure. As the control ball starts to relieve, system pressure pushes against the unloading piston and forces it off its seat. This takes all pressure off the top of the relief valve poppet. The pump unloads to tank at 25 to 100 psi until system pressure drops approximately 15%. After that drop, spring force pushes the unload piston back and pump flow goes to the circuit again.
The accumulator dump valve blocks fluid from going to tank while the pump is running and opens to discharge stored energy when the pump shuts down. The accumulator dump valve is a high ratio (up to 200:1) pilot-to-close check valve that is held shut by the pump's unloaded or work pressure. With a 200:1 area ratio between the poppet and the pilot piston, 25-psi pressure at the pilot port will stop as much as 5000 psi at the poppet shut off. This keeps fluid in the accumulator circuit until the pump is shut down. Then, all stored pressurized fluid flows to tank quickly and safely. (One supplier offers the unloading relief valve and the accumulator dump valve in a single body. This combination simplifies piping while offering the same effect.)
Other accumulator applications
Accumulators are also used for systems where thermal expansion could cause excessive pressure. Cylinders with blocked ports in a high ambient heat area can go to high pressure if there is no place for expanding fluid to go.
Another use for accumulators is as a barrier between two different fluids. The pump that uses hydraulic fluid keeps pressure on a circuit that uses water or another incompatible medium.
One supplier offers low-pressure accumulators as breathing devices for sealed reservoirs. This keeps airborne contaminants out of the hydraulic oil as the fluid level rises and falls.
For more circuits and other information on accumulators, see the author’s upcoming e-book Fluid Power Circuits Explained.