Page 155 - Valve Selection Handbook

142 Valve Selection Handbook

retain lubricants and contaminants and therefore display an increased
tendency to gall when in sliding contact. For this reason, the seating
faces of stainless steel valves are usually very finely machined rather
than polished.
If a high galling tendency is expected, as when handling dry gases, one
of the seating faces may be faced with stellite, which is known to provide
good resistance to a wide range of corrosives.
Seizing of the valve stem in the yoke bush is commonly avoided by
choosing dissimilar materials for the yoke bush and the stem. A material
frequently used for the bush in stainless steel valves is Ni-Resist ductile
iron D2, which provides complete freedom from galling due to the
graphite in its structure. If the bush is made of stainless steel, the free-
machining grade type 303 provides remarkable freedom from galling in
conjunction with stainless steel grades type 304 and 316 for the stem.

Light-Weight Valve Constructions

Efforts in the United States to reduce the cost of stainless steel valves
led to the development of standards for 150 Ib light-weight stainless steel
valves. The pressure ratings specified in these standards apply only to
valves made from austenitic materials.
The flanges to these standards are thinner than the corresponding full-
rating carbon steel flanges and have plain flat faces. Many users object to
the light-weight flanges and request full-rating flanges with a raised face.
The light-weight bodies are, of course, more flexible than the bodies
of full-rating carbon steel valves. This is particularly important for gate
valves, in which body movements can unseat the disc. Experience has
shown that plain solid wedges may be used only for sizes up to DN 100
(NFS 4). Larger valves are satisfactory only if the wedge is of the self-
aligning type.
Standards Pertaining to Stainless Steel Valves

Appendix C provides a list of standards pertaining to stainless steel
valves.