2007 New York City Steam Explosion - Cause

Cause

As of August 2007, the cause of the disaster remained under investigation, and the investigation was expected to last three months. Engineers were unable to access the crater to assess the damage until the tow truck was removed on July 22, and the on-site investigation was complicated by the asbestos contamination in the crater.

Steam hammer was suggested as a possible cause. It is a phenomenon that can occur when cold water comes in contact with a hot steam pipe, causing the steam inside to condense into liquid water, resulting in multi-phase flow which can damage piping. Runoff from the heavy rain that day, or possibly a water main break, have been suggested as possible triggers. There was a broken water main at the bottom of the crater, near the broken steam pipe, but it is not known if this was a cause, or an effect. The utility initially reported that instrument readings did not indicate any pressure buildup prior to the explosion. They subsequently revised this assessment saying the ruptured pipe "experienced a sudden internal over-pressurization, due to water hammer, at least five times greater than normal."

Cold water impinging on the cast iron pipe may have also put excess stress on the metal causing it to fail. The age of the pipe, and the difficulty of inspecting underground infrastructure, made corrosion a possible factor as well, but a New York State safety official reported at a Public Service Commission meeting in September 2007 that there was "no indication that the pipe was deteriorated or weakened by corrosion."

There had been persistent steam leaks and related repair activity in the area the weeks and months prior to the explosion, and utility crews had checked the site only six hours before. Preliminary investigation results showed that a leaking seam repaired just before the disaster was found intact, and the pipe burst suddenly at a location approximately 10 feet (3 m) away.

Robert Caligiuri, an engineer for Exponent Engineering, blamed the rupture on a "crack-like flaw" in a welded seam in the pipe wall, in a preliminary engineering report released on October 23, 2007 that was commissioned by attorneys representing the tow truck driver badly burned in the explosion. He stated, "The observed crack-like flaw appears to be old and is large enough that, in my opinion, Con Ed should have detected it prior to the rupture. Once detected, good and accepted practices would have required that this pipe section be immediately replaced." The utility called the report "misguided" and said in a statement that, "our independent consulting engineers have determined that there was no degradation in the condition of the pipe prior to the rupture," and "the weld in this instance in no way contributed to the rupture. The weld was forced open when the rupture occurred; it was not the cause of the rupture."

The official report released by the utility company on December 27, 2007 cited a combination of these factors, accepting responsibility for deficient repair work done by a contractor that ultimately led to the rupture. The report said that excess sealant, previously used to repair a leaking joint, migrated to two steam trap valves used to drain excess condensed water, clogging them. Then when heavy rains on the day of the event cooled the pipe causing excess condensate to collect in the steam pipe, the valves could not remove it. As a precaution, the utility replaced more than 1600 similar valves throughout the system, but did not find any other clogs.