“If I had a nickel for every time this happened, I’d have two nickels. Which isn’t a lot, but it’s weird that it happened twice. Right?”  — Dr. Heinz Doofenshmirz

Friends and acquaintances recently flooded my inbox with stories about a series of sewer explosions near the north riverfront of St. Louis. It launched several manhole covers. Fortunately, there were no injuries. Nonetheless, a 200-lb cast iron flying disc can do some serious harm, depending on where it lands, and on whom.

I know, because I was consulted on a similar incident that occurred several years ago at a chemical facility.

Launching a Manhole Cover

This is a simplified version of what happened in that previous incident: The plant operated a chemical sewer system, separate from its stormwater sewer system and its sanitary sewer system. An operator was discharging a less than full 55-gallon drum of spent solvent, with the intention of sending it through the chemical sewer system to the on-site chemical treatment unit. An error lining up the valves, however, sent the solvent to the stormwater sewer system instead. From there it began flowing to the nearby POTW (publicly owned treatment works).

Meanwhile, outside of the plant, a crew was working on repairing some downed power lines. Somehow there was an arc from the cable near the entrance to a culvert. The arc ignited the flammable liquid floating on top of the stormwater. A flame front then travelled upstream in the sewer and back into the plant.

A series of explosions launched three manhole covers into the air, one after another. The third manhole cover to launch came down on a different operator who was walking from one process unit to another, striking the operator on the head.  The operator was not killed but suffered permanent brain injury.

“Was This Considered in the HazOp?”

Early in the incident investigation, someone asked whether the HazOp team had considered a release of hazardous chemicals to the stormwater sewer. They had. The team considered the potential for ignition of the released material as unlikely (there were no ignition sources in the sewer) and determined that the most likely consequence would be a reportable discharge to the POTW. The discharge would require notification; a reportable release, which the plant considered a severe consequence.

During the incident, the plant did, in fact, call the POTW to let them know that there had been an accidental release, what the identity of the flammable material was, and how much had been released. Personnel at the POTW indicated that they could handle it.

What the HazOp team had not considered was the likelihood of a flame front that travelled through the sewer, launching manhole covers as it travelled, with one eventually striking an operator and causing permanent brain damage. In truth, if someone had suggested that as possibility, the team would have laughed at what was obviously intended as a joke. “Yeah, right. And the plant might get hit by an asteroid.”

More Common Than You Think?

Then I heard about the recent incident in north St. Louis. Twice in just a few years? Perhaps sewer explosions that launch manhole covers are more common than we think.

CBS News has a page devoted to sewer explosions that launch manhole covers, just in New York City. It links to articles about 18 separate incidents, going back to June 15, 2013. That works out to about 1.6 incidents per year in that one city. The go-to explanation from CBS News is that the fuel is sewer gas—a mixture of methane, carbon monoxide, hydrogen sulfide, and ammonia, all of which have a flammability range—and that the ignition source is a spark from underground wiring, which corrodes in the winter when the street department puts down salt to deice roadways.

Sewer explosions that launch manhole covers are not unique to New York City, however. In addition to the explosion in St. Louis on 29-Aug-2024, there were news reports in 2024 for sewer explosions in at least seven other cities:

These explosions are not simply attributed to corrosion caused by deicing salt. The sewer explosion and launch of a manhole cover in Hastings, Nebraska, for instance, was attributed to residents dumping acetone and gasoline into the sewer.

Flammable Liquids in the Sewer

Perhaps the most historic instance of sewer explosions that launched manhole covers was the 1929 Ottawa, Ontario sewer explosion. The series of explosions went on for 25 minutes, following the sewer line to the point where it emptied into the Ottawa River. A consensus has emerged that the fuel for the explosions came from fuel stations and mechanics’ shops in the area, which routinely dumped used motor oil and other flammable waste into the sewers.

Another instance was the 1981 Louisville, Kentucky sewer explosions. Ralston-Purina used hexane to extract oil from soybeans at their Louisville plant. On the evening of February 12, the hexane recovery system failed, releasing the flammable solvent into the sewer system. The next morning, after spreading through the sewer system, the hexane found an ignition source. A series of explosions began at 5:16 am and continued until 3:45 pm, destroying 13 miles of sewer lines and the streets under which the sewers travelled.

On August 11, 2001, sewer explosions in Baltimore, Maryland launched manhole covers. After the explosions, city workers pumped almost 3,000 gallons of tripropylene from the stormwater sewer system. Less than a month earlier, a train derailed in a downtown Baltimore tunnel, spilling thousands of gallons of tripropylene that ignited and burned in the tunnel for 4 days. At the time, authorities believed that all of the tripropylene had burned off in the fire.  Apparently not.

About the recent sewer explosion in St. Louis, Captain Dave Neighbors, of the St. Louis Fire Department, had this to say: “From the best we can tell, something, probably from the south, got into the sewer system.” The stretch of the Mississippi River frontage north of downtown St. Louis is a heavy industrial neighborhood, although the “something” could have come from residential neighbors, as with the Hastings incident, or it could have been deliberately and illegally dumped by someone who drove into the area.

More Than Two Nickels

The next time a HazOp identifies the potential for releasing flammable liquids to the stormwater sewer, perhaps it would be wise to consider the safety consequences as well as the environmental consequences. It would seem that transforming the 200-lb disks that we call manhole covers into projectiles is far more common, and far easier, than most of us would imagine.

And what goes up must come down.

 

Author

  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.

    View all posts