Missing instructions caused deadly gas explosions, NTSB report says

SOUTH LAWRENCE, Mass. — Inadequate planning and instructions led to a Columbia Gas contractor over pressurizing a distribution system in the Merrimack Valley and causing a series of explosions and fires that killed one person and left dozens injured, according to an initial review from the National Transportation Safety Board.

Minutes before the explosions sparked widespread evacuations and left three towns reeling, an alarm went off in the Columbia Gas Monitoring Center in Columbus, Ohio. What’s called a low-pressure gas system in South Lawrence was suddenly over pressurized.

At 4:04 p.m., monitoring crews in Ohio knew something was wrong, but they weren’t able to manually open or close the valves, the NTSB report notes.

A series of 14 regulator stations control the pressure of the natural gas being delivered through the South Lawrence low pressure system. The regulators will open or close based on sensors located throughout the distribution mains.

A contractor crew was working on replacing an old cast-iron main that dates back to the 1950s. When the crew disconnected the old main, the NTSB says they were not told to disconnect the sensor. That sensor, being disconnected from the system and sensing no gas moving through it, triggered the regulator stations to increase pressure on the system, according to the report. %



“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 NTSB report states. “The work was performed in accordance with steps laid out in the work package.”

Thirty minutes after monitors in Ohio sounded the initial pressure alarm, Columbia Gas crews shut down the regulator causing the over pressurization.

The cast-iron main replacement is part of the company’s plan to replace the whole system with a modernized plastic piping due to ‘system integrity concerns.’

The report released Thursday is preliminary and will be updated later in the investigation.

MORE: Timeline of events during Merrimack Valley gas explosions