The importance of Inspecting Forced Draft and Induced Draft Fans

As part of Thielsch Engineering’s condition assessment and balance of plant inspection programs, we include the examination of the forced draft and induced draft fan components. These items are often overlooked as they are tucked away in the back of the station or external to the main powerhouse. However, these components represent a significant safety hazard, weighing as much as a ton with the ability to reach blade tip speeds of over 800 miles per hour at full operation, these components generate a tremendous amount of stress. Photographs of a typical power utility fan are provided in Fig. 1.

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Thielsch Engineering has performed more than 150 fan inspections over the last fifteen years, 20 of which involved utility fan failure investigation, which resulted in catastrophic failure with significant collateral damage. The failure mechanisms have ranged from poor design considerations, improper welding techniques, bearing failures and normal operational deterioration. The fan airfoils and shroud materials are typically produced from high strength, hardened alloy steels, made in accordance to ASTM A-517 covering “Pressure Vessel Plates, Alloy Steel, High Strength, Quenched and Tempered”.

Specific designs result in high stressed joint locations mainly at the airfoils to center plate welds. During operation, these fabricated components are subjected to varying load capacities, seasonally temperature fluctuations and particulate damage, these factors also contribute to fatigue damage mechanisms.

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