With sustained economic growth fueling Indias electricity demand where thermal power plants presently account for almost 60% of Indias installed power generation capacity, many coal-fired power generating stations are continually focusing on improving their efficiency and productivity. This calls for increasing unit availability, reliability and operational life of critical equipment, particularly ensuring uninterrupted operation of boilers, that produce steam to propel turbines for generating power. Power plants are under ever increasing pressure to reduce their production cost to stay competitive and offset the investments needed in complying with stringent environmental norms around the harmful NOx and SOx emission control. Any operational disruption will not only hamper the production but also the profits for each MW-hr sold.
One of the primary causes of forced outages in fossil plants today is the erosion on components like Boiler Tubes and Coal Nozzle Tips (CNT), stemming from continuous operation of boilers. Erosion can be defined as the progressive loss of original material from a solid surface due to its interaction with the impinging fluid or solid particles. In other words, erosion is caused by the impact, cutting action or abrasive wear of small solid particles freely immersed in the direction of flow that frequently undercut portions of the material they strike. It is considered as the most problematic, hard to predict and seemingly increasing mode of failure in a power plant.
Let us take a moment to dwell into the metallurgical properties of surfaces that exhibit erosion. Studies have yielded that when high erosion — resistant particles such as Tungsten Carbide exist in low erosion or soft matrix, the impacting particles can undercut and remove portions of the material. However, if the same high erosion resistant particles are densely packed in a matrix material that causes the impacting particles to impinge on a greater percent of the hard particle, the erosion resistance increases dramatically.
Kennametal offers an innovative solution by way of Conforma Clad tungsten carbide, proven to demonstrate highest erosion resistance. The Conforma Clad material, which is an infiltration brazed tungsten carbide in nickel-chrome-boron matrix, has an erosion resistance particle percentage of close to 70%.
The graphs below demonstrate the results from a high temperature erosion test performed by EPRI (Electric Power Research Institute) where six materials commonly used for the protection of boiler tubes were selected for analyses, of which two samples were with conforma cladding. All the data was obtained using ASTM G76 standardized erosion test. The samples were as follows:
• Conforma Clad WC — Infiltration Brazed
• Nickel alloy 625 — GMAW
• Inconel 622 — GMAW
• 309L stainless steel — GMAW
• Stellite 6 — GMAW
• Amstar 880 – HVCC
Laboratory Erosion Test Results
Table – 1 shows lab test results of 6 materials tested in order of erosion resistance.
Chart – 1 shows graphically the lab test results.
Laboratory Erosion Test Summary
The results indicate that among the 6 alloys tested, Conforma clad outperformed all other samples in terms of erosion resistance.
Having proven its technological superiority in both laboratory tests and field trails, Conforma Cladding is undoubtedly the obvious choice to avoid failures of boiler components that undergo wear and tear, thereby keeping the boilers running continuously and optimizing production costs. Kennametal has successfully deployed this technology at various units of NTPC, other state electricity boards and many captive power plants in severe erosive environment, where its performance has been satisfactory even in a challenging conidtions.
Uniqueness of Conforma Cladding includes
1) Uniform carbide distribution and dense loading of carbide
2) Strong metallurgical bonding
3) High bond strength over 1,00,000 PSI as against 10,000 PSI with other coating
4) High hardness – over 70 HRc
Components that can be value added with Cladding Technology
• Coal Nozzle Tips: More than doubles the life of tips with better heat rates, reduced coal consumption, lower emissions, reduced maintenance costs and reduced thickness of base material.
• Boiler Tubes: High mechanical bond strength in comparison to thermal sprays. Extends life by over 5 times in Indian coal conditions and eliminates the requirement of protective shields / studs.
• Fan Liners: Extend life by 4 times as compared to existing hard facing solutions.