LEARNING THE PROCEDURE WITHOUT UNDERSTANDING A PROPANE GAS “FLARE”
BY STEPHEN L. HERMANN
In the process of training our hazardous-materials emergency responders, we may be overemphasizing classroom knowledge at the expense of common sense and field-acquired skills. We already spend an inordinate amount of time making students memorize chemical formulas and concepts that have very little real-world application. But an incident involving an overturned propane cargo tank on Interstate 10 indicates we may be guilty of teaching procedures without ensuring students really know how they work.
A 2,500-gallon propane cargo tank had overturned on an exit road from Interstate 10, just outside a small, central Arizona town. Responders found the truck was discharging a minor amount of gas from a hose, but they were unable to control the leak. (The driver of the tank was inexperienced, and the owner and manager of the firm were out of town.)
Emergency responders were not familiar with the plumbing on the truck. They examined the tank and determined that it had been damaged and that the contents should be removed before uprighting the tank. No truck to which the propane could be off-loaded could be located. The responders, therefore, determined the best course of action was to “flare” the propane contents of the tank. This procedure involves hooking up a hose to the tank and connecting piping to the end of the hose. Propane then is flowed through the hose and ignited at the end of the piping.
The hose and piping were laid out, the propane flow started, and the discharging at the end of the pipe was ignited. Responders had been working the incident most of the night and determined they weren`t needed at the scene. So they departed.
When a liquefied compressed gas, such as propane, converts from liquid to vapors in the tank, an “autorefrigeration” effect occurs. The liquid remaining in the tank is cooled by this vaporization process.
As the flaring continued, the remaining liquid propane got colder and colder and approached the boiling point of the product, -44°F. At the same time the temperature of the liquid dropped, so did the pressure inside the tank.
Pressure and product temperature are directly related. At a temperature of 115°F, the internal tank pressure is 213 psi. At
100°F, the pressure is 173 psi; at 70°F, the pressure is 110 psi; at 0°F, the pressure is 34 psi; and at -44°F, the pressure drops to zero. Once the pressure inside the tank dropped to near zero, the flow of propane gas stopped at the end of the pipe, and the fire went out.
It was about sunrise. When the sun came up, it heated the tank, thus warming the remaining propane liquid and increasing the internal tank pressure. Now significant amounts of unignited propane were flowing out of the pipe within a few yards of the Interstate 10 exit.
Presumably, the responders had figured that, when all the propane was gone, the flare fire would go out and it then would be safe to lift the tank. They simply were unaware of the autorefrigeration effect, in which propane becomes cooler as vapors are consumed. The remaining propane liquid would cool enough to drop the tank pressure sufficiently to interrupt the gas flow to the flame.
Their instructor, who probably had never flared product, taught them what “flaring” was but didn`t know enough to teach them how to do it right. Fortunately, no tragedy occurred as a result of this improper procedure. I arrived on the scene shortly thereafter.
Too much of our hazardous-materials instruction is conducted by well-intentioned, classroom-educated individuals who have insufficient field, practical working experience to be teaching the subject. As many agencies and institutions emphasize the “academic” credentials of their instructors, they tend to ignore what is more important in this field–the amount of real-world emergency response experience the instructor has.
More emphasis should be placed on utilizing experienced, qualified instructors to avoid repeats of incidents such as this “flaring” incident. n
STEPHEN L. HERMANN is hazardous-materials coordinator for the Arizona Department of Public Safety, Arizona`s senior state on-scene hazardous-materials coordinator, and the former national chairman of U.S. DOT`s COHMED.
