A trunnion type ball valve having valve seats that are laterally moveable within seat recesses of the valve and establish sealing engagement with a valve ball member that is trunnion supported for rotational movement between open and closed positions. The valve seat members are self-centering for optimum sealing engagement with the spherical sealing surface of the ball member. The internal geometry of the seat assemblies are designed to permit optimum flow through the valve mechanism even under circumstances where the seat members are located in laterally off-center relation with respect to the centerline of the flow passages due to self-centering seat movement. The ball valve is readily adapted for fire-safe application via the use of heat resistant seals and metal-to-metal sealing of components to prevent or minimize leakage in the event of seal destruction or degradation by excess heat.