Page 154 - Valve Selection Handbook

Manual Valves 141

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STAINLESS STEEL VALVES -

Corrosion-Resistant Alloys

The least corrosion-resistant alloy is normally thought of as steel AISI
type 304 (18 Cr, 10 Ni). Stainless steel AISI type 316 (18 Cr, 12 Ni, 2.5
Mo) has a wider range of corrosion-resistance than type 304, and valve
makers often endeavor to standardize on type 316 as the least corrosion-
resistant alloy. If the valve is to be welded into the pipeline, the low car-
bon grades (less than 0.3% carbon) are better than stabilized grades.
Flanged valves require welding only when a casting defect has to be
repaired. Because the repair is done prior to the 1100°C (2000°F) water
quench solution anneal, standard carbon grades are quite satisfactory for
flanged valves.

Crevice Corrosion

Practically all corrosion-resistant alloys are susceptible to crevice cor-
rosion. Good valve designs therefore avoid threading any component that
comes in contact with the corrosive fluid. For this reason, valve seats are
normally made an integral part of the valve body. An exception is body
designs in which the seat is clamped between two body halves, as in the
valve shown in Figure 3-12. However, the gaskets between the seat and
the body halves must be cleanly cut to avoid crevices.
If the valve is to be screwed into the pipeline, seal welding will
improve the performance of the screwed connection. Alternatively,
thread sealants, which harden after application, are helpful in combating
crevice corrosion in threaded joints. Flanged facings, which incorporate
crevices such as tongue and groove, should be avoided.
The points of porosity in the valve body that are exposed to the corro-
sive fluid can likewise produce crevice corrosion. The body may thereby
corrode through at the point of porosity and produce gross leakage, while
the remainder of the body stays in good condition.
Galling of Valve Parts

Published information usually shows that stainless steel in sliding con-
tact, particularly austenitic grades of like compositions, are susceptible to
galling. This galling tendency diminishes considerably if the fluid has
good lubricity and the seating surfaces can retain the lubricants and pro-
tective contaminants. Polished surfaces have only a limited ability to