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Before and after photos for the explosion from a surveillance video from a nearby custom work truck shop shows the power of the explosion and the effect on the building.
Photo from the Kranz video

A six-inch ring left from the original pressure vessel at the Loy Lange Box Company in St. Louis, MO, had thinned so much, it was just a matter of time before a disaster would occur.

That was one of the conclusions found in a “Factual Investigative Update” from the April 3 catastrophic rupture of a pressure vessel at the Loy Lange Box Company that killed four people and left another in critical condition.

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There was a history of leaks in the pressure vessel, which was part of a steam generation system, according to the ongoing investigation by the Chemical Safety Board (CSB).

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The time bomb started ticking in 2012 when the vessel underwent repair after water was leaking from the bottom of its tank. In an “emergency repair,” a portion of the bottom of the tank was replaced with a custom made center section.

Four days before the blast, 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. At 7:20 a.m., 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 (0.25”) thick, this specific area had been worn down to eight hundredths of an inch (0.08”).

The 1952-pound, 30-inch diameter by 17-1⁄2-feet long steel tank, called a Semi-Closed Receiver (SCR) contained about 510 gallons condensed steam (water at about 330°F and 100 psig.),

As the pressure in the tank suddenly dropped due to the failure of the tank bottom, a portion of the water in the SCR instantaneously exploded into steam, resulting in an increase in volume of about 75 times the volume of the SCR. A steam explosion of this type is extremely hazardous.

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 Faultless Healthcare Linen’s site. This was a massive explosion, releasing energy equivalent to about 350 pounds of TNT.

The City of 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.

Click here for a full factual investigative update.

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