[Note that this article is a transcript of the video embedded above.]

On September 13, 2018, a pipeline crew in the Merrimack Valley in Massachusetts was hard at work replacing an aging cast iron natural gas line with a new polyethylene pipe. Located just north of Boston, the original cast iron system was installed in the early 1900s and due for replacement. To maintain service during the project, the crew installed a small bypass line to deliver natural gas into the downstream pipe while it was cut and connected to the new plastic main line. By 4:00 pm, the new polyethylene main had been connected and the old cast iron pipe capped off. The last step of the job was to abandon the cast iron line. The valves on each end of the bypass were closed, the bypass line was cut, and the old cast iron pipe was completely isolated from the system. But it was immediately clear that something was wrong.

Within minutes of closing those valves, the pressure readings on the new natural gas line spiked. One of the fittings on the new line blew off into a worker's hand. And as they were trying to plug the leak, the crew heard emergency sirens in the distance. They looked up and saw plumes of smoke rising above the horizon. By the end of the day, over a hundred structures would be damaged by fire and explosions, several homes would be completely destroyed, 22 people (including three firefighters) would be injured, and one person would be dead in one of the worst natural gas disasters in American history. The NTSB did a detailed investigation of the event that lasted about a year. So let’s talk about what actually happened, and the ways this disaster changed pipeline engineering so that hopefully something like it never happens again. I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking about the 2018 Merrimack Valley natural gas explosions.

Like many parts of the world, natural gas is an important source of energy in homes and businesses in the United States. It’s a fossil fuel composed mostly of methane gas extracted from geologic formations using drilled wells. The US has an enormous system of natural gas pipelines that essentially interconnect the entire lower 48 states. Very generally, gathering lines connect lots of individual wells to processing plants, transmission lines connect those plants to cities, and then the pipes spread back out again for distribution. Compressor stations and regulators control the pressure of the gas as needed throughout the system. Most cities in the US have distribution systems that can deliver natural gas directly to individual customers for heating, cooking, hot water, laundry, and more. It’s an energy system that is in many ways very similar to the power grid, but in many ways quite different, as we’ll see.

Just like a grid uses different voltages to balance the efficiency of transport with the complexity of the equipment, a natural gas network uses different pressures. In transmission lines, compressor stations boost the pressure to maximize flow within the pipes. That’s appropriate for individual pipelines where it’s worth the costs for higher pressure ratings and more frequent inspections, but it’s a bad idea for the walls of homes and businesses to contain pipes full of high-pressure explosive gas. So, where safety is critical, the pressure is lowered using regulators.

Just a quick note on units before we get too far. There are quite a few ways we talk about system pressures in natural gas lines. Low pressure systems often use inches or millimeters of water column as a measure of pressure. For example, a typical residential natural gas pressure is around 12 inches (or 300 millimeters) of water, basically the pressure at which you would have to blow into a vertical tube to get water to raise that distance: roughly half a psi or 30 millibar. You also sometimes see pressure units with a “g” at the end, like “psig.” That “g” stands for gauge, and it just means that the measurement excludes atmospheric pressure. Most pressure readings you encounter in life are “gauge” values that ignore the pressure from earth’s atmosphere, but natural gas engineers prefer to be specific, since it can make a big difference in low pressure systems.

The natural gas main line in the Merrimack Valley being replaced had a nominal pressure of 75 psi or about 5 bar, although that pressure could vary depending on flows in the system. Just for comparison, that’s 173 feet or more than 50 meters of water column. But, the distribution system, the network of underground pipes feeding individual homes and businesses, needed a consistent half a psi or 30 millibar, no matter how many people were using the system. The device that made this possible was a regulator. There are lots of different types of regulators used in natural gas systems, but the ones in the Merrimack valley use pilot-operated devices, which are pretty ingenious. It’s basically a thermostat, but for pressure instead of temperature. The pilot is a small pressure regulating valve that supports the opening or closing of the larger primary valve. If the pilot senses an increase or decrease in pressure from the set point, it changes the pressure in the main valve diaphragm, causing it to open or close. This all works without any source of outside power just using the pressure of the main gas line.

Columbia Gas’s Winthrop station was just a short distance south of where the tie-in work was being done on the day of the event. Inside, a pair of regulators in series was used to control the pressure in the distribution system. One of these regulators, known as the worker, was the primary regulator that maintained gas pressure. A second device, called the monitor, added a layer of redundancy to the system. The monitor regulator was normally open with a setpoint a little higher than the worker so it could kick in if the worker ever failed, and, at least in theory, make sure that the low-pressure system never got above its maximum operating level of about 14 inches of water column or 35 millibar. But, in this worker/monitor configuration, the pilots on the two regulators can’t use the downstream pressure right at the main valve. For one, the reading at the worker would be affected by any changes in the downstream monitor. And for two, measuring pressure right at the valve can be inaccurate because of flow turbulence generated by the valve itself. It would be kind of like putting your thermostat right in front of a register; it wouldn’t be getting an accurate reading. So, the pilots were connected to sensing lines that could monitor the pressure in the distribution system a little ways downstream of the regulator station.

The worker and monitor regulators were both functioning as designed on September 13, and yet, they allowed high pressure gas to flood the system, leading to a catastrophe. How could that happen? The NTSB’s report is pretty clear. Tying a natural gas line while it’s still in service, called a hot tie-in, is a pretty tricky job that requires strict procedures. Here are the basic steps: First a bypass line was installed across the upstream and downstream parts of the main line. Then balloons were inserted into the main to block gas from flowing into the section to be cut. Once the gas was purged from the central section, it was cut out and removed while the bypass line kept gas flowing from upstream to downstream. The line to be abandoned got a cap, and the new plastic tie in was attached to the downstream main. Once the tie-in was complete, the crew switched the upstream gas service from the old cast iron line over to the new plastic line and deflated the last balloon so that gas could flow. The upstream cast iron line was still pressurized, since it was still connected to the in-service line through the bypass. But, as soon as the crew closed the valves on the bypass, the old cast iron line was fully isolated, and the pressure inside the line started to drop, as planned.

What that crew didn’t know is that when that plastic main line was installed 2 years back, a critical error had been made. The main discharge line at the regulator station had been attached to the new polyethylene pipe, but the sensing lines had been left on the old cast iron main. It hadn’t been an issue for the previous 2 years, since both lines were being used together, but this tie-in job was the first of the entire project that would abandon part of the original piping. Within minutes of isolating the old cast iron pipe, its pressure began to drop. To a regulator, there’s no difference between a pressure drop from high demands on the gas system and a pressure drop from an abandoned line, and they respond the same way in both cases: open the valves. In a normal situation, the increased gas flow would result in higher pressure in the sensing lines, creating a feedback loop. But this was not a normal situation. It’s the equivalent of putting your thermostat in the freezer. Even as pressure in the distribution system rose, the pressure in the sensing lines continued to drop with the abandoned line. The regulators, not knowing any better, kept opening wider and wider, eventually flooding the distribution system with gas at pressures well above its maximum rating.

By the time things went sideways, the crew at the tie-in had taken most of their equipment out of the excavation. But as one worker was removing the last valve, it blew off into his hand as gas erupted from the hole. The crew heard firefighters racing throughout the neighborhood and saw the smoke from fires across the horizon. The overpressure event had started a chain of explosions, mostly from home appliances that weren’t designed for such enormous pressures. The emergency response to the fires and explosions strained the resources of local officials. Within minutes, the fire departments of Lawrence, Andover, and North Andover had deployed well over 200 firefighters to the scenes of multiple explosions and fires, and help from

neighboring districts in Massachusetts, New Hampshire, and Maine would quickly follow. The Massachusetts Emergency Management Agency activated the statewide fire mobilization plan, which brought in over a dozen task forces in the state, 180 fire departments, and 140 law enforcement agencies. The electricity was shut off to the area to limit sources of ignition to help prevent further fires, and of course, natural gas service was shut off to just under 11,000 customers.

By the end of the day, one person was dead, 22 were injured, and over 50,000 people were evacuated from the area. And while they were allowed back into their homes after three days, many were uninhabitable. Even those lucky enough to escape immediate fire damage were faced with a lack of gas service as miles of pipelines and appliances had to be replaced. That process ended up taking months, leaving residents without stoves, hot water, and heaters in the chilly late fall in New England.

NTSB had several recommendations stem from their investigation. At the time of the disaster, gas companies were exempt from state rules that required the stamp of a licensed professional engineer on project designs. Less than three months after NTSB recommended the exemption be lifted, a bill was passed requiring a PE stamp on all designs for natural gas systems, providing the public with better assurance that competent and qualified engineers would be taking responsibility for these inherently dangerous projects. And actually, NTSB issued the same recommendation and sent letters to the governors of 31 states with PE license exemptions, but most of those states still don’t require a PE stamp on natural gas projects today. There were recommendations about emergency response as well, since this event put the area’s firefighters through a stress test beyond what they had ever experienced.

NTSB also addressed the lack of robustness of low pressure gas systems where the only protection against overpressurization is sensing lines on regulators. It’s easy to see in this disaster how a single action of isolating a gas line could get past the redundancy of having two regulators in series and quickly lead to an overpressure event. This situation of having multiple system components fail in the same way at the same time is called a common mode failure, and you obviously never want that to happen on critical and dangerous infrastructure like natural gas lines. Interestingly and somewhat counterintuitively, one solution to this problem is to convert the low-pressure distribution system to one that uses high pressure. Because, in this kind of system, every customer has their own regulator, essentially eliminating the chance of a common mode failure and widespread overpressure event.

Most importantly, the NTSB did not mince words on who they found at fault for the disaster. They were clear that the training and qualification of the construction crew, or the condition of the equipment at the Winthrop Avenue regulator station were NOT factors in the event. Rather, they found that the probable cause was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the project.

To put it simply, they just forgot to include moving the sensing lines when they were designing the pipeline replacement project, and the error wasn’t caught during quality control or constructability reviews. NiSource, the parent company of Columbia Gas (of Massachusetts), estimated claims related to the disaster exceeded $1 billion, an incredible cost for weak engineering management. Ultimately, Columbia Gas pleaded guilty to violating federal pipeline safety laws and sold their distribution operations in the state to another utility. They also did a complete overhaul of their engineering program and quality control methods.

All those customers hooked up to natural gas lines didn’t have a say in how their gas company was managed; they didn’t have a choice but to trust that those lines were safe; and they probably didn’t even understand the possibility that those lines could overpressurize and create a dangerous and deadly condition in the place where they should have felt most safe: their own homes. The event underscored the crucial responsibility of engineers and (more importantly) the catastrophic results when engineering systems lack rigorous standards for public safety.