Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1275515
ML www . machinerylubrication.com | July - August 2020 | 13 ough often not shown on the schematic symbol, most pneumatic regulators are of the relieving type. e 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. e 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. 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. ey are quite common in paper machines, for example, where it is necessary to maintain precise force, even if some imper- fection is encountered that forces a cylinder to retract slightly. e 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 pres- sure in the pilot line forces the arrow down to release hydraulic fluid to tank. 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. is 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. ese valves are often replaced unnecessarily because the drain line has become plugged. e new valve will function for a short period until bypassed oil collects in the valve and keeps it from shifting. ese 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. ML ML Figure 3. Increased pressure Figure 4. External drain line Figure 5. Bypass check valve Figure 1. Symbols Figure 2. Secondary regulators