Experts identify cause of fatal district steam system explosion in New York City

Con-Edison has released a report by independent consultants on what caused the fatal steam pipe explosion in Manhattan, New York last July.
The district steam system — which stretches 105 miles — ruptured at Lexington Avenue and East 41 Street on July 18, killing one person and severely burning two others. The disruption cost businesses in the area millions of dollars.
The experts have found that the explosion was caused by a bubble-collapse water hammer that generated a momentary force against the pipe’s wall. The force produced was more than seven times greater than the pipe’s normal operating pressure.
The pipe itself, though dating from 1924, was found to be in good condition, had no significant corrosion and did not contribute to the event.
According to the findings, heavy rains that morning led to water accumulating around the buried steam pipe, cooling it and causing above-normal condensate. A Con-Edison safety patrol team inspected the pipe at around 11 a.m. but found no vapour.
Unknown to the patrol, two of the steam traps intended to drain condensate were compromised because they were blocked by epoxy debris. The loose resin probably originated when an outside contractor sealed a leaking flange a few months earlier. The compromised traps could not drain the large amount of condensate produced when the pipe was surrounded by rainwater.
Just before the rupture at 5:56 p.m a routine adjustment elsewhere on the steam system changed steam flow, prompting some steam to enter the cooled condensate that had already collected in the main, creating a steam bubble.
According to ConEdison, “The steam bubble’s contact with the cooled condensate caused the steam to condense to water very rapidly, creating a void in the pipe and causing the surrounding water in the pipe to rush to fill the void left as the steam bubble collapsed. The rushing water slammed against water rushing from the other side, creating a large momentary pressure pulse, likely in excess of 1,200 pounds per square inch. This event, called a bubble-collapse water hammer, caused the pipe rupture.”
By the time the expert reports by ABS Consulting Group and Lucius Pitkin were released at the end of December, Con Edison had already implemented several steps in response to their findings. These steps included replacing all 1,654 steam traps in the system. The company is also enhancing its procedures in response to rain, enhancing its protocols for overseeing repairs, and researching new steam trap design.
See www.coned.com.