The natural gas ignited at June 28, 2021, at 3:35 p.m. and exploded after workers inserted an in-line tool (or pig) into a launcher, said officials at the National Transportation Safety Board (NTSB) in a report. Seven workers from Atmos, FESCO, Ltd. (FESCO), and Bobcat Contracting, L.L.C. (Bobcat) were on-site at the time of the incident performing work for Atmos.
The explosion occurred while the workers were removing the pig insertion tool during the sixth of a series of in-line inspections, according to the report.
Along with the fatalities and injuries, Atmos estimated property damage at the site to be $580,000.
The five surviving workers described the events leading up to the explosion in interviews with NTSB investigators.
Shortly before the explosion, workers used a portable flaring system to vent natural gas from the launcher.
A worker opened the 1-inch valve that connected the flare flow line to the launcher and allowed natural gas to vent from the launcher to the flare tip, where it was successfully ignited. As natural gas pressure in the launcher decreased and less natural gas flowed to the flare tip, the work crew observed the flame die down and extinguish.
Atmos Energy Corporation accident site where two workers died and two others suffered injuries.
Source: Wylie Fire Department
The Atmos workers, who were overseeing the contractor personnel, did not expect gas to be leaking from the mainline valve, but they did note in interviews they thought the flare system would provide a safe path for gas leaking past the mainline valve to vent to atmosphere if any leakage happened to occur.
The workers were not using any form of gas monitoring other than observing the flare tip.
Workers began performing the remaining steps needed to load the pig at 3:28 p.m.: photographing the pig, opening the launcher door, lifting the pig into position with an excavator, attaching a grounding cable to the launcher and insertion tool, manually inserting the pig into the launcher, sliding the pig further into the launcher with the excavator, and removing the insertion tool. Before the insertion tool was completely removed, workers at the site heard a loud sound, and one worker observed a flash near the open launcher door.
An Atmos worker called 9-1-1 at 3:35 p.m. The Collin County Assistant Fire Marshal was the first emergency responder on the scene; he arrived at 3:41 p.m., determined two of the four injured workers were deceased, and began to assist the two injured survivors. He observed no fire.
NTSB determined the probable cause of the explosion was a leaking mainline valve that allowed natural gas to enter the launcher where it mixed with air, creating a flammable gas–air mixture ignited by an undetermined source.
Contributing to the explosion and its severity were Atmos Energy Corporation’s procedures and training practices that did not prepare workers to recognize and safely respond to abnormal operating conditions.