1. The trailer involved in the incident used a frangible burst disk based upon the proprietary metal compound designated as Inconel #600. Random sampling of similar pressure relief devices from the same trailer showed that all of them failed at pressures below design specification, indicating that all were adversely affected by exposure to the combination of stresses and the product lading (hydrogen). Examination of all other hydrogen trailers in the supplier's fleet confirmed that different (Carpenter 20-based) pressure relief devices were in service.
2. There has been no specific industry guidance on the type of pressure relief device materials in terms of their metallurgical makeup, but only the pressure ratings associated with the DOT rating of the tubes to which they are attached. This is based upon 5/3 of the marked DOT service pressure of the tube (e.g., 2400 psi tube X 5 รท 3 = 4000 psi pressure relief device rating).
3. The cause of the frangible disk failure was an anomaly. All frangible disks on the trailer were replaced. Prior to placing tube trailer back into hydrogen service, all tube trailer appurtenances were examined for leaks using nitrogen at two succeeding pressures and standard leak detection fluid. A third and final examination was performed at full settled pressure before releasing the tube trailer back into hydrogen service.