Pressure surged through several miles of natural gas lines on the afternoon of Sept. 13, fueling explosions and fires in three Merrimack Valley towns.The disaster killed one person and injured at least 21 others, including two firefighters. The explosions damaged 131 structures, including five homes that were completely destroyed. According to a preliminary report from the NTSB, a contracted work crew was replacing an old cast iron natural gas distribution main with a new plastic one in South Lawrence when they failed to account for the location of a sensor designed to maintain appropriate pressure in the distribution system. The sensor, which was attached to the cast iron main that was being abandoned, sent input to the regulators that control system pressure. Officials identified a rise from 0.5 pounds per square inch inside the Columbia Gas distribution system to at least 6 psi. That's about 12 times the pressure the aging gas distribution system was intended to hold, according to a letter from U.S. Sens. Ed Markey and Elizabeth Warren. Feeney Brothers Utility Services confirmed it had a crew of four workers contracted by Columbia Gas and working under the oversight of an inspector from the utility company at the intersection of South Union and Salem streets on the day of the disaster.Eight days after the disaster, Feeney Brothers issued a statement confirming the crew was interviewed by the NTSB. They also wrote that the crew was interviewed by Columbia Gas officials on the morning after the explosions. "Columbia Gas reported that our crew was solid and had done nothing wrong," they wrote.The pressure spike triggered alarms at a Columbia Gas monitoring center in Ohio at 4:04 p.m. and 4:05 p.m., and the center reported the alarms to counterparts in Lawrence at 4:06 p.m.The first 911 emergency call about a gas explosion or fire came from a Lawrence resident at 4:11 p.m., the NTSB said.Dispatchers in the three communities were inundated by the 911 calls. According to Gov. Charlie Baker, at least 150 calls were received, and dispatchers described a tidal wave of pleas for help."It was scary at first. It was overwhelming at first. But then, you know, you kind of take it for what it is at the moment and you do what you can," said Andover dispatcher Melissa Kurkjian.Early in the disaster, a home exploded about 1 mile from where the Feeney Brothers crew was working. The blast caused a chimney to topple onto a vehicle parked outside, killing 18-year-old Leonel Rondon. He had just received his driver's license hours earlier."What happened in the Merrimack Valley yesterday was a tragic incident. We are saddened to learn of the death of a young man as a result of these events," Columbia Gas said in a statement issued on Sept. 14.“He was a good kid. Just like any other regular kid from Lawrence, was in school trying to better himself,” friend Anthony Gil said.In the immediate aftermath of the disaster, gas customers throughout Lawrence, Andover and North Andover were told to evacuate. Hundreds of utility workers went door to door shutting off gas service and clearing properties of natural gas.Customers have since returned home, but gas service could only be restored for those outside the area serviced by the 48 miles of affected pipeline. Columbia Gas and state officials originally set a deadline of Nov. 19 to rebuild the distribution system and restore service to customers, but that deadline was extended. As of late October, Columbia Gas had replaced most of the distribution pipeline but was falling behind on making customers' homes and appliances ready for the restoration of service. While they wait, customers are reliant on alternative forms of energy to heat their homes, cook their food and create hot water. Teams of National Guard soldiers began distributing electric hot plates about nine days after the disaster and electric heaters a few days later. Columbia Gas also announced that it would pay customers' claims for the cost of converting permanently to electric or oil-fueled appliances. In the wake of the disaster, the Massachusetts Department of Public Utilities has also announced plans to hire an independent evaluator to examine the natural gas distribution system statewide. How natural gas distribution systems work:Natural gas is pumped through interstate transmission lines at extremely high pressures, but several local mechanisms reduce those pressures to a level that is safe for powering appliances. According to the American Gas Association, pressure in the interstate lines can range from 1,500 to 200 pounds per square inch. Before entering the local distribution system, the fuel must pass through a gate station. The AGA says these stations reduce the pressure to 200 psi or lower, depending on the local distribution system, and add an odorant to the normally scentless gas as a safety measure. Distribution mains are the pipes of varying sizes that bring natural gas from the gate station to customers. "Distribution mains are interconnected in multiple grid patterns with strategically located shut-off valves," the AGA wrote. "These valves minimize the need for customer disruption to service during maintenance operations and emergencies." Pressure can vary in these systems and is adjusted by remote-controlled pressure regulators. In the low-pressure Columbia Gas system in the Merrimack Valley, the NTSB said 14 regulator stations reduced the pressure from 75 psi to about 0.5 psi. Service lines lead from the gas mains to natural gas meters at homes or businesses. In high-pressure systems, those meters are the final pressure regulator before gas enters a home. Homes in the affected area of the Merrimack Valley did not have that safety measure. The AGA said gas pressure in a home may be as low as 0.25 psi, which is "less than the pressure created by a child blowing bubbles through a straw in a glass of milk."Gas pressure inside an appliance, like a stove, only needs to be slightly higher than the surrounding air pressure for the gas to flow out of the system and ignite.