Hydrocarbon Processing - June 2022 - 49

Maintenance and Reliability
lower boiling point materials. The second
stage is employed to further crack
the first-stage effluent into
the desired boiling range
products. Reactor effluent
passes through feed/effluent
exchangers and product
condensers into a high-pressure
separator where hydrogen
recycle is taken out. The
net liquid is then flashed at
a lower pressure and temperature
before being debutanized
and fractionated into the desired
products.2
The typical process schematic
is shown in FIG. 4.
Problem. In one incident, the recycle oil
piping (8 in., 15 m) of the hydrocracking
process was ruptured and caused an
explosion and large fire. The ruptured
portion of the piping is depicted in FIG. 5
because it is not detailed properly in the
overall process schematic (FIG. 4). When
the inspection team and the author arrived
at the site, the ruptured parts of
the damaged piping were cut as if with a
sharp knife (FIG. 6). The burging (swelling)
part of the left end piping seemed to
be caused by fire and explosion (secondary
damage) rather than by the original
damage mechanism.
Cause analysis. The material group
and the inspection team began investigating
the cause of the accident. The operating
condition of the damaged piping
was checked with the API RP941 Nelson
chart, since the carbon steel line was exposed
in the hydrogen environment. The
chart indicates the material's service limAmine
absorber
Sweet
gas
Frequent leaks (five times
in one year) occurred due to
piping corrosion
Lean amine
CW
Rich
Sour gas
feed
amine
Fuel
Lean
amine
Amine regenerator
(stripper)
Reboiler
Steam
FIG. 9. Amine system: Amine absorber and amine regeneration unit (ARU).
FIG. 10. Amine corrosion in the rich amine
piping.
Hydrocarbon Processing | JUNE 2022 49
H2S to SRU
Amine
flash drum
it to avoid decarburization and fissuring,
which is used in the high-temperature
methane by permeating into the steel
inside and reacting with dissolved carSubstantial
savings can be obtained using corrosionresistant
construction materials. Close cooperation
between the inspection (corrosion) engineer and process
and design personnel is required before a plant is built
to eliminate or reduce maintenance costs in many cases.
hydrogen environment. Operating personnel
confirmed that hydrogen reverse
flow through this carbon steel piping
could be formed during turndown operation
and startup due to the temporary
stop of the recycle oil pump. The actual
operating conditions and design values
of the line are shown in TABLE 1.
Considering 95% of the hydrogen
content in the recycle hydrogen, the
actual operating point was marked on
the Nelson chart shown in FIG. 7. The
point exceeded the limit of carbon steel
and low-alloy materials, which are susceptible
to high-temperature hydrogen
attack (HTHA). Stainless steel should
have been selected for this application.
As described in FIG. 5, the 14-in. bypass
line from the temperature control valve
(TV) was designed as stainless steel, but
the section of the 8-in. main line downstream
of the TV was made with carbon
steel, which showed inconsistency in design
since both lines are exposed to hightemperature
hydrogen.
In equipment that is exposed to hightemperature
and high-pressure hydrogen
gas of > 200°C, hydrogen creates
Pressure
Amine
filters
Lean/rich
amine heat
exchanger
Amine
regenerator
Temperature
bon or metal carbide in dislocation, inclusion,
gross discontinuity, lamination
or other inside voids, according to the
reaction in Eq. 3:
Fe3C + 2H2
→ 3Fe + CH4
(decarburization reaction)
(3)
Because a molecule of methane is
large, it is hard to diffuse. The methane
pressure increases by accumulating in
the grain boundary or the boundary between
the grain and nonmetal inclusion.
Decarburization then progresses continuously,
creating bubbles and fissures
(minute cracks) in the steel inside. There
is no change in the thickness of metal, but
micro cracks are connected and there is
lower tensile strength and ductility by
decarburization and fissuring. As a result,
hydrogen attack occurs. To confirm
the cause of this accident, a replica test of
metal microstructure was performed by
TABLE 1. Actual operating conditions
and design values of the line
Design case Operating case
260 kg/cm2,g 180 kg/cm2
370°C
,g
343°C
Rich amine
Hydrocarbon Processing - June 2022

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