Abstract: A conduit closure apparatus comprising a main frame adapted for connection to a conduit in a position generally in registry with the conduit, a movable blade slidably mounted in the frame, means for moving the blade back and forth between an open position in which the blade is outside the conduit and a closed position in which the blade blocks the conduit, a slot-like port through which the blade slides when it is moved between its open and closed positions, and an inflatable sealing ring that is operable, when inflated, to press against the face of the blade at the blade's periphery when the blade is in the closed position, thereby creating a seal between the blade and the frame, is improved by having the main frame define a channel which holds the edges of the blade when in the closed position, and having the inflatable ring being carried by a second frame, smaller than the main frame, which slidably fits in the channel of the main frane and which can be withdrawn from the main frame through the blade port