Hydrocarbon Processing - May 2021 - 50

Special Focus

Maintenance and Reliability
S. CHIDAMBARAM, Central Mechanical Research Institute,
Durgapur, Indi

Thick-wall reactor shell repair procedure after damage
detection at an interface in a hydrocracking unit
The vessel and reactor operating in a
refinery may undergo various deteriorations due to their operating conditions.1
Therefore, to protect the vessel and reactor from such deteriorations, the inner
surface of the wall should be cladded by
stabilized austenitic stainless-steel grades
like SS321 and SS347. However, reactors and/or vessels are often composed
of mostly chromium molybdenum (CrMo) alloy steel grades, which is enough
to maintain structural stability during
operations at full capacity load. The combination of low-alloy steel with stabilized
austenitic steel grades was obtained by
dissimilar welding processes between
clad plates or by a weld overlay.2
One such similar combination (i.e., a
vessel or reactor wall made up of dissimilar welding) was affected by lamination
defects after a few years of operation. The
lamination-like defects were observed
on a weld overlay of the vessel shell during non-destructive testing (NDT). The
cause of this defect was identified as dis-

bonding, which is defined as the failure
of a coating to adhere to the substrate to
which it was applied.3 These disbonding
defects were observed between stainlesssteel (SS309) layers-i.e., the first layer of
the weld overlay and the parent shell wall
of the reactor vessel. It was found that,
during initial fabrication, there were no
defects observed at these interface junctions, and these defects were initiated
during long-term exposure of service.
A schematic representation of the
reactor shell configuration, heavy-wall
thick alloy steel (parent metal) and weld
overlay within a reactor shell in transverse
direction is shown in FIG. 1. An arbitrary
disbonding defect parallel to the inner
wall surface is shown in FIG. 2. The flaw
is represented in a general manner in the
current study, but actual flaw dimensions
(e.g., flaw shape, size and orientation) are
dependent on many factors, such as microstructures at interfaces, fracture toughness at interfaces and material properties.
These all play a vital role in structural

integrity. A schematic of the vessel shell
adjacent to the internal load bearing is
shown in FIG. 3.
Repair procedure. There are two sce-

narios to consider for repair procedures,
and these are 1) if the defect has not damaged or affected the parent base metal
shell,4 or 2) if the defect has damaged the
parent base metal (and if the defect damaging the vessel shell is assumed to be affected by hydrogen damage).4
Repair that has not affected the parent base metal. If the defect has not affected the parent base metal, then the
following repair steps should be implemented:
* In-situ metallography: Prior
to the repair process, an insitu metallographic study is
recommended on the weld overlay
region to access the nature and type
of material behavior induced in the
material during long-term exposure
of service. If any crack is discovered
in the grinding process during

3 mm
X mm

6 mm

Disbond
SS347
SS309Nb
SA336 F22 Class 3

Defect
Weld overlay
Weld overlay
Base metal

FIG. 1. Schematic of the reactor wall in transverse direction and corresponding material of
construction.

MAY 2021 | HydrocarbonProcessing.com

FIG. 2. Schematic view of the arbitrary
disbonding defect in parallel to the base metal
at the vessel's wall surface, with (a) and (b)
representing arbitrary defect sizes.

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Hydrocarbon Processing - May 2021

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