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Generator Failure and Meltdown

A catastrophic failure occurred in the stator of a Westinghouse generator at a large power plant on the East Coast. Aptech was asked by counsel for the power company to perform an independent engineering investigation of the cause of this failure. Aptech wrote an inspection and test protocol and carried it out with the assistance of plant personnel, other general consultants and a team from the original equipment manufacturer. These inspections focused on the through-bolts. These bolts were used to hold the stator's laminations together. We found that three of the through-bolts were melted. Both ends of the bolts had arced over to the grounded steel end plates. Six of the generator's intact through-bolts were removed to inspect and test them. Epoxy/fiberglass bushing and washer assemblies on the ends of each bolt were intended to insulate the generator's end plates from the steel washers behind the nuts. We found that all of the epoxy/fiberglass bushings on the ends of all of the bolts had separated from their washers. Our opinion was that tightening the nuts by a re-winding contractor created an excessive bending stress on the joint between the epoxy/fiberglass bushing and washer. This caused them to break apart at the joint. This separation exposed a portion of the inside faces of the steel washers, which are at the generator's induced voltage, to the generator end plates, which are grounded. A short circuit eventually occurred and high temperature electrical arcs were created on the ends of the bolts, producing the flow of molten metal. This failure would have been avoided if the bushing assemblies had been designed such that torquing the nuts would not have caused them to break apart.

Satish Almaula		Plant failure and root cause analysis; process and design engineering analysis; process technology development and testing; plant engineering, operations, maintenance and safety management; plant and process control system management.

Michael Cronin, P.E.		Stress analysis, design evaluation, engineering mechanics

Kimble Clark, Ph.D.		Failure analysis, heat transfer & thermodynamics, process plant equipment failures & explosions, fuel science, combustion, industrial fires and explosions.

Eric Sullivan, P.E.		Metallurgy, welding, failure analysis, fire and explosion investigations, equipment, piping.

Steve Paterson		Metallurgy, welding, boiler pressure part and machinery failure analysis.