Hydrocarbon Processing - August 2022 - 86

Maintenance and Reliability
equivalent stress range (P + Q + F) at the
various SCL locations. The maximum accumulated
fatigue of 13.5% was calculated
at SCL-11 and was found acceptable,
as it was within the specified limits. The
SCL-11 corresponds to the PSA vessel's
skirt section, which was also not a part of
the vessel's pressure boundary shell.
The damage factors were calculated
based on the stress-fatigue analysis
(Ke
the calculations), and all SCL locations
in the adsorber vessels were found acceptable
for continued operation well
beyond the previously established fatigue-design
life. However, due to embedded
flaws, the effect of crack growth
has been considered in the following section,
which provides the additional criteria
to limit the fatigue design life. This
= 1.0 and Kf
= 1.2 were assumed for
shall be based on the crack growth from
its size (yr) to its critical size.
Crack criticality assessment. The
PSA vessels in pressure cycling service
containing the crack-like flaw are subject
to loading conditions that may result in
crack growth. As a result, the Level 1 and
2 assessment procedures in API-5791
Part 9 do not apply as conditions are not
satisfied. As the embedded flaw is anticipated
to be subjected to subcritical crack
growth during future operation, it has
been evaluated using a Level 3 assessment
procedure that provides the best estimate
of the structural integrity of a component
with a crack-like flaw. A Method A assessment
has been utilized, such that the FAD
was utilized for the acceptance criteria
with user-specified partial safety factors
(PSFs) based on a risk assessment.
The flaw characterization
rules
API-5791
FIG. 8. SCLs in finite element model at top flange.
in
will allow existing or postulated
crack geometry to be modeled by
a geometrically simpler one to make the
actual crack geometry more amenable to
fracture mechanics analysis. As a result,
these linear indications were categorized
as embedded cracks [FIG. 16 (cylinder-
embedded crack, longitudinal direction
and elliptical shape)] and [FIG. 17 (cylinder-embedded
crack, circumferential
direction and elliptical shape)] of API5791
.
The crack depth, length, angle and
distance to other embedded cracks were
typically required to be determined using
the angle beam ultrasonic testing (UT)
examination techniques, such as time of
flight diffraction (TOFD) and pulse-echo
techniques, as shown in TABLE 3.
The Mode I stress intensity factor soFIG.
9. SCLs in finite element model at top head, local.
lution representing a fourth-order polynomial
stress distribution (KPECE2) was
chosen to represent the cylinder-embedded
crack, elliptical shape in the longitudinal
direction and circumferential
direction. For the FAD, the Level 2 recommended
curve was used in the assessment
(Level 3).
Stress analysis results from the previous
section were used in a crack-like
flaw assessment, such that the numerical
model did not include the crack explicitly.
This approach entails post-processing the
stress values in such a way as to tie into
existing KI
and σref
FIG. 10. SCLs in finite element model at top head, general.
86 AUGUST 2022 | HydrocarbonProcessing.com
solutions in Annex C
and Annex D, respectively. This approach
was applied because it is best suited to the
case where the PSA vessel's shell could
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