Great Molasses Flood of 1919

Kim Smiley


Cause Mapping

In 1919, a tank holding about 2 million gallons of molasses collapsed in the North End neighborhood of Boston, Massachusetts. The resulting flood of molasses was two stories tall and killed 21 people, injured 150 more, and did 100 million dollars of damage (in today’s dollars). The rescue efforts went on for days and cleaning up this disaster is estimated to have taken 87,000 man-hours.

The word molasses is more likely to bring to mind baked goodies rather than destruction or danger, but a flood of molasses is actually more dangerous than a comparable flood of water. Molasses is a non-Newtonian fluid and a wave of molasses does not behave the same as a wave of water. Once the tank collapsed, the molasses flowed out relatively quickly (think ketchup when it finally flows out of a glass bottle), moving fast enough that that people were swept up in the flood, and then it quickly settled into thick goo that made it very difficult for people to escape and rescue workers to reach victims.

After one wraps their mind around the fact that molasses can be deadly, the next natural question is: what caused this accident? As with nearly all accidents of this scope, there isn’t a single, simple root cause, but rather a number of causes that led to the accident. A Cause Map, a visual format for performing a root cause analysis, can be created to help understand the many causes that contributed to this issue. In order to create a Cause Map, ‘Why’ questions are asked and answers are visually laid out to instinctively show the cause-and-effect relationships. To view an intermediate level Cause Map of this accident, click on the thumbnail above.

So why did the tank collapse? The pressure inside the tank exceeded the strength of the tank (ability to hold the pressure). Investigators think the pressure in the tank may have increased in the days preceding the accident because the temperature had increased significantly (from 2°F to 41°F). As the temperature increases, molasses may ferment more, creating more carbon dioxide in the tank and causing the internal pressure of the tank to increase. More significantly, investigators determined that the strength of the tank was inadequate. It was calculated that the tank walls were only half as thick as they should have been. There was no engineer involved in the design of the tank and it was basically built as quickly and cheaply as possible. There have also been questions about whether or not the construction was done properly. Investigators also determined that basic safety testing, such as filling the tank with water before use, was not performed because of schedule pressure.

Another important element worth understanding in this disaster is that warning signs that all was not well with the tank were repeatedly ignored. There are many reports that the tank was known to rumble, creak and make ominous noises. The tank also leaked badly, to the point that it was common for local residents to use buckets to collect the molasses for household use. The company’s response to the leaks? They painted the tank brown so that the molasses leaks would be less noticeable.

The size and location of this tank were additional factors that played into this disaster. The tank was huge, holding about 2 million gallons of molasses at the time of the accident. The tank was also located in a neighborhood. The location was convenient because it was close to the harbor that brought the molasses from Cuba and near the railroad used to transfer the molasses to a distillery. The location made sense in that regard, but this incident highlights the risks associated with locating a large tank (especially one known to be leaky) in a populated area.

The findings from the investigation into this accident were one of the factors that prompted Massachusetts, and eventually other states, to pass laws that required all plans for construction projects to be approved and signed off by an engineer or architect and filed with the appropriate government body. I would argue that although this is one of the more bizarre engineering failures in American history, the lessons learned from studying it can be applied across industries. Even if your business has nothing to do with building codes or storing products, it’s a good reminder of the importance of taking action when you see warning signs. Simply covering up the symptoms, i.e. painting the leaking tank brown, will not make the real problems go away.

If you are interested in this topic, you can click here to read our blog about another molasses spill in Honolulu Harbor in Hawaii that occurred in 2013.

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