The accidental release of a toxic chemical mist that prompted the evacuation of 2,500 people in October 2008 around Petrolia, Butler County, was caused by a worker's failure to shut off a pump that lacked tank overflow controls, according to a federal Chemical Safety Board report.

The 14-page report issued yesterday also faulted INDSPEC Chemical Corp. for not having an automatic safety shut-off system in place to prevent the highly hazardous chemical oleum -- also known as fuming sulfuric acid -- from overflowing a 38,000 gallon tank.

The report said that to save time on weekends, operators routinely ran two oleum transfer pumps simultaneously, using the regular pump equipped with a safety shut-down device, and an auxiliary or emergency power supply installed in the 1970s did not have that safety device.

On the day of the accident, a worker began transferring oleum at 11:45 a.m., using both pumps. He shut down the regular pump but not the auxiliary pump before leaving work at 2:15 p.m. One of the tanks overflowed and an acid mist began escaping from the tank building by 4:30 p.m.

Facility personnel were unable to control the release and the 30 employees were evacuated. The evacuation was quickly extended to Petrolia and the surrounding towns and lasted for about 12 hours. No one at the plant was injured, but three people went to the hospital with respiratory problems.

The chemical plant, owned by Occidental Petroleum Corp. and located 50 miles northeast of Pittsburgh, uses oleum to make resorcinol, a chemical compound used in the tire and rubber industries. When liquid oleum is spilled, it reacts with water vapor in the air to create a dense cloud that hugs the ground and has a sulfuric smell, like rotten eggs.

In high concentrations it can be lethal, and in lower concentrations it can cause lung damage and irritation.

Dave Dorko, INDSPEC plant manager, said the installation of tank filling shut-off equipment on the auxiliary pump was done last October, shortly after the accident and before the plant resumed operations.

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