| Summary: | An accidental gas explosion occurred at the Hilton Bayfront hotel in San Diego, California on May 19, 2008, while the hotel was in the final stages of construction. The mechanical and plumbing contractor had finished installing natural gas plumbing to the primary mechanical room on the fifth floor, and workers were in the process of purging the air in these lines with natural gas. This was accomplished by disconnecting a coupling from the primary gas manifold to a bank of water boilers and establishing gas flow directly into the fifth floor mechanical room. The contractor did not install a temporary vent line to the outside and did not monitor natural gas concentration within the area where the gas was venting. Instead, workers relied upon the distinctive odour of natural gas to determine when the lines had been purged of air.
Gas records indicate the flow of gas continued for approximately one hour, releasing 113.3 m3 (4000 ft3) ofnatural gas into the relatively confined mechanical room. Since the steel gas line was completely new, the Tetrahydrothiophene (THT) and Tetra-Butyl Mercaptan (TBM) based odorant in the gas was completely adsorbed by the pipe wall and none of the workers smelled gas during the purging operation. The resulting explosion caused considerable damage to the hotel, 15 workers were injured and construction was delayed for 1 year. Exponent conducted an engineering investigation of the accident, examined the gas delivery lines, and reviewed gas service records from the utility company. Our findings confirmed that, even though the natural gas was odorized to required levels at the meter, the gas became deodorized after it came in contact with new steel pipes within the hotel. The amount of gas that was vented by the workers was sufficient to cause the explosion.
Post-incident investigation by various regulatory agencies, including the Occupational Safety and Health Administration, suggested that, while the actions of the contractor violated certain requirements, there were no explicit regulations or standards that governed gas purging. Due to this explosion and similar incidents at a power plant in Connecticut and a food manufacturing facility in North Carolina, the United States Chemical Safety Board recommended that the National Fire Protection Association formulate a new standard for fire and explosion prevention during cleaning and purging of flammable gas piping systems. The primary author is a member of the Technical Committee of this new NFPA standard, and the lessons learned section of this paper includes corrective measures required by this standard, which should prevent similar incidents.
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