Page 517 - Corrosion Engineering Principles and Practice

484 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 485

Condenser

Reflux

Skip weld
Column wall
External
stiffening
A A ring
Feed Crevice where external
SCC agents can
accumulate
Section A–A
Process-side cracking
External cracking
Tails Both sides cracking
FIGURE 12.6 Typical cracking locations in distillation columns [7].

Column packing is also especially susceptible to SCC. Expanded
metal packing or mesh contain a great deal of cold-work and very
high stress levels, rendering it particularly susceptible to SCC.
Although both types of packing can be stress-relieved, alone or with
the column itself, a preferred method is to manufacture them from
crack resistant alloys.
Other areas of corrosion in columns are trays, especially valve
trays and bubble caps which may have high residual stress are also
primary areas for environmental cracking. Tray posts, clips, fasteners,
and other hardware may be highly stressed and susceptible to SCC.
Sieve trays are usually manufactured by cold-punching, and bubble
caps and valves are cold-formed. The latter particularly, unless stress-
relieved or of resistant-alloy construction, may lose their restraining
legs and fall off the trays, leaving only an extremely large “sieve” tray
of inefficient characteristics.

12.4 Key Performance Indicators
The importance of meaningful management information is often
highlighted when serious problems arise. However, it is operationally
a much better practice to prevent these detrimental processes. Key
performance indicators (KPIs) may be used in this context to optimize
the need and sequence of inspection and other maintenance activities.