LISBON — In a recent update, the National Transportation Safety Board said investigators are looking closely at aluminum protective housing covers used on three of the tank cars carrying vinyl chloride on the train that derailed a month ago in East Palestine.

According to the report issued Thursday, damage assessment inspections were completed Feb. 22 on the 11 hazardous materials tank cars that derailed, including the five cars carrying vinyl chloride.

The tank car wreckage was released to Norfolk Southern on Feb. 24.

Based on those inspections of the vinyl chloride tank cars, the NTSB expressed concern that aluminum protective housing covers on some tank cars melted or were consumed when pressure relief devices vented burning gas while functioning as designed to relieve tank pressure.

Preliminary examination suggests that melted aluminum may have dripped into some PRDs, possibly degrading their performance.

“When a tank car is exposed to fire conditions and its contents are heated, the pressure inside the tank rises. This can lead to loss of tank shell strength and eventually a breach. To protect emergency responders and the public from the possibility of catastrophic tank failure under fire conditions, the pressure inside tank cars must be controlled. PRDs are intended to regulate internal pressure by releasing small quantities of material and reclosing after normal conditions are restored. This reduces the probability of a breach in the tank shell and limits the amount of energy a breach can release if one occurs. Properly functioning PRDs thus reduce the potential for catastrophic tank failure,” the press release states.

The NTSB is looking for industry data to determine the number of cars that might have aluminum protective housing covers. Three of the derailed vinyl chloride tank cars were made in the 1990s with aluminum protective housing covers.

PRD and valve assemblies recovered from the five vinyl chloride tank cars will be examined further during the week of March 13 for conditions that might have affected their operation following the derailment.

The NTSB is continuing to work on determining the probable cause of the derailment to issue any safety recommendations to try to prevent future derailments. Urgent recommendations can also be made any time during the investigation. The NTSB is not involved in air monitoring, testing of water quality, environmental remediation or evacuation orders.

Additional information is available on the NTSB investigation web page.

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