Abstract: A process is disclosed for controlling movement of a vertically movable plug valve with respect to a bottom open end of a riser conduit of a catalytic conversion apparatus during startup and shutdown. The method involves sensing the pressure between the communicating surfaces of the plug valve and the riser conduit as the riser conduit expands or contracts due to thermal changes experienced during startup and shutdown. When the riser conduit expands and causes the pressure to exceed a preselected range, the position of the plug valve with respect to the riser conduit is lowered to reduce the pressure to within the range. When the riser conduit contracts and causes the pressure to fall below the preselected range, the position of the plug valve with respect to the riser conduit is raised to prevent catalyst flow through the valve.