In hydraulics, a pressure reducing valve serves the same purpose as a "pressure regulator" valve in a compressed air system. It can be described as a 2-way, normally open valve which is piloted from its own outlet. It is one of a variety of pressure control valves available for hydraulic circuits. It is always used in a branch circuit, never in the full pump flow line. It will reduce the pressure in one branch while allowing other branches to operate at full system pressure. A knob or screw allows adjustment of the outlet pressure over the working range of the valve. It will accept a flow of high pressure oil on its inlet and deliver the same flow from its outlet at a constant reduced pressure, as long as the inlet pressure is higher than the value set on the adjustment knob.
A description of operation of the valve itself will be found in "Industrial Fluid Power - Volume 2" published by Womack Machine Supply Co.
Low Pressure Branch Circuit
Figure 1. In this 2-branch circuit, the working cylinder operates at full system pressure, but the clamp cylinder must operate at reduced pressure if full pressure would distort or crush the part being clamped.
If, on the other hand, the clamp must operate at full pressure and the work cylinder pressure must be reduced, the reducing valve would be placed in the feed line to the work cylinder.
Pressure reducing valves should never be used on all branches of a system. Pressure to the branch requiring the highest pressure should be set on the system relief valve.
Reducing and Relief Valve Combination
Figure 2. On most applications a relief valve is not needed on the outlet of the reducing valve; the system relief valve, upstream of the reducing valve will give sufficient protection to components downstream of the reducing valve. However, an application occasionally appears in which at certain times during the machine operation, the load exerts a mechanical reaction which may cause pressure behind the cylinder piston to rise higher than the pressure setting on the reducing valve outlet. If this is a sudden reaction or an impact, a relief valve should be added as shown in Figure 2.If needed, a direct-acting relief valve should be used and should be adjusted to a cracking pressure 300 to 500 PSI higher than the reducing valve outlet to prevent discharge of oil to tank during normal operation of the cylinder.
Some manufacturers supply a 3-port reducing valve which includes a built-in relief valve on the outlet port.
All reducing valves have an external drain port, and on all applications, without exception, a drain line must be run directly to tank. A reducing valve drain can usually be combined with the drains from other reducing valves or sequence valves but should never be teed into a main tank return line. Pressure surges which may appear at times in a tank return line would cause similar surges to appear in the outlet of the reducing valve.
While main tank return lines should always discharge below the oil level in the tank, drain lines are often discharged on top of the oil to prevent siphoning oil out of the reservoir if a drain line should be disconnected at the valve.
Remote Control of Reducing Valves
If the reducing valve is the pilot-operated type (and practically all of them are), it may be remotely adjusted for outlet pressure level from a remote control position up to 10 feet away, sometimes from a greater distance.
Figure 3. A 2-way shut-off valve, manual or solenoid operated, can be placed in series with the external drain. When de-energized, it will permit the reducing valve to drain normally and deliver a reduced pressure on the outlet. When energized, it will block the spool of the reducing valve and cause full pressure to appear on its outlet.