A housing, connectable to a pipe, has therein an opening, a chamber, a stationary sealing seat, and a movable sealing seat displaceable toward and away from the stationary sealing seat. A slide plate is movable from an open position, exterior of the housing, through the opening to a closed position between the sealing seats to block the pipe. A closing element closes the opening and seals the chamber when the slide plate is in the open position.