Hydrocarbon Processing - June 2022 - 50

Maintenance and Reliability
the materials group with the sample of
the damaged piping. Numerous microfissures
were found (FIG. 8).
Additionally, as hydrogen atoms are
located in a high-stress area, hydrogen
attack occurs easily because it diffuses
well. So, a heat affected zone (HAZ) is
weaker against HTHA than base metal
or weld metal, and decarburized areas
and fissuring are often found there. This
explains why the line was sharply cut at
the points of the piping's HAZ.
Solution. The refinery owner claimed
inappropriate piping design and an incorrect
piping spec break against the engineering
company. Consequently, the
damaged piping and 10 m of the recycle
oil line upstream of the TV were planned
to be replaced with stainless steel material.
The installation of check valves
to prevent reverse hydrogen flow was
also considered.
Lessons learned. To clearly identify
the damage mechanism of the corrosion
incident, cooperation among concerned
teams (operations, process technical
services, inspection, R&D, etc.) is vital.
Additionally, corrosion and material experts
with sound knowledge and experience
of various aspects of corrosion, as
well as inspection tools and methodologies,
are required. For process engineers,
a spec break review of piping and equipment
is critical by those who are aware
of process stream operating conditions
and stream components. After a joint
review with corrosion engineers, the
proper materials for piping and equipment
can be selected.
CASE 3: REPEATED
CORROSION IN THE
PIPING OF THE AMINE
REGENERATION UNIT (ARU)
An amine system removes H2
carbon dioxide (CO2
units (SRUs) and lean amine solution is
returned to the amine absorber.
Problem. In an amine unit, operation
and maintenance personnel noticed five
leaks in one year in the piping circuit-
oddly, the leaks occurred only in the rich
amine line between the lean/rich amine
heat exchanger and the amine regenerator
(within the dotted red line in FIG. 9).
The piping material was carbon steel
while the tube material of the amine heat
exchanger was stainless steel.
S and
) from various
fuel gases produced in a refinery to prevent
air pollution when the gases are
burned. Amine plants consist of two
units (FIG. 9): the absorber in which the
unwanted gas constituent is removed,
and a regenerator in which the amine
solution is reactivated. In the absorber,
gas flows up through the column and
the amine cascades down, providing intimate
contact. Sweet gas exits the top,
and rich amine leaves through the bottom.
In the regenerator, H2
S and CO2
FIG. 11. The piping layout and leak points
around the caustic tank.
are expelled by heat and stripping steam.
The acid gases go to sulfur recovery
Cause analysis. To determine the
cause of the leaks, an inspection team
investigated the inside of the piping after
cutting the line. Internally, severe pitting
corrosion with uniform thickness
loss was found in the rich amine piping
(FIG. 10). This is a typical type of amine
corrosion. In this case, the proper material
is austenitic stainless steel or duplex
stainless steel; the corroded piping was
made with carbon steel, which is weaker.
Furthermore, a temperature rise and
the current speed are accelerating factors
for this type of corrosion, particularly
when turbulence is created by acid
gas (H2
S and CO2) flashing in rich amine
due to a temperature rise (from 52°C to
100°C) through the tube side of the lean/
rich amine heat exchanger and a pressure
drop across the flow control valve at the
inlet of the amine regenerator. Erosion
corrosion can be also shown in this area.
As a reference check, the material of rich
amine in other amine units was investigated.
The rich amine piping downstream of
the heat exchangers in other amine units
was made with stainless steel material. So,
the investigation teams concluded that
this repeated corrosion occurred due to
incorrect material selection.
Solution. The rich amine piping after
the lean/rich amine heat exchanger was
replaced with stainless steel material.
FIG. 12. Sample analysis of the damaged caustic line.
50 JUNE 2022 | HydrocarbonProcessing.com
Lessons learned. This amine unit had
been designed by a local engineering
company that had insufficient experience
with amine processes. The owners
or engineering companies should have
consulted with major license companies
or major engineering companies. Referencing
piping and instrumentation diagrams
(P&IDs) for other amine units is
also an effective and preventive measure.
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