Page 84 - Valve Selection Handbook

Manual Valves 71

Figure 3-25. Parallel Slide Gate Valve
with Converging-Diverging Flow
Passage ana Follower Eyepiece.
(Courtesy of Hopkinsons Limited.)

the seat bore, so heavy wear may occur in the seat bore and on the outer
edge of the disc. To keep the seating stress and corresponding seating
wear within acceptable limits, the width of the seatings must be made
appropriately wide. Although this requirement is paradoxical in that the
seating width must be small enough to achieve a high seating stress but
wide enough to keep seating wear within acceptable limits, the fluid
tightness that is achieved by these valves satisfies the leakage criterion of
the steam class, provided the fluid pressure is not too low.
Parallel slide gate valves have other excellent advantages: the seatings
are virtually self-aligning and the seat seal is not impaired by thermal
movements of the valve body. Also, when the valve has been closed in
the cold condition, thermal extension of the stem cannot overload the
seatings. Furthermore, when the valve is being closed, a high accuracy in
the positioning of the discs is not necessary, thus an electric drive for the
valve can be travel limited. Because an electric drive of this type is both
economical and reliable, parallel slide gate valves are often preferred as
block valves in larger power stations for this reason alone. Of course,