Merrimack Valley Gas Explosions: What Really Happened?

Merrimack Valley Gas Explosions: What Really Happened?

A summary of one of the worst natural gas disasters in US History.
🌌Get Nebula using my link for 40% off an annual subscription:
✈️Watch Neo’s exclusive on the Tenerife disaster:

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. 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.

Watch this video ad-free on Nebula:

Signed copies of my book (plus other cool stuff) are available here:

Practical Engineering is a YouTube channel about infrastructure and the human-made world around us. It is hosted, written, and produced by Grady Hillhouse. We have new videos posted regularly, so please subscribe for updates. If you enjoyed the video, hit that ‘like’ button, give us a comment, or watch another of our videos!


Please email my agent at

This is not engineering advice. Everything here is for informational and entertainment purposes only. Contact an engineer licensed to practice in your area if you need professional advice or services. All non-licensed clips used for fair use commentary, criticism, and educational purposes.

This video is sponsored by Nebula.
Stock video and imagery provided by Getty Images, Shutterstock, Pond5, and Videoblocks.
Music by Epidemic Sound:
Tonic and Energy by Elexive is licensed under a Creative Commons Attribution License
Video by Grady Hillhouse
Edited by Wesley Crump
Written and Produced by Ralph Crewe
Production Assistance from Josh Lorenz
Graphics by Nebula Studios

You may also like...

52 Responses

  1. Practical Engineering says:

    🌌Get Nebula using my link for 40% off an annual subscription:
    🛠What engineering disaster would you like to see me cover next?

    • Tony Weaverly says:

      The way you pronounced Winthrop really bothers me. Lmao..

      Yeah I remember this first hand. I live in a neighboring city..

      I remember when all they were talking about was terrorism, and that’s one of the biggest reasons why New Hampshire and Maine in every neighboring city hopped in.

      I remember seeing even all the way down to nearly Boston, people were displaced and forced to move south where there was more housing, and everyone was afraid to move in somewhere with similar infrastructure owned by the same company after the dust settled.

      That was super scary.

    • TheBitter73 says:

      What is the method/process/algorithm/checklist that should have been done to let someone know that the sensor needed to be moved? How was someone in the management chain supposed to know that the sensor needed to be moved? Or that it was there? Also, why doesn’t the value have a top end to prevent an accidental over pressure? If someone punches a hole in the line and the pressure keeps dropping no matter how much that valve opens, that seems like a recipe for disaster. I understand that a sign off by a licensed PE is recommended but what are they doing differently? Are there some standards they are using to examine the plans against to know to ask “what about the sensors?”

    • Dan Griggs says:

      Deepwater Horizon Disaster

    • Generic Scottish Channel says:

      leave nebula

  2. Xeonerable says:

    I can’t say I remember hearing about this back then. Imagine being a gas utility worker* doing that job and you realize the whole town started blowing up, that must be awful. Seeing that map of all the locations that were on fire was frightening.

  3. Tom Leach says:

    Thank you for this video. I was one of the many firefighters from New Hampshire that responded down there. Was one of the scariest days in my career.

  4. Ken Chorney says:

    As a former controls programmer, I am a bit surprised that there were no high pressure shutdowns (measurement and valves) built into the discharge side of the regulators.

    • zyeborm says:

      You would think they would have a “short loop” control as well as the long loop. Set it to whatever the absolute max pressure rating the low side supply has. Turbulence should never trip that, and the cost vs risk of failure…
      But then again it’s easy to say these things from our armchairs after the fact.

    • Bruce Lytle says:

      I too program control systems. The hardest, most time consuming part is to program what to do when things don’t work the way it’s supposed to.

    • Jeffrey Bue says:

      @zyeborm Well stated sir.

    • Zachary says:

      I work in this industry, and am pretty familiar with Merrimack.

      A large number of our control systems are purely mechanical, only relying on control lines and regulators. We have some SCADA, but it’s really only for monitoring. Most of the controls are on the large points of delivery sites that operate at the 500-1000 PSIG range. Benefit to the mechanical ones is that they operate 24/7 regardless of conditions or power. Most modern regulators have great turndown, so as long as there’s even some pressure differential from the high to low side they will regulate well enough.

      Modern regulator stations are typically monitor operator, with two downstream points to measure pressure. The operator reg actually controls the pressure, and the monitor is there to take over if the operator fails, as they usually fail open. Downstream of that is a relief valve, usually a pop relief that blows at a set pressure and prevents downstream buildup, or, more rare now, a security valve (slam shut valve is what i’ve also heard these called) that deteces overpressure and shuts off all flow.

      The kicker with all of this is that you can hop the fence at any major regulating station in any rural state, clamp a control line (which tricks the reg into thinking it’s 0 PSIG downstream) and dump 600 PSIG into a town. Lots of stations out there with absolutely 0 security.

  5. Denis Beaudoin says:

    I was one of the electricians who worked on restoring all this and making sure houses were safe. All over Lawrence and Andover. It was crazy times

  6. Phillip Neitzel says:

    As an oil and gas operator, this is one of my biggest fears. Not tying in the sensing instruments to the control loop. Especially dangerous when commissioning a new line/equipment. Really not sure how this would be missed though. Hot Taps usually have very intensive oversight.

    • Jehty says:

      Why aren’t there more safety features?
      For example a pressure gauge that automatically shuts off the line when an overpressure is detect may have prevented or at least softened this event.

    • CptJistuce says:

      ​@Jehty Only if it was hooked to the right place. The existing pressure regulators weren’t malfunctioning, they were just monitoring an abandoned pipe.

      Any additional mechanisms installed before the new line was laid would likely also be monitoring the old line, and this would probably still have been overlooked during planning to switch over to the new line.

    • WildRapier says:

      I’ve hot tapped a water line at 120 psi, I would not want to hot tap a gas line. But, how else are you going to get the job done? Don’t envy you!

    • Highwalker says:

      Intensive oversight can be so expensive though

    • zyeborm says:

      ​@CptJistuce putting distributed (electronic) pressure sensors throughout the network would be how I’d do it. Not just one or two mind, think more like 20 for every regulator. They phone home their data every once in a while or instantly for a fault. One giving a high reading gives a warning 10 giving high readings triggers an automatic shutdown of the segment. The sensors don’t have to be $10k a pop either. Few hundred $ will suffice as failures of any sensor don’t really matter.
      As a bonus you’ll get detailed data for the whole network, probably find some leaks that’ll pay for the cost in a few months i’d wager.

  7. ClebyHerris says:

    I can’t imagine being the guys installing the pipe and just hearing and seeing explosions in the distance and just know that something went wrong

    • _-- says:

      I feel bad for those workers thinking they messed up bad when in reality, it was the poor management and boneheaded engineering practices of the firm

    • XonX says:

      Shouldn’t the workers themselves understand pressure regulators and pressure sensing lines?

    • ClebyHerris says:

      @XonX that’s like getting an oil change then the wheels exploding. Just not near each other.
      This kinda works

      But yeah they probably know about how they work but they are doing what they are told

    • Sean Roland says:

      Seriously! Talk about your heart sinking

    • ElvinGearMaster Irma says:

      ​@Sean Roland Id probably be mentally preparing to be fired-
      Like it wasnt their fault but they didnt know that

  8. Christopher G says:

    The scary part is that there are still up to 30 states that haven’t fixed the insufficient oversight by removing the excemptions.

    • inpressi75 says:

      The list is in the NTSB report, contact your legislators to fix it:

      …..Currently 34 states have exemptions and 16 states and the District of Columbia do not. The State of
      New York is in the process of removing the exemption. Table 4 outlines the P.E. industrial
      exemption by state.37
      Table 4. P.E. industrial exemption for infrastructure project practices.
      State Exempt(Yes or No) If Yes, action required for change
      Alabama Yes Amend statute
      Alaska Yes Amend statute
      Arizona Yes Amend statute
      Arkansas Yes Amend statute
      California Yes Amend statute
      Colorado Yes Amend statute
      Connecticut Yes Amend statute
      Delaware No
      District of Columbia No
      Florida Yes Amend statute
      Georgia Yes Amend statute
      Hawaii No
      Idaho Yes Amend statute
      Illinois Yes Amend statute
      Indiana No
      Iowa Yes Amend statute
      Kansas No
      Kentucky Yes Amend statute
      Louisiana Yes Amend statute
      Maine Yes Amend statute
      Maryland Yes Amend statute
      Massachusetts No Legislation passed and signed into law
      Michigan No
      Minnesota Yes Amend statute
      Mississippi Yes Amend statute
      Missouri Yes Amend statute
      Montana Yes Amend statute
      Nebraska Yes Amend statute
      Nevada Yes Amend statute
      New Hampshire No
      New Jersey No
      New Mexico Yes Amend statute
      New York Yes Amend statutea
      North Carolina Yes Amend statute
      North Dakota No
      Ohio Yes Amend statute
      Oklahoma No
      Oregon No
      Pennsylvania Yes Amend statute
      Rhode Island No
      South Carolina Yes Amend statute
      South Dakota Yes Amend statute
      Tennessee No Amend statute

    • xsk8rat says:

      @inpressi75 The rest of the states:
      Texas Yes Amend statute
      Utah Yes Amend statute
      Vermont No
      Virginia Yes Amend statute
      Washington No
      West Virginia No
      Wisconsin Yes Amend statute
      Wyoming Yes Amend statute

    • Bigga Nigga says:

      Not needed, overregulation

    • KS says:

      I stated this in a separate comment, but: Just because you have a PE does not mean you are a competent/qualified engineer. It means you passed a test. Heck, there could be a PE Engineer signing off on natural gas drawings but said PE Engineer has no natural gas experience. Ask me how I know…

    • Jack Linde says:

      Lobbyists are scary things. And where there’s money at stake, there are corners to be cut.

  9. Dave A says:

    It is good to see the NTSB didn’t try to throw the trades under the bus and showed that management totally dropped the ball.

    • Michael Steeves says:

      In serious incident investigation, management has a share of the blame in almost every situation. For example: if untrained workers do a slipshod job — who is really to blame?

    • Heptex says:

      ​@Michael Steeves Could dig a bit deeper, Maybe the certification issuers did a slipshod job actually examining their candidates ability. Can’t always blame management

    • The Eh Team says:

      I think if they had the backup sense line already attached to the new line it wouldn’t have ended this way.

    • Michael O says:

      Thats not the NTSBs way. They know hanging people out to try leads to coverups and that makes things unsafe. All they care about is making things safer and preventing the same things from happening again.

    • spicy bean says:

      @Heptex That’s still management. It’s their job to make sure people are actually qualified, too.

  10. Hal Hearst says:

    I live in North Andover MA, one of the towns impacted by this. Thankfully, my family was not impacted, but the day was very surreal. Fires, power turned off, traffic from evacuations, school closures, schools turned into shelters were all experienced. Thank you to all the firefighters and other first responders who helped that day. And, Grady, thanks for making this video. I never really understood the cause until now.

Leave a Reply

Your email address will not be published. Required fields are marked *