A natural gas explosion heavily damaged a meat processing factory. A large section of the building collapsed killing 4 workers. 4 others were critically burned and 71 people were sent to the hospital including 3 firefighters who were exposed to toxic anhydrous ammonia from the plant’s refrigeration system damaged by domino effect. 18,000 pounds of ammonia were released to the environment and 100,000 square feet of the plant were damaged. The costs tops 100 000 dollars.

The accident occurred during the installation of a new fuel gas-fired industrial water heater in an interior utility room of the plant. A new section of piping was tied into a natural gas supply line located on the roof. The new natural gas piping ran horizontally over 120 feet along the roof and then descended into the utility room.

A worker from the water heater manufacturer was attempting to purge the new gas line by using natural gas to directly displace the air. According to the company’s normal practice, and common practice, he removed threaded fittings, creating one or more pipe openings near the heater and opened a quarter-turn valve to control the release of purged gases.

The purged fuel gas was vented indoors into the utility room, which was ventilated by an exhaust fan, but no assessment was made of the adequacy of the ventilation in comparison to the rate of the gas release. Because of the difficulties in lighting the water heater, personnel perceived that the gas line was not effectively purged of air. Therefore, purging was conducted intermittently over a period of up to 2h30.

No appropriate combustible gas detectors were used to warn of a potential accumulation of gas in the building. Instead personnel relied primarily on the sense of smell to determine when the piping had been effectively purged of air and whether or not an unsafe release of natural gas occurred.

Some employees noticed the gas odor but considered the purging activity to be a normal part of the start-up process. The employees were not aware that as a result of the purging, a dangerous accumulation of natural gas had occurred into the building, exceeding the lower explosive limit.

The vicinity of the utility room contained numerous potential ignition sources, including a number of unclassified electrical devices. Nonessential personnel were not aware of the water heater start-up or instructed to leave the plant during the gas purging activity. Over 200 people who had no role in the installation were in the building when the gas exploded at 11:25 a.m.

Following the explosion, the operator established new procedures for gas purging : direct venting of purged gases via a hose or piping to a safe location outdoors, exclusion of personnel and ignition sources from the vicinity of the vent, continuous air monitoring using combustible gas detectors, and evacuation of nonessential personnel from the facility. The operator closed the factory a few months later.