FIELD OF THE INVENTION
The present invention relates to an improved method of thermal spraying in
boilers. More particularly, the Invention relates to an improved method of
prevention of erosion in circulating Fluidlsed Bed Combustion boiler.
BACKGROUND AND PRIOR ART OF THE INVENTION
The continuous fluidization of solid substances (ash, coal, limestone etc.) in
fluidized-bed systems results in abrasion, erosion and even corrosion of the
Individual equipment components. These effects are pronounced to varying
degrees, depending on the geometry of the component, the flow conditions and
the solids particles concerned. In Circulating Fluidised Bed Combustion (CFBC)
Boilers, wastage of tubes and other materials can occur anywhere along the gas
path due to entrained solid particles passing through the boiler. One area of
special concern regarding erosion Is the refractory Interface in Combustor water
wall in the CFB boiler. The Figure 1 shows the typical tube above the refractory
lining. Erosion has commonly been found in lower furnace walls immediately

above the refractory interfaces, on water walls near cyclone Inlets and on
furnace roof tubes. Figure 2 shows the gouging of the tubes due to erosion .
These wear marks above the refractory lead to continuous erosion and
eventually leakage of tubes
Maintenance costs for replacing worn tubes are very high and the downtime
associated with unscheduled breakdowns caused by the failure of tubes is a
source of lost revenue. The environment is also very erosive due to a number of
sources.
To overcome the above erosion problems in CFBC one way is to replace the
eroded tubes during the shut down, which is quite cumbersome process in
practice as it takes more time and more labour and even the tube might get
damaged. Further these replacements of tubular panels are not totally effective
during prolonged exposure to erosion.
Japanese patent JP2185961 discloses a method to improve the corrosion and
erosion resistances of a steel tube for a boiler coated with a self-fluxing alloy
such as MSF Ni 1-45 or MSF Co 1 standardized by JIS by previously forming an Al
coated layer on the steel tube. The outer surface of a steel tube is coated with Al
and a self-fluxing alloy containing. Ni-Cr-B-Si-C-Fe-Co, Ni-Cr-B-Si-C-Fe-Co-Cu, Ni-

Cr-B-Si-C-Fe- Co-Mo-Cu or Ni-Cr-B-Si-C-Fe-Co-Mo-W is flame sprayed and fused
on the resulting Al coated layer to obtain a steel tube for a boiler.
Chinese patent CN1858293 relates to surface processing material technology, in
particular, a nano grade modified wear resistant and erosion resistant arc
sprayed wire material. The wire material has coating of Fe or Ni, and powdered
core comprising high carbon ferrochrome 45-60 wt%, Ni 10-20 wt%, ferrosilicon
4-10 wt%, ferroboron 5010 wt%, Cr3C2, TiB2 or TiC 10-15 wt%, micron level TiB2
5-15 wt%, nanometer level Al2O3 5-15 wt%, and optional wollastonite 1-3 wt%,
ZrO2 2-5 wt% and TiO2 5-15 wt%. The material is used mainly for coating the
surface of blower blade, tube wall in power station boiler etc. and the coated
layer can increase the surface life of the said parts.
Chinese patent CN1814852 discloses an invention related to surface engineering
field used in the protection of power station boiler pipelines including welding a
layer of hardness alloy on a cleaned metal matrix with self protection metal core
wire, in which, the weld current is 180-220A, voltage is 18-22V, the welded
metal matrix is sprayed with sand to eliminate the oxidized skins, rust and the
old coating and expose the metallic brightness when a compound layer of anti-
erosion and anti-abrasion is sprayed on the matrix by an arc spraying method.

Canadian patent CA1067354 discloses a method of boiler tube coating wherein
steel tubes or pipes, for example, steel boiler tubes and/or integrated panels of
steel boiler tubes, are provided with a fused overlay of corrosion and erosion
resistant coating comprised of a refractory hard component, e.g. tungsten
carbide, dispersed through a corrosion resistant matrix alloy.
US patent US7066242 discloses a method of prevention of erosion of boiler
tubes. The refractory shield assembly comprises: a semi-circular, elongate, metal
shield; a plurality of spaced apart anchors protruding from the front surface of
the shield; a layer of abrasion-resistant refractory material overlying the surface
and extending between and engaging the anchors, whereby the refractory
material is held on the shield by the anchors; and means, such as clamps, for
securing the shield on a boiler tube. The refractory shield assembly functions to
protect the underlying boiler tube from erosion by a stream of hot combustion
gas containing particulates.
Accordingly, there remains a need in the art for quicker and improved coating
methods and coating compositions that provide effective protection against
erosion resistance, such as is required for CFBC boiler components. Improved
coating methods and/or coating compositions on regions of boiler components
desirably need coatings with a combination of high erosion resistance, low
residual stresses, and higher thickness to provide a coating with long life and

high erosion resistance under Indian CFBC boiler operating conditions having
coal with high ash content or lignite .
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose an improved method of
prevention of erosion in CFBC boilers which is capable of resulting in increased
life of CFBC boiler tube against erosion.
Another object of the invention is to propose an improved method of prevention
of erosion in CFBC boilers which enables in situ recoating of the boiler tube and
repairing localised defects inside the boiler even during a scheduled outage.
An yet another object of the invention is to propose an improved method of
prevention of erosion in CFBC boilers which enables significant operational cost
savings.
A further object of the invention is to propose an improved method of prevention
of erosion in CFBC boilers which is capable of producing a dense, low porosity
coating of high bond strength.

At the outset of the description, which follows, it is to be understood that the
ensuing description only illustrates a particular form of this invention. However,
such a particular form is only an exemplary embodiment and the teachings of the
Invention are not intended to be taken restrictively.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - schematically shows a prior art process of not providing any protection
coating over the tube for erosion protection.
Figure 2 - schematically shows the tube above the insulation lining which has
worn out or gouging of the tube
Figure 3- shows a taper section coating layer by thermal spraying according to
invention
DETAILED DESCRIPTION OF A PREFERRED EMBODIMNET OF THE
INVENTION
This invention relates to the vertically positioned water wall tube or combustor
wall such as found in circulating fluidized bed boiler. The water wall has a upper
end and lower end and is formed from a set of vertically positioned parallel metal
tubes interconnected by plates or webs. The combustor wall has fire side in front

of the plate/web and a cold side behind the web. A vertically positioned
refractory wall is positioned against the bottom side of the fire side of the water
wall tube, the wall covers the lower end of the tubes. During the boiler
operation, it has been found that the significant gouging (erosive wear) of the
exposed boiler tubes occur just above the refractory wall as shown in fig.2. As
shown in Fig.3 in this invention, the improvement is the water wall tubes with a
section of increased side wall thickness (4) just above the combustor wall. The
section of side (fire side) wall thickness of atleast 0.25 mm or atleast 250
microns and smoothly and gradually tapers down to a section of tubes having
uniform wall thickness. The taper region(4) extends over distance of more than
300 mm upwardly tapering in any geometry from the thickest coating section to
zero.
Thermal spray coatings is an effective approach that offers advantages because
they allow in situ recoating of the boiler tubes with the additional ability to repair
localized defects inside the boiler. The ability to recoat boilers during a scheduled
outage is especially attractive because downtime can be minimized, which
translates into significant operational cost savings.
Referring now to the drawings as shown in fig.l, no coating is applied over the
tube in the prior art. The erosion resistance coating on a boiler panel is shown in
Figure 3 is to be made with the least possible outlay by the process according to
the invention. As per Figure 3, the coating is completely made by making use of

only High velocity Oxyfuel (HVOF) thermal spray process using the powder of
following chemistry in the tapered section. Of all the different prior art deposition
processes, HVOF yield the most dense erosion resistant coatings and as such, is
generally preferred for forming erosion resistant coatings.
The HVOF (High Velocity Oxy-Fuel) process efficiently uses high kinetic energy
and controlled thermal output to produce dense, low porosity coatings that
exhibit high bond strengths. This process uses an oxygen-fuel mixture.
Depending on user requirements, LPG gas may be used as the fuel in gas-fueled
spray systems and kerosene as the fuel in liquid-fueled systems. The coating
material, in powdered form, is fed axially through the gun, generally using
nitrogen as a carrier gas. The fuel is thoroughly mixed with oxygen within the
gun and the mixture is then ejected from a nozzle and ignited outside the gun.
The ignited gases surround and uniformly heat the powdered spray material as it
exits the gun and is propelled to the workpiece surface. As a result of the high
kinetic energy transferred to the particles through the HVOF process, the coating
material generally does not need to be fully melted. Instead, the powder
particles are in a molten state and flatten plastically as they impact the
workpiece surface. The resulting coatings have very predictable chemistries that
are homogeneous and have a fine granular structure. The smooth, as-sprayed
surface, uniform chemistry and low porosity of the coating can be finished to
very smooth surface profiles.

Initially for the establishment of the HVOF spraying procedure, spraying is
carried out in thin carbon steel trips of 1.2 mm thickness. Prior to spraying, the
grit blasting is done on the sheet metal and procedure adopted is summarized in
Table 1.
Table-1 Typical approach for surface preparation prior to coating

HVOF spraying is carried out with the gas based system and the typical
parameters used for spraying is summarized below:


For assessing the quality of the spraying of chromium carbide over the
substrate, first the sprayed sheet samples from low carbon steel substrate of 25
mm, 150 mm, 1.2 mm are subjected to bond test and bend test as per AWS C
2.18-2002 specification on thermal spraying. Bond test was carried out to check
the cohesive or adhesive strength of the coated specimen as per ASTM C633.
The bond test values of HVOF samples are reported in Table 3 and it is found
that the HVOF coated sample value is good. The bend test was carried out in the
standard low carbon steel substrate of 25 mm, 150 mm, 1.2 mm and 5 bend test
samples were prepared and tested. All the five samples exhibited no spalling or
cracks and indicated that the integrity of coating is good. Subsequently, for
ensuring the taper section coating through HVOF on the combustor walls, a
special procedure was developed by spraying on the boiler tube samples.
Table 3 . Bond strength measurement for establishing the procedure

The important aspects for HVOF spraying of a coating of chromium carbide
(75%) with nickel chromium (25%) in CFBC combustor wall is the taper section
(4) of fig 3 of the coating in the erosion prone area which is the embodiment of
the invention. As shown in the fig.3 the section of the outer surface of the tube

tapers off from the area of maximum wall thickness in a direction away from the
refractory wall. This is done by substantially building up the side wall thickness of
the tube by HVOF coating to a level of 300 microns which gradually and
smoothly tapers to the outer diameter of the tube. The side wall thickness
increase provides material for forming characteristic pattern which in turn alters
the localized erosion. This in turn reduces further erosion and slows down
significantly on going tube damage and possible premature failure.
For production of taper section coating layer, the coating layer was built up layer
by layer There is a requirement to build as high as 300 micron thickness and
each pass around 20 micron thickness coating layer is built up. To create for a
length of 300 mm taper section, first pass of coating is done throughout the
length thus creating a coating layer of 20 micron and then the second pass of
coating is done and stop little ahead of 300 mm length and this procedure is
extended till the layer of 300 micron is built in one end and gradually reducing to
the other end with one pass layer of 20 micron. Thus the taper section layer is
developed in the tube.
Subsequently, with the established procedure, the spraying is carried out in a
CFBC installation in the combustor wall corners where lignite is used as fuel and
the performance of the coating is good.
The advantages of the invention can be seen, inter alia, in the fact that the
HVOF spraying can be done by making use of simple manual spraying

equipment and can be taken to any elevation in the CFBC boiler site for
combustor walls. Further, the old method of not providing the coating on the
erosion prone area requires lot of lead time in the replacement of the combustor
walls and also costly. Moreover, this method will be suitable for providing life of
more than two years.
The maximum thickness of coating of 300 microns is on the fire side of the tube
directly facing the erosion environment in the fluidized boiler. The fluidized bed
boiler having a plurality of tubes. The tubes are provided after thermal spraying
an increased sidewall thickness of at least 250 micron at locations where the
tubes in the fluidized boiler are susceptible to gouging. The tubes are provided
with a tapered coating sections with increased thickness section smoothly and
gradually tapers away from the locations where the tubes susceptible to gouging
to nil over a distance of at least 300 mm. This method of thermal spraying
results in increased life of CFBC boiler tube of more than 2 years against erosion
in actual operation.

WE CLAIM
1. An improved method of prevention of erosion in circulating Fluidised Bed
Combustion boiler comprising the steps of:
spraying a coating of chromium carbide with nickel chromium in CFBC
Combustor wall;
spraying the first coating of at least 20 microns thickness for a length of at
least 300 mm;
spraying second coating of at least 20microns for a length less than the
first;
spraying third coating of at least 20 microns for a length less than the
second;
continuing the spraying process;
characterised in that spraying continues till a taper section of coating of at
least 300 microns at one end and at least 20 microns at the other end
develops,
2. An improved method as claimed in claim 1, wherein composition of
coating is 75% chromium carbide and 25% nickel chromium.

3. An improved method as claimed in claim 1, wherein 300 microns of
thickness of coating is on the fire side of the tube.
4, An improved method as claimed in claim 1, wherein the coating is high
velocity oxyfuel thermal spray process.

An improved method of prevention of erosion in CFBC boiler consists of HVOF
thermal spraying a first coating of at least 20 microns for a length of at least 300
mm and second coating of at least 20 microns for a length little less than the
first process of spraying continues till a taper section of coating of at least 300
microns at one end and at least 20 microns at the other end develops. The
composition of coating is 75% chromium carbide and 25% of nickel chromium.