Hydrocarbon Processing - October 2022 - 60A

Maintenance
and Reliability
S. LEE, SK Energy, Ulsan, South Korea; and
O. KWON, Quest Integrity, Wellington, New Zealand
Case studies: Evaluation of the soundness and
remaining life of PSA vessels with weld defects
Multiple defects were found in pressure
swing adsorption (PSA) vessels during
an outage of one of four PSA process
units. The option for immediate repair
during the shutdown was infeasible due
to time constraints, particularly without
new catalysts for replacement. The concerns
were whether the equipment was
safe to start up again after the outage and
how long the vessels could continue to be
operated. A fitness for service (FFS) assessment
and remaining life assessment
were undertaken to assess the current status
of the cracks (defects), and to assist in
future outage plans and inspection based
on the remaining life results.
A 3D finite element (FE) model of a
PSA vessel was generated to determine
the stress distribution of the critical areas
where the defects were found, and a fracture
mechanics analysis was carried out
for the FFS assessment and remaining life
assessment in accordance with API 579
TABLE 1. Summary of the major
geometry of the PSA vessel
Item
Cylinder internal diameter
Cylinder wall thickness
Elliptical heads 2:1 thickness
Distance between tangent
lines (TL) of heads
Internal diameter of N2 top nozzle
Outer diameter of N2 top nozzle
Internal diameter of N1 bottom nozzle
Outer diameter of N1 bottom nozzle
Radius of internal diameter
of nozzle to shell
Radius of outer diameter
of nozzle to shell
Values,
mm
3,300
40
45
6,030
483
633
318
488
25
50
FIG. 1. FE model of a PSA vessel (A) geometry and (B) FE mesh.
Hydrocarbon Processing | OCTOBER 2022 60A
and BS7910 standard procedures. This
article describes the procedures of FFS
and remaining life assessments and their
results in detail in the following sections.
FINITE ELEMENT
STRESS ANALYSIS
FE models. A simplified FE model of a
PSA vessel (including top and bottom
nozzles) was generated, as shown in FIG. 1,
and a summary of vessel geometry is presented
in TABLE 1. The model consisted of
37,164 quadratic brick elements with a
global mesh seed size of 50 mm, a minimum
of 16 elements around a circle, and
three elements through the wall thickness.
This model was then treated as a generic
model that was used for other vessels;
therefore, the geometry, boundary
conditions and loads remained the same.
Boundary condition and loading
condition. A kinematic constraint was
applied to the location of the vessel
where the connection to the skirt would
be. Constrained to an arbitrary reference
point but free in the radial direction, the
reference point (in turn) had all degrees
of freedom constrained. Kinematic constraints
were additionally applied to the
top and bottom nozzles modeled. Both
were constrained to individual reference
points, centered flush with the end
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