Multi-pressure relief valves
Pilot-operated relief valves have a vent port. In Figures 18-21 and 18-22, the vent port is piped to a single, remote direct-acting relief for adjusting pressure remotely. Figures 18-23 through 18-25 show the vent port connected to directional valves and multiple remote reliefs. These circuits allow changes from maximum pressure to several preset or infinitely variable limits during a cycle.
Figure 18-23 shows a pilot-operated relief valve with the vent port connected to a 3-position directional control valve. With the directional valve centered, it blocks the vent port on the relief to keep system pressure at the setting of the main relief. An open-center directional valve would vent the main relief, lowering pressure to a 20- to 70-psi range.
Some manufacturers offer a relief valve with the remote pilot heads and solenoid valve built into the valve body. This eliminates external piping but is less flexible than piping the vent of a standard pilot-operated relief valve to standard directional valves.
In Figure 18-24, solenoid A1 is energized. This connects the vent to the left remote direct-acting relief, dropping system pressure to a maximum of 350 psi. Energizing solenoid A1 keeps pressure from going above the setting of the left direct-acting relief. The main relief valve always limits maximum system pressure.
Figure 18-25 shows solenoid A1 energized, allowing system pressure to go to 700 psi. In this condition the right remote direct-acting relief controls system pressure. Set remote reliefs at any pressure lower than the main relief valve.
Figure 18-26 shows a pilot-operated relief controlled by an infinitely variable proportional or servovalve. Using a variable-flow valve to control a pilot-operated relief valve gives infinitely variable pressure. The control signal may come from a rheostat, a programmable controller, or a computer.
Purchase an infinitely variable relief valve as an assembled unit or pipe one remotely. In each case, the pilot head relief controls maximum pressure, while the servo or proportional valve only sets a lower pressure.
Unloading relief valves
An accumulator circuit using a fixed-displacement pump must have some way to unload the pump after reaching maximum pressure. A normally open solenoid-operated relief valve controlled by a pressure switch is one way to unload a pump. Chapter 1 shows this circuit and explains its operation.
Some accumulator circuits use a special type valve called an unloading relief valve. This relief valve eliminates the need for electrical, high and low pressure switches and a solenoid-operated dump valve to unload the pump. Only a few manufacturers make an unloading relief valve. Two of these operate at preset pressure differentials and may not be suitable for some accumulator circuits. One manufacture makes an unloading relief valve with adjustable pressure differentials.
Several companies make a relief unload-and-dump valve combination with other features. Operation is the same as an unloading relief valve but includes a back-flow check valve and an accumulator dump valve in the same body. See Chapter 1, Figure 44, for an explanation of this accumulator unload and dump valve.
Figures 18-27 through 18-30 schematically depict an unloading relief valve in an accumulator circuit. Figure 18-27 shows the circuit after the pump starts. Normally closed relief valve A forces fluid to the accumulator and the circuit. Pressure increases as fast as the pump fills the accumulator. When the accumulator and circuit reach set pressure of 3000 psi, pilot pressure opens relief valve A and unloads the pump to tank.
In Figure 18-28, the accumulator is at pressure and the pump is unloading. The relief valve is fully open or vented because the control piston pushes the pilot control piston off its seat. Without a control piston, relief valves relieve excess pump flow at set pressure, generating a lot of heat. This unloading relief valve has a preset differential of 15% between unloading and reloading the pump.
When system pressure drops to approximately 2550 psi, as in Figure 18-29, its spring force reseats the pilot control poppet again. This forces pump flow into the circuit. This action repeats as long as the pump runs. With a tight circuit and the machine not cycling, the pump unloads approximately 80% of the time.
Figure 18-31 shows a cutaway view of an unloading relief valve. It is similar to a standard relief valve but has an extra control piston in its head. There is an approximate 15% difference in the area of the control piston and the pilot-control poppet seat. As pressure builds, it pushes against both sides of the control piston and against the pilot-control poppet. Nothing moves until pressure starts to force the pilot-control poppet off its seat, as in Figure 18-32. Pressure drop in front of the control piston lets it move and force the pilot-control poppet completely off its seat, Figure 18-33. Forcing the pilot-control poppet off its seat unloads the pump at 20 to 70 psi. The pilot-control poppet stays open until system pressure drops approximately 15%, then closes to force pump flow into the circuit again. When pressure rises to maximum, the pilot-control poppet is pushed off its seat and unloads the pump. This action continues anytime the pump runs.