Hydrocarbon Processing - June 2022 - 51

Maintenance and Reliability
CASE 4: REPEATED PIPING
CRACKS OF CAUSTIC
MAKEUP LINES DUE TO
CAUSTIC STRESS CORROSION
CRACKING (SCC)
Caustic (NaOH) is used widely in
refining and petrochemical units for the
neutralization of acid constituent and pH
control of cooling water or boiler feed water.
However, it can cause SCC to carbon
steel welds, according to the concentration
of caustic and temperature (refer to
API RP571: NACE caustic soda chart).
Stress-relieved carbon steel or nickel alloys
can be more resistant to caustic SCC.
Problem. At the makeup line of fresh
caustic to the caustic tank in the alkylation
unit, five leaks occurred over a period
of 3 mos (FIG. 11). The service of the 3-in.
piping was 25 wt% fresh caustic and it was
installed with steam tracing. This caustic
was used for neutralization at the caustic
wash drum and for acid gas scrubbing
at the blowdown vapor scrubber in the
alkylation unit.
Cause analysis. An inspection team
collected the piping sample at a fifth leak
welding part. A crack was visually detected
at the circumferential direction of the
welding part and several white stains were
noticed on the surface of piping due to
caustic leak (FIG. 12, left top). Through a
radiography test (RT), it was determined
that the crack penetrated the weld metal
beginning at the weld toe (FIG. 12, left bottom);
its microstructure was investigated
after cutting the sample piping.
Intergranular cracking was observed
along grain boundaries (FIG. 12, right).
Intergranular stress-corrosion cracking
is typically shown in the alkaline SCC,
such as caustic SCC and amine SCC.
Intermittent caustic supply to the tank
was required about once a week, while
steam tracing on the piping was under
continuous service. The actual operating
temperature of the makeup line
fluctuated 30°C-75°C, according to the
temperature gauge, while the normal
temperature was shown as 38°C on the
document. At 38°C, caustic SCC cannot
exist; therefore, it was possible that the
piping temperature rose due to continuous
heat flux from steam tracing, causing
the increase of concentration of the
remaining caustic solution in the piping.
The temperature to cause caustic
SCC in carbon steel is 50°C-80°C or
more, according to caustic concentration
(refer to the API RP571). Accordingly,
the welded part of carbon steel
that is exposed to this temperature range
must be executed post-weld heat treatment
(PWHT, stress-relieving) to avoid
SCC. Through a hardness check of the
sample, stress-relieving on some parts
seemed improperly performed. By the
above mentioned analysis results, the inspection
team reported that the frequent
leaks happened due to caustic SCC.
Solution. To fundamentally
prevent
caustic SCC, a material upgrade to " Ni
alloy " could be considered. Alternatively,
precise temperature control of the caustic
line through electric tracing rather than
steam tracing could be selected. Both
improvements are costly. The technical
staff checked the freezing point of the
25 wt% caustic (FIG. 13) and compared
it (-20°C) with the regional lowest temperature
in the city (-8.5°C ). It was not
possible that the caustic solution would
reach freezing point in that area. Finally,
they eliminated stream tracing from the
caustic makeup line and have not experienced
this type of piping damage since.
Lessons learned. By carefully reviewing
process conditions and chemical
properties, optimum solutions to corrosion
issues can be found. Alternative
countermeasures can be considered, but
engineers should also consider and compare
their costs and effectiveness.
Takeaways. This article has highlighted
two key lessons for engineers:
* Close cooperation between
corrosion engineers and process
and design engineers is essential
to determine the best and most
economical solution for a corrosion
and material issue.3
The process
engineer should provide exact
process conditions and operating
concepts, including normal
operations and abnormal cases
(shutdown, startup, turndown
and emergency), for each area of
concern. This is a starting point
for trouble-shooting.
* All discipline engineers (process,
mechanical, piping, inspection,
materials, etc.) should ensure
GI WON PARK works in the
upstream division of Kuwait Oil Co.
in Kuwait. He has 26 yr of experience
in process design and operation
and holds two U.S. patents and
one international patent in process
technologies. He has worked on
various projects and revamps, including three billiondollar
projects for grassroots refining (SK Energy) and
petrochemical plants as a Senior Process Engineer
and Engineering Team Leader. As a corrosion engineer,
he is also involved in corrosion failure investigations
and reliability improvement task forces. Mr. Park holds
a BS degree in chemical engineering.
Hydrocarbon Processing | JUNE 2022 51
1
2
3
FIG. 13. Freezing point curve for caustic soda
(NaOH) solutions.
consistency of design documents
at each step, such as basic design
by license companies, front-end
engineering and detailed engineering
by engineering companies, and
vendor design. Careful review is
required on material, operating
and design conditions, stream
compositions (including corrosives),
piping spec breaks, etc. Reference
document checks, such as P&IDs
and equipment drawings from
other companies, are helpful if no
similar process or equipment is in
use in existing plants. Equipment
inspection must be performed
by owner-side engineers at the
construction stage; they are
responsible for a new process or
equipment as an asset owner.
LITERATURE CITED
American Iron & Steel Institute, " The role of stainless
steels in petroleum refining, " April 1977.
UOP, " Corrosion and fouling management course, "
Des Plaines, Illinois, June 1995.
Fontana, M., Corrosion engineering, 3rd Ed., McGrawHill
Companies, 1986.
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