What is the purpose of a thermal relief valve in a closed-center hydraulic system?

In a closed-center hydraulic system, a thermal relief valve protects the system from overpressure caused by thermal expansion of the hydraulic fluid. A closed-center system is characterized by its valves blocking flow to the actuators when in the neutral position, trapping fluid in the lines. When the hydraulic system is inactive, and the temperature rises, the hydraulic fluid expands. Because the fluid is confined within the closed circuit, this expansion creates increased pressure. Without a relief mechanism, this pressure can exceed the system's pressure rating and damage components such as hoses, cylinders, or valves. The thermal relief valve is designed to open at a predetermined pressure slightly above the normal operating pressure but below the maximum allowable pressure of the system. When the pressure due to thermal expansion reaches this set point, the valve opens, relieving the excess pressure by allowing a small amount of fluid to flow back to the reservoir. This prevents over-pressurization and protects the system components. Once the pressure drops below the set point, the valve closes again. For example, if a hydraulic system on a piece of heavy machinery is left in direct sunlight, the fluid temperature can rise significantly, causing the pressure to increase. The thermal relief valve will then open to prevent damage.