Abstract: A closure member for a pipe or pressure vessel, comprising a door adapted to fit an aperture in the body of the pipe or pressure vessel, a plurality of blocks slidably mounted adjacent the periphery of the door and movable from a position at least substantially wholly within the said periphery to a position in which a part of each block protrudes beyond the periphery for engagement with the said body, the said parts in the blocks together forming a protrusion extending substantially entirely round the periphery of the door, and means for moving the blocks from the withdrawn to the protruding position.