Rotatable valve assembly includes a mounting mechanism for rotatably mounting the valve in a housing, the mounting mechanism including a shaft having an outside end extending through the housing. A conversion mechanism converts fluid pressure in the housing into torque exerted on the shaft. A release mechanism is located outside the housing for preventing rotation of the shaft and valve when the torque exerted on the shaft is below a selected magnitude. The release mechanism allows the shaft to rotate when the torque exerted on the shaft exceeds a selected magnitude. A seal is provided for sealing the gap between the housing and the valve in the closed position. The seal has a first portion secured to the housing on a first side of the rotational axis of the valve, a second portion secured to the valve on a second side of the rotational axis, a first transitional portion coinciding with the rotational axis, and a second transitional portion coinciding with the rotational axis. The seal is unsecured to the valve or the housing and the transitional portions so that the seal may flex in the transitional portions as the valve rotates between the closed position and the open position. The first and second portions of the seal define a seal plane in the closed position of the valve and the second portion of the seal moves out of the seal plane with the valve when the valve is moved out of the closed position.