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Corrosion Blamed in Fatal Plant Blast
Stress corrosion cracking and a lack of inspections likely caused the fatal explosion at an Illinois manufacturing plant in 2009, the U.S. Chemical Safety Board has concluded.
One person 650 feet away was killed and one was injured in the blast, which originated in a vessel growing synthetic quartz at extremely high temperatures and pressures for Nihon Dempa Kogyo (NDK) Co. Ltd., in Belvidere, IL.
The company produces and sells crystal-related products used in cell phones and wireless Internet devices.
'Falling Through the Cracks'
The CSB published its draft investigation report Thursday (Nov. 14) in the form of a Case Study. The board also released a video, Falling Through the Cracks, which uses computer animation to show the accident's sequence of events and findings of the investigation.
During the explosion Dec. 7, 2009, a piece of steel was blown off the building, striking and killing a driver who was walking back to his car at a gas station 650 feet away
A vessel fragment weighing more than 8,000 pounds blew through the facility's wall, crossed over a parking lot, and struck the wall of an automotive supply company where 70 people were working, injuring one.
No NDK employees were injured.
NDK immediately shut down all operations at the plant and has not resumed them. The Illinois facility was Japan-based NDK's only U.S. production site.
Direct Warnings Ignored
In NDK's vessels, raw mined quartz, or silica, was mixed with corrosive sodium hydroxide solution at high pressures and temperatures. This resulted in a reaction with the steel vessels that formed a layer of iron silicate, called acmite, on the inside of the vessel walls, which the company believed would protect the vessels from corrosion.
Despite warnings that corrosion might still be occurring under the acmite, NDK kept operating the vessels without performing inspections, the Chemical Safety Board report says.
"After a review of the metallurgical testing data, the CSB found strong evidence of cracking on and near the inner diameter of the vessel fragment," Johnnie Banks, CSB lead investigator, said in a press release. "The cracks reduced the vessel material toughness, which eventually led to large flaws, resulting in the catastrophic failure.
"Stress corrosion cracking was the likely failure mechanism that caused the cracks. Had NDK conducted regular inspections, it would have discovered that the acmite coating was not protecting the vessel walls," said Banks.
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The CSB's Falling Through the Cracks uses animation to show the events leading up to the explosion. |
Temper embrittlement, or another form of heat treatment embrittlement, may have been a contributing factor in addition to the stress corrosion cracking, the report says.
The board also found that the company ignored a direct warning from a third-party safety auditor sent by its insurance carrier after a hot, caustic material leaked from the lid of a similar pressure vessel in January 2007.
The auditor said the leak had stemmed from the vessel's improper design, fabrication and material selection. He said that four vessels at the facility were showing stress corrosion cracking and advised against returning any of the facility's eight vessels to service.
The auditor said "far more catastrophic scenarios are possible" and specifically warned of public death or injury at the gas station where the truck driver was later killed, the CSB found.
However, NDK neither established an internal vessel inspection program nor tested the vessels before returning them to service.
'Near-Inevitable Catastrophe'
"Over the years, NDK ignored safety recommendations and warnings that it should regularly inspect the interior walls of the vessels," CSB Chairperson Rafael Moure-Eraso said.
In addition, the Illinois Board of Boiler and Pressure Vessel Safety incorrectly designated the vessels for non-corrosive service and did not conduct internal inspections, the CSB said. In 2003, in 2006, and three months before the 2009 explosion, the state conducted three inspections of the vessel that later failed, but it focused only on accessible external surfaces.
"Because the vessels did not meet the requirements of the widely recognized national pressure vessel code, the State of Illinois allowed installation of three of the vessels under a special exemption. In doing so, the vessels were incorrectly put into 'non-corrosive' service.
"The manufacturer of the vessels—including the one that exploded years later—recommended annual inspections, but neither NDK nor the state performed the inspections. This set in motion a near-inevitable catastrophe," Moure-Eraso said.
Nor did the vessels meet American Society of Mechanical Engineers standards, the report found. NDK's vessel walls were eight inches thick, while the recommended limit is seven inches, making them possibly too thick for proper heat treatment during manufacturing, according to the CSB.
Zoning Questions
The CSB also examined the laws that allowed NDK to be built near the Illinois Tollway gas station and other nearby businesses, discovering that the property was zoned "light industrial" when it should have been considered "heavy industrial."
"NDK was not identified as a potential risk to the nearby community when it was built in 2001," Moure-Eraso said. "In this and other CSB investigations, we express concern that potentially hazardous chemical facilities are permitted to be built near, or to continue to operate near public facilities.
"Particularly where the public might be endangered, companies should pay all the more attention to near misses and other warnings to preclude catastrophic accidents."
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