Chemical Engineering October 2020 - 32

FIGURE 3. Weld overlay involves applying layers
of corrosion-resistant materials onto the selected
surfaces
sion and corrosion of the furnace and
wall tubing. Both types of damage
can be exacerbated by chloriderich
ashes that inhibit the growth of
a dense, duplex oxide scale on the
boiler's surface, which would ordinarily
act as a diffusion barrier to prevent
or reduce corrosion and erosion. Additionally,
alkali metals, lead and zinc
can react readily with chlorine and
further contribute to the transport of
chloride-rich ashes to the boiler tube's
metal surface, increasing the corrosivity
of the deposit. Finally, heat flux in
boilers leads to the rapid diffusion of
corrosive substances.
All of these factors serve to decrease
the useful life of boilers in
processing facilities. Nonetheless,
proper boiler maintenance prevents
these issues from escalating and
causing cracks, ruptures, thermal
fatigue or other factors that can further
shorten the unit's lifespan, affecting
reliability and performance of all
plant operations. Also, highly effective
maintenance strategies can help
facilities by optimizing performance
and productivity.
In accordance with these goals,
the waste-to-energy plant scheduled
regular maintenance of its three boiler
lines to maintain optimal boiler and
plant performance, and also planned
a turnaround for one of its boiler lines.
However, the facility experienced
an unexpected failure on that
line
well before its scheduled downtime.
To avoid having two periods of
downtime on the same equipment
within one year, it was necessary to
conduct the planned maintenance
activities at the same time as the
emergency repair.
A team of maintenance engineers
assessed the condition of the boiler,
32
which was found to be eroded and
corroded (Figure 2). After abrasive
grit-blasting operations were conducted
on the structure to remove
impurities from the eroded surfaces,
the team evaluated the state of preservation
of the vessel. Together with
tube-wall thickness readings, the engineering
team could offer a quantitative
analysis on the level of corrosion
and erosion in the equipment and determine
the best course of action.
Further work found heterogeneous
conditions across the tubes,
with areas where the thickness was
compromised, but above 2.5-mm
thickness, and others that were more
heavily eroded or corroded, with
thicknesses as low as 2 mm.
To restore the structural integrity of
60 m2 of membrane walls within the
boiler, where thicknesses above or
equal to 2 mm were found, the suggestion
was made to clad, or weld overlay,
the structures (Figure 3). This method
relies on the application of layers of
corrosion-resistant materials, such
as Inconel 625, an austenic nickelchromium-based
superalloy containing
niobium, on damaged areas in
existing units
Inconel 625 is particularly durable
and resistant to corrosion, erosion and
oxidation, even in harsh environments
with elevated temperatures, such as in
boilers. More precisely, the chromium
component in the material provides
resistance to oxidizing chemicals,
while the high nickel and molybdenum
content make the alloy resistant to
non-oxidizing environments.
Molybdenum also makes the material
particularly resistant to pitting and
crevice corrosion. The added niobium
stabilizes the alloy against sensitization
during welding, thereby preventing
subsequent intergranular attack.
Also, the high nickel content protects
against stress-corrosion cracking
caused by chloride ions, which are
typically found in incinerated waste.
In addition, Inconel 625 exhibits high
fatigue strength, even when it is present
in the form of thin sheets.
Moreover, the thermal conductivity
of Inconel 625 is lower than that of
carbon steel, so the application of thin
layers is ideal to avoid any increase in
surface temperature that would promote
corrosion while optimizing heat
exchange for elevated boiler performance.
As a result, this alloy would
allow the boiler's surface to better
withstand its extreme operating conditions
and increase its lifespan.
The power of automation
To streamline the process, the maintenance
engineers leveraged automation
to perform the weld overlay
procedures. By doing so, the project
could be completed at least three
times faster than using manual cladding,
helping the plant to reduce
the shutdown interval and re-start
operations faster.
For automating such a task, robotic
arms can be used in welding activities
to create weld beads on a surface
that needs repairing (Figure 4). The
robot is moved along the damaged
area on a carriage, which travels on a
laser-leveled track system fixed to the
boiler's surface. All the weld-overlay
process parameters, such as carriage
speed and bead thickness, are
controlled by a programmable logic
controller (PLC; Figure 5).
The control unit also stores different
weld procedures, meaning that
operators can easily select and monitor
the best procedure on the basis
of the intended application via a human-machine
interface (HMI), without
the need to qualify and test new
procedures. Establishing new weld
procedures could otherwise require
up to four weeks. So, utilizing proven,
standardized setups helps substantially
reduce downtime and address
emergency shutdowns as quickly
as possible.
FIGURE 4. To restore the integrity of the boiler's wall,
a robotic arm was used to move a welding torch
and an oscillator, creating weld beads. The robot is
moved along the damaged area by a carriage, which
travels on a laser-leveled track system fixed to the
surface that needs repairing
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