Page 100 - Valve Selection Handbook

Manual Valves 87

wedge is carried on valve opening initially on the seat until the fluid load
has become small enough for the body ribs to carry the wedge. 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 begin 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 sec-
tion of the wedge may be able to tilt the wedge into the downstream seat
bore. This support requirement 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 consid-
erable 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 forming the T-slot may crack.
For critical applications, guides in wedge gate valves are machined 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 machined guide ribs welded to the valve body.
The wedge is permitted in 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 in
machined grooves of the valve body. When wear has taken place in the
guides, the original guide tolerance can be restored by adjusting the
thickness of a spacer ring between the two wedge plates.
Valve Bypass

Wedge gate valves may have to be provided with bypass connections
for the same reason described for parallel gate valves on page 77.

Pressure-Equalizing Connection

In the case of wedge gate valves with a self-aligning double-seated
wedge, thermal expansion of a fluid locked in the valve body will force