Hydrocarbon Processing - August 2022 - 87

Maintenance and Reliability
be approximated by simple shapes, such
as a cylinder addressed in Annex C and
Annex D of API-5791
. The stress results
were linearized based on the crack location
within the section thickness-this
linearization method employed KI
mation, as the stress distribution was uniform.
The reference stress σref
estiwas
determined
from the membrane and bending
stresses derived from the linearization of
stress results. The lower-bound estimate
of fracture toughness 69.141 ksi√in. used
in this assessment was based on the API5791
equation
F.54 (Eq. 1):
KIC
= 33.2 + 2.806 exp
[0.02(T - Tref
+ 100)]
(1)
The primary stress, material fracture
toughness and flaw size should be modified
using the PSF. However, as the conservative
estimate of lower-bound fracture
toughness has been established, the
applicable PSF equal to 1.0 would have
normally been permissible. However, for
conservatism, a PSF = 1.3 was used in
this assessment.
The toughness ratio (Kr
) was calculated
based on the applied stress intensity
due to the primary stress distribution and
was plotted as the ordinate of the above
FAD assessment point. The load ratio was
calculated using the reference stress for
primary loads (LP
r) and plotted as the abscissa
of the above FAD assessment point.
The FAD plot in FIG. 18 shows that all existing
flaws are well within the acceptable
region and were deemed non-critical.
Crack growth assessment. It is anticipated
that these cracks may grow by
fatigue when the adsorber vessels continue
operating, witnessing a PSA pressure
cycle that results in cyclic stresses. Each
increment of crack extension correlates to
a certain increment of stress cycles.
The following basis was adopted for the
crack growth assessment methodology:
1. During the crack growth, crack
dimensions a and c will grow
simultaneously, with the a/c ratio
being assumed to remain constant
during crack growth of 12.5%.
2. Crack dimension d1
has been
taken from the closest of the two
edges, such that d1
≤ 0.5, and is
assumed not to change during
crack growth.
3. 1st limitation on crack growth (for
those cracks that exceed the a/d1
FIG. 13. Stress plots: bottom head.
Hydrocarbon Processing | AUGUST 2022 87
≥ 0.8 criteria before exceeding the
cut-off limit for steel): During the
crack growth, crack dimension a
will increase, resulting in a possible
highest a/d1
face nears the thickness boundary,
a becomes = d1
≥ 0.8. As the crack
. This limiting/
highest crack ratio a/d1
= 0.8 will
correspond to the critical size.
4. 2nd limitation on crack growth
(for those cracks which exceed
the cut-off limit for steel before
exceeding a/d1
≥ 0.8 criteria):
During the crack growth, crack
dimension a will increase,
resulting in a FAD position in the
unacceptable region. The crack
dimension will correspond to the
FIG. 11. Stress plots: top and bottom heads.
FIG. 12. Stress plots: flange and nozzle neck.
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