NTSB Releases Preliminary Report On Merrimack Valley Explosions

BOSTON (CBS) -- The National Transportation Safety Board has released the preliminary results of their probe into the Merrimack Valley explosions of Sept. 13. That afternoon, dozens of fires and explosions erupted across Lawrence, North Andover, and Andover after high-pressure natural gas was released into a low-pressure gas system.

The incident killed an 18-year-old man, sent 21 people to the hospital, and damaged 131 buildings, completely destroying at least five homes, according to the NTSB.

A Columbia Gas inspector and contracted workers were replacing a pipe at South Union Street and Salem Street in South Lawrence at the time. "The distribution main that was abandoned still had the regulator sensing lines that were used to detect pressure in the distribution system and provide input to the regulators to control the system pressure. Once the contractor crews disconnected the distribution main that was going to be abandoned, the section containing the sensing lines began losing pressure," said the report.

Workers then increased the pressure to the maximum amount when they saw it drop.

"Columbia Gas developed and approved the work package executed on the day of the accident. The work package did not account for the location of the sensing lines or require their relocation to ensure the regulators were sensing actual system pressure."

The first related 911 call from Lawrence came in at 4:11 p.m.. Six and seven minutes prior, a monitoring system received two high-pressure alarms but that system did not have the ability to close the valves.

"Columbia Gas shut down the regulator at issue by about 4:30 p.m. The critical valves of the involved natural gas distribution system were closed by 7:24 p.m." Starting around midnight, crews began shutting off meters at individual homes.

Columbia Gas is working to replace all of the affected piping with "new high-pressure plastic mains with regulators at each service meter," according to the NTSB.

NiSource, the parent company of Columbia Gas responded to the report saying that they were cooperating with the investigation and that safety is their top priority.