A work crew replacing an old cast iron natural gas distribution main with a new plastic one in South Lawrence did not account for the location of a sensor designed to maintain appropriate pressure in the system, a mistake that the National Transportation Safety Board said is to blame for gas explosions and fires in the Merrimack Valley last month.
A preliminary NTSB report into the Sept. 13 explosions and fires that left one man dead and damaged 131 structures said the incidents occurred "after high-pressure natural gas was released into a low-pressure gas distribution system."
On Sept. 13, a crew contracted by Columbia Gas and a Columbia Gas inspector were working at the intersection of Salem and South Union streets in South Lawrence on a project to replace a cast iron gas main with a plastic one. The cast iron main, which was to be abandoned, was still connected to the sensors that monitor pressure in the system and send input to the regulators that 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," the NTSB wrote in its report.
The agency said that the system regulators responded to the drop in pressure by opening further and increasing the system pressure.
"Since the regulators no longer sensed system pressure they fully opened, allowing the full flow of high-pressure gas to be released into the distribution system supplying the neighborhood, exceeding the maximum allowable pressure," NTSB said.
The change in system pressure triggered two high-pressure alarms at the Columbia Gas monitoring center in Columbus, Ohio, at 4:04 p.m. and 4:05 p.m., the NTSB said. The center did not have the ability to control system pressure but at 4:06 p.m., the monitoring center reported the high-pressure alarms to its counterparts in Lawrence, which the NTSB said was "following company protocol."
The first 911 emergency call about a gas explosion or fire came from a Lawrence resident at 4:11 p.m., the NTSB said.
"The company is fully cooperating with the NTSB and provided information to assist in its ongoing investigation into relevant facts related to the event, the probable cause, and its development of safety recommendations," Joe Hamrock, president and CEO of Columbia Gas parent company NiSource Inc., said in a statement Thursday, adding that he cannot discuss the apparent cause of the incident. "However, we can say that, because safety is our top priority, in the hours immediately after the incident we suspended similar work and enhanced procedures related to our low pressure systems. We saw these as responsible steps to take in the aftermath of the incident and while the facts were being gathered."
The NTSB said that the work being done on Sept. 13 was designed and approved by Columbia Gas and that it was carried out in accordance with the steps laid out in the approved work package. But the NTSB found that 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."
In all, the overpressurization damaged 131 structures, including five homes that were completely destroyed, NTSB said. One man was killed and at least 21 other people were hospitalized as a result, the agency said.
Many of the affected homes and businesses in the Merrimack Valley are still without gas service. Columbia Gas brought retired Navy Capt. Joe Albanese on board to lead its restoration efforts. The company and public officials said gas service is expected to be restored to all affected homes and businesses by Nov. 19.
Robert Sumwalt, chairman of the NTSB, has previously said it could take up to two years for his agency to finalize its investigation and present safety recommendations.
