Page 99 - Valve Selection Handbook

86 Valve Selection Handbook

bottom, sides, and the top lateral faces. When the valve is being closed,
the rubber rings seal against the bottom and the sidewalls of the valve
body and, by a wedging action, against the seat faces at the top.
The wedge of the valve shown in Figure 3-45 is completely rubber
lined and forms part of a diaphragm, which separates the operating
mechanism from the flowing fluid. When the valve is being closed, the
bottom of the wedge seals against the bottom of the valve body and the
body seats. The valve may also be lined with corrosion-resistant materi-
als, and is therefore widely used in the chemical industry.

Connection of Wedge to Stem

The wedge-to-stem connection usually consists of a T-slot in the top of
the wedge, which receives a collar on the stem. According to API stan-
dard 600, this connection must be stronger than the weakest stem section
so that the wedge cannot become detached from the stem while operating
the valve.
The T-slot in the wedge may thereby be oriented in line with the flow
passage, as in the valves shown in Figure 3-38, Figure 3-39, and Figure
3-41, or across the flow passage as shown in Figure 3-36 and Figure 3-
42. The latter construction permits a more compact valve body design
and, therefore, has become popular for economic reasons. Also, this con-
struction favorably lowers the point at which the stem acts on the wedge.
However, the T-slot must be wide enough to accommodate the play of
the wedge in its guide, allowing also for wear of the guide.
There are also exceptions to this mode of wedge-to-stem connection,
as in the valves shown in Figure 3-34 and Figure 3-37, in which the stem
must carry the entire thrust on the wedge. For this reason, this construc-
tion is suitable for low pressure applications only. In the valve shown in
Figure 3-43, guide play is virtually absent, allowing the stem to be cap-
tured in the wedge.

Wedge Guide Design

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 machined for reason of low cost con-
struction. However, the rough surface finish of such guides is not suited
for carrying the travelling wedge under high load. For this reason, the