ly used thermal-spray coatings. In addition, using Conforma Clad cladding allows substituting less-expensive carbon steel screens rather than using stainless steel or ultra-hard alloys such as Inconel for building screens. One coal-fi red power plant installed stainless steel
screens to protect its SCR equipment. In less than six months the screens were torn apart and had to be replaced. After installing Conforma Clad screens, the components showed little wear after nine months. Other types of protection may guard against one form of
wear, but Conforma Clad tungsten carbide cladding provides unmatched resistance against abrasion, impact, erosion and corrosion and also creates a true metallurgical bond. Conforma Clad popcorn screen capabilities include screens up to 4 × 12' (1.2 × 3.7 m) (longer lengths may be possible), with screen openings 1/4" (6.35 mm) and larger. Screen opening size may need to be adjusted to accommodate coating thickness.
Valve Coatings for Supercritical Steam Operations Recently, a major US-based power company experienced new challenges with the supercritical steam drain and vent lines in one of its plants. Kennametal Stellite together with Velan (Montreal, QC, Canada), a world leader in valve design and engineering for a broad range of industrial applications, were engaged to resolve the issue. MSBVs (metal-seated ball valves) were being used on drain and vent lines to extract a large quantity of condensate during plant startup in order to get dry, superheated steam rapidly. MSBVs must remain steam-tight to prevent energy loss and maximize plant effi - ciency. This particular fossil-fuel power station runs continu- ously from May to October due to high power demand to run air-conditioning systems. For the balance of the year, the plant only runs when the demand calls for more power during peak usage times. This presented a unique situation. As the plant operation
+ 20% NiCr coating. On balls exposed to supercritical steam, valve coatings were failing after a year and less than 500 mechanical cycles in service, with deterioration extending to regions where there is no contact between ball and seats. Visual examination of damaged components revealed minor frictional wear and a typical stress/fatigue pattern. Testing revealed the dominant failure mechanism to be coating embrittlement due to chrome carbide precipitation in
C2
changed from base load to peaking, every time the plant shuts down or starts up, MSBVs are operated and exposed to su- percritical steam operating conditions. The MSBVs in question were of a fl oating ball design with a fi xed seat, manufactured from forged Inconel 718 PH and coated with a HVOF 80% Cr3
Supercritical steam traps for power plants are made safer and more effi cient with new innovative coatings.
service across the coating binder. Brand-new samples were produced with three new Kennametal Stellite coatings. A bat- tery of tests was performed to assess their behavior for this specifi c application.
Once the cause was known, further tests revealed a spray-and-fuse NiWCrBSi coating more suitable to support high bearing loads when facing thermal shock. This coating is less sensitive to aging at high temperatures and therefore more durable under these very specifi c conditions. An enhanced HVOF coating, produced from mixed (W,Cr) C and WC carbides in a nickel matrix, also emerged success- ful from the tests. While it showed similar ductility compared with the original HVOF Cr3
C2 -NiCr coating, it consistently
provided longer in-service life under 540ºC. Armed with these new coating technologies, new valves
were installed in the plant. They have worked fl awlessly for the past year. As a result, Velan’s MSBV line for the power industry now features two coatings: one designed for regular service—HVOF (W,Cr)C-Ni—and one specially designed for applications involving severe thermal shocks—S&F NiWCrB-
Si—all in keeping with Velan’s motto: “Quality That Lasts.”
85 — Energy Manufacturing 2015
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