Hydrocarbon Processing - May 2021 - 25

Maintenance and Reliability

Beam maximum temperature, °C

confirm how far the influence
would be through this modeling.
* Temperature profile of the
radial direction: Conversely, the
temperature effect in the radial
direction was very limited. If the
hot spot was only 150 mm away
from the reactor wall, the wall
temperature could be reduced
below the design temperature
(FIG. 3). This is closely related
to the directional flow of the
feedstock, which seems to
rapidly reduce the range of hightemperature areas as feedstock
flows from the top to bottom.
This means that, even if the hot
spot location is close to the wall,
the temperature increase of the
wall does not have to be a concern.
In fact, in the authors' experience,
an increase in wall temperature
was not detected.
In the structural analysis of the
reactor wall, the hot spot case at
50 mm was considered. A peculiar
item is that the point where the
maximum temperature appears
is not the area of the hot spot
occurrence. Since the maximum
temperature area appeared
below approximately 1.8 m, it
is necessary to take this into
account when checking the actual
shell temperature. This can be
confirmed in the CFD temperature
profile on the right side of FIG. 3.
To summarize the CFD results, the
temperature effect is much larger in the
direction below the hot spot, while the
effect is very small in the radial direction.
This seems to be closely related to the directional flow of the feedstock.
Based on this, the master curve-
1,150
1,050
950
850
750
650
550
600

which can be used for various hot spot
sizes-is prepared to estimate the exposure temperature of the lower structure,
by distance, according to the hot spot
size and temperature (FIG. 4). The exposure temperature of the lower structure is
50°C-160°C lower than the hot spot indicator temperature. For example, with a
hot spot of 900°C, the exposure temperature of the lower structure can be predicted to be as high as 750°C-840°C.
Evaluation of the shell and interior
structure stability through FEA. A

structural stability evaluation was performed on the reactor wall, interbed
beam and outlet collector. Only 800°C
and 500-mm diameters were considered
for the shell and outlet collector, while
500 mm, 1,000 mm and 1,500 mm were
considered for the interbed beam. The
standard ASME VIII, Div. 2 Part 5 specification was applied to the analysis.
The shell's material comprises 2.25CrMo steel with thermal conductivity of 35
W/mK, and the internal structures are
composed of Tp347 steel with thermal
conductivity of 25 W/mK.
Evaluation results. The following are
the results of the evaluation:
* Reactor wall: Consider when a
hot spot mass with a temperature
of 800°C and a 500-mm diameter
exists 50 mm away from the
wall. As a result of the analysis,

the maximum stress caused by
the hot spot was 322 MPa-and
there were no problems, as it was
below the acceptance criteria of
331 MPa. It was analyzed that the
maximum stress was applied inside
of SCL-A-2-L1; whereas, it was
found that lower stress was applied
at the maximum temperature
point (SCL-A-2-L2). The previous
CFD analysis indicated that the
maximum surface temperature of
the wall was located at the lower
part of a certain distance, not at the
point of the hot spot occurrence.
The stress analysis results were
similar (FIG. 5).
* Interbed beam: The top of the
interbeam's center showed the
highest temperature (750°C),
with a stress of 48.6 MPa (less
than the allowable stress of 230
MPa). Conversely, the maximum
stress at the bottom of the beam's
center was 172 MPa (less than the
allowable stress of 315 MPa), and
the temperature was 638°C. The
maximum temperature was at the
top, but the maximum stress was at
the bottom. Although the surface
temperature of the upper part of
the beam was at its maximum due
to the hot spot, it was in line with
the expectation that the maximum
bending stress would be the

Wall temperature, °C
(P) Predeﬁned Field-1
530.54
519.02
507.50
495.98
484.46
472.94
461.42
449.90
438.38
426.85
415.33
403.81
392.29

S, Mises
(Avg: 75%)
350.54
327.10
303.96
280.82
257.68
234.54
211.40
188.26
165.12
141.98
118.84
95.70
72.56

Stress check

MPa
SCL-A-L1
SCL-A-L2
Out
In
Out
In
SA336-F22

Location
Material
Wall
temperature, °C 413.46
Cal. stress
118.47
(PL + Q)
Allowable stress 390.7
Result
OK

497.64

442

530.54

321.95

77.8

197.41

331.0
OK

388.9
OK

296.7
OK

SCL-A-L1
700

800

900 1,000 1,100 1,200 1,300

Hot spot temperature, °C

SCL-A-L2

500-mm diameter 1,000-mm diameter 1,500-mm diameter
y = 0.6827x + 126.93 y = 0.7627x + 94.906 y = 0.8632x + 54.726
R2 = 1
R2 = 1
R2 = 1

FIG. 4. Maximum surface temperature of the
internal structure according to hot spot size
and temperature.

FIG. 5. Stress at the reactor wall.
Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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