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Gas-Fired Utility Boiler Explosion

Explosions occurred in the furnace and backpass of a 170 MW gas-fired utility boiler during shutdown. The utility company's insurance carrier retained APTECH to do a third-party investigation of the origin and cause of the incident. The boiler was a 1950's-vintage, Combustion Engineering unit with front-wall mounted burners. The burners did not have flame scanners, so the boiler was operated under the NFPA's manual operation guidelines. APTECH experts examined the operational data and found that alarms for carbon monoxide and combustibles had occurred prior to the explosions. We walked down the entire boiler and documented the damage pattern. The furnace buckstays and the walls of the backpass were bowed outward. The outward deflection of the walls caused the economizer to fall into the hopper below. The superheater screen tubes were severed at the roof. Most of the severed screen tubes were found deflected into the superheater, but one of them fell to the furnace floor. The evidence indicated that there were at least two separate, but temporally closely spaced, explosions - one originating in the furnace above the burners and the other in between the superheater and reheater. Metallurgical examinations of the screen tubes showed that they had preexisting hydrogen damage. The examination also showed that the tubes had been severed when the furnace explosion lifted the roof. The reaction forces from the steam jets accounted for their final positions. APTECH determined that the fuel that exploded was raw natural gas from unignited burners. The bottom row of burners had been extinguished when they became too fuel-rich as the fuel, but not the air, was shut off to the upper burners. The unignited burners were not detected due to the absence of flame scanners. APTECH provided an origin and cause report to the client.

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.

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

Richard Schreiber, P.E.		Boiler explosions, boiler and machinery failure analysis, industrial fires and explosions.

Steve Lefton		Boiler explosions, power plant design, operation and maintenance, boiler pressure part and machinery failure analysis.