Page 101 - Valve Selection Handbook

88 Valve Selection Handbook

the upstream and downstream seatings into still closer contact and cause
the pressure in the valve body cavity to rise. A similar situation may arise
with soft-seated wedge gate valves in which the wedge is capable of pro-
ducing an upstream seat seal. Thus, if such valves handle a liquid with
high thermal expansion, or if revaporation of trapped condensate can
occur, they may have to be provided with a pressure-equalizing connec-
tion as described for parallel gate valves on page 77.
Case Stud/ of Wedge Gate Valve Failure

Figure 3-46 through Figure 3-48 show components of a DN 300 (NFS
12) class 150 wedge gate valve to API standard 600 that failed on first
application.
The valve was mounted in a horizontal line under an angle of 45° from
the vertical for ease when hand-operating a gear drive. This operating
position required the wedge to ride on the body rib. Unfortunately, the
guide slot in the wedge had sharp edges. As the wedge traveled on the
rib, the sharp edges of the guide slot caught on the rough rib surface,
causing the wedge to rotate until contacting the opposite body rib. At this
stage, the wedge was jammed. Further closing effort by the valve opera-
tor produced the damage to the valve internals shown in Figure 3-46
through Figure 3-48.
Figure 3-46 shows the damage to the wedge guide that was riding on
the body rib and Figure 3-47 shows the damaged body rib. Figure 3-48
shows the bent valve stem and damage to the stem surface around the
stem guide bush.
Further inspection of the valve showed also that play in the wedge
guides was larger than the possible travel of the wedge on the stem.
Thus, the stem had to carry the wedge for part of its travel in a tilted
position. Furthermore, the lengths of body rib and wedge guide were far
too short to adequately support the wedge during all stages of travel.
This valve failure was not isolated but was typical for a high percent-
age of all installed wedge gate valves. Finally, all suspect valves had to
be replaced prior to start-up of the plant.

Standards Pertaining to Wedge Gate Valves

Appendix C provides a list of U.S. and British standards pertaining to
wedge gate valves.