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Merrimack Valley Gas Incident-The Importance of Natural Gas Safety Practices to Prevent and Minimize Risk of Accidents

The Merrimack Valley gas explosions occurred on September 13, 2018, when excessive pressure in natural gas lines owned by Columbia Gas of Massachusetts caused a series of explosions and fires in as many as 40 homes, with over 80 individual fires, in the towns of Lawrence, Andover, and North Andover, all within the Merrimack Valley, in Massachusetts, United States. 22 people (including three firefighters) would be injured, and one person would be dead.


The cause of the over-pressurization was due to a failure in the procedure set out by Columbia Gas for replacing some of the low-pressure piping. The procedure failed to include the transfer of a regulator’s pressure sensing line from the old, disused piping to the new. As a result, when the old pipe was depressurized, the regulator sensed zero pressure on the low-pressure side and opened completely, feeding the main pipeline’s full pressure into the local distribution network.


The National Transportation Safety Board concluded that the probable cause was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main. This resulted in a low-pressure natural gas distribution system being operated without adequate overpressure protection.


(used with author's permission)


The bypass involving the old low pressure cast iron gas main and the new high pressure plastic main looked like this:



The NTSB's final report established the Probable Cause as:


The National Transportation Safety Board determines that the probable cause of the over-pressurization of the natural gas distribution system and the resulting fires and explosions was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main. Contributing to the accident was a low-pressure natural gas distribution system designed and operated without adequate overpressure protection.


The NTSB recommended the following changes:



Having complete and accurate procedures, designs by qualified and experienced engineers, and risk management are essential when conducting bypass and other operations on gas facilities. The gas utility involved did not have personnel that were setup to react immediately to mitigate the abnormal operating condition of high pressure gas going into the low pressure system until it was too late to prevent catastrophic damage.


If your gas or propane company would like help in reducing your risk of accidents related to your gas/propane operations please contact The Oak Tree Group to see how we can help your company.

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