What is in this article?:
- BOOK 2, CHAPTER 16: Reducing valves
- Two-pressure circuit with a pressure-reducing valve
- Remotely operating a pressure-reducing valve
- Modular pressure-reducing-relieving valves
Remotely operating a pressure-reducing valve
Pilot-operated reducing valves have a remote pressure-control port. Connecting this port to other pressure valves allows pressure to be changed from a remote location. For example, Figure 16-12 shows a reducing valve with a directional valve and two remote relief valves connected to the remote-control port. With the directional control valve in its center position, set the pressure with the knob on the reducing valve. This setting is always the highest reduced pressure for the circuit.
Energizing solenoid A1 of the directional valve, as in Figure 16-13, connects the remote pilot port to the remote relief valve SET 350 psi. Pressure in the system now drops to and holds at 350 psi. Energizing solenoid B1 of the directional valve, as in Figure 16-14, connects the remote pilot port to the remote relief valve SET 700 psi. Pressure in the system now rises to 700 psi and holds at that level.
Figure 16-15 shows the reducing valve’s remote port connected to an infinitely variable electrically modulated relief valve. An electronically controlled relief valve changes the reduced pressure infinitely with a remote electrical controller.
When it is possible for an external force to increase pressure in a reduced-pressure circuit, use a reducing-relieving valve. Most modular valves now have the reducing-relieving function. When in doubt, specify reducing-relieving valves where they are required.
Figure 16-16 shows a large-bore cylinder opposing a smaller-bore cylinder. With a standard reducing valve, oil in the cap end of the 2-in. bore cylinder (CYL1) is blocked after reaching reduced pressure. With a 6-in. bore CYL2 opposing CYL1, pressure could increase to 9000 psi in its cap end. Pressures this high could cause machine damage and be unsafe.
Figure 16-17 shows a reducing-relieving module installed. Now, pressure in the end of cylinder CYL1 only increases to 430 psi. At 430 psi, the relief function takes over and the cylinder retracts.
A cylinder in a high-temperature location may have a similar problem. (Normally, hydraulic systems are not installed in areas with excessive heat, but it is a possibility.) With the cylinder extended at reduced pressure, as in Figure 16-18, heat could cause pressure at a conventional reducing valve outlet to increase and cause failure. Figure 16-19 shows how a reducing-relieving valve allows any heat-expanded oil to relieve to tank.
With slow heat build-up in a location with conventional ambient temperatures, oil expansion that raises pressure is slow enough to pass through the normal drain function.
All pilot-operated reducing valves have a drain line that bypasses control oil. There is always a small amount of oil passing through it. When drain flow is sufficient to handle backpressure from outside forces or heat, a reducing-relieving valve may be unnecessary. If in doubt, specify a reducing-relieving valve for safety’s sake.