CHEMICAL DATA NOTEBOOK SERIES #53
Acetylene is a flammable, unstable, narcotic, colorless, tasteless gas. Pure acetylene is odorless, but commercial acetylene generated from calcium carbide has a garlic-like odor. The most visible use of acetylene is in oxyacetylene welding, cutting, and heat treating. However, just under 20 percent of the acetylene produced in the United States is used for these purposes; the rest is used in the synthesis of other very common chemicals, such as acetaldehyde, acetic acid, acetic anhydride, acetone, and vinyl chloride. Most acetylene is produced by the reaction of water with calcium carbide (see Fire Engineering, May 1987), and the rest is produced by either the thermal or arc “cracking” of hydrocarbons or by the partial combustion of methane with oxygen.
Acetylene is a very flammable gas— with an extremely wide flammable range of from 2.5 percent to 82 percent in air and an ignition temperature as low as 581° F. It has a vapor density of 0.91 and a molecular weight of 26.04. It has a reported boiling point of — 103°F, has a freezing point of — 116°F, and is soluble in water. The boiling and freezing point information is generated at pressures above atmospheric pressure, since solid acetylene will sublime at — 118°F at 14.7 psig. Its molecular formula is C2H2.
Acetylene’s main hazard is that it is such a flammable gas. Its wide flammable range and low ignition temperature are the reasons for this hazard, but its structural formula gives rise to another hazard: its instability under pressure or when shocked. Some references say that acetylene’s flammable range stretches to 100 percent, since there have been reported explosions when an ignition source contacted the essentially pure acetylene. This phenomenon occurs because of the instability of acetylene, especially under pressure. The gas inside the cylinder ignites and explodes, because at any pressure above 6 psig any rapid input of energy will cause a molecular decomposition, releasing greater amounts of energy and causing the gas to detonate. Whether this is technically normal combustion (rapid oxidation of the fuel) or molecular decomposition, the result is the same —an explosion.
The structural formula of acetylene shows a triple covalent bond between its two carbon atoms. It is this triple bond that is so unstable. When energy is put into the structure, whether as heat or as a shock to the molecule, the triple bond “comes apart,” releasing tremendous amounts of energy. This instantaneous release of energy then impacts the other acetylene molecules, and a detonation results. This is another reason to handle cylinders of acetylene with care. Protect any container of any compressed gas against falls or other impacts that might cause the walls or one of the containers to fail. An additional hazard is that an impact from a fall or collision might shock the gas so that the container detonates.
While it is not necessary to understand the chemistry of covalent bonding, it is important to know that hydrocarbon compounds containing only single covalent bonds between carbon atoms (so-called saturated hydrocarbons) are relatively chemically inactive, while hydrocarbons having a double covalent bond between at least one pair of carbon atoms (unsaturated hydrocarbons) are relatively active chemically. By the time a hydrocarbon is created with a triple covalent bond (also unsaturated), the structure becomes very unstable and subject to catastrophic breakdown.
The triple bond also is the reason for acetylene’s extremely high flame temperature. Once energy input into acetylene molecules begins the combustion process, the tremendous amount of energy tied up in the triple bond is released, resulting in flame temperature reaching 2,4l7°F in air and 5,710°F when mixed with pure oxygen, as in an oxyacetylene welding or cutting operation. This is hotter than any other fuel gas and shows just how much energy is stored in the triple bond.
IDENTIFICATION NUMBERS AND RATINGS
(Chemical Abstract Services)
(Registry of Toxic Effects of Chemical Substances)
(United Nations/North America)
(U.S. Department of Transportation)
NFPA 704 Rating
Acetylene may be liquefied or solidified as may other gases, but it is not advisable to do so, since the fragility of the triple bond seems to be increased by the liquification and freezing process. Liquefied or solid acetylene is extremely dangerous, and because of the threat of detonation, no acetylene may be transported in these forms. Even gaseous acetylene is not transported over very large distances. The principal raw material, calcium carbide, is what is shipped around the country to acetylene generation operations, where water is added to produce the gas. The rough treatment of the cylinders on the back of a truck may be enough to shock the acetylene into detonation.
Due to the instability of the triple bond, acetylene may not be pressurized above 15 psig or it will detonate. So how can acetylene be put into cylinders where the pressure gauge reads 250 psig? The inside of the acetylene cylinder is filled with a very porous mineral, and into this is poured a liquid hydrocarbon, acetone. The acetone fills all the nooks and crannies of the porous mineral, and then the acetylene is pumped into the container. The acetylene immediately dissolves in the acetone, and any undissolved acetylene that exists above the liquid is so .segregated by all the tiny pores of the mineral that the danger of high pressure is eliminated. So an “empty” acetylene container is really “filled” with acetone, a flammable liquid (see Fire Engineering, June 1987).
Never store acetylene in a container other than the one for which it is intended. The penalty for pumping acetylene into the wrong container will be detonation of the container at some pressure not far above 15 psig.
Acetylene is mildly toxic when inhaled and is narcotic when breathed in high concentrations. Symptoms include dizziness, gastric distress, headache, and slight intoxication.
Acetylene is incompatible with many materials—some explosively — such as brass, copper and copper salts (especially copper carbide), cobalt, halogens, hydrides, mercury and mercury salts, potassium, silver and silver salts, trifluoromethyl hypofluorite, and all oxidizers. And because of its very wide flammable range, any mixture of acetylene in air must be regarded as explosive.
Acetylene is most commonly shipped, stored, and used in cylinders ranging in capacity from 10 to 850 cubic feet. A special cylinder of 1,400cubic-feet capacity is called a “lighthouse” type cylinder and is used as a marine navigation aid. Each cylinder must be marked with DOT specification numbers, a serial number, an identifying symbol, the date of the manufacturer’s test, and the tare weight. These cylinders may be found on large and small trucks, rail cars, and water traffic vessels of all kinds. Acetylene cylinders should not be transported in nonventilated compartments of passenger vehicles.
These cylinders must never be stored near oxygen or any other oxidizing agent. All federal, state, and local regulations must be followed for the safe handling, storage, and use of acetylene. Also, consult NFPA 51, Standard for the Design and Installation of Oxygen-Fuel Gas Systems for Welding, Cutting and Allied Processing, 1987 Edition, for further information.
Acetylene containers, like most other pressurized gas containers, have a safety relief device. In acetylene’s case, the safety relief device is a fusible (meltable) plug. This device is designed to melt between 208°F and 220°F (212°F nominal). Any time the temperature of the metal in these plugs reaches these temperatures, the plug melts and the acetylene leaves the container. What’s left is an atmosphere filled with an explosive gas and a container holding a flammable liquid (acetone) with a flash point of 0°F.
Any acetylene that escapes from its container poses a serious explosion risk. When such a leak occurs or is expected, all exposed life and property are in jeopardy. Evacuate all occupants within a half-mile radius of the leak and for a minimum of one-half mile downwind immediately. The degree of hazard is directly proportional to the amount of acetylene present and, of course, the degree and type of exposure.
If an accident occurs involving a vehicle carrying acetylene containers, depending on the severity of the accident, the cylinders certainly could have been stressed. It is possible to knock the valves off one or more containers and vent a large amount of acetylene into the air. Since the vapor density of acetylene is 0.91 (and air is nominally 1.0), the gas will disperse fairly quickly into the air. Since acetylene cylinders are always shipped (and stored) in an upright position, the gas will be vented upward, assuming the cylinder is still standing.
As the pressure of a gas drops, its temperature drops. Therefore, when any gas under pressure escapes its container (to the lower pressure of the atmosphere), its temperature will drop dramatically. Since the density of a cold gas is higher than a (relatively) warm gas, the vapor density of cold acetylene will be higher than 0.91 and may not rise as quickly as one would expect. This is especially important to know if the leaking container is on its side and the gas is escaping horizontally or even in a downward direction. There is always the possibility that acetylene gas may be found unexpectedly near the ground.
Approach any suspected leak of acetylene from upwind after making sure that there are no ignition sources in the vicinity. Exclude all unauthorized and nonessential emergency personnel from the incident scene. The incident commander may decide to approach the scene using a highpressure water spray to disperse the vapors, keeping in mind that rapidly moving water (in streams) can product static electricity.
If large amounts of acetylene are being vented to the atmosphere, it may be best to not intervene in the incident beyond enforcing evacuation and securing the area. Notify the shipper and consignee of the load of the incident, as they can offer advice on properly handling the situation.
If there is a suspected leak inside a building, the Compressed Gas Association, Inc. (an excellent source of information on the safe handling of acetylene and other gases) suggests moving the cylinder to an open space, well away from any possible ignition sources, and tagging the cylinder. It even recommends opening the valve (outside in a safe environment) and letting the acetylene bleed off very slowly. In any situation involving leaking acetylene, exercise the utmost caution in mitigating the situation.
Whenever acetylene cylinders are exposed to fire, a great danger exists for anyone nearby. The energy sufficient to melt the container’s safety relief device may come from radiated heat from a fire and certainly from any impinging flame. At the point of operation of the safety relief device, a new flame from the burning, newly released acetylene will add greatly to the total heat of the fire. Acetylene’s very high flame temperature will immediately endanger any other cylinders or any exposures. In addition, the flammable liquid, acetone, will endanger anyone near the containers. Since the fusible plugs will melt at approximately 212°F, it is unlikely that the containers will BLEVE. However, the possibility of BLEVE cannot be ruled out, since the heat of combustion of acetylene is so high. It is also possible that a safety relief device might malfunction, or that the pressure might rise so fast that the device is overcome. There is also the possibility that the hot, unstable acetylene might detonate inside its container due to the increased energy input. If this occurs, the resulting explosion of the cylinder with the attending fireball and shrapnel may indeed look like a BLEVE.
If possible, use water to cool any exposed containers as long as it can be delivered by unmanned appliances and will not endanger emergency responders (or anyone else). Hot acetylene, in the container under increased pressure, will now be very sensitive. The slight bumping together of the containers against each other by the force of water being applied in a straight stream may be enough to cause detonation.
The rules for fighting acetylene gas fires are the same as for any other gas, with the additional warning that hot acetylene is unstable and might detonate in its container. As in any other situation where gas escaping from its container is burning, the flame must not be extinguished until the flow of gas is stopped.
If gas is escaping from a leaking valve, you might be able to turn the valve and stop the flow of gas. However, keep in mind that the tremendous amount of heat energy released by burning acetylene will (by radiation) raise the temperature of the cylinder to the point where the fusible plug will melt, producing another, possibly longer tongue of flame emanating from the cylinder itself. An unsuspecting emergency responder who tries to stop the flow of fuel may be caught in an unexpected trap.
Of course if the flame from the leaking valve (or any other part of the system) is impinging on the cylinder itself, the venting of the cylinder due to the activation of the safety relief device will happen much quicker, possibly before any responders can become involved. The safest approach is not to approach at all.
In any incident involving burning acetylene, the fire will be so hot that all exposures will be threatened. You can try to protect exposures if no one is put in danger by being near the threatened acetylene containers or put in any other dangerous situation.
For inhalation, move the victim to fresh air and keep calm and warm. If the victim’s breathing has stopped or has become labored, administer artificial respiration. Keep in mind that such action might expose the first-aid giver to the material in the victim’s lungs and/or vomit. Seek immediate medical attention.
There is no special danger to the . eyes or skin from acetylene. Also, since it will be encountered as a gas, it is not likely to be ingested.
PROTECTIVE CLOTHING AND EQUIPMENT
No special encapsulating suit or other protective clothing, other than regular turnout gear, is necessary to protect the body from acetylene. However, it is necessary to wear positive-pressure, self-contained breathing apparatus to protect the respiratory system.