A metal alloy upgrade

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A US refinery identified a problem with its second stage desalter and
their overhead accumulator vessels during a routine inspection.

Anickel-copper thermal spray coating had been applied in both of these
vessels approximately 20 years ago.
Initially, the coating performed as expected,
however after some time, localised damage was
 evident on the bottom third of both vessels.
Initially, various organic coatings were chosen
for patch repairs to the bottom of the shell.
These coatings needed to be applied repeatedly
 every 4-5 years. In 2014, the inspection revealed
 areas where both the historical nickel-copper
thermal spray (TS) coating and organic patch
repairs had worn away completely, leading to
deep pitting and metal wastage beyond existing
corrosion allowance.

Moreover, when weld repairs were attempted
 adjacent to the failing TS, a crack had formed
on its heat affected zone. In 2017, the refinery’s
engineers decided they needed a more 
permanent solution.

EVALUATION
IGS inspected the vessels and proposed a
 High Velocity Thermal Spray (HVTS) cladding
solution to stop corrosion for the expected life
of the asset, without any further maintenance 
anticipated for at least the next 15+ years.
HVTS technology utilises next generation
 alloy materials, which offer erosion-corrosion 
protection, even in HT/HP service up to 1371°C/2500°F. Both organics and weld overlay
 were evaluated as potential options by the
refinery and, given the upgraded metallurgy
 and the time savings of HVTS compared with
overlay, the refinery project manager welcomed
 this proposal and HVTS work was scheduled for
the spring of 2019.

APPLICATION

IGS started by clean blasting each unit, allowing
the refinery’s QC to inspect and address any
issues with the vessels’ substrate.

The Overhead 
Accumulator required no mechanical work,
whereas the First Stage Desalter needed to be
 welded due to the deep pitting that occurred
due to the poor quality of the previously applied
 coating.

TS COATING FAILURE

Typical Thermal Spray (TS) coatings are not
suitable for internal protection of mission critical 
process equipment due to their permeability,
 weaker bond strength and propensity to
cracking. TS systems cannot produce flat and
tightly-packed particle sizes or nano-scale grain 
structures, leading to the coating’s failure due to
corrosion and/ or permeation.
In addition, TS materials’ cladding particles 
form surface oxides in flight. These oxides then
 form part of the applied TS structure and are
permeable, creating a pathway for corrosive media to penetrate through the coating 
leading to subsequent substrate corrosion, and 
premature TS failure.
Moreover, being applied at high thicknesses, TS
 structures are inherently highly stressed, and such
highly stressed coatings have a high propensity to
 crack and subsequently fail in service.

COMPLETED APPLICATION

HVTS application was completed on time
and within schedule, even accounting for
unexpected delays. The bottom third of the
overhead accumulator, including the stem
 pipe with a vortex breaker and a flange, were
protected with HVTS. With regards to the crude
 desalter, HVTS was applied from the five o’clock
to the seven o’clock position of the vessel and
 the manway.

INSPECTION
Final inspection of the work scope involved
the generation of a cladding thickness record,
with mapped electromagnetic stand-off gauge
(MLO) readings on a defined reference grid 
across the area protected. This record will be
used for future inspection and verification of
the cladding’s integrity. HVTS cladding systems 
can be readily inspected visually for any signs
 of deterioration, and thickness measurements 
taken with a magnetic liftoff gauge.
IGS has invested heavily in the development
and optimisation of its High Velocity Thermal
Spray (HVTS) process technology, materials
 technology, application procedures and field 
personnel capability. Decades of successful 
cladding applications, coupled with fast 
mobilisation and turnkey applications, are
 helping position IGS as a go-to service provider
for oil, gas and petrochemical sites worldwide. ■

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