CRYOGENIC LIQUID OXYGEN CONTAINER EXPLOSION: AN INVESTIGATION
Noon traffic on Interstate 10 in Mobile, Alabama, was sparse and moving in routine fashion on Monday, July 26, 1993. The pattern of vehicles and people was the usual-families returning home from vacation, delivery trucks, and workers returning to their jobs from lunch. Then, without warning, the unexpected occurred-the structural failure and rupture of a cryogenic liquid oxygen container and the subsequent launching of the container, much like an unguided missile, accompanied by an intense fire that destroyed a privately owned pickup truck.
The first report was received in the Mobile County E-911 Center at 12:40 p.m. It indicated a truck explosion and fire with injuries on I-10 westbound at the Dauphin Island Parkway exit in the south part of the city. Engine 16, Rescue 24, and Medic 103 were assigned the initial response. Engine 16 arrived on the scene at 12:44 p.m. and reported a truck fully involved. The two victims received burns and traumatic injuries and were treated on the scene by Rescue 24 prior to being transported to local hospitals. The truck fire was quickly suppressed and a joint investigation by the Mobile police and fire departments commenced.
PRELIMINARY REPORT
The preliminary investigation revealed that the involved 1974 pickup truck was parked on the emergency shoulder while the driver was changing a flat tire. An unknown type of vessel was being transported in the bed of the truck and exploded, causing a flash fire secondary to the blast.
Debris from the explosion was scattered within a radius of 60 feet. There was some collateral damage to other vehicles in the vicinity of the explosion, but no other injuries had occurred. Other property damage was minimal.
A circular stainless-steel vessel base found approximately 75 feet southeast of the explosion site provided the first clue to the nature of the explosion. This base had a clearly defined initial rupture point in the approximate center, which opened to a Vshaped split that extended out to the edge of the base. The edge condition indicated that it had sheared cleanly away from the rest of the vessel, whose whereabouts were unknown. The vessel base apparently had traveled horizontally after the blast and struck the windshield of an eastbound vehicle.
Other indications of an explosion were found in and around the vehicle. The sides of the bed were completely sheared away by the force of the explosion, and the cab of the truck was blown forward over the engine compartment. The epicenter of the blast was determined to be the front center of the bed, directly over the transmission bell-housing. This determination was based on the equidistant deformity of the frame at this point and the evenly distributed crack across the top of the transmission bell-housing. The driveshaft and exhaust pipe at the point directly below the blast site were significantly deformed.
A careful examination of the debris around the outside burn mark on the pavement located a ⅝-inch brass fitting of the type used exclusively on liquid oxygen containers. All debris and the truck remains were removed from the scene to the police impound lot to be retained for investigative purposes.
SUBSEQUENT INVESTIGATION
The focus of the investigation then shifted to an apartment complex located approximately three-eighths of a mile to the south of the incident location from which the E-911 Center received a report of the smell of gas in one of the apartments at 2 p.m. on the day of the incident. The caller, a resident of the complex, had just returned home from a three-week vacation and opened her front door to a scene of destruction resembling that of a war zone. The resident found that a blaek painted container approximately six feet long and 24 inches in diameter had crashed through the ⅜ roof of the apartment, shattering 2by Pi O-inch rafters and breaking water and gas lines. The cylinder had damaged the bathroom and kitchen. This “missile” was the missing container from the explosion site. Examination revealed there was an internal stainless-steel vessel inside a black-painted outer steel shell. The stainless-steel vessel matched the end found along the interstate. This container also was impounded as evidence.
Interviews with the driver and passenger of the pickup truck disclosed that the container had been purchased at a local scrap yard. The container had been repainted black to conceal its original green color. The owner of the truck said the container had been filled at a second local recycling/scrap yard earlier the same day. The yard selling the liquid oxygen apparently allowed customers to fill their own containers, relying on the customers’ judgment regarding container capacity. Liquid oxygen sales at this yard were based not on weight but on the pressure indicated on the container pressure gauge.
The pieces of the puzzle began to fit more closely together when a motorist passing the site immediately prior to the explosion reported that he saw what he believed to be a ring of white frost around the bottom of the container, giving us a more certain indication that it had been leaking immediately before the explosion. ‘ITie report of the white frost provided crucial evidence regarding the condition of the container.
The container, as designed, consisted of a stainless-steel vessel within a steel exterior shell. It was intended to have a vacuum between the stainlesssteel vessel and the outer shell, to prevent the transmission of heat from outside the shell. The air temperature on the day of the incident was 95°F, and the container was exposed to direct sunlight for a period of at least 20 minutes before the explosion. T he lack of a vacuum, coupled with a container. All agree leaking container, would cause condensation frost to form on the lower shell surface.
A second, and perhaps the most critical, piece of evidence was recovered from the debris in the kitchen of the apartment where the container came to rest. Careful sifting of the area around the final location of the top of the container disclosed a T-fitting of the type used on liquid oxygen containers. Both the pressure gauge and the relief valve had been replaced with plugs. Thus, there was no way the owners could determine the amount of liquid oxygen that had been pumped into the container at the point of purchase. Neither was there any safety device that could have activated prior to the explosion. Examination of the steel exterior shell indicated ridges in the skin, the apparent result of the tremendously overpressurized stainless-steel vessel’s stretching some two inches beyond design specifications, causing the ridges of the pressure vessel to indent the steel outer shell.
We consulted with local gas suppliers to determine the point at which the same type of container would suffer structural failure. The suppliers contacted the manufacturer’s representatives, some of whom were flown in to Mobile to examine the failed that the stainless-steel vessel had remained intact according to design specifications.
BLAST SCENARIO
Investigation findings suggest the following approximate blast scenario:
- The owner of the container had filled it well beyond its design limits. A contributing factor in this situation was the lack of supervision by the seller during the filling process.
- The container was defective at the time of filling and developed a slow leak.
- The container sat in direct sunlight for an extended period of time. Since there was no vacuum between the stainless-steel vessel and the steel shell, heat was transferred to the internal vessel.
- The container could not vent the overpressure since a relief valve had not been installed. The stainless-steel vessel ruptured at the base, causing a violent impact with the steel shell bottom.
- The steel shell bottom blew out, causing violent contact with the truck bed, followed by penetration of the bed and rupture of the truck’s gas line. (Simultaneously, the container launched vertically and the stainlesssteel vessel’s end launched horizontally.)
- The truck became well-involved in fire after the explosion.
- The container vertically descended and crashed through the roof of the apartment three-eighths of a mile south.
Police explosives ordnance demolition experts and Bureau of Alcohol, Tobacco and Firearms agents were involved in the initial investigation to ensure that no explosive device or initiator had caused the incident. The family of one of the truck’s occupants attempted to disseminate to the news media false information concerning the cause of the explosion. This effort was quickly neutralized by the timely and joint disclosure of all available information by police and fire public information officers.
The investigators’ final report identified human error as the primary contributing cause of this incident. The owners of the container failed to follow Department of Transportation (DOT) regulations regarding the transporting of hazardous materials. In addition, the supplier of the liquid oxygen failed to follow DOT regulations regarding the sale of hazardous materials. No fault or responsibility for the failed container was assigned to the manufacturer.
LESSONS LEARNED
The following lessons can be learned from this incident:
- Local authorities must ensure that suppliers of hazardous gases obey all laws regarding the storage and sale of such materials.
- Law enforcement agencies must be alert to transportation violations, particularly those pertaining to camouflaged and inappropriately used “surplus” containers. Communities should investigate the possibility of enacting local laws prohibiting the sale of containers that have been scrapped.
- All debris associated with a container blast must be recovered and carefully sifted through for tank valves, gauges, piping, shell components, and so on.
- Investigators must make every effort to locate and interview all witnesses to such incidents to maximize the amount of data available.
- A joint police-fire investigation allows each agency to pursue its particular area of responsibility and provides each with resources not ordinarily available.
- Investigators should make maximum use of local experts. These resources are readily available. In this instance, they provided technical information not available from any other source.
- The news media should be given factual information on a regular and timely basis. All investigative agencies should coordinate their public relations efforts and release joint press releases.
The design and safety features of modern containers preclude the possibility of catastrophic failure. Human error, however, makes such failures a very real possibility. A slight alteration of any of the circumstances surrounding this incident could have resulted in a loss of lives and significant property damage. Code enforcement and thorough post-blast investigation can prevent such incidents
