Hydrocarbon Processing - October 2022 - 60F

Maintenance and Reliability
whereas vessels D01, D10 and D12 have
relatively shorter lives (i.e., 1 yr, < 1 yr
and 2.3 yr, respectively).
The graph in FIG. 9 shows an example
of how the current cracks grow by fatigue
prior to becoming critical.
Takeaways and recommendations.
The FFS assessment and remaining life
assessment were carried out for the PSA
vessels D01-D12, where defect indications
were discovered in the welds during
the inspection. The main objective
TABLE 8. Cyclic stresses used for remaining life calculations
Cyclic stress = Max op. - min op.
Location
Top nozzle, N02
Bottom nozzle, N01
Top head weld, H01
Middle body weld, H02
Bottom head weld, H03
Upper vertical weld, V01
Lower vertical weld, V02
Membrane stress,
Mpa
94.2
89.3
40.6
82
40.6
59.9
59.9
Bending stress,
MPa, internal
15.5
11.7
1
-1.08
1
10.4
10.4
Bending stress,
MPa, external
-15
-9.9
-1
1.08
-1
-10.4
-10.4
of the assessments was to determine
whether the vessels with the defects were
fit for service and how long the vessels
can be operated. The following conclusions
and recommendations were made
from the assessment:
* The selected critical cracks
assessed for vessels D01-D12
fell within the FAD envelope;
therefore, the cracks are acceptable
for continuing operation.
* The cracks in vessels D01 and D10
were found be the most critical.
* The remaining life of each vessel
was determined based on the
assumption that the current crack
continues to grow by fatigue as
a result of cyclic loading until it
reaches critical flaw size.
* The remaining life of most of the
PSA vessels (i.e., D02, D03, D04,
D05, D06, D07, D08, D09 and
D11) ranges from 3 yr-5 yr.
* Vessels D01 and D12 have
relatively shorter lives: 1 yr and
2.3 yr, respectively. Vessel D10
has the shortest life: 302 d.
* It is recommended to prioritize
vessels D10 and D01 for repair,
particularly the top nozzle weld
(N02 weld) in vessel D10 and
the middle body weld (H02 weld)
in vessel D01.
FIG. 8. A refined FAD of the D01-H02-2 defect and the D10-N02-2 defect.
* If normal operating conditions
change (especially the frequency
of cyclic loading and maximum
pressure), or there is a departure
from the operating conditions
assumed in this assessment
that could be detrimental, it
is recommended to carry out
repeated assessments under these
revised operating conditions to
ensure the vessel remains fit for
service.
SANG-MO LEE has more than 26 yr of experience as a
Fixed-Equipment Engineer at SK Energy. His main tasks
include root cause analysis (RCA), troubleshooting,
FFS evaluation, maintenance procedures and technical
support for the refining and petrochemical business.
He earned a BS degree in mechanical engineering
and is qualified as a Professional Engineer for welding
and metallurgy by the South Korean government.
FIG. 9. Fatigue crack growth of the initial crack size to the limiting flaw size for D12-N02-3.
60F OCTOBER 2022 | HydrocarbonProcessing.com
OHGEON KWON is a Principal Consultant and Structural
Integrity Team Lead at Quest Integrity. Dr. Kwon has
more than 25 yr of experience and expertise in hightemperature
creep, FFS and remaining life assessments
for various components in the petrochemical, refining
and power generation industries. He earned his PhD at
Imperial College, London.

http://www.HydrocarbonProcessing.com

Hydrocarbon Processing - October 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - October 2022