A large eastern utility’s outage-to-outage production goals for its 2,052-megawatt coal-fired generating station once were unattainable because of 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 unpredictable forced outages, coming as frequently as every three to five months.

Unscheduled outages seemed inevitable, because fly ash constantly bombarded the hollow blades of the induced draft fan. This bombardment erodes the airfoil, eventually blowing a hole in the blade. Then the fly ash fills the hole, causing a significant weight imbalance. As a result, the fans trip and the boiler shuts down.

During a typical outage lasting 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 had to replace nearly 600 megawatts of electricity. And they paid as much as $200 per megawatt.

Recognizing 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 remedies met with failure.

Earlier testing by EPRI (Electric Power Research Institute) came to the utility’s attention. The test results cited Conforma Clad’s tungsten carbide coating as the most resistant to fly ash erosion of nearly thirty materials tested. Conforma Clad® coatings consist of precisely size-controlled tungsten carbide particles embedded in a matrix of braze alloy (primarily nickel and chromium). The particles bond metallurgically to each other and to the substrate, with bond strength up to 70,000 psi. The carbide gives superior resistance to abrasion, while the matrix provides ductility and resistance to erosion and corrosion.

Airfoil erosion shields typically consist 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. A welded seam joins the two pieces. The utility determined the highest erosion was at the junction of the centerplate and airfoil. 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.

The Conforma Clad solution differed in two important ways. First, precise measurements taken directly from the fan showed that a one-piece liner would protect both the centerplate and airfoil. This design provided a continuous coating, even in the vulnerable junction of the centerplate and airfoil, without the need for an exposed weld seam. The second major difference was weight. The Conforma Clad coating is much lighter than typical weld overlays, and the process is much less dilutive of the substrate material. Therefore you can apply it to thinner substrates. The resultant weight reduction meant the utility 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.

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 causing of an unplanned outage. Thus, Conforma Clad eliminated a primary cause of expensive, unplanned outages.