Page 71 - Valve Selection Handbook

58 Valve Selection Handbook

Flat seatings (see Figure 3-15a) have the advantage over other types of
seatings in that they align readily to each other without having to rely on
close guiding of the disc. Also, if the disc is moved onto the seat without
being rotated, the seatings mate without friction. The resistance of the
seating material to galling is therefore unimportant in this case. Deforma-
tion of the roundness of the seat due to pipeline stresses does not interfere
with the scalability of the seatings as long as the seat face remains flat. If
flow is directed from above the seat, the seating faces are protected from
the direct impact of solids or liquid droplets travelling in the fluid.
By tapering the seatings, as shown in Figure 3-15b, c, and d, the seat-
ing stress for a given seating load can be greatly increased. However, the
seating load can be translated into higher uniform seating stress only if
the seatings are perfectly mated; that is, they must not be mated with the
disc in a cocked position. Thus, tapered discs must be properly guided
into the seat. Also, the faces of seat and disc must be perfectly round.
Such roundness is sometimes difficult to maintain in larger valves where
pipeline stresses may be able to distort the seat roundness. Furthermore,
as the seatings are tightened, the disc moves further into the seat. Tapered
seatings therefore tighten under friction even if the disc is lowered into
the seat without being rotated. Thus the construction material for seat and
disc must be resistant to galling in this case.
The tapered seatings shown in Figure 3-15b have a narrow contact
face, so the seating stress is particularly high for a given seating load.
However, the narrow seat face is not capable of guiding the disc squarely
into the seat to achieve maximum sealing performance. But if the disc is
properly guided, such seatings can achieve an extremely high degree of
fluid tightness. On the debit side, narrow-faced seatings are more readily
damaged by solids or liquid droplets than wide-faced seatings, so they
are used mainly for gases free of solids and liquid droplets.
To improve the robustness of tapered seatings without sacrificing seat-
ing stress, the seatings shown in Figure 3-15c are tapered and provided
with wide faces, which more readily guide the disc into the seat. To
achieve a high seating stress, the seat face in initial contact with the disc
1
is relatively narrow, about 3mm ( A in.) wide. The remainder of the seat-
bore is tapered slightly steeper. As the seating load increases, the disc
slips deeper into the seat, thereby increasing the seating width. Seatings
designed in this way are not as readily damaged by erosion as the seat-
ings in Figure 3-15b. In addition, the long taper of the disc improves the
throttling characteristic of the valve.