The Type D pressure sustaining/surplus valve is designed to sustain and control the upstream pressure and pass surplus mediums into a zone where they can perform useful work or be recovered and conserved.

The type D has a larger piston area for the pressure to act upon, compared to a standard relief valve lid. This ensures a smoother operation, which is required due to the fact a sustaining valve is designed for constant use, unlike a relief valve.

As both the valve seat and small piston area of the spindle are of equal areas, any fluctuation in the back pressure does not affect the set relieving pressure.