Recently, Con Ed of New York concluded that the cause of the deadly steam explosion in Midtown Manhattan on 18 July last summer was the result of faulty maintenance that led to a pipeline condition known as a bubble-collapse water hammer. The afternoon rush-hour blast, which caused one fatality and sent dozens to emergency rooms, sent a spectacular geyser of steam and debris hundreds of feet into a high-rise canyon adjacent to Grand Central Terminal, setting off a panic that sent the city into red-alert status.
We covered the steam pipe explosion in a Tech Talk item the following day (please see "New York City Rocked by Steam Pipe Blast") and a backgrounder on the technology of district steam power generation in our August online issue, "Source of New York Steam Blast Is a Literal Mystery".
In a prepared statement from Con Ed on the steam pipe blast, the utility said that the accident 'was caused by a bubble-collapse water hammer that generated a momentary force against the pipe's wall that was more than seven times greater than the pipe's normal operating pressure'. The finding was the result of months of analysis by two consulting firms that specialize in such investigations. The company stated that it had already conducted inspections throughout its district steam system to check for similar conditions to prevent further occurrences. It also said it had instituted new policies and procedures for the maintenance of the system going forward.
â''We are committed to operating the steam system in a safe and reliable manner,â'' said William G. Longhi, Con Ed's senior vice president of central operations. â''We are also committed to applying what weâ''ve learned to enhance the safety of our equipment in a complicated underground environment.â''
Upon hearing of the announcement from the company, New York Mayor Michael Bloomberg responded: "I'm glad to see that Con Ed at least did an investigation and that investigation showed that it was their actions that caused it, and that's the first step in making sure it doesn't happen again."
The maintenance work blamed for the chain of events leading to the explosion was performed by Team Industrial Services, of Alvin, Tex. According to Con Ed, workers for the contractor's New Jersey affiliate applied too much epoxy to a leaky flange on a steam trap that vented condensate (or hot water) from the pipeline section buried deep beneath the intersection of Lexington Avenue and East 41st Street. Some of the epoxy material ended up inside the trap.
On the afternoon of 18 July, torrential rain swamped Manhattan. The rainwater soon settled into the earth the district steam pipes are surrounded by. The seeping cool water then came into contact with the compromised main line and created a "steam bubble" inside, which set off the water hammer problem, according to Con Ed.
In its words:
The steam bubbleâ''s contact with the cooled condensate caused the steam to condense (change) 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.
Further details on the analysis of the spectacular accident are being posted to Con Ed's Website, the company reported. It also said that its new action plan--which calls for replacement of all 1654 steam traps in the system, enhanced rain response procedures, new repair oversight protocols, research and development on steam trap design, and other measures--will be immediately made public.
This will most likely come as too little, too late for most of the individuals and enterprises severely impacted by the accident. Many lawsuits have been filed in the intervening months, and more will surely follow as circumstances are sorted out. In the meantime, the engineers who conducted the investigation have provided a foundation for understanding how such trivial matters as properly sealing a valve can have such devastating consequences. Once again, the old adage of "taking care of the little things" has proven to be time-tested advice.