Which type of pressure control valves require external drains?

Pressure control valves that require external drains are primarily the sequence and pressure reducing valves. These types of valves are designed to regulate pressure within a hydraulic system and often need to vent excess fluid or relieve pressure that exceeds certain thresholds. The external drain allows for the safe dispersion of fluid back to the reservoir or another designated point in the system, preventing pressure build-up that could cause damage or unsafe conditions.

In contrast, check valves are designed to allow fluid flow in only one direction and do not require drainage since they do not have a relief or regulating function. Relief valves automatically release pressure when a set level is exceeded but typically operate without needing an external drain unless specified for particular applications. Flow control valves manage the speed of actuators but also do not necessitate external drains as part of their function. Thus, sequence and pressure reducing valves are distinct in their requirement for external drains to maintain proper system pressure and operation.