Claims:WE CLAIM:
1. A shield for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace that is operably connected to the waste heat recovery boiler, the shield comprising:
a metal guard adapted at a predetermined position on an outer surface of the boiler tube, an inner surface of the metal guard and the outer surface of the boiler tube defining an enclosed space therebetween when the metal guard is adapted on the boiler tube; and
a thermally conductive material disposed in the enclosed space so as to allow heat from the flue gases in the vicinity of the metal guard to be transferred to the boiler tube.

2. The shield as claimed in claim 1, wherein a pair of metal plates is provided in a parallel relation, each plate extending along the length of the boiler tube for allowing the metal guard to be adapted to the boiler tube, and wherein the inner surface of the metal guard, the outer surface of the boiler tube and a portion of each of the metal plates near to the boiler tube forms the enclosed surface within which the thermally conductive material is disposed.

3. The shield as claimed in claim 1, wherein the metal guard has an arcuate cross-section.

4. The shield as claimed in claim 1, wherein the material of the metal guard is steel.

5. The shield as claimed in claim 1, wherein the metal guard has a thickness in the range of 3 mm - 6 mm.

6. The shield as claimed in claim 1, wherein the thermally conductive material is a mixture of a heat conducting compound and aluminium powder wherein the heat conducting compound comprises of clay, silicon, grog and water.

7. The shield as claimed in claim 6, wherein ratio of heat conducting compound to aluminium powder is within the range of 50:50 to 70:30.

8. The shield as claimed in claim 1, wherein the thermally conductive material has a thickness in the range of 2.5 mm to 3.5 mm.

9. A method for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace that is operably connected to the waste heat recovery boiler, the method comprising:
identifying corrosion susceptive zones on an outer surface of the boiler tube;
applying a thermally conductive material on the identified zones of the boiler tube; and
adapting a metal guard on the tube applied with the thermally conductive material such that the material is disposed between the tube and the metal guard, the thermally conductive material allowing heat transfer from the flue gases in the vicinity of the metal guard to the boiler tube.

10. A method for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace that is operably connected to the waste heat recovery boiler, the method comprising:
identifying corrosion susceptive zones on an outer surface of the boiler tube;
adapting a metal guard on the identified zones defining an enclosed space between an inner surface of the metal guard and the outer surface of the boiler tube; and
disposing a thermally conductive material in the enclosed space formed between the metal guard and the boiler tube, the thermally conductive material allowing heat transfer from the flue gases in the vicinity of the metal guard to the boiler tube.

Dated this 7th day of March 2017

Hindalco Industries Limited
By their Agent & Attorney

(Adheesh Nargolkar)
Of Khaitan & Co
Reg No IN/PA-1086
, Description:FIELD OF THE INVENTION
[001] The invention relates to a shield for preventing erosion and corrosion of a water boiler tube, and more particularly to a shield for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace.

BACKGROUND OF THE INVENTION
[002] In a typical waste heat recovery boiler, water circulates through tubes exposed to hot flue gases arising out of smelting and converting furnaces, gas exhausts, incinerators etc. When hot flue gases pass over the tubes, heat exchange takes place between the surface of the tube and flue gases. Such heat is ultimately transferred to water circulating in the tubes. For the purpose of recovering heat from flue gases and to withstand high temperatures of the flue gases, tubes are generally made of steel.
[003] Flue gases from smelting and converting furnaces generally contain particulates such as fly ash, metal particles, etc. generally in the form of fine solids suspended in the flue gases and partially smelted concentrates along with elements like arsenic, fluorine, zinc, lead, SO2, etc. Particulates pass over the tube surface and cause erosion of the tubes over a period of time. Partially smelted concentrates get deposited on the surface of tube and undergo oxidation resulting in smelting reaction. Such oxidation is exothermic in nature and therefore causes localized overheating of the tubes. As a result of localized overheating of the tubes, SO2, present in the vicinity, undergoes oxidation forming SO3. Due to particular flow pattern of the flue gases, certain cold zones are created inside the boiler. As a result, after the oxidation of SO2, SO3 hydrates to form H2SO3 – sulfurous acid which may further oxidize to H2SO4 – sulfuric acid. Such hydration is further promoted by the presence of lead and zinc which lowers the hydration temperature of SO3.
[004] Sulfurous acid and sulfuric acid are highly corrosive mineral acids which when come in contact with steel cause corrosion. Accordingly, these acids when present in the environment of the boiler, cause corrosion of the boiler tube and may quickly eat away the surface of the boiler tube. Additionally, arsenic and fluorine present in the flue gases also react with steel thereby speeding up the corrosion of tubes. As a result, thickness of the tube decreases at the affected localized areas making the tube vulnerable to puncture.
[005] Since boilers operate at high pressure, chances of boiler tubes getting punctured at the affected localized areas due to corrosion and erosion increases. A punctured tube causes water leakage from the boiler tubes thereby leading to lowering of boiler drum level and further increases other associated problems. In addition to this, leaking water reacts with corrosive elements present on the boiler tube surface to cause further corrosion of the nearby tubes. Moreover, maintenance of such leakage areas requires the boiler to be shut down for a long period of time which creates huge loss of plant uptime and productivity.
[006] To obviate the problem of corrosion, duplex steel is generally proposed as a material for boiler tubes. While such duplex steel demonstrates overall better corrosion resistant properties than standard steel, however cost of such material is substantially high and moreover requires redesigning of the boiler to compensate for reduction in coefficient of thermal conductivity due to change in material.
[007] Alternatively, omega type tube construction over panel construction may be proposed to minimize exposure of the tubes to flue gases thereby reducing corrosion. However, such construction will require redesigning of the boiler to compensate for reduction in available surface area for heat transfer.
[008] Thus, there is a need to develop measures that obviates at least some of the above noted drawbacks, which is economical and does not inhibits the heat transfer of the flue gases to the inside of the boiler tubes.

SUMMARY OF THE INVENTION
[009] Accordingly, the present invention in one aspect provides a shield for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace that is operably connected to the waste heat recovery boiler. The shield having a metal guard adapted at a predetermined position on an outer surface of the boiler tube. An inner surface of the steel metal guard and the outer surface of the boiler tube define an enclosed space therebetween when the metal guard is adapted on the boiler tube. The shield further includes a thermally conductive material disposed in the enclosed space so as to allow heat from the flue gases in the vicinity of the metal guard to be transferred to the boiler tube.
[010] In an embodiment of the invention, a pair of metal plate is provided in a parallel relation wherein each plate extends along the length of the boiler tube for allowing the metal guard to be adapted to the boiler tube, and wherein the inner surface of the metal guard, the outer surface of the boiler tube and a portion of each of the metal plates near to the boiler tube forms the enclosed space within which the thermally conductive material is disposed.
[011] In another embodiment of the invention, the metal guard is made of steel and has an arcuate cross section. Further, the metal guard has a thickness in the range of 3 mm - 6 mm.
[012] Yet in another embodiment of the invention, the thermally conductive material has a thickness in the range of 2.5 mm to 3.5 mm and is a mixture of a heat conducting compound and aluminium powder wherein the heat conducting compound comprises of clay, silicon, grog and water. In this regard, ratio of the heat conducting compound to aluminium powder is within the range of 50:50 to 70:30.
[013] In another aspect, the present invention provides a method for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace that is operably connected to the waste heat recovery boiler. The method includes the steps of - identifying corrosion susceptive zones on an outer surface of the boiler tube, applying a thermally conductive material on the identified zones of the boiler tube, and adapting a metal guard on the tube applied with the thermally conductive material such that the material is disposed between the tube and the metal guard. The thermally conductive material allows heat transfer from the flue gases in the vicinity of the metal guard to the boiler tube.
[014] Yet in another aspect, the present invention provides a method for preventing erosion and corrosion of a waste heat recovery boiler tube due to flue gases extracted from a smelter furnace that is operably connected to the waste heat recovery boiler. The method includes the steps of - identifying corrosion susceptive zones on an outer surface of the boiler tube, adapting a metal guard on the identified zones defining an enclosed space between an inner surface of the metal guard and the outer surface of the boiler tube, and disposing a thermally conductive material in the enclosed space formed between the metal guard and the boiler tube. The thermally conductive material allows heat transfer from the flue gases in the vicinity of the metal guard to the boiler tube.

BRIEF DESCRIPTION OF DRAWINGS
[015] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Figure 1 is a perspective view of a section of a waste heat recovery boiler tube showing a shield adapted on the tube, in accordance with an embodiment of the present invention;
Figure 2 is a cross sectional view of the shield adapted on the waste heat recovery boiler tube of Figure 1;
Figure 3 is a perspective view of a boiler panel of a waste heat recovery boiler, in accordance with an embodiment of the present invention; and
Figure 4 is a cross sectional view of the boiler panel of the waste heat recovery boiler of Figure 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE PRESENT INVENTION
[016] The present invention relates to a shield for preventing erosion and corrosion of a waste heat recovery boiler tube from hot flue gases extracted from smelting furnaces (not shown) which is operably connected to the waste heat recovery boiler.
[017] Figure 1 shows a section of a waste heat recovery boiler tube 110 having a shield 120 adapted on the tube 110 in accordance with an embodiment of the invention. A pair of metal plates – a first metal plate 130a and a second metal plate 130b is provided in a parallel relation such that each plate extends along the length of the boiler tube 110. Preferably, each of the metal plate 130a, 130b extends along an axial direction of the tube 100 and welded to an outer surface of the tube 110 to form a unit. Plurality of such units are joined together along the length of the metal plates 130a, 130b to a form a boiler panel 300 as shown in Figures 3 and 4. The boiler panel 300 as shown in Figures 3 and 4 has plurality of tubes 110 placed in a spaced-apart parallel relationship. Such boiler panel 300 generally forms wall of the boiler.
[018] Hot flue gases arising out of a smelting and converting furnace enter the waste heat recovery boiler in a turbulent flow pattern which changes to a laminar flow or a straight flow. Such flue gases travel through a passageway defined by the walls of the boiler to pass over the boiler panels 300. Due to such typical flow pattern of the flue gases, all tubes 110 on a boiler panel 300 may not be exposed to the flue gases and therefore may not be susceptible to corrosion. Even entire surface area of a single tube 110 may not be exposed to the flue gases due to typical flow pattern of flue gases. Accordingly, corrosion susceptive zones are identified on an outer surface of the boiler tube 110 in advance. In this regard, the shield 120 is provided on the predetermined positions on an outer surface of the tube 110 thereby preventing any erosion and corrosion of the tube 110 due to flue gases resulting in enhancing the life of the tube 110.
[019] More particularly, the shield comprises a metal guard 140 and a thermally conductive material 150. The metal guard 140, which is preferably arcuate in shape according to one embodiment of the present invention, is adaptable on an outer surface of the tube 110 and the thermally conductive material 150 is sandwiched between the metal guard 140 and the tube 110. As shown in Figure 2 and Figure 4, the metal guard 140 has an outer surface 140a, an inner surface 140b, a first side edge 140c and a second side edge 140d. When the metal guard 140 is placed at the predetermined positions of the outer surface of the tube 110, each metal plates 130a, 130b receives and adaptably engages with the corresponding side edges 140c, 140d of the metal guard 140. The inner surface 140b of the metal guard 140, the outer surface of the tube 110 and a portion of each of the metal plates 130a, 130b near to the boiler tube 110 forms an enclosed space within which the thermally conductive material 150 is disposed. In this regard, as shown in figure 2, the side edges 140c, 140d of the metal guard are welded on the metal plates 130a and 130b respectively to secure the metal guard 140 on the tube 110.
[020] In an embodiment of the invention, the metal guard 140 is made of same material as that of the tube 110 for an effective heat recovery from flue gases. Accordingly, the material of the metal guard 140 is steel such as SA 210 Grade C, SA 210 Grade A1, etc. In another embodiment of the present invention, for efficient heat recovery and better erosion and corrosion resistance, thickness of the metal guard 140 is preferably in a range of 3 mm to 6 mm. The above noted embodiments are just for illustration purpose and there could be further obvious modifications to these embodiments made by a person skilled in the art. All these embodiments should be considered to be within the scope of the present invention.
[021] As described above, the thermally conductive material 150 is sandwiched between the metal guard 140 and the tube 110. As shown in Figures 1 and 2, the thermally conductive material 150 is disposed in the enclosed space formed between the inner surface 140b of the metal guard 140, the outer surface of the tube 110 and a portion of each of the metal plates 130a, 130b near to the boiler tube 110. It is important to note here that any heat recovered from the flue gases by the metal guard 140 should be effectively transferred to the tube 110. Accordingly, the thermally conductive material 150 should be chosen to allow transfer of heat from the flue gases in the vicinity of the metal guard 140 to the tube 110. For this reason, it is preferable to have a mixture of a heat conducting compound and metallic powder as a thermally conductive material 150.
[022] In an embodiment of the invention, the heat conducting compound is a mixture of clay – 75%, silicon – 7%, and water – 18%. Additionally, the clay may be a mixture of china clay, fire clay and grog. In a further embodiment of the invention, the metallic powder can be any metal such as aluminum, copper, etc. In other embodiments, the ratio of heat conducting compound to the metallic powder is preferably within a range of 50:50 to 70:30. Additionally, thickness of the thermally conductive material 150 is preferably in a range of 2.5 mm to 3.5 mm to ensure an efficient heat transfer and to avoid any air gap that may damage the thermally conductive material 150 due to such heat. The above noted embodiments are just for illustration purpose and there could be further obvious modifications to these embodiments made by a person skilled in the art. All these embodiments should be considered to be within the scope of the present invention.
[023] The present invention also relates to a method for preventing corrosion of a waste heat recovery boiler tube 110 due to hot flue gases. In the first step, corrosion susceptive zones on the outer surface of the boiler tube 110 are identified. To do so, flow pattern of the flue gases is taken into account to identify such corrosion susceptive zones. In the second step, the thermally conductive material 150 in a paste form is applied on the identified zones of the boiler tube 110. The paste is evenly spread on the identified outer surface of the boiler tube 110. Finally, the metal guard 140 is adapted on the tube 110 such that the thermally conductive material is disposed evenly between the tube 110 and the metal guard 140.
[024] As an alternative to the above method embodiment, the metal guard 140 is adapted on the identified corrosion susceptive zones thereby forming an enclosed space between the inner surface 140b of the metal guard 140, the outer surface of the boiler tube 110 and a portion of each of the metal plates 130a and 130b near to the boiler tube 110. Thereafter, the thermally conductive material 150, preferably in a form of powder or liquid, is disposed in the enclosed space such that no air gap is left.
[025] In both the above noted method embodiments, the metal guard 140 and the thermally conductive material 150 so adapted is preferably left for curing in dry condition for a predetermined period of time. The shield 120 so formed is further flame heated to remove moisture before welding of the metal guard edges 140c and 140d with corresponding metal plates 130a and 130b, before putting it to use during the waste heat recovery boiler operation.
[026] Advantageously, the shield 120 in accordance with the present invention provides better corrosion resistant capability and good heat transfer rate. Furthermore, such shields may be implemented in any existing boiler without undergoing any modification of the boiler.
[027] While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.