We have seen a lot of confusion about pressure reducing valves and regulators in fluid power systems. This confusion is largely due to the nomenclature and partly because the schematic symbols are not always as intuitive as they could be. The valves themselves are actually fairly simple. In a pneumatic system, the valve is called a regulator. In a hydraulic system, it is called a pressure reducing valve. Notice that the symbols are quite similar, because their function is the same, only with a different medium. In Figure 1, the two symbols are shown side-by-side. As in most fluid power pressure controls, the regulator and the pressure reducing valve are characterized by a single square with a single arrow drawn inside. We can see that the only difference between the two is that, in the pressure reducing valve, the arrowhead is filled in, whereas in the regulator there is only the outline of an arrowhead. This is to illustrate that the regulator is designed to control pressure in a gas medium while the pressure reducing valve is intended to operate with a liquid.
Both of these valves are normally open, which is indicated by the arrow touching both the inlet and the outlet ports. On the bottom of the symbol is a jagged line representing a spring. If the spring is adjustable, a diagonal arrow will be drawn across it. A good way to think of the valve when tracing flow on a schematic is to consider the spring pushing the arrow up and holding the valve open. But, in order to reduce the pressure, the valve must close off to some degree.
Notice the downstream pilot line. System pressure is measured downstream of the valve and applied to the top of the arrow, pushing it down and partly closing the valve. When the two forces equalize, i.e. spring tension on the bottom and air or hydraulic pressure on the top, a balance is achieved and the pressure is reduced.
Figure 1. Symbols
The air valve is called a regulator, but even though the name suggests that it can either increase or decrease the pressure, like the pressure reducing valve, it can only decrease the inlet pressure. In a pneumatic system, the regulator is the primary pressure control. The compressor will determine the maximum pressure and the regulator reduces the pressure to a level that is safe and usable, opening and closing as necessary to keep the pressure stable. While there may also be secondary regulators to lower the pressure further for use in branch circuits as shown in Figure 2, there will always be a primary regulator to stabilize and set the main system pressure. In a hydraulic system, pressure reducing valves are used to lower the system pressure for use in circuits that require less pressure than the maximum system pressure. This extends the life of the lower pressure circuits and conserves energy.
Figure 2. Secondary regulators
Though often not shown on the schematic symbol, most pneumatic regulators are of the relieving type. The advantage of this type of regulator is that it not only can reduce pressure in the system but can also allow any excess to escape. With a non-relieving regulator, it is possible for air to become trapped in the circuit, keeping the valve from reducing pressure. In the relieving type, if the downstream pressure ever exceeds the spring tension of the valve, a vent opens to release the trapped air. The non-relieving type are usually only found in systems using gases that are either too toxic or too expensive to be released to atmosphere.
The corresponding hydraulic valve is called a pressure reducing relieving valve.
Figure 3. Increased pressure
The reducing relieving valve is used in hydraulic circuits where it is essential that the downstream pressure never exceed the spring setting but opposing forces may act against it. They are quite common in paper machines, for example, where it is necessary to maintain precise force, even if some imperfection is encountered that forces a cylinder to retract slightly. The pressure then builds briefly downstream of the valve, causing it to shift into its relieving mode once the pressure exceeds about 3-5% of the spring tension. In Figure 3, imagine the downstream pressure being maintained by the spring tension of the valve, but a reverse force raising the pressure briefly causes the valve to shift to its relieving mode. In the case of the air regulator, the arrow gets pushed down past the vent port and air is released immediately to atmosphere. In the hydraulic pressure reducing relieving valve, the increased pressure in the pilot line forces the arrow down to release hydraulic fluid to tank.
Figure 4. External drain line
Another characteristic to note in both the hydraulic pressure reducing valve and pressure reducing relieving valve is the external drain line as shown in Figure 4. This drain line must be present in all hydraulic reducing valves so that oil bypassing the internal spool has a flow path to tank. Whenever the reducing valve’s outlet pressure is below its inlet pressure, drain oil flows. If the external tank line becomes plugged, the valve cannot shift freely and pressure will not be controlled. Whenever the valve is replaced, the drain line must be inspected to ensure that it is clear. These valves are often replaced unnecessarily because the drain line has become plugged. The new valve will function for a short period until bypassed oil collects in the valve and keeps it from shifting.
Figure 5. Bypass check valve
These pressure controls are designed to function in one direction only. If they are installed in a line where fluid may be directed in either direction, there must be a bypass check valve to allow free flow in the opposite direction as shown in Figure 5. When troubleshooting the valve, be sure to inspect the check valve (if it is present) for trash. Typically, when check valves fail, they fail open. If the check valve becomes stuck open, the reducing valve will no longer be able to control pressure.