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Crystal plant accident caused by stress corrosion cracking

The U.S. Chemical Safety & Hazard Investigation Board last week released its report on a 2009 explosion at a quartz crystal manufacturing plant in Illinois operated by NDK America. CSB determined that a corrosive environment led to cracks in a pressure vessel’s steel walls, resulting in its failure. The blast blew a piece of steel 650 feet to a nearby gas station, where it fatally injured one truck driver.

When they were in operation, the six 50-foot tall crystallization vessels at the plant were loaded with raw mined quartz, 800 gallons of 4% sodium hydroxide in water, a “small amount” of lithium nitrate, and seed crystals of pure quartz, the CSB report says. Once sealed, they were heated to 370 °C and pressurized to 29,000 psig for 100 to 150 days.

The vessels’ steel walls were eight inches thick. Sodium hydroxide and silica will react with iron in steel to produce a layer of sodium iron silicate, or acmite. ADK believed that the acmite layer would prevent corrosion of the steel, but neither the company nor the state ever inspected the vessels’ interiors. In 2007, one of the vessels leaked through a connection in its lid. A consultant hired by NDK determined that the leak was caused by stress corrosion cracking and found cracks in three other lids. The company continued to operate the remaining vessels without inspections, the CSB report says.

For more detail and for CSB’s recommendations to the American Society of Mechanical Engineers, the National Board of Boiler & Pressure Vessel Inspectors, the Office of the Illinois State Fire Marshal, and NDK, see CSB’s report. The agency also produced its usual excellent summary video:

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