Abstract: A valve for sealing a pipe opening with a seal plate movably attached to the pipe opening and a valve body fixed to the pipe opening. Moving the seal plate towards the pipe opening closes the valve, and moving the seal plate away from the pipe opening opens the valve. The valve body includes a plate stop to limit the movement of the seal plate relative to the pipe opening. For instance, the seal plate, when forced fully away from the pipe opening, is forced to define an oblique plane that is oblique to the plane of the pipe opening. A method of sealing and then unsealing a pipe opening incorporating such a seal plate is also included.