The Los Angeles County Metropolitan Transportation Authority (LACMTA) has a large fleet of CNG buses and several CNG refueling sites. Filtration canisters are installed in the fueling systems to remove incidental particulate matter in the gas stream.
On three occasions, refueling system filter canisters ruptured and caused damage to nearby buses. This paper describes the failure investigation that was performed after one of these incidents. The elements of the investigation included determination of the root cause of failure, metallurgical examination of both incident and exemplar canisters, and non-destructive testing of similar components still in service. It also describes a hazard analysis, which was performed to help identify and rank other process hazards associated with CNG refueling operations. Since the completion of this investigation, all similar filters in the MTA system were replaced with improved canisters, constructed from higher strength materials.
The investigation concluded that the filter canister ruptured due to a fatigue crack, having been subjected to approximately 100,000 pressurization/depressurization cycles. Upon inspection, an exemplar canister was found to exhibit a manufacturing defect that could reduce its fatigue life from 10,000,000 cycles (several hundred years) down to as little as three years. The hazard analysis uncovered no potential hazards that were ranked unacceptable or undesirable.