Improved testing safeguards fan reliability
New NDT inspection services and materials help power plant managers avoid costly fan failures
By William C. Chedister, Circle Systems Inc., and Jeffery Long, MQS Inspection
A single broken or disconnected fan blade can induce vibration that renders a fan useless. While power plant fan repair and replacement costs can rise as high as $500,000, the cost is small compared to lost revenue from a long unscheduled outage to complete the job. It is not surprising then that non-destructive testing (NDT) is essential in detecting system component trouble. New-generation test materials and highly experienced service specialists are substantially improving inspection effectiveness.
Fan examination is an exercise in weld inspection. Magnetic particle testing (MT) is the NDT method inspectors most commonly employ, and it is well suited to detect fine cracks that form at or slightly under ferromagnetic welds and surfaces. Inspectors also use ultrasonic testing and radiographic testing to find internal flaws well below the metal surface. Some manufacturers perform NDT on every fan fabrication weld.
Fine surface cracks found with MT are among the clearest impending failure indicators. Cracks are usually on and adjacent to welds that mount the numerous fan blades and around the housing fabrication (Figure 1). Fan operating temperatures near 850 F and rotational speeds up to 3,600 rpm create the stresses largely responsible for the cracks.
While proper design, installation, preventive maintenance and inspection help ensure long fan life, even the best designs are dependent on fabrication quality. Manufacturers develop elaborate welding procedures for the specific objects and materials being assembled, and inspectors use MT to verify workmanship throughout fabrication. For example, a full-penetration weld attaching a blade can require three welding passes, each requiring MT. Inspection in the fabrication phase focuses on detecting inclusions and cracks caused by a variance from one or more welding parameters. Likewise, welds for assembling and installing fans and fan housings at the power facility must receive the same scrutiny. Any installation failure can bear the same consequences as a fan failure.
Power generation systems are subject to temperature and load cycles which, over time, can stress fans. Periodic inspection (Figure 2) often reveals finite cracks that propagated during system use, but were too subtle to be detected during fabrication and installation.
Many utilities use contractors to conduct component inspections. The plant managers rely on these inspectors to be well trained, certified and to stay apprised of recent advances in inspection technology, thus avoiding in-house labor and training costs. The contractor works closely with the utility to ascertain which areas to inspect and which NDT methods to employ. In addition, the utility defines critical indication criteria.
The inspection job itself is normally a two-day undertaking. Access to inspection areas can be difficult, with minimal space for the inspector to magnetize the weld, apply the MT solution and use the light source for ultra-violet (UV) inspection. In addition to the necessary lights and magnetizing yokes, inspectors must drag power cords through every opening and passage.
Because working in tight spaces and around rotating equipment can be dangerous, service company inspectors must be well versed in current Occupational Safety and Health Administration (OSHA) requirements and increased safety awareness. To avoid inhaling hazardous materials, inspectors may even monitor air quality, particularly in the presence of combustibles common in some inspection materials.
New inspection materials
Recent MT technology advances now make the inspector?s job less difficult and improve accuracy. For example, a new water-bath aerosol MT product for conventional UV inspection eliminates oil from the spray, allowing inspectors to work without respirators and providing easy clean-up. Another product contains red particles that stand out in visible light and are fluorescent under UV light (Figure 3), eliminating inspection in the dark and allowing repair personnel to locate defects easily.
Inspectors compare crack indications or other defects to criteria supplied by the power plant managers. Defects that exceed the utility?s allowable size criteria require excavation or removal by grinding. Once further examination determines that the crack has been fully removed, the weld is ready for repair. If the repair weld displays any defect, the entire examination-excavation-repair cycle is repeated until the weld is suitably repaired.
NDT is a critical process that ensures reliable components and cost-effective power plant operation. Using specialized inspectors and the latest technology provides maximum test accuracy, safety and productivity. END
William C. Chedister is vice president of Circle Systems Inc., a manufacturer of magnetic particle inspection materials. Bill is currently an officer of the American Society of Non-Destructive Testing Technical Council and Chair of the MT Methods Committee. He is also a member of the American Society for Testing and Materials E-7 Committee on NDT and is a registered professional engineer.
Jeffery Long is the manager of the Los Angeles Laboratory for MQS Inspection Inc. MQS provides training and services for all NDT disciplines and serves more than 80 percent of the domestic utility industry from 25 laboratories. Long is a certified Level III in MT and four other NDT methods with more than 10 years of field experience.
Figure 2. A certified inspector utilizes MT yoke and spray particles to detect cracks in a fan weldment.
Figure 3. Magnetic particles indicate a toe crack in a weldment under visible and ultraviolet light.