Abstract: In a ball valve, springs urging opposed annular valve seats into engagement with the ball press against heat sensitive material, and bushes between trunnions and sockets in the ball are made of heat sensitive material, so that, in the event of fire reaching a ball valve in a pipeline, and the valve being closed, said heat sensitive material and said bushes disintegrate, and the ball is moved by fluid pressure relative to the trunnions in the downstream direction and pushes the downstream valve seat into sealing engagement with the valve housing.