Page 24 - Valve Selection Handbook

Fundamentals 11

VALVE SEATINGS

Valve seatings are the portions of the seat and closure member that
contact each other for closure. Because the seatings are subject to wear
during the making of the seal, the scalability of the seatings tends to
diminish with operation.

Metal Seatings

Metal seatings are prone to deformation by trapped fluids and wear
particles. They are further damaged by corrosion, erosion, and abrasion.
If the wear-particle size is large compared with the size of the surface
irregularities, the surface finish will deteriorate as the seatings wear in.
On the other hand, if the wear-particle size is small compared with the
size of the surface irregularities, a coarse finish tends to improve as the
seatings wear in. The wear-particle size depends not only on the type of
the material and its condition, but also on the lubricity of the fluid and
the contamination of the seatings with corrosion and fluid products, both
of which reduce the wear-particle size.
The seating material must therefore be selected for resistance to ero-
sion, corrosion, and abrasion. If the material fails in one of these require-
ments, it may be completely unsuitable for its duty. For example, corro-
sive action of the fluid greatly accelerates erosion. Similarly, a material
that is highly resistant to erosion and corrosion may fail completely
because of poor galling resistance. On the other hand, the best material
may be too expensive for the class of valve being considered, and a com-
promise may have to be made.
Table 2-1 gives data on the resistance of a variety of seating materials
to erosion by jets of steam. Stainless steel AISI type 410(13 Cr) in heat-
treated form is shown to be particularly impervious to attack from steam
erosion. However, if the fluid lacks lubricity, type 410 stainless steel in
like contact offers only fair resistance to galling unless the mating com-
ponents are of different hardness. For steam and other fluids that lack
lubricity, a combination of type 410 stainless steel and copper-nickel
alloy is frequently used. Stellite, a cobalt-nickel-chromium alloy, has
proved most successful against erosion and galling at elevated tempera-
tures, and against corrosion for a wide range of corrosives.