Wedge Guide Design
[author：topsvalve] [date：10-11-11]
[heat：]
The body guides commonly consist either of ribs, which fit into slots of the wedge, or of slots, which receive ribs of the wedge. Figure 3-42 and Figure 3-43 illustrate these guiding mechanisms.
The body ribs are not normally machmed for reason of low cost construction. However, the rough surface finish of such guides is not suited for carrymg the travellmg wedge under high load. For this reason, the wedge is carried on valve openmg mitially on the seat until the fluid load has become small enough for the body ribs to carry the wedgge. This method of guiding the wedge may require considerable play in the guides, which must be matched, by the play in the T- slot for suspending the wedge on the stem.
Once the body ribs begm to carry the wedge upon valve opening, the wedge must be fully supported by the ribs. If the length of support is insufficient, the force of the flowing fluid acting on the unsupported section of the wedge may be able to tilt the wedge into the downstream seat bore.This support requkement is sometimes not complied with. On the other hand. some valve makers go to any length to ensure full length wedge support.
There is no assurance that the wedge will slide on the stem collar when opening the valve. At this stage of valve operation, there is considerable friction between the contact faces of the T-slot and stem collar, possibly causing the wedge to tilt on the stem as the valve opens. If, in addition, the fit between T-slot and stem collar is tight, and the fluid load on the disc is high, the claws formmg the T- slot may crack.
For critical applications, guides m wedge gate valves are machmed to close tolerances and designed to carry the wedge over nearly the entire valve travel, as in the valves shown in Figure 3-42 and Figure 3-43.
In the valve shown in Figure 3-42, the wedge grooves are hard-faced and precision-guided on machmed guide ribs welded to the valve body.The wedge is permitted m this particular design to be carried by the seat for 5% of the total travel.
in the valve shown in Figure 3-43, the wedge consists of two separate wedge-shaped plates. These carry hard-faced tongues that are guided m machmed grooves of the valve body. When wear has taken place in the guides, the original g;uide tolerance can be restored by adjusting the thickness of a spacer ring between the two wedge plates.