Hydrocarbon Processing - December 2021 - 28

Catalysts
FIG. 8. Expansion skirt plug weld.
FIG. 10. Plug weld: Expansion skirt assembly.
FIG. 9. Catalyst support t-beam.
the plug weld of the expansion skirt rings welded to the reactor
shell (FIG. 8) were found to be slightly above the maximum
allowable of 1.5Sm
.
The maximum stress values obtained for the reactor internal
structural components were acceptable and well within the
design limits, corresponding to an 85-psid pressure drop across
the first bed.
Further simulations for the Case 3 (116 psid) and Case 4
(159 psid) values of pressure drops across the catalyst structure
and the expansion skirt structure of the reactor first bed
were completed.
It was found that the stress (PL
+ Pb) at the expansion skirt
welded connection (SCL-4) location does not increase beyond
37 ksi even with an increased value of the pressure drop, which
does not correlate to a primary load. It was found that the pressure
drop load does not have a direct effect on values of the
primary local (membrane + bending) stresses at SCL-4. Based
on this finding, an important aspect was realized that the bending
stress at SCL-4 in reality should be classified as " secondary "
(Q), although membrane stress is classified as " primary " (PL
).
As a result, evaluating (P + Q) at SCL-4 at its design-value load
is no longer valid with a lower allowable stress limit of 1.5Sm
. In
the case of SCL-4, to evaluate the (P + Q) criteria, the more appropriate
operating value loads were applied and a corresponding
higher allowable stress limit of 3.0Sm was utilized. The
maximum simulated stress values of the respective SCL-4 location
were found to be well within the allowable limit of 3.0Sm
For SCL-6 and SCL-7 in the longest catalyst support beam,
.
2
as shown in FIG. 9, the stresses increased to a level that exceeded
the corresponding maximum allowable stress intensity value
of 16.9 ksi (Sm
(1.5Sm
for primary general-membrane + bending).
Takeaway. For the expansion skirt weld, at SCL-4 location on
the internal expansion skirt weld as shown in FIGS. 8 and 10, it
was found that the 9.5-in. thick reactor shell undergoes an insignificant
amount of hoops deformation due to the internal pressure
of 2,480 psi. The 1-in. thick internal expansion skirt rings
are not designed for hoops stress, as the reactor shell internal
28 DECEMBER 2021 | HydrocarbonProcessing.com
for primary general-membrane) and 25.23 ksi
pressure acts hydrostatically on all internal surfaces of expansion
skirt rings. Owing to the weld between the internal expansion
skirt rings and the shell, the shell imposes its hoop's deformation
on the internal expansion skirt rings at their connection
(SCL-4). Although this hoop's deformation is insignificant for
the 9.5-in. thick reactor shell, but it becomes significant for the
1-in. thick internal expansion skirt ring, causing the internal expansion
skirt ring to preferentially bulge at the welded connection
(SCL-4) location, resulting in the above 36 ksi.
It has been found that the pressure drop load does not have
a direct effect on values of the primary local (membrane +
bending) stresses at SCL-4, but the stresses at the SCL-4 location
are related to only the hoop's deformation imposed by
the shell on the internal expansion skirt rings at their connection
(SCL-4). Therefore, the membrane + bending stresses
are purely " secondary " (Q) in nature. In conclusion, the stress
situation of 36 ksi will continue to exist at this magnitude with
the presence of internal design pressure, irrespective of change
in the magnitude of the pressure drop load.
For the catalyst support beam, an additional simulation was
performed with a reduced pressure drop of 142 psid across
the first bed, which resulted in a limiting value of the linearized
stress intensity of 16.8 ksi at SCL-7 (in the direction of the
beam axis) location (FIG. 9).
The analysis established that the longest catalyst support
beam limits the pressure drop across the first bed of the reactor
vessel to 140 psid. For operation of the hydrocracker reactor
vessel, this pressure drop could be considered as the limiting
value.
LITERATURE CITED
1
American Society of Mechanical Engineers (ASME), " Boiler and pressure vessel
code ASME Sec-VIII Div-2.
American Society of Mechanical Engineers (ASME), " Boiler and pressure vessel
code: Material properties, " ASME Sec-II.
ASLAM KITTUR is a Mechanical Engineering Specialist for
refinery static equipment in the technical services division of
Saudi Aramco. He has more than 20 yr of experience in the
energy industry (upstream, downstream) and specializes in
performing stress analyses to understand the complex failure
mechanisms of critical equipment. He focuses on establishing
root causes for repetitive failures, and developing solutions
to ensure reliable design for any given operating conditions. Mr. Aslam earned
an MS degree in solid mechanics and employs FEM-based structural and thermal
simulations that are often required for a Level-III API-579 fitness-for-service
evaluation. Prior to joining Saudi Aramco, he participated in a similar capacity
for 10 yr in the Canadian oil sands facilities.
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