At what pressure are hydraulic system thermal relief valves generally set to open?

In hydraulic systems, thermal relief valves are designed to protect the system from excessive pressure caused by thermal expansion of the hydraulic fluid. When the temperature of the hydraulic fluid rises due to mechanical work or environmental conditions, the fluid can expand, leading to increased pressure within the system.

The thermal relief valve is set to open at a pressure that is lower than the set pressure of the system relief valve. This differentiation is essential because the system relief valve is intended to prevent catastrophic failure by venting excess pressure from the system, while the thermal relief valve serves as an additional safety mechanism to manage pressure due to fluid expansion.

Setting the thermal relief valve to open at a lower pressure ensures it functions effectively to relieve pressure before it reaches the critical level where the system relief valve would need to act. By doing so, the thermal relief valve maintains a safer operating environment and ensures the system remains within safe pressure limits even during temperature increases.

As a result, the correct answer is that thermal relief valves are generally set to open at a lower pressure than the system relief valve to properly handle thermal expansion and protect the integrity of the hydraulic system.