An explosion that occurred December 7, 2009, at the NDK Crystal manufacturing company in Belvidere, Ill., resulted from corrosion in the walls of a pressure vessel that went uninspected for years.

The findings were released in a draft investigation report from the U.S. Chemical Safety Board and set for consideration at a public meeting to be held today in Rockford, Ill., a few miles from the facility. At the meeting, CSB will present its case study report and vote on whether or not to adopt its contents and safety recommendations. The CSB also released a safety video on the accident, “Falling through the Cracks,” which utilizes a computer animation to depict the sequence of events and investigation findings.


Explosion Results from Stress Corrosion Cracking in Vessel
The investigation concluded that the failure most likely resulted from undetected stress corrosion cracking on the inside walls of the vessel (shown in a still from the video).
The investigation concluded that the failure most likely resulted from undetected stress corrosion cracking on the inside walls of the vessel (shown in a still from the video).
Photo courtesy of CSB

Explosion Resulted from Stress Corrosion Cracking in Vessel

In the CSB case study on the accident, investigators note that NDK mixed raw mined quartz, or silica, with a corrosive sodium hydroxide solution at high pressures and temperatures. The resulting reaction with the steel vessels 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.

On the day of the accident, the violent rupture occurred in the Number 2 vessel, which was growing synthetic quartz crystals at extremely high pressure and temperature. One piece of steel from the building was blown 650ʹ, striking and fatally injuring a truck driver at a nearby gas station on the Illinois Tollway. Also, a vessel fragment weighing more than 8,000 lb, tore through a wall at NDK Crystal and skipped across a neighboring parking lot, striking the wall of an automotive supply company where 70 people were working. One person was injured in that facility.

“Over the years, NDK ignored safety recommendations and warnings that it should regularly inspect the interior walls of the vessels,” says CSB Chairperson Rafael Moure-Eraso. “Because the vessels did not meet 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.”


Safety Auditor Raised Concerns Potentially Catastrophic Outcomes

During its investigation, the CSB found that had NDK Crystal ignored a direct warning by a third-party safety auditor called in by its insurance carrier following an uncontrolled leak of hot, caustic material in January 2007 from the lid of a similar pressure vessel. The consultant concluded the accident resulted from the vessel’s improper design, fabrication and material selection, determined four vessels were experiencing stress corrosion cracking, and advised against returning any of the facility’s eight vessels into service.

In his report, the insurance consultant cautioned “far more catastrophic scenarios are possible,” specifically naming danger of public death or injury at the gas station where the truck driver would be killed in 2009. But NDK never established an internal vessel inspection program and did not perform testing on the vessels before returning them to service. In fact, despite warnings that in fact corrosion might be occurring, resulting in a phenomenon known as stress corrosion cracking (SCC), NDK continued to operate the vessels without performing the recommended inspections.

CSB lead investigator Johnnie Banks said, “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. 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.”


CSB Findings Point to Stress Corrosion Cracking

The CSB found that the Illinois Board of Boiler and Pressure Vessel Safety — after incorrectly designating the NDK vessels for non-corrosive service — did not conduct internal inspections. The state conducted three inspections of the vessel that failed in 2003, 2006 and 2009 (less than three months prior to the incident) but these inspections focused only on accessible external surfaces and did not look for inside corrosion.

The investigation determined that the vessels did not meet requirements of the American Society of Mechanical Engineers’ (ASME) Boiler and Pressure Vessel Code, which provides codes and standards that are adopted by state and federal regulators, including those in Illinois. The report found NDK vessel walls, 8ʺ thick, exceeded the recommended limits (7ʺ), making them possibly too thick for proper heat treatment during manufacturing.

Investigator Lucy Sciallo-Tyler said, “Evidence from the 2007 and the 2009 accidents suggests the excessive wall thickness at NDK resulted in improper manufacturing and contributed to the metallurgical damage mechanism that led to the catastrophic rupture.”

The investigation also examined zoning laws that permitted NDK Crystal to be built near the Illinois Tollway Oasis and other nearby businesses, including the automotive supply company. The CSB found that NDK’s property was zoned “light industrial,” for businesses such as car repair shops or lumberyards. Investigators determined that NDK should have been considered “heavy industrial,” in the category of oil refineries, metal and ore manufacturing and fertilizer manufacturing. However, the CSB found no evidence NDK informed the City of Belvidere that its pressure vessels would operate at extremely high and potentially dangerous temperatures and pressures typical of heavy industry.

Chairperson Moure-Eraso said, “NDK was not identified as a potential risk to the nearby community when it was built in 2001. 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.”

Lead investigator Banks said, “Our report lists eight key findings, which in summary point to the results of regulatory ambivalence and a culture of not inspecting for problems in the face of clear warnings.” Among the failings noted:

  • NDK did not verify the integrity of the vessel coating.
  • Regulators incorrectly designated vessels as noncorrosive.
  • NDK did not examine vessels even after being told of corrosion.
  • The company did not perform inspections even after a recommendation to do so by the vessels’ designer, who knew the equipment better than anyone else.

The CSB report makes eight safety recommendations to pressure vessel code and regulatory authorities, and to NDK. The company has not resumed operations since the accident. But should it plan to restart, the CSB recommends:

  • The Illinois State Fire Marshal office prohibit NDK the use of its existing pressure vessels for crystal-growing operations.
  • NDK must ensure that any restarted process is rigorously demonstrated to be inherently safer than the existing one.
  • NDK should commission a facility-siting study of all potential off-site consequences, addressing emergency planning and response programs relevant to each failure scenario and to provide a copy of the study to the city and state.
  • The marshal’s office should see that the state’s pressure vessel approval process identifies corrosion and similar deterioration mechanisms and ensures regular inspections.

In addition, the CSB recommended that the American Society of Mechanical Engineers revise its Boiler and Pressure Vessel Code to include specific material thickness limitations to prevent environmentally induced damage.

The report also recommends NDK Crystal implement a program to ensure the ongoing integrity of the interior coating of vessels, and implement an annual inspection and corrective action program in accordance with the pressure vessel inspection code of the American Petroleum Institute.