lower area the guide and bearing pipe 13 possesses a vertical slot 17 which is parallel to the axis and at its upper end an obliquely rising circumferentially disposed slot 18 a projection 19 a guide roller for instance is fastened internally to the bearing bushing 14 and engages in this slot 17 18 thus when the pressure medium drive 10 moves downwardly the guide and bearing pipe 13 also moves downwardly together with the bracket 11 and the closure body 8 supported thereupon also the projection 19 moves in the vertical slot 17 which is parallel to the axis and at first secures against rotational motion the guide and bearing pipe 13 however once the downwardly moving projection 19 reaches oblique slot 18 it rotates or swivels the guide and bearing pipe 13 around axis 15 so that the closure body 8 initially simultaneously moves downwardly and then after completion of the downward movement swivels from its position underlying the aperture 7 to a displaced position shown in fig2 this position completely opens the bottom aperture 7 this open position can be seen in fig2 where the swivelled and lowered closure body 8 is shown in dotted lines as indicated at 8 39 in the open position of closure body 8 the.