Which valve types require a drain line?

The correct response pertains to the pressure reducing valve and the direct operated relief valve because both of these valve types require a drain line for proper operation and safety.

A pressure reducing valve is designed to maintain a set downstream pressure that is lower than the inlet pressure. During its operation, any excess pressure that occurs must be discharged to prevent overpressurization in the system. This discharge can lead to unwanted fluid in the system, hence requiring a drain line to safely route this fluid away.

Similarly, a direct-operated relief valve is there to protect the hydraulic system from excess pressure. When the system pressure reaches a predetermined level, this valve opens to allow fluid to escape, ensuring that pressure doesn't rise dangerously high. Again, a drain line is necessary to handle the fluid discharged by this valve, directing it away safely from the system.

The sequence valve, while crucial in hydraulic circuits for controlling the order of operations, does not inherently require a drain line in standard applications. Its role is to allow connections to manage multiple actuators or functions based on pressure conditions but doesn’t directly deal with managing excess or discharge fluid in the same manner as the other two valves.

Thus, understanding the function of each valve highlights why the pressure reducing valve and the direct operated relief valve specifically