Page 92 - Valve Selection Handbook

Manual Valves 79

Standards Pertaining to Parallel Gate Valves

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

Applications

Duty:
Stopping and starting flow
Infrequent operation
Service:
Gases
Liquids
Fluids with solids in suspension
Knife gate valve for slurries, fibers, powders, and granules
Vacuum
Cryogenic

WEDGE GATE VALVES

Wedge gate valves differ from parallel gate valves in that the closure
member is wedge-shaped instead of parallel, as shown in Figure 3-34
through Figure 3-45. The purpose of the wedge shape is to introduce a
high supplementary seating load that enables metal-seated wedge gate
valves to seal not only against high, but also low, fluid pressures. The
degree of seat tightness that can be achieved with metal-seated wedge
gate valves is therefore potentially higher than with conventional metal-
seated parallel gate valves. However, the upstream seating load is not
normally high enough to permit block-and-bleed operation of metal-to-
metal seated wedge gate valves.
The bodies of these valves carry guide ribs, or slots, in which the disc
travels. The main purpose of these guides is to carry the wedge away
from the downstream seat except for some distance near the closed valve
position so as to minimize wear between the seatings. A second purpose
of the guides is to prevent the disc from rotating excessively while travel-
ling between the open and closed valve positions. If some rotation
occurs, the disc will initially jam on one side between the body seatings
and then rotate into the correct position before travelling into the final
seating position.