Hydrocarbon Processing - December 2021 - 26

Catalysts
FIG. 5. Temperature distribution.
FIG.3. Load contribution on weld.
modes were only considered due to the hydrocracker unit startup
and shutdowns cycles. The FEM with continuum elements
provided the total stress distribution for evaluation and the
simulated stresses were compared with stress limits defined in
ASME Sec-VIII Div-2. This established the maximum load bearing
capacity of the structure and, consequently, the maximum
allowable pressure drop that the reactor can safely sustain.
Nomenclature used here includes:
P = Primary stress
Pm
= General primary membrane equivalent stress
PL = Local primary membrane equivalent stress
Pb = Primary bending equivalent stress
Q = Secondary equivalent stress
F = Peak stress produced by a stress conc. or a thermal stress
over and above the nominal (P + Q) stress level
S = von Mises equivalent stress
Sm
Sy = Yield strength
ΔSn
FIG. 4. Geometry of the FEM model.
Finite element modeling. The reactor was modelled as a 90°
3D-solid model based on the cyclical symmetry shown in FIG. 4.
This model enables the accommodation of the geometric details
for applying the appropriate loads and boundary conditions.
The catalyst weight, self-weight of the support beams and the
grids, operating liquid static head, velocity head, pressure drop
mechanical loading and temperature distribution were considered
while establishing the mechanical strength of the shell
integral support ring. Whereas, for establishing the mechanical
strength of the expansion skirt plug weld to shell, the self-weight
of the expansion skirt and the lower liquid distribution tray, liquid
collection tray assembly, mixing chamber, rough liquid distribution
tray assembly, the operating liquid static head, velocity
head and pressure drop mechanical loading on each of the three
trays, and temperature distribution, were considered.
Similarly, the mechanical strength of the individual structural
components of the reactor internals was also established based on
individual weight, fluid and temperature distribution loads. The
temperature-dependent mechanical properties for reactor shell
base metal A336 GrF22, weld overlay A312 Gr347, and shell internals
A312 Gr321 were obtained from ASME Sec-II Part-D.2
Analysis methodology. The reactor internal structural components
were evaluated primarily for protection against plastic
collapse. Despite the steady nature of operating pressure and
temperature of the reactors, the ratcheting and fatigue failure
26 DECEMBER 2021 | HydrocarbonProcessing.com
= Stress intensity range.
Temperature. A thermal stress was simulated for the reactor
vessel as the thermal gradient along the radial direction was
40°F per 9 in. = 4.45°F/in., as shown in FIG. 5. The thermal gradient
across the entire height (longitudinal) of the reactor was
8°F per 5 m = 0.04°F/in., which could be ignored as it was insignificant
toward generating secondary stresses.
The FEM model in FIGS. 6 and 7 shows the quality of the
mesh on the stress classification planes (SCPs) and the location
of the stress classification lines (SCLs) selected at the critical
locations of gross and local structural discontinuities for this
analysis. The FEM model has been meshed using hexahedrons
(20-node brick elements), which can effectively simulate the
stress and deformation fields in solids in a structural analysis.
The stress components were obtained from the linear elastic
stress analysis and were then integrated along the SCLs through
the wall thickness to compute the equivalent linearized membrane,
bending stresses and peak stress components.
Catalyst bed support ring at Bed #1 (FIG. 6):
* SCL-1 is the shell thickness at the general area
(Pm
and Pb
)
* SCL-2, -3 and -4 are the gross discontinuity
of the shell (PL
and Q).
Expansion skirt assembly at Bed #1 (FIG. 7):
* SCL-1 is the gross discontinuity of the shell (PL
and Q)
* SCL-2 and -3 are the concentration plug weld-root,
connecting the top and bottom rings (F)
* SCL-4 is the local discontinuity of the plug weld
connecting the top and bottom rings (PL
and Q).
= Allowable stress intensity
Salt = Alternating stress intensity
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Hydrocarbon Processing - December 2021

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