Rotary selector valve is a multi-port valve usually provided with 2-8 ports. This valve can be used as quiet replacement for solenoid valves. It internally supports a rotatable trunnion having an elbow passage between a nozzle portion of the trunnion and a lower end outlet that communicates with a test port of the valve body. The outer end of the nozzle has an axially reciprocable hollow seal piston with a seal support ring whose outer face is formed with an endless groove to receive an O-ring. It can control the communication of a plurality of conduits with an outlet opening into a chamber of a stationary valve casing. It can be used in installations for the detection of fuel-element-casing failure or whenever variable communications between a certain numbers of inlet passageways traversed by a fluid and one or more outlet passageways are desired.