Dr Gary Heath looks at products available to support maintenance managers in their daily fight against wear
Erosion, abrasion, corrosion and cavitation are the main wear mechanisms reducing lifetime of machinery components in power plants. The most important factors influencing these wear types are: the content, the mass, the hardness, the relative velocity, humidity, heat and the angle of attack of the particles.
Working alongside plant maintenance managers, many coating technologies have been developed through the years using diverse processes which have enabled these coating to reach unimaginable wear resistances.
Thermal spray processes
Together with its maintenance experts, Castolin Eutectic has developed ranges of products for most thermal spray processes to support maintenance managers in their daily fight against wear. High Velocity Oxy/Fuel (HVOF), Arc spray, cold and hot powder flame spraying, plasma transferred arc welding (PTAW) or cold polymeric materials, are some coating processes that have been successfully used for wearfacing in power plants.
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Depending of the part and its working conditions, different coating processes are recommended:
Spray and fuse: is the application of a special Ni-based hard metal powder by a flame spray process, followed by the subsequent fusion of this alloy to the substrate forming a metallurgical bond. Castolin Eutectic has developed a dedicated process and metal powder ranges. Successful power plant applications are drive shafts (rebuilding worn parts and replacing hard chrome), protection boiler tubes against erosion wear and corrosion, providing a smooth wear resistant surface in feeders and pumps, ball valves, ash screw feeders, etc.
PTAW: Plasma Transferred Arc Welding is a welding process using metal powder where a high-energy electrical arc is created between the torch head and the base metal. The powder is melted forming a thick liquid metal coating that fuses and metallurgical bonds to the substrate. PTAW offers up to 15kg/hr deposition rates and high quality, low dilution coating. Castolin Eutectic has developed a dedicated equipment and powders and applications include Cobalt coatings on valves, transport piping, etc.
HVOF: High Velocity Oxy/Fuel (HVOF) is a 'cold spray' thermal spray process that uses high gas pressure and extremely high velocity to produce a very dense coating. These are the hardest coatings with tungsten carbide HVOF is increasingly being used as hard chrome replacement. HVOF coating can be performed on all substrates including aluminium, cast iron, complex steels and does not effect their properties. Power plant applications include erosion resistant coatings of turbine blades, boiler tube panels, coal feeders, burner nozzles, etc.
Arc Wire Spray: using a pair of wires which are melted by an electric arc. No combustion gases are involved, so it is often performed on site and it also has the highest spray rates and lowest running costs of all the thermal spraying processes. Castolin Eutectic has developed a dedicated equipment for on site and workshop work. Applications include protection of exhauster fans, superheater tubes, mill housings, cyclones, etc.
Extensive laboratory testing and field usages have shown that innovative coating techniques have produced cost effective maintenance systems and are providing long-term financial and performance benefits.
Coating technologies have established themselves as one of the best option for the maintenance manager to achieve their daily targets reducing operating costs and improved availablity by extending machineries lifetime and minimising production shutdowns.
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Dr. Gary Heath is with Castolin Eutectic, Lausanne, Switzerland. www.castolin.com or www.eutech.com
