Hydrogen was found to be leaking from a vent line during cryogenic loading operations. The leak was attributed to a cracked weld on a hydrogen vent line that consisted of (1) double wall aluminum piping and (2) slotted spacers between the inner and outer line to provide a hydrogen gas blanket for insulation. The weld that failed was repaired using a "clamshell" over the area of the failed weld in order to support continued operations. A portion of the failed weld was removed for analysis prior to the repair. After operations, the clamshell repair was excised from the non-vacuum-jacketed double wall piping to allow further analysis of the failed weld. It was later replaced with a new half shell piping section.
An investigation team was formed to determine the cause of this anomaly. The team performed a fault tree analysis which identified the following five main paths or gates: fault in the original weld, age, component not built per design intent, overstress, and design error. This led to 48 events on the fault tree, all of which were investigated. The investigation, analysis and testing determined that the first path, fault in the original weld, was the primary source of the failure.
The fault tree analysis pointed to three probable and two possible contributors of the failure. The probable contributors included incorrect filler rod selected by welder, insufficient fusion in the weld, and insufficient weld penetration. The possible contributors included incorrect power level setting and incorrect heat level applied during welding.
Materials laboratory work confirmed that improper filler metal (4043 Al) was used to weld the 5083 Al alloy vent pipe. The 5083/4043 combination is not recommended for Al welding per either current or past standards. In addition, the failed weld was a very poor weld, most likely a field weld. NDE data indicated a higher number and severity of defects in this weld compared to other welds in the vent line. According to the material lab's failure analysis report, the weld failure resulted from tensile overload originating at the bottom surface of the vent line during tanking operations and was likely the result of a single overload event.
Vent line inspection was performed to identify other suspect welds. Inspection methods included the following: visual inspection, conductivity tests on both sides of each weld to identify the pipe metal (5000 or 6000 series Al), helium leak check of welds, non-destructive evaluation (NDE) using a combination of x-rays and ultrasonic testing, and silicon etch tests to identify weld material. The silicon etch test procedure was successfully developed in the course of this effort to identify 4043 weld filler material in the field.
Instrumentation attached to vent lines during subsequent operations has provided data for the ongoing weld stress analysis and screening criteria to determine which welds required clamshell repairs. Weld defect growth/propagation can now be monitored at periodic intervals.
An unexpected temperature difference of ~200 deg F between the top and bottom of the piping was found to occur during tanking operations in the same area of the failed weld. This temperature difference was responsible for the high thermal stress seen there. Data indicates that the peak stress in that pipe section occurred at about the same time that the weld failed and was higher than predicted.