The U.S. Chemical Safety Board released a “Factual Investigative Update” on the April 3 catastrophic rupture of a pressure vessel at the Loy Lange Box Co. in St. Louis that killed four people and left another in critical condition.

The CSB’s ongoing examination of the incident has identified a history of leaks in the pressure vessel, which was part of a steam generation system. In 2012, the vessel was repaired when it was discovered that water was leaking from the bottom of its tank. In what was termed an “emergency repair,” a portion of the bottom of the tank was replaced with a custom-made center section.

On March 31, employees again noticed a leak from the bottom of the vessel. Photos taken by the employees revealed leaks coming from at least two distinct sections of a 6-inch ring of original tank material that had been left surrounding the replacement center section of the vessel in 2012.

Three days later, on April 3, Loy-Lange started up the steam generation system and the vessel ruptured in the area of that ring.

In examining the vessel post-incident, investigators found the metal in the rupture area extremely thinned from its original state. While the thickness of the metal should have been a quarter of an inch thick, this specific area had been worn down to eight hundredths of an inch.

The immediate cause of this incident is the sudden mechanical integrity failure of the entire ring of the original bottom of the pressure vessel. This rupture separated the bottom of the tank from the rest of the pressure vessel. This created the unique conditions for a steam explosion, launching the vessel through the building about 520 feet before landing at the Fautless Healthcare Linen’s site.  This was a massive explosion – releasing energy equivalent to about 350 pounds of TNT.

St. Louis is required to inspect the pressure vessel by its ordinance; however, the CSB has received no evidence of inspection.

The investigation will continue with mechanical analysis and additional document reviews, interviews, research and analysis.  A full factual investigative update can be found here.