EMERGENCY RESPONSE TO ACETYLENE CYLINDERS
BY PETER M. STUEBE
Responses involving compressed gas cylinders are among the most potentially dangerous. We respond to leaking and burning cylinders that may contain one of hundreds of different gases, pure or in mixtures, that can cause great injury in numerous ways. In addition to the “standard” hazardous characteristics of a product involved in a haz mat response–toxicity, flammability, and reactivity–cylinder responses also pose hazards from cryogenics, BLEVE (boiling-liquid, expanding-vapor explosion), oxygen enrichment or depletion, rocketing cylinders, and catastrophic container failure, to name a few. Compounding the problem is that there seems to be a lack of readily available information for responders on how to handle compressed gas cylinders.
The prevalence of acetylene makes it one of my unit`s most common cylinder responses. Sometimes the cylinder is leaking without any fire, and sometimes the cylinder is burning. Additionally, it is not unusual to find acetylene cylinders abandoned in vacant commercial facilities, to be discarded in a “midnight dumping” operation, or to have an occupant call the fire department to report that he doesn`t know what to do with a cylinder that has been on the premises for years.
Acetylene presents a problem to firefighters because it is common in every community, is extremely flammable and unstable, and–for us and other departments–is a frequent response. It has a flammable range of 2.5 percent to about 80 percent, which is extremely wide. Additionally, some references list the upper flammable limit as 100 percent. Although this is not technically true, acetylene possesses the hazard of spontaneous decomposition. During this decomposition, the acetylene breaks down into carbon and hydrogen with the evolution of heat. This process can be initiated by heat or mechanical impact and can occur in the absence of air, which is the environment inside the cylinder. Hence, the upper limit is sometimes stated as 100 percent.
As in any haz mat response, one of the first objectives at a cylinder incident is to attempt to identify the product. One of the identifying clues of a container is its shape. A cylinder made for low-pressure, liquefied compressed gas service generally has broad and fat proportions. A high-pressure cylinder containing a compressed gas (one that is nonliquefied, called a permanent gas) is relatively taller and thinner and is bullet shaped. Low-pressure cylinders generally have a foot ring at the bottom; high-pressure cylinders do not. Acetylene, since it is shipped as a liquefied gas in solution, is found in the broad and fat type of cylinder with a foot ring.
For acetylene being transported, examining the shipping papers, or manifest, is a definitive method of identification. The manifest should list the number of the full and empty cylinders being carried and should be updated at each stop by the driver.
Another identification method is the standard four-inch Department of Transportation (DOT) label, which may be affixed to the cylinder. For cylinders containing flammable and nonflammable gases transported by private or contract motor carriers, a color-coded 114-inch label may be used in lieu of the four-inch DOT label. Sometimes, additional information, such as hazard warnings or precautionary statements, must be included on this smaller label. Whereas the DOT mandates labeling while the product is in transportation, the Occupational Safety and Health Administration (OSHA) mandates, under the Hazard Communication standard, that these same labels be on the container while it contains any product. This allows the cylinder to be identified when it is no longer being transported and is a tremendous help to emergency responders.
Haz mat technicians should be familiar with cylinder markings, which give a lot of information quickly and readily. DOT-approved cylinders are required to have certain information regarding manufacture and use stamped on them. This information is usually stamped into the shoulder of the cylinder, the sloping part near the top that leads up to the neck, the point at which the valve is threaded in. On cylinders that have a valve guard, such as propane cylinders, the information is usually stamped into the valve guard. Required cylinder information that must be stamped into the cylinder includes the cylinder specification number, the service pressure in psig (pounds per square inch gauge), manufacturer`s identification number, manufacture date, hydrostatic test date, and identification of the tester. Of this information, the most important to emergency responders are the specification number and the service pressure. Either the letters “ICC,” indicating the Interstate Commerce Commission, or “DOT,” indicating the Department of Transportation, precede the spec number. If “ICC” precedes the spec number, the cylinder was built to ICC specs and is at least 30 years old, since the ICC was replaced by the DOT in 1967. If “DOT” precedes the spec number, the cylinder is newer and was manufactured some time since 1967. The letters and numbers after ICC or DOT give the manufacturing specifications. Some of the more common specifications (not including those for acetylene service) are 3A, indicating a seamless carbon-steel cylinder for high-pressure service above 1,800 psi; 3AA, an alloy steel high-pressure cylinder; 3AL, seamless aluminum alloy; and 4B, 4BA, and 4BW, low-carbon steel with service pressures of about 240 psig. The next number gives the service pressure, the pressure in psig to which the cylinder is charged when filled. (In the future, the service pressure will be indicated in bars, to comply with international regulations.) For example, the marking “DOT 3A 2216” i
Under DOT regulations, a cylinder manufactured to one specification can be authorized for shipping many different gases, and a particular gas can be authorized for shipment in a number of different cylinders. Although this may narrow down the possibilities of what is in the cylinder, it does not identify it. There are hundreds of gases and mixtures but only a relatively small number of cylinder types.
Acetylene is the one exception to this DOT policy. Acetylene can be shipped only as a dissolved gas in ICC/DOT-8 or ICC/DOT-8AL cylinders, which are unique. Additionally, these cylinders cannot be charged with any gas except acetylene. If a cylinder bears the spec number DOT-8 or DOT-8AL, the only gas it may hold is acetylene dissolved in acetone. Even if a cylinder has no shipping paper, label, or other means of identification, knowing this information will identify the product.
An acetylene cylinder is welded carbon steel manufactured to the single authorized service pressure of acetylene, 250 psig. (For this reason, the service pressure is normally not stamped into the DOT 8 and 8AL cylinders, as they are always filled to 250 psig.) Such cylinders may have a welded seam running either the length or the circumference of the container, and usually there is a foot ring at the bottom. The cylinder will have fusible-plug type pressure relief devices. These devices are designed to melt when their temperature reaches about 2127F. When a fusible plug is activated, the entire content is expelled. There will be at least one such device at the top of the cylinder, either as part of the valve assembly or adjacent to it. Additionally, there will also be a fusible plug at the bottom. Although this will vary with the size of the cylinder, this is the basic arrangement.
The inside of an 8 or 8AL acetylene cylinder is not hollow like other cylinders but contains a honeycomb-like matrix that is asbestos, diatomaceous earth, or clay. Although it looks solid, it is actually about 90 percent air space. These tiny spaces limit the volume of gas that may collect. The cylinder is filled with liquid acetone; then the acetylene gas is pumped into a pressure of 250 psig, which dissolves in the acetone. This arrangement keeps the acetylene stable for transportation and use. When in use, the cylinder valve is opened, and the dissolved acetylene comes out of solution as the pressure drops, much like when a can of soda is opened and the carbon dioxide is released. Because acetylene is spontaneously combustible at pressures above 22 psi, regulators are used to prevent its use above 15 psi, a safety factor that prevents spontaneous ignition under normal circumstances.
A leaking cylinder without a fire is dangerous because the gas could find an ignition source and explode. If an acetylene cylinder is suspected of leaking, check it to determine if this is the case, and place a hoseline in position to vent the escaping gas. When checking for leaks, do not use a combustible gas indicator (CGI). Acetylene gas burns so hot it can burn out the catalytic filament in the Wheatstone bridge, thus destroying the sensor. Also, the fumes of burning acetylene will coat the filament in the Wheatstone bridge, which will reduce the instrument`s accuracy for future readings. Based on our various methods of identification, we should know what the product is. When we know it is flammable, there is no reason to use a CGI to test for a leak. Instead, use a simple soap-and-water solution from a squeeze bottle on the cylinder; it will harmlessly determine the location of a leak.
Typical leak points are the packing nut, the valve threads, the collar, the pressure relief device, the regulator, and other attachments. Tighten attachments from which acetylene is leaking. Remember that the regulator connection is a left-handed thread and must be turned to the left to be tightened. Acetylene cylinders are normally equipped with a “packed valve.” If the cylinder leaks from the valve stem, try shutting the valve and tightening the packing nut under the valve handle. This compresses the packing material and should stop the leak. If the valve continues to leak, move the cylinder to an open outside area and allow the gas to slowly vent until the cylinder empties. The area should be monitored, and a hoseline should be used to dissipate the gas. Never overpack a leaking acetylene cylinder into any type of containment or “coffin” device. If a leaking cylinder is overpacked, the pure acetylene released will become pressurized inside the containment device and can spontaneously ignite as the pressure rises above 22 psi.
Fires are frequently caused by hot slag that falls on top of the cylinders during welding operations. The heat melts the fusible plug, resulting in a fire that blows from the fusible plug opening. This is frequently described as an explosion because of the noise. Depending on the cylinder`s pressure, the flames can reach 10 to 12 feet into the air. As the gas escapes and the pressure inside the cylinder drops, the size of the flame will correspondingly diminish. In this situation, it is important to get a hoseline on the cylinder and cool it. Remember that the cylinder has fusible plugs: If the cylinder heats up and water is keeping just the plugs cool, the plugs may not function and the cylinder may rupture. Cool the body of the cylinder, avoiding the fusible plugs. Check for other cylinders in the area that could become an exposure problem. Move them if you can do so safely. If not, cool those cylinders also. Do not move a burning cylinder; it is hazardous. Decide whether to allow the cylinder to burn out as a hoseline cools it during the entire time or to extinguish the fire and plug the leak.
The standard operating procedure in my unit (FDNY Haz Mat 1) is to extinguish the fire and plug any leak from the fusible plug. A fire can easily be extinguished with a dry chemical extinguisher or by slapping a wet glove on the fire. Once the fire is extinguished, our procedure is to carefully screw a self-tapping screw into the opening, thus stopping the leak. I realize this goes against the generally accepted principle of never overriding a pressure-release device, but the reasons for doing it are sound. If allowed to vent, there is always the possibility that a flammable mixture will ignite. It could take a tremendous amount of time to empty a cylinder, depending on its pressure and the size of the leak. Units will be tied up for the entire time venting the escaping gas and monitoring the situation. Importantly, there are several fusible plugs on a cylinder. If we plug one, others can still activate should a second problem occur. Assuming the cylinder will be turned over promptly to competent personnel for repair, I feel that this is a safe procedure.
A fire from a leaking valve stem can also occur. One method of handling this situation is to extinguish the fire and then treat the cylinder as a leaker. However, if such a fire is extinguished, there is always the possibility that tightening the packing material will not stop the leak. In this case, the product will continue to release and must be vented with the area monitored for any accumulation of gas.
Another possibility with these cylinders is decomposition of the acetylene inside the cylinder, which is accompanied by the release of heat. This results from the cylinder`s being exposed to heat, even briefly, from such sources as a welder`s torch or a fire that involves the cylinder or one in proximity to it. You can recognize this situation by the following signs: a bulge or a discoloration on the outside of the cylinder; a spot that is hot to the touch, even after cooling with a hoseline; a spot that dries faster after water has been applied; or a hot spot revealed by a thermal imaging camera. Close the valve of such a cylinder. This will prevent more acetylene from being drawn to the hot spot inside, and the reaction should eventually cease. The heat given off by the decomposition is dissipated to the outside wall and is conducted away by the matrix inside. However, in this situation, the cylinder should be kept cool by a hoseline or immersion in water and the hot spot monitored until the decomposition ceases after the valve is closed.
Acetylene cylinders can be a major problem for fire departments. I recommend a visit to compressed gas suppliers to talk with the people who deal with them every day. These people have handled numerous problems over the years and are valuable sources of information. Get to know the components of the cylinders and how they operate. Obtain a copy of the Compressed Gas Handbook and other Compressed Gas Association pamphlets and industry safety information to learn more. Safety material from gas suppliers contains information on cylinders and their contents and reports on how they have caused death, injury, and destruction. As always, training is the best solution to a common but potentially deadly problem. n
n PETER M. STUEBE is a 20-year veteran of the City of New York Fire Department (FDNY), the captain of Hazardous Materials Company 1, a member of the Westchester County (NY) Hazardous Materials Response Team, and a former haz mat instructor at the FDNY Bureau of Training. He has a bachelor`s degree in business from Marist College, a master`s degree in economics from Pace University, and a master of public health degree in environmental science from Columbia University. He is an adjunct instructor at the National Fire Academy.