According to a press release, the safety board has been looking closely at the aluminum protective housing covers that were used on three of the vinyl chloride tank cars that derailed. Some of these covers are believed to be almost 30 years old.
On Feb. 22, NTSB investigators finished their damage assessment inspections of the 11 hazardous materials tank cars that derailed, including the five carrying vinyl chloride.
NTSB investigators discovered that three of the examined derailed vinyl chloride tank cars were made in the 1990s with aluminum protective housing covers.
Based on damage assessment inspections of the cars, the NTSB is worried that aluminum protective housing covers on some tank cars melted when pressure relief devices (PRDs) vented burning gas to relieve tank pressure. The preliminary examination showed signs that melted aluminum may have dripped into some PRDs, which would have affected their performance.
Properly functioning PRDs thus reduce the potential for a large-scale tank failure. When a tank car is exposed to fire conditions and its contents are heated, the pressure inside the tank rises, which can weaken the tank shell. PRDs are intended to regulate internal pressure by releasing small quantities of material and reclosing under normal conditions. This reduces the probability of a breach in the tank shell and limits the amount of energy a breach can release if one occurs.
Although the number of tank cars in flammable gas service with aluminum protective housing covers is presently unknown, the NTSB has requested industry data to determine the size of the potentially affected fleet.
Pressure relief devices and valve assemblies recovered from the five vinyl chloride tank cars are going to a testing facility, which investigators will examine starting March 13.
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 is not involved in air monitoring, testing of water quality, environmental remediation or evacuation orders. Questions on environmental issues should be referred to the Environmental Protection Agency.