Hydrocarbon Processing - May 2021 - 30

Maintenance and Reliability
undertaken of the region adjacent
to the gouge, using a load of 100 g.
6. Duplicate tensile tests were
carried out on the shell's
material away from the crack to
ensure conformance with the
requirements of the ASME SA516

Grade 60 specification.
7. Chemical analysis of the
shell, nozzle, reinforcement
pad materials and both weld
metals were carried out, using
direct-spark optical emissions
spectrometry, and the results were
compared to the requirements
of the ASTM 516-17 Grade 60
specification.
Results. An initial examination revealed
two crack-like features on the inner surface of the shell. The larger of these was
around 70 mm. Ultrasonic testing inspection revealed that the two features were

FIG. 2. Photographs of the nozzle and location
of the crack.

FIG. 3. Photographs of part of the fracture
faces: (A) nozzle side of the fracture, and
(B) the fracture face from the opposite side
of the nozzle, showing a macroscopically flat
surface and the presence of ratchet marks.
Note: This is in mm scale.

30

MAY 2021 | HydrocarbonProcessing.com

FIG. 4. SEM of the fracture surface, showing
the absence of cleavage fracture or microvoid
coalescence. The flat featureless surface
is consistent with fatigue. Magnification is
indicated by the micron marker.

FIG. 5. (A) Metallographic section through the
reassembled fracture. The nozzle parent metal
is on the left, the upper plate is the shell,
and the lower plate is the reinforcement pad;
(B) as-built drawing schematic representing
the joint details for comparison with the
metallographic section.

part of a larger crack around 180 mm in
length (FIG. 2). At this stage in the investigation, the outer surface of the shell could
not be inspected due to the presence of
the reinforcement pad.
Approximately half of the crack was
examined after breaking it open. This revealed that the fracture face had multiple
flat, smooth regions, which are consistent
with fatigue cracking. FIG. 3 shows both
halves of the part of the fracture (before
cleaning) that was broken open.
Scanning electron microscopy (SEM)
confirmed the failure mode to be multiinitiation fatigue from the outer surface of
the shell. The fracture surface was characteristically smooth and flat (FIG. 4).
A photo macrograph of the metallographic section is shown in FIG. 5.
The crack had initiated from the root of
Weld 1. An examination of this section
revealed that the reinforcement pad had
been attached to the shell plate and then
the main weld had been performed, both
from the inside of the shell. A 2-mm gap
existed between the reinforcement pad
and the shell parent materials. The crack
was initiated at the corner where the shell
parent material side of this initial weld
bead met at a stress concentrating feature.

FIG. 6. (A) Microscopic image of Weld 1
(between the shell and the nozzle), showing
the connection to the reinforcing plate and
the location of the crack; (B) detail of the
crack mouth region with magnification
indicated by the micron marker.
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Hydrocarbon Processing - May 2021

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