Pressure reducing valves, normally open pressure relief devices, throttle part of a circuit to a predetermined level while allowing the inlet pressure to rise to full system pressure if required. They are available in two configurations: direct acting or pilot operated, with the additional option of an integral relief valve to limit the controlled pressure against external forces.

 

Pressure relief valves, on the other hand, are pressure limiting mechanisms. They are activated as soon as the inlet pressure exceeds the preset pressure of the valve, which is determined by the counteraction of the pressure against a spring. This activation point is known as the opening pressure and the spring tension can be changed to achieve the desired pressure threshold. Pressure relief valves are available in various designs, each of which fulfils a different purpose.