INVISIBLE FLAME Fire in a Liquid Hydrogen Tanker
FIRE REPORTS
The potential consequences of a fire involving a cryogenic liquefied flammable gas tanker are well known to most fire fighters. The awesome sight of a BLEVE, a boiling liquid expanding vapor explosion, sticks in the minds of those who have seen the pictures in training programs and considered the possibilities. This potential seemed very real to Phoenix fire fighters on the morning of Mar. 30 as they faced a burning tanker carrying 13,000 gallons of liquefied hydrogen.
The truck driver and his helper had just finished connecting the hoses to fill a 4500-gallon stationary tank at a semiconductor plant when fire enveloped the rear end of the tanker. Both workers received first and second degree burns, although their injuries were not critical. Invisible flames of burning hydrogen came from the tanker’s hose connections located in a large compartment at the rear of the trailer, filling the 10-foot-wide area between the rear of the truck, the stationary tank, and a 4500-gallon liquefied nitrogen tank. This placed three cryogenic liquid tanks within the immediate area of fire involvement.
The Phoenix Fire Department dispatched a first-alarm assignment of four engine companies and two ladder companies plus the hazardous materials response team, a paramedic engine company and its medical support unit (equipped for mass casualty incidents) and two command officers to the 8 a m. call.
Photos by lack Iordan, Phoenix F.D.
First-arriving Engine 10 noted the involvement of the hydrogen tanker and sized up the risk factors, noting two industrial plants, an apartment complex, a day-care center and a busy street within a 1000-foot radius. Deciding to buy evacuation time by cooling the tank with water, Engine 10 laid in with a 3 ½-inch supply line and directed their Stang gun on the area of flame impingement. Within the next 10 minutes, five master streams were operating on the tank to keep the shell cool while fire fighters and police evacuated the area.
Members of the hazardous materials response team donned reflective proximity suits and, under cover of the water deluge, approached the tanker. They evaluated the possibilities of shutting valves, disconnecting hoses and moving the tanker farther away from the other exposed tanks. Shutting the valves proved to be impossible.
While they were in the immediate area, one of the rupture disks blew, sending a plume of flame 30 to 40 feet up from the vent stack. The flame was only occasionally visible as it took on an orange tint; at other times only heat waves were visible.
A decision was made to pull all personnel out of the area because of the danger of a potential BLEVE. A second alarm had been called and the companies were placed in a staging area over a quarter-mile from the scene. The master streams were left unattended and all personnel retreated to find safe positions, waiting to see if the tank would survive or BLEVE.
Attempts were made to obtain engineering data on the truck from the company’s local personnel. The engineer in charge of equipment maintenance and development for the hydrogen supplier, who was intimately familiar with this truck, flew in from San Francisco and arrived at the scene five hours after the ignition. Fie felt that he could shut off the leaking fuel at the house connections, move the truck away from the tanks and then switch the safety relief venting over to the secondary system. If it worked, this would hold the pressure and stop both the leak and the fire. If not, it would also rupture this disk and the fire would continue to burn until all of the hydrogen was consumed hours later.
The expert donned a proximity suit and went in with haz-mat team members who held corn brooms in front of them to test for invisible flame. Within minutes the flames were extinguished as the entire plan went together perfectly. The expert knew exactly what to do and was able to do it with assistance from haz-mat team personnel.
The truck was moved later that day to a large vacant area and the remaining hydrogen was allowed to vent to the atmosphere Extensive investigations were begun to pinpoint the exact cause of the problem, but it was obvious to all concerned that the truck was well built to survive its fiery experience. The only visible damage was some blackened paint and melted plastic labels. The nitrogen tank was also scorched, but not significantly damaged.
The lesson provided many tense moments and some valuable experience for Phoenix fire fighters. The experience gained in dealing with hydrogen and this type of tanker was particularly significant Officials of the company involved were very happy with the performance of both the fire fighters and the truck. Two significant lessons were learned by the department.
- When there is danger of a BLEVE, unattended master streams must be set up and abandoned very rapidly.
- Fuel tanks on pumpers need to be checked and topped off every morning (one of the pumpers ran out of fuel while pumping in an unattended spot).
The Tanker and Its Contents
Each gallon of liquefied hydrogen expands 850 times when changing from a liquid to a gas. As a gas it is lighter than air and highly flammable. To maintain it in a liquid state for more compact transportation, hydrogen is kept at a temperature lower than 400 degrees below zero. Cryogenic hydrogen is one of the most potentially dangerous cargoes carried over the road, and the design reflected the industry’s level of concern. Escaping gas when mixed with air to the lower flammable limit of 4 percent, forms a combustible mixture with a volume 21,000 times that of the original liquid.
The truck has a double shell, stainless steel tank. The space between the shells is maintained as a vacuum for insulation purposes and filled with a new superinsulating foil wrap. The tank is designed to maintain the necessary low temperatures for up to 224 hours in transit.
The tanker had twin safety relief systems described to vent the gaseous hydrogen in case of a problem. Each safety system included a safety valve and a rupture disk sized to allow the gas to vent as quickly as it was produced from the liquid. The truck was only two months old and incorporated the latest state-of-the-art safety features. What if it hadn’t?
