A relief valve has been designed to limit the differential pressure between two fluids that are not allowed to be mixed. This valve offers advantages over both simple check valves and simple relief valves. A simple relief valve vents a fluid at the preset absolute relief pressure, but offers no means for limiting the differential pressure between two fluids. A simple check valve can be used to limit the pressure between two fluids, but when the valve opens to relieve differential pressure, one fluid flows into the other one; thus, a simple check valve is not suitable for a system in which the two fluids must not be mixed.

alt
This Differential-Pressure Relief Valve vents the fluid that enters the lowermost port when its pressure exceeds the reference pressure by a preset amount. There is no mixing of the higher-pressure and lower-pressure fluids.

The present valve (see figure) includes two pistons, one of which is spring-loaded and slides in response to the balance between the differential pressure and spring forces. The other piston rests against a hard stop provided by a socket-head cap screw, which is used to adjust the spring load and thus the differential-pressure relief setting. O-ring seals on the pistons prevent mixing and unintended venting of the fluids.

The lower-pressure fluid is deemed to be at the reference pressure and is connected via the uppermost (in the figure) port. The higher-pressure fluid is connected via the lowermost port. When the higher pressure exceeds the reference pressure by the preset amount, the piston moves upward, uncovering a vent hole and thus allowing excess higher-pressure fluid to escape without mixing with the lower-pressure fluid.

This work was done by Joseph Robert Trombley of Johnson Space Center and James Dwight Baker and James Everett Bryan of McDonnell Douglas Corp. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category. MSC-22607