By Chris Harley
A large mid-Atlantic utility's new outage-to-outage production goals for its 2,052-megawatt coal-fired generating station once were thought to be unattainable due to unscheduled down time. The problem was erosion on induced draft fan airfoils (blades). The erosion had been increasing in recent years due to a combination of factors, including worn separation equipment, the use of lower grades of coal, and reduced combustion temperatures. The result was an increase in unpredicted forced outages, coming as frequently as every three to five months.
The hollow blades of induced draft fans in a coal-fired generating plant are subjected to constant bombardment by particulate matter passing through the system. Because these particles have a higher melting point than the operating temperature in the boiler, they remain intact after the coal has been burned, and fly through the boiler's air system in the form of fly ash. The fly ash impacts the induced draft fan eroding the airfoil, which in time can actually blow a hole in the blade. The fly ash quickly enters the hole, filling the airfoil with ash, which produces a significant weight imbalance. The utility has no choice but to shut down the fan and boiler.
Unplanned outages may have been invisible to the consumer, but they were extremely expensive for the utility. The power necessary to satisfy consumer demand had to be purchased from other sources. During a typical outage, which routinely lasted up to four days, the utility lost the use of the affected boiler (each of the generating plant's three boilers has two induced draft fans). For every inactive boiler hour, the utility was forced to replace nearly 600 megawatts of electricity. At rates ranging from $60 to $200 per megawatt, the cost of replacing lost production during the outages was staggering.
The frequency of the unplanned outages, and the resultant costs, prompted the utility to seek a solution to the problem. Recognizing that it was impossible to prevent particulate matter from passing through the system, the utility decided to concentrate its efforts on making the system more resistant to damage from fly ash. Several alternatives were tried, without success. The last of these trials involved the application of a spray-and-fuse coating. Blade liners that underwent this treatment resisted the fly ash for three to four months Ð a duration only slightly better than that of the standard carbon steel airfoils themselves.
A representative of Howden Fan who was familiar with the per cent coatings available from Conforma Clad brought them to the utility's attention in August 1997. Based on the results of testing by the EPRI (Electric Power Research Institute), which cited them as the tungsten carbide coating most resistant to fly ash erosion of nearly thirty materials tested, Conforma Clad's engineers were confident they could help the utility solve its problem.
A look at Conforma Clad's unique process explains its success in difficult applications where other methods fail. The process involves the creation of high-density cloth composed of millions of size-controlled particles of tungsten carbide. Constituent particles, density and thickness are precisely controlled to provide the required protection exactly where it is needed. A cloth made from engineered braze alloys (primarily nickel and chromium) is made in a similar manner, and applied to the substrate material over the tungsten carbide cloth. The coated part is then placed in an atmospherically controlled furnace, where infiltration brazing takes place.
The result is a cladding of tungsten carbide particles imbedded in a matrix of braze alloy. The particles are bonded metallurgically to each other and to the substrate, with bond strength (up to 70,000 psi) Ð five to seven times stronger than is found with most spray coatings. The coating is virtually 100 per cent dense, with no interconnected porosity. The carbide gives superior resistance to abrasion, while the matrix provides ductility and resistance to erosion and corrosion.
Using this unique process, Conforma Clad's engineers contributed another, unexpected solution to the utility. Airfoil erosion shields found in induced draft fans typically are made of two heavy pieces of steel. Hardface weld overlays or sprays protect the shields. One piece is a liner that conforms to the geometry of the airfoil, while the other is a horseshoe-shaped piece for centerplate protection. The two pieces are joined by use of a welded seam. Utility personnel determined that the highest erosion occurred at the junction between the centerplate and airfoil pieces. Regardless of the wear protection material used, all previous installations had one thing in common: the area most vulnerable to erosion had the least protection. This was inevitable, given the low carbide loading, low bond strengths, heat affected zones, porosity, and check cracking inherent with most weld overlays.
Our solution differed in two important ways. First, precise measurements taken directly from the fan convinced us that a one-piece liner that would protect both the centerplate and airfoil was feasible. More importantly, the proposed design would allow for a continuous coating, even in the vulnerable junction between the centerplate and airfoil, without the need for an exposed weld seam.
The second major difference was weight. The coating is much lighter than typical weld overlays, and the process is much less dilutive of the substrate material, therefore it can be applied to thinner substrates. The resultant weight reduction gave the utility a choice it never had before. It could reduce the weight of the entire installation, thus improving efficiency, or it could use the savings to extend wear protection to areas not previously protected. The utility opted for the latter, and was able, for the first time, to provide additional protection in the form of liners for the trailing edge.
We were so confident our design would improve the situation that we offered an 18-month guarantee. Within two weeks of its on-site measurements, the first set of new liners was ready for installation. After six-months of uninterrupted operation, the utility was so pleased with the results that it authorized further installations.
Fans protected with Conforma Clad have been in operation for more than 18 months now, without being the cause of an unplanned outage. One of this utility's primary causes of expensive, unplanned outages has been eliminated. The application of Conforma Clad's protection is not limited to induced draft fans. The utility has also installed the company's liners on all of its scrubber booster fans.
The next target of investigation is the facility's ash conveying pipe. The fact is, the coatings can extend the life of almost any metallic component that is exposed to the effects of abrasion, erosion or corrosion.
We are actively working with utilities throughout North America to identify and implement cost-effective solutions to the problems of premature wear.
Chris Harley is a senior applications engineer with Conforma Clad Inc. ET