TYPES OF VALVESDOE-HDBK-1018/2-93ValvesSplitWedgeFigure 7 Split Wedge Gate ValveSplit wedge gate valves, as shown in Figure 7, are of theball and socket design. These are self-adjusting and self-aligning to both seating surfaces. The disk is free toadjust itself to the seating surface if one-half of the diskis slightly out of alignment because of foreign matterlodged between the disk half and the seat ring. Thistype of wedge is suitable for handling noncondensinggases and liquids at normal temperatures, particularlycorrosive liquids. Freedom of movement of the disk inthe carrier prevents binding even though the valve mayhave been closed when hot and later contracted due tocooling. This type of valve should be installed with thestem in the vertical position.ParallelDiskThe parallel disk gate valve illustrated in Figure 8 isdesigned to prevent valve binding due to thermaltransients. This design is used in both low and highpressure applications.The wedge surfaces between the parallel face disk halves are caused to press togetherunder stem thrust and spread apart the disks to seal against the seats. The taperedwedges may be part of the disk halves or they may be separate elements. The lowerwedge may bottom out on a rib at the valve bottom so that the stem can develop seatingforce. In one version, the wedge contact surfaces are curved to keep the point of contactclose to the optimum.In other parallel disk gates, the two halves do not move apart under wedge action.Instead, the upstream pressure holds the downstream disk against the seat. A carrier ringlifts the disks, and a spring or springs hold the disks apart and seated when there is noupstream pressure.Another parallel gate disk design provides for sealing only one port. In these designs,the high pressure side pushes the disk open (relieving the disk) on the high pressure side,but forces the disk closed on the low pressure side. With such designs, the amount ofseat leakage tends to decrease as differential pressure across the seat increases. Thesevalves will usually have a flow direction marking which will show which side is the highpressure (relieving) side. Care should be taken to ensure that these valves are notinstalled backwards in the system.ME-04Rev. 0Page 12
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