Patent Application
20160305651
1. Field of the Invention
This invention relates to a ceramic coating composition for use in boilers, furnaces, and the like, particularly for use on boiler tubes, and to a method for coating boiler tubes and other surfaces exposed to high temperatures to extend the service life and increase the thermal conductivity thereof, the coating being characterized by resistance to heat, erosion, corrosion, slagging, and fouling.
2. Description of Related Art
Boiler tubes generally have a relatively short life due to corrosion and erosion problems which exist in high temperature boilers. The high boiler temperatures together with the flow of hot gases carrying particles, soot and other wear and erosion-causing materials generally result in aggravated wear through the tube walls. Complete tube stacks or panels have been known to require replacement in a matter of months.
In order to avoid or postpone the need to replace boiler tubes and other components exposed to the high temperature environment of a boiler, these components are often coated with a protective coating. The protective coatings used vary, but they all must periodically be reapplied once the coating has corroded and/or eroded to an unsafe condition. The only way to determine the remaining coating thickness is to shut down the boiler, then construct scaffolding and measure the remaining coating with a magnetic lift off device, which is costly and time consuming especially if all surfaces are measured and evaluated to determine high erosive and/or corrosive areas. Alternately the conditions in the boiler could cause slag and fouling materials to deposit on the coating, which decease the thermal efficiency of the boiler and should be removed to restore and maintain boiler performance. During removal of these deposits, the protective coating may be thinned or damaged. Accordingly, the thickness of the protective coating should be checked and measured to determine if the protective coating should be reapplied. During scheduled shut down periods identification of tube wear is of great concern to boiler operators.
An object of this invention is to avoid and overcome the defects and deficiencies of the prior art practices by providing a ceramic composition and method for coating surfaces, such as boiler tubes, to extend the useful life and/or extend the service intervals thereof. Another object is to develop a ceramic coating system, which provides for a quicker and less expensive method of inspecting the thickness of the protective ceramic coating in a boiler. Other objects of the invention will clearly appear when taken in conjunction with the following disclosure and the accompanying drawings.
The present invention is directed to, in general, a dual color, sprayed-on ceramic coating composition and system that when applied to properly prepared fireside boiler tubing will not only reduce corrosion and erosion, caused from burning corrosive or erosive fuels, but will allow for a visual inspection of the remaining ceramic coating thickness.
In several embodiments, the invention is a dual color sprayed on composite ceramic coating configured and formulated to protect fireside boiler tubing while also acting as a visual inspection aid. The coating is an environmentally safe, non-reactive, water soluble composite ceramic that will withstand operating temperatures up to 2000° F. (1093° C.). This coating is ideal for fluidized bed boilers and coal fired utility boilers experiencing tube erosion, corrosion and slagging. This sprayed-on ceramic coating system allows for fast application rates reducing equipment down time. The composite ceramic coating is a high solids system that will withstand severe thermal cycling from −300° F. (−185° C.) to 1600° F. (871° C.). This composite system is applied in two distinct colors. The base (white) coat is formulated to provide high mechanical bonding and is erosion and corrosion resistant. The base coat can be applied at least up to 0.008 thick (203 microns). The top (green) coat can be applied at least up to 0.012 thick (304 microns). This allows for thicker applications to applied in areas of higher wear such as roof tubes, refractory interface areas and corners.
The top coat forms a strong chemical bond to the base coat, has exceptional erosion and corrosion resistance and high emissivity and is thermally conductive. After a significant period of operation, the dual color ceramic coating will allow operators and inspectors to visually see any areas of erosion or corrosion. These areas will begin to show the white base coat. Wear areas can then be addressed before tube thinning or failure occurs. Coating thickness can be easily evaluated using a simple magnetic lift off device. Due to the coatings' high bonding characteristics, if required, it can be brush blasted and re-applied to specification very quickly with limited down time and without removal of any existing ceramic coating.
In one embodiment, the coating is applied in two colors. The base layer is applied at a minimum thickness of 0.006 thick and is white. The top coat can be applied at least up to 0.012 thick and is green. After each layer is applied, the coating is measured with a magnetic lift off device and the thickness of both the white layer and the green layer are recorded separately. As the top (green) layer wears away (due to corrosion or erosion) the bottom (white) layer will be exposed. This composite coating can then be visually inspected with a hand held light from a distance. If the white base layer has not been exposed, then there is no need to address the coating thickness until the next scheduled boiler outage.
In another embodiment, the invention is a method of forming a composite ceramic coating for high temperature environments, including the steps of providing a first ceramic composition comprising: sodium silicate, potassium silicate, zirconium dioxide, and aluminum oxide; providing a second ceramic composition comprising: sodium silicate, potassium silicate, zirconium dioxide, and aluminum oxide, applying the first ceramic composition to a substrate, creating a first ceramic coating having a first color, applying the second ceramic composition to the first ceramic coating of the first color, creating a second ceramic coating having a second color, wherein the first color and the second color are visually distinct from each other. In yet another embodiment, the first ceramic composition further comprises dolomite. In still another embodiment, the second ceramic composition further comprises silicon carbide. In another embodiment, the second ceramic composition further comprises chromium(III) oxide. In another embodiment, the first ceramic composition further comprises dolomite, and the second ceramic composition further comprises silicon carbide and chromium(III) oxide. In yet another embodiment, the first ceramic composition is applied to the substrate as a water based composition. In another embodiment, the second ceramic composition is applied to the substrate as a water based composition. In still another embodiment, the first ceramic composition is applied to the substrate as a water based composition, and wherein the second ceramic composition is applied to the substrate as a water based composition. In yet another embodiment, the dolomite in the first ceramic composition comprises 1-5% by weight of the total solids in the first ceramic composition.
In another embodiment, the invention is a composite ceramic coating including, a first ceramic coating having a first color, the first ceramic coating comprising: sodium silicate, potassium silicate, zirconium dioxide, and aluminum oxide, a second ceramic coating having a second color, the second ceramic coating comprising: sodium silicate, potassium silicate, zirconium dioxide, and aluminum oxide, wherein the first ceramic coating is adhered to a substrate, wherein the second ceramic coating is adhered to the first ceramic coating, and wherein the first color and the second color are visually distinct from each other. In another embodiment, the first ceramic coating further comprises dolomite. In yet another embodiment, the second ceramic coating further comprises silicon carbide. In still another embodiment, the second ceramic coating further comprises chromium(III) oxide. In another embodiment, the first ceramic coating further comprises dolomite, and the second ceramic composition further comprises silicon carbide and chromium(III) oxide. In yet another embodiment, the first ceramic coating is applied to the substrate as a water based composition. In another embodiment, the second ceramic coating is applied to the substrate as a water based composition. In another embodiment, the first ceramic coating is applied to the substrate as a water based composition, and wherein the second ceramic coating is applied to the substrate as a water based composition. In another embodiment, the first ceramic coating comprises 1-5% dolomite by weight. In another embodiment, the first ceramic coating comprises 2-4% dolomite by weight. In another embodiment, the first ceramic coating comprises about 3% dolomite by weight.
These and other features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claim with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”, “longitudinal”, and derivatives thereof shall relate to the invention as it is oriented in the drawings. However, it is to be understood that the invention may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting.
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The charts below show several exemplary coating composition formulations according to the present invention.
While various embodiments of the present invention were provided in the foregoing description, those skilled in the art may make modifications and alterations to these embodiments without departing from the scope and spirit of the invention. For example, it is to be understood that this disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. Accordingly, the foregoing description is intended to be illustrative rather than restrictive. The invention described hereinabove is defined by the appended claims and all changes to the invention that fall within the meaning and the range of equivalency of the claims are to be embraced within their scope.
