The Inflatable Seal Mechanical Interlock system for the Roto-Flate spherical valve (photo) enhances reliability and causes the valve to fail closed and inflated in the case of electric or pneumatic loss. The mechanical interlock prevents the inflatable seal from inflating before the valve dome is in the closed position. When utilized with a spring-return pneumatic actuator and normally open seal solenoid, the Roto-Flate valve now closes and inflates upon electrical power loss, regardless of the position of the valve at the time power is lost. The interlock also prevents unintended or premature inflation of the inflatable seal, thereby decreasing risk of damage during startup. For applications where the valve is required to also close on pneumatic loss, the Roto-Flate can be provided with a valve arrangement that will inflate the seal from a reserve air-source, and keep it inflated for extended periods. Applications that require a fail-open or fail-last disposition can also be easily accommodated. — Roto-Disc, Inc., Erlanger, Ky.