Hydrocarbon Processing - May 2021 - 28

Maintenance and Reliability
may occur. While HTS is a well-known
corrosion phenomenon within the typical operating range of a hydroprocessing
unit, corrosion data above the design
temperature of the reactor (454°C) is
limited, especially for austenitic stainless
steel. The corrosion rate of Tp347 stainless steel at 500°C-600°C has a maximum of 60 mils/yr. However, this value
has a low confidence level because it is
calculated by extrapolation.
In the authors' case, the process ran
for about a month after the hot spot
occurred. As a result of the inspection
of the internal structure during shutdown, a severe corrosion phenomenon
was found on part of the mesh screen of
the outlet collector and on several thermowell pipes on the wall. Even with the
austenitic stainless-steel material, it was
found that corrosion may occur within
a short period of time under hot spot
conditions where the temperature rises
rapidly. As shown in FIG. 8, the mesh
screen had a wider gap, and some of the
mesh was completely corroded. If exposed for a longer period, the catalyst
above may have been lost. Although the
thermowell pipe was 3-mm thick, it was
completely corroded within a short period of time, and high-temperature corrosion was accelerated by carburization
and metal dusting phenomena. Based on
the authors' inspection data, the amount
of corrosion was acceptable when the
exposed metal temperatures were below
700°C, and the duration of high-temperature exposure was less than 1 mos.
In the case of PASCC, it is known
that sensitization occurs first and contact with polythionic acid is required.
Sensitization occurs more frequently
with higher temperatures. According to
literature, austenitic 347 stainless steel
can be exposed at 495°C for 10,000 hr,
but at 530°C for only 1,700 hr. In the
authors' case, the microstructure of the
bolting bar-which is a part of the bottom collector assembly-was inspected
using the filed replication technique.
It was found that there were no signs
of sensitization, even though corrosion was spotted at the nearby mesh.
Note: The microstructural review was
conducted after replacing corroded/
damaged internal parts. Therefore, the
authors believe that the remaining part
of the reactor internal, which was not
replaced, still maintains enough PAS28 MAY 2021 | HydrocarbonProcessing.com

CC resistance because the temperature
and duration of the hot spot were not
enough to cause sensitization.
In summary, the possibility of HTS
corrosion should be kept in mind, even
if the hot spot period is a short duration.
Furthermore, high-temperature exposure
may cause sensitization of austenitic stainless steel, but the level of damage should
be carefully reviewed by metallography.
Considering the exposure temperature, time and area, the authors decided
to maintain a temperature of 650°C.
Corrosion testing under the same conditions-high temperatures with high sulfur-was not easy. Only a hardness comparison was made through simple heat
treatment experiments, and some of the
650°C limit was reflected.
Takeaway. CFD modeling of hot spot
cases in the RDS reactor catalysts was
performed to analyze temperature profiles for catalysts, reactor walls and internal structures. Accordingly, structural
stability was evaluated through FEA for
structures where temperature increases
locally. Corrosion under this condition
was also identified, and the following
conclusions were drawn:
* The larger the hot spot size or the
higher the internal temperature,
the larger the temperature area
of the axial direction-and the
temperature of the lower internal
structure increases locally. In the
downward direction, the influence
of temperature was confirmed
to about 7,000 mm, as it was
affected by the directional flow
of feedstock.
* Conversely, the effect of
temperature in the radial direction
was relatively small. Even if it
is only about 150 mm away, it
has been cooled rapidly below
the design temperature. This
means that, even if an internal
hot spot occurs, the temperature
of the reactor wall does not rise
significantly.
* It was analyzed that the
temperature of the internal
structure increased locally due
to the hot spot. Compared to the
500-mm distance, the structure
was evaluated to be 50°C-150°C
lower than the hot spot
temperature.

* As a result of evaluating the
structural stability of the wall and
internal structure of the reactor
reflecting CFD, there was no
indication that the structural
stability was unsatisfactory, even
if the design temperature was
exceeded locally.
* Two types of corrosion must be
considered: HTS and PASCC.
In this case, corrosion by HTS
was observed on the mesh
screen and thermowell pipe. For
HTS, the exposure temperature
is important, and the authors
suggested a short-term allowable
excursion temperature of 650°C.
No PASCC was found on the
remaining internal parts by
field metallography because the
temperature and duration were not
enough to cause sensitization.
This study is the result of reviewing the
hot spot case of the RDS reactor. It is difficult to apply these results to all hot spots;
however, when similar hot spots occur, it
is believed that reasonable decisions can
be made by referring to them.
SANG-MO LEE has more than
26 yr of experience as a
Fixed-Equipment Engineer at
SK Energy. His main tasks include
root cause analysis (RCA),
troubleshooting, fitness-for-service
(FFS) evaluation, maintenance
procedures and technical support for the refining
and petrochemical business. He earned a BS degree
in mechanical engineering, and is qualified as a
professional engineer for welding and metallurgy
by the South Korean government.
SUN HYUK BAE is a Chief
Researcher for SK Innovation.
His background is in transport
phenomena and reaction
engineering. He has designed over
30 commercial and pilot reactors.
Prior to joining SK Innovation,
Dr. Bae was a senior researcher at LG Chemical.
After joining SK Innovation, he was the leader of the
computational engineering group from 2009-2016.
Dr. Bae earned a BS degree in chemical engineering,
along with his PhD, from the Korea Advanced
Institute of Science and Technology (KAIST).
DONG-SIK LEE is a Senior Engineer
and a Non-Destructive Examination
(NDE) Specialist at SK Energy.
He has more than 21 yr of
experience in the inspection
department, and supports the
technologies of NDE, corrosion
control and failure analysis in refineries, especially
hydroprocessing and residual fluid catalytic
cracking units. He earned a BS degree in mechanical
engineering from Yonsei University in South Korea.

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