Shut=off slider with two=part shut=off plate insertable between two sealing seats - has ventilation elements each with two independently operating pressurised medium circuits used to raise plate parts from seats

Each ventilation element has a reserve pressurised medium circuit which replaces the normal circuit should the latter fail. Two compressed air pipes (17,18) open into a ring chamber (16) between the outer and inner cylinders with their openings (19,20) axially spaced from each other. Between these openings there is an axially displaceable ring piston (21) set inside the chamber (16) and biased with pressurised medium either through the opening (19) in normal operation or through the other opening (20) with the reserve operation. The ring piston can be sealed from the outer and inner cylinders (11, 13) by ring seals (33, 34). A further ring seal (35, 36) for sealing the ring chamber from the atmosphere can be set between the two ring collars (12, 14) on one side and the outer and inner cylinder on the other. USE/ADVANTAGE - Reliable function of shut-off slider using only relatively low number of ventilation elements.