Thermal relief valves are usually smaller as compared to system relief valves. They are used in systems where a check valve or selector valve prevents pressure from being relieved through the main system relief valve.

Figure 7-25 shows a typical thermal relief valve. As pressurized fluid in the line in which it is installed builds up to an excessive amount, the valve poppet is forced off its seat; this allows excessive pressurized fluid to flow through the relief valve to the reservoir return line, as shown in view B of figure 7-25. When system pressure decreases to a predetermined pressure, spring tension overcomes system pressure and forces the valve poppet to the closed position, as shown in view A.

Relief valve maintenance is limited to adjusting the valve for proper relieving pressure and checking the valve for leakage. If you think a relief valve is leaking internally, a flexible hose maybe connected to the return port of the valve and the drippings, if any, caught in a container. The opening and closing pressure of the valve may also be checked in this manner provided an external source of rower is used.

Figure 7-24.System relief valve.



Figure 7-25.Typical thermal relief valve.

 To adjust the opening pressure of a relief valve, turn the adjusting screw clockwise to increase opening pressure and counterclockwise to decrease opening pressure.

Do not attempt to adjust a relief valve while it is installed on an aircraft. This action will result in an incorrect pressure setting. The valve must be removed and adjusted on a test stand to ensure proper pressure settings.

All hydraulic systems do not have shutoff valves incorporated; however, in some systems a shutoff valve is installed in the fluid supply line between the reservoir and the engine-driven pumps, and other places where shutting off the fluid is desirable. These valves, like other valves, may be electrically or manually controlled, depending upon the design of the valve.

The purpose of shutoff valves differ according to their installation. All shutoff valves control the flow of fluid; however, they may isolate troubles by shutting off a complete system or subsystem, or they may control the speed a component moves by partially closing the valve (manual type).