Potential brake valve failures on cars in long term storage
Date: January 2019
This bulletin is being released following the issuance of a Transportation Safety Board Rail Safety Advisory Letter on July 19, 2018. The letter relates to an incident on January 10, 2018, where the crew of a Canadian National (CN) unit coal train lost control of its speed on the Luscar Industrial Spur near Cadomin, Alberta, while proceeding southward down the steep mountain grade.
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The train's braking system was fully charged and despite a full service brake application and full dynamic brake, the train continued to accelerate. An emergency brake application was further initiated, however the train continued to accelerate until reaching the bottom of the grade where it slowed down and eventually stopped.
Upon inspection it was determined that 27 of the train's 58 cars did not have their brake cylinder pistons extended, indicating that the brakes had not applied or had leaked off after being applied.
The cars had previously been in storage for about 21 months before being returned to service on November 10, 2017.
Testing and analysis concluded that there were two failure mechanisms, one relating to the emergency portion and the other to the service portion.
The DB-20 emergency portion failure was the result of rubber setting of the vent valve k-ring seal, in combination with cold weather shrinkage of the seal. This led to excessive leakage during the extreme cold and a subsequent failure to propagate the emergency brake application command. Setting occurs when rubber sits for an extended period of time leading to the evaporation of elastomers, the loss of rubber memory and permanent deformation. The rubber setting was the result of the associated rail cars being in storage for 21 months.
The failure of the service portion led to the unintentional release of the brakes. It was the result of the same type of known failure that was the subject of the Association of American Railroads (AAR) Circular letter C-12027 in 2013. In this instance, worn rubber seals from the bottom cover exhaust port of the DB-10 service portion could shrink during extreme cold and result in auxiliary reservoir leakage, leading to the unintended release of the brakes.
Recommended actions relating to the emergency portion failure
Transport Canada is of the view that railway companies and car owners should examine the complete valve functionality of cars that have been placed in long term storage, where the potential for rubber setting of valve seals is possible, before placing the cars in service. This is of particular concern if cars are to be used in cold weather service.
For additional information on this Bulletin, please communicate with Transport Canada's Rail Safety Head Office
Ottawa head office
330 Sparks St
Ottawa ON K1A 0N8