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: About like that, 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. Just like natural gas projects require
strict oversight to protect the public, the same is true for aviation. One example is
the Tenerife airport disaster. Why would a fully loaded 747 venture onto the runway of a small
airport in the Canary Islands without permission, leading to the deadliest crash in aviation
history? My fellow content creator, Neo, recently released a documentary about the disaster
with incredible 3D animations to tell the story. Maybe you’ve noticed what I have over the past few
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fascinating and thought-provoking than any of the old tv shows I miss. Neos video (along with a
ton more excellent content) is only available on Nebula, the streaming platform built by and for
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