NTSB examines pressure relief devices on derailed tank cars

National Transportation Safety Board (NTSB) investigators discovered anomalies with the function of some pressure relief devices (PRDs) removed from the five vinyl chloride monomer (VCM) tank cars involved in the Feb. 3 derailment of a Norfolk Southern Railway train in East Palestine, Ohio.

PRDs regulate the internal pressure of rail tank cars by releasing material when under pressure, reducing the potential for a catastrophic tank failure, NTSB officials said in a press release. The board's investigators recovered the PRDs and tested them at a Texas facility last week.

Their examination and testing revealed:

• NTSB testing found anomalies with the function of some PRDs that may have compromised their pressure relieving capability. This will require further testing and evaluation to assess the impact on the operation of the PRDs.

• According to the manufacturer’s part specifications, one of the installed PRD’s internal springs was coated with aluminum, which is not compatible with vinyl chloride.

• While aluminum debris from melted protective housing covers entered the PRD discharge areas, there was no evidence that melted aluminum entered the tank. The NTSB continues to assess if the debris impacted the PRD operation.

The board is continuing to investigate the derailment's probable cause. An update is available here.