1352721 Valves APV CO Ltd 11 Aug 1972 [23 Aug 1971] 39429/71 Heading F2V In a fluid flow control device having three flow ducts 12 whose terminal portions 13 are coplanar, a movable plate element 26 supports a U-shaped pipe 27 for connecting a selected pair of ducts 12, the remaining duct being then closed by a member 79 carried by a strut which also serves to support the U-shaped pipe 27, (the plane of the latter is orthogonal to the plane of the drawing). A first power drive in the form of a fluid pressure operated piston 16, acts to move a piston-rod 41 to which the strut is connected, such movement being axial but having a rotary component superimposed thereupon by the interengagement of a cam member carried by the rod and a compound cam track carried by the device housing. A second power drive in the form of a fluid pressure operated piston 66 controls a latch member 64 which when engaged with a recess 61 in the piston-rod 41, acts to maintain the rod in such a position that pipe ends 27 and closure 79 are all firmly engaged with seals 14 circumadjacent ducts 12. Accordingly, piston 16 does not then need to be pressurized. To effect a changeover of the connected ducts 12, piston 16 is first caused to move to the right, as seen in the drawing, whereupon piston 66 is allowed to raise latch member 64. This then permits piston-rod 41 to be moved to the left and hence plate 26 to be moved axially and rotationally to effect this desired changeover. Sensors such as 71 and 72 may detect the position of the various movable members and may also act to control the operation of the power drives.