Hydrocarbon Processing - August 2022 - 85

Maintenance and Reliability
TABLE 2. Loading values
Loads (based on 360° model) for Operating Case:
Internal Operating Pressure
Longitudinal pressure (Boundary stress) @ circumferential cross-sectional area of shell (At mid-section)
Gasket Reaction Load. The gasket reaction varies with the operating load cycle.
Axial force P, inwards along global-X on Nozzle N2 (Gas Inlet Nozzle on Bottom Head) (Non-cyclic)
Moment M1 on Nozzle N2 gas Inlet nozzle on Bottom Head (Non-cyclic)
Moment M2 on Nozzle N2 Gas Inlet Nozzle on Bottom Head (Non-cyclic)
Axial force P, inwards Along global-Y on Nozzle M1 Manway / Gas Outlet Nozzle on Top Head (Non-cyclic)
Moment M1 on Nozzle M1 Manway / Gas Outlet Nozzle on Top Head (Non-cyclic)
Moment M2 on Nozzle M1 Manway / Gas Outlet Nozzle on Top Head (Non-cyclic)
Operating Fluid Load (Non-cyclic)
Catalyst Weight (Non-cyclic)
Vessel Weight - Top half (Non-cyclic)
Flange tightening loads on Ø24 " #150, WNRF, Nozzle M1 Manway / Gas Outlet Nozzle on Top Head
(Non-cyclic). This bolt load remains constant throughout the operating pressure cycle. (Non-cyclic)
assessment diagram (FAD) approach to
evaluate the combined effects of fracture
and plastic collapse per API-5791
Part 9.
The stress values were post-processed
to tie into existing KI
and σref
solutions,
as the geometry of the embedded flaws
were approximated by simple shapes addressed
in Annex C and Annex D of API5791
.
The crack growth model based on
the Paris equation was employed to estimate
the remaining life of the vessels with
a crack-like flaw based on the linear elastic
fracture mechanics approach, where each
increment of crack extension shall correlate
to a certain increment of stress cycles.
FIGS. 5-10 show the FEM model and
the quality of the mesh on the stress classification
planes and the location of the
stress classification lines (SCLs) selected
at the critical locations of gross and local
structural discontinuities for this analysis.
The stress components will be integrated
along the SCLs through the wall thickness
to compute the equivalent linearized
membrane, bending stresses and peak
stress components.
The cases for analyses performed have
been developed based on the objective of
assessment and methodology for the two
scenarios encountered during operation.
The stress analysis was performed for the
below scenarios:
* CASE I: OPE-MAX internal
pressure of 285 psig
* CASE II: OPE-MIN internal
pressure of 16 psig.
The stress results from Cases I and
II were combined to determine the
FIG. 6. SCLs in finite element model at bottom head, local.
ΔSn,k (primary + secondary, equivalent
stress range) and ΔSp,k
(primary + secondary
+ peak, equivalent stress range).
The fatigue analysis was completed for
the scenario that the PSA vessels of both
trains have witnessed 480,000 pressure
cycles over the 15 yr in operation based
on the available operating data for the
uptime of the H2
plant.
RESULTS-ELASTIC STRESS
ANALYSIS
The equivalent stress consisting of
primary + secondary + peak was derived
from the highest value across the thickness
of the section (head, nozzle, flanges,
shell and skirt) and was produced by
specified operating pressures and other
mechanical loads, including the effects
of gross and local structural discontinuiFIG.
7. SCLs in finite element model at bottom
head, general.
ties. The stress distribution plots have
been provided in FIGS. 11-13. The graphical
comparison of the simulated values of
equivalent stress range (P + Q) at the various
SCL locations has been illustrated in
FIG. 14, whereas FIG. 15 shows the values of
Hydrocarbon Processing | AUGUST 2022 85
Symbol
PIO
PIOL
WgO
Fn2
M1n2
M2n2
Fm1
M1m1
M2m1
Wf
Wc
WvT
Wb
Case-I
285 psig
11,849 psi
154,933 lbs
Case-II
16 psig
665.9 psi.
300,439 lbs
22,400 N
17,185 lbf-ft.
13,217 lbf-ft.
7,554 lbf
463,956 lbf-in
356,856 lbf-in
1,200 lbs.
92,323 Lbs
53,930 lbs
309,094 lbs

Hydrocarbon Processing - August 2022

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