Safety agency examining rail car component in US train derailment investigation
06 March 2023
The US National Transportation Safety Board is looking closely at aluminium protective housing covers used on three of the vinyl chloride tank cars that derailed on February 3 in East Palestine, Ohio.
Figure 1: Tank car OCPX80235 with missing aluminium protective housing cover and metallic debris contained within the protective housing (left), and OCPX80370 with open steel protective housing cover (right) - Image: NTSB
On February 22, NTSB investigators completed damage assessment inspections of the 11 hazardous materials tank cars, including the five carrying vinyl chloride, that derailed. The tank car wreckage was released to Norfolk Southern Railway on February 24.
The five derailed railcars carrying vinyl chloride were DOT-105J300W tank cars. Based on damage assessment inspections of these tank cars, the NTSB is concerned that aluminium protective housing covers on some tank cars melted or were consumed when pressure relief devices (PRDs) vented burning gas while functioning as designed to relieve tank pressure. Preliminary examination suggests that melted aluminium may have dripped into some PRDs, possibly degrading their performance (see Figure 1).
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. Figure 2 illustrates PRD function during the East Palestine derailment.
NTSB investigators discovered that three of the examined derailed vinyl chloride tank cars were manufactured in the 1990s with aluminium protective housing covers. Although the number of tank cars in flammable gas service with aluminium protective housing covers is presently unknown, the NTSB has requested industry data to determine the size of the potentially affected fleet.
Figure 2 (left): Energetic pressure relief from a vinyl chloride tank car. After releasing material, the PRD closed and remained closed - Image: NTSB
PRD and valve assemblies recovered from the five vinyl chloride tank cars are being delivered to a testing facility where investigators will further examine them during the week of March 13, for conditions that might have affected their operation following the derailment in East Palestine, Ohio.
NTSB is conducting a safety investigation to determine the probable cause of the derailment and issue any safety recommendations, if necessary, to prevent future derailments. The NTSB can also issue urgent recommendations at any point during the investigation. All questions regarding the safety investigation should go to NTSB.
Additional information is available on the investigation webpage.