Hydrocarbon Processing - May 2021 - 26

Maintenance and Reliability
bottom point of the beam (FIG. 6).
* Outlet collector: As a result of the
evaluation of the outlet collector,
the maximum temperature area
was 551.6°C with the structure's
top, but the calculated stress
value was 82 MPa (less than the
allowable stress of 354 MPa),
which was enough (FIG. 7).
Conversely, the bottom area had
a maximum calculated stress of
337.2 MPa (less than the allowable
(P) SIZE1POINT5meter
749.84
720.69
691.53
662.38
633.23
604.07
574.92
545.77
516.61
487.46
458.31
429.15
400

stress of 396 MPa). This area was
where the maximum bending
stress was applied, as shown in
the inter-beam analysis. In other
words, the surface temperature at
the top of the structure exceeded
the design conditions due to the
hot spot, but it was not the area
where the maximum stress was
applied, so it could be assessed that
there was no significant impact on
the stability of the structure.
Stress check

Beam temperature, °C

MPa

Material
A
Point
Metal
635.5
temperature, °C
Cal. stress
163.8
Allowable stress 326.9
Result
OK

SA182 F347
B
C
749.8

400.08

172.6
315
OK

48.6
230
OK

146.7
397.5
OK

S, Mises
(Avg: 75%)
172.55
158.85
145.16
131.46
117.76
104.07
90.37
76.68
62.98
49.29
35.59
21.9
8.28

D
C

638.4

FIG. 6. Stress at the interbed beam.

Beam temperature, °C
551.6
543.6
535.7
527.7
519.7
511.7
503.7
495.8
487.8
479.8
471.8
463.8
455.9
447.9
439.9
431.9
423.9
416
408
400

FIG. 7. Stress at the outlet collector.

MAY 2021 | HydrocarbonProcessing.com

Stress check
Material
Point
Metal
temperature, °C
Cal. stress
Allowable stress
Result
S, Mises
(Avg: 75%)
342.66
314.14
285.61
257.08
228.56
200.03
171.51
142.98
114.46
85.93
57.4
28.88
0.35

MPa
A

SA182 F347
B

445.5

551.6

425.8

337.2
396
OK

82
354
OK

127.5
396
OK

Summarizing the structural stability
evaluation results, the surface temperature of the shell wall, interbed beam and
outlet collector structure exceeded the
design temperature by the hot spot. It
was decided that there were no structural stability problems, even if the temperature of some of the shell and internal
structures exceeded the design standard
due to the hot spot effect. The cause
seemed to be that the maximum temperature point and the maximum stress
point did not coincide in the structure.
In other words, due to the hot spot, the
temperature exceeded the design conditions locally, but there was no problem
with the stability of the structure.
Corrosion analysis. A review of the

corrosion environment is essential, even
if structural stability assessment results
are satisfied. The internal structure is
composed of a part supporting the load,
a beam, a supporting bar and a mesh
screen that simply prevents catalysts
from passing through. A thermowell pipe
is installed to measure the temperature.
If these parts are severely damaged by
corrosion or cracks, then catalyst leaks
or structure collapse may occur during
operation, and repair or replacement
may be long and expensive. In particular, the thin mesh screen is susceptible
to damage and, if the gap is widened and
the catalyst escapes, normal operation is
not possible.
In normal operating conditions, corrosion of the internal parts (Tp347 steel)
rarely occurs, so no special care is taken.
However, the problem changes at high
temperatures above 500°C. If this temperature is exceeded, high-temperature
sulfidation (HTS) corrosion and/or
polythionic acid stress corrosion cracking (PASCC), induced by sensitization,
Thermowell pipe

Screen of outlet collector

C
B

FIG. 8. Corrosion at the internal parts.

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Hydrocarbon Processing - May 2021

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