 A specialized liquefied hydrogen cargo tank belonging to a competitor is used to off-load 11,500 gallons of the highly flammable, super-cold liquefied gas. |
IT WAS ARound midnight WHEN the driver of a cargo tanker carrying 11,500 gallons of highly flammable, liquefied, refrigerated, hydrogen gas along Interstate 40 in Arizona some 25 miles west of Flagstaff discovered a leak during a routine check he was required to make every two hours. The incident involved a line that was cracked at a welded repair and couldn’t be “valved off.” The gas is shipped at a temperature of less than 14237F.
 The driver who discovered the leak wrapped the cracked pipe weld with a wet rag, which was frozen by the leaking gas, significantly reducing the volume of escaping flammable gas. (Photos by author.) |
Although the leak was relatively small, liquid hydrogen expands 850 times as it converts from a liquid to a gas. The gas vapor has an extraordinarily wide flammable range with a lower explosive limit (LEL) of 4.1 percent and an upper explosive limit (UEL) of 93.9 percent. The driver applied a wet rag to the leak, which quickly froze because of the product’s extremely cold temperature.
The driver immediately contacted the Arizona Department of Public Safety (DPS), which dispatched a highway patrolman to the scene. The DPS is Arizona’s primary hazardous materials emergency response agency and quickly dispatched a senior commercial vehicle safety specialist (CVSS), who acts as the agency’s state on-scene coordinator.
The CVSS officer determined that it would take several hours for the company that owned the leaking trailer to determine how to best resolve the incident. Since no empty trailers were immediately available to come to the scene to transfer the load, controlled venting of the hydrogen to the atmosphere was considered. It was possible that the leak could quickly get dramatically worse and begin dumping the trailer’s contents.
The CVSS officer, also a member of the Arizona Army National Guard, which operates Navajo Army Depot, located just a few miles from the leaking trailer on Interstate 40, had the trailer escorted to the depot and parked in an unoccupied area of the installation-away from structures and people.
At the same time, the trucking firm that operated the trailer dispatched a safety officer from its closest terminal in Phoenix. The corporate safety officer, several states away, and the manager of cryogenic operations boarded a company jet and headed for Flagstaff. The DPS dispatched a hazardous materials specialist, who coordinated with the firm’s Phoenix safety officer.
Several hours later, the firm’s two safety officers met with the DPS at the leaking trailer and reported that they had contacted another firm that operated cryogenic highway cargo tank vehicles for refrigerated hydrogen service, which is a very rare service. The firm had a half-empty tanker several hundred miles away. It was to be sent to Phoenix to be unloaded and then dispatched to the incident scene.
The special austenitic chromium-nickel stainless steel cargo tanks have an inner pressure vessel, surrounded by a highly efficient vacuum insulating space and a 3/16-inch to 1/2-inch outer jacket. Although newer vessels are plumbed slightly differently than older tanks, most have a “road” safety relief valve, a secondary spring-loaded safety relief valve, and a final rupture disk emergency vent in case the tank loses its vacuum insulation. Because of the liquid’s extremely cold temperature and the rapid and massive expansion from the liquid to vapor phase, were it not for the large emergency vent, a tank could rupture following insulation loss.
Examination of the temporary patch showed that although the crack was still discharging hydrogen, because of the estimated five-mile-per-hour wind, the concentration was out of the flammable range more than a few inches from the leak. The trailer was moved to a remote location about one-half mile away, where the closest abandoned, uninhabited structures were several hundred yards away. It was repositioned to await the arrival of the off-loading vehicle.
MONTORING/VENTING/UNLOADING
Cryogenic hydrogen is typically transferred without pumps using a pressure-building coil on the transport vehicle. The super-cold liquid is valved into a long, folded coil assembly, where it vaporizes, expands, and creates pressure that is piped back into the tank. This pressure then forces off the liquid load into the unloading tank.
The pressure-reducing hydrogen venting was monitored with a combustible gas indicator to determine downwind flammable vapor travel. Even though hydrogen is significantly lighter than air, its extremely cold temperature and the turbulent effect of the wind caused some vapors to be detected within a few feet of ground level. During the maximum pressure-reducing venting, no concentration of flammable gas higher than 40 percent of the LEL was detected downwind.
Almost continuous monitoring disclosed that the condensed moisture vapor in the released gas provided an excellent visual check on the physical location of the cloud. The super-cold venting gas condensed the normal water vapor in the air, producing a visible white cloud. This cloud became invisible from 20 to 100 feet from the trailer. The venting stack is on the top of the cargo tanks, so the vapor concentration typically reached only 20 percent of the LEL around 30 feet from the trailer and had dissipated to below five percent of the LEL within another 20 feet.
Product transfer commenced around 1:30 p.m. and was completed by 4:25 p.m. An estimated 500 gallons of the liquid product were lost from the initial leak and normal venting during the transfer operation. After the liquid was removed from the leaking trailer, it was vented down to zero pressure. The valving was then closed, and the pressure-building coil system was used to ensure that all liquefied hydrogen had been vaporized. The tank’s internal pressure built slowly back to around two psi within 45 minutes. This pressure was then vented down to zero before the tank was allowed to continue to a repair facility.
The operation was concluded at 5:30 p.m., a little more than 17 hours after the driver had first detected the leak. The cooperation between the two transport firms and the technical expertise of the three industry safety officers made it possible to transfer 11,000 gallons of highly flammable hydrogen without incident.
LESSONS LEARNED AND REINFORCED
- As demonstrated by the driver’s application of a wet rag to freeze and stop the leak, experienced industry personnel may frequently have specialized knowledge unknown to public safety agency responders.
- Competitors in the hazardous materials transportation business will frequently help each other out in an emergency.
- The lack of availability of highly specialized equipment may delay the resolution of some incidents.
- Because of changes in temperature or wind conditions, flammable vapor clouds may not behave as normally as would be expected.
STEPHEN L. HERMANN is hazardous materials coordinator for the Arizona Department of Public Safety and Arizona’s senior state-on-scene coordinator for hazardous materials emergency response. He is past national chairman of COHMED, the national organization of state and local hazardous materials enforcement officers, and past chairman of the Commercial Vehicle Safety Alliance Hazardous Materials Committee. Hermann has a bachelor of science degree in explosive technology and is a graduate of the U.S. Naval School Explosive Ordnance Disposal, U.S. Army Command; General Staff College; and the U.S. Army War College and is a hazardous materials specialist for his agency and a state Division of Emergency Services Hazardous Materials technical course graduate.
