Abstract: The invention provides a closure suitable for use on pipelines and pressure vessels, and which can resist high pressure but which can be opened quickly when the pressure differential across the closure has been reduced to zero. The closure comprises a door (3) which fits into an aperture of a hub (1) in a member (2) to be closed. A number of arcuate locking members or segment blocks (30) disposed around the periphery of the door for locking the door (3) to the hub (1). The locking members (30) are constrained by guide pins (34) to move along guide slots (40) in the door. A drive plate (101) is rotatably mounted on the door (3) and includes a plurality of drive slots (122) which engage with the guide pins (34).