Tom Corporandy

Fire emergencies involving flammable gas cylinders are anything but “routine.” There is, however, one characteristic that we can always count on and that is the dynamics of each event. Acetylene emergencies are unlike most flammable gas emergencies in that their chemical and physical characteristics are so unusual. Chemically, it is their triple bonded carbons that create extreme instability (Each set of electrons which are “out of the plane” lends to this instability because of the energy associated with them. In addition, because only one pair of electrons has a “firm” hold, the remaining two pair are loosely held – see figure 1). Physically, acetylene actually sublimes and therefore does not always conform to what we have come to accept as the all inclusive gas laws. (The melting point and boiling point are listed as -119 degrees F, which is its sublimation point. Interestingly, the molecular weight is listed as 26.04. Since the molecular weight of air is 29, this indicates a gas, which is lighter than air. In fact the vd, or vapor density of acetylene is 0.91). It should also be understood that acetylene is not just for welding/cutting operations as we have come to know. It is also used in the synthesis of numerous organic materials including ethyl alcohol, synthetic fabrics and rubber, plastics, etc.

Typically, it is the carbon-hydrogen content that gives hydrocarbon fuels their heat energy. The larger the hydrocarbon, the lower the ignition temperature. Comparing methane (CH₄) with an ignition temperature of 1,004 degrees F. to that of kerosene (C₁₆H₃₄) with an ignition temperature of 444 degrees F., we see that the greater the hydrogen content, the lower the ignition temperature. Though acetylene (C₂H₂) has only two hydrogens, its ignition temperature is only 514 degrees F., evidence of the effect of the triple bond. In addition, acetylene’s heat output is over 4,200 degrees F.!

Unlike oxygen cylinders, acetylene cylinders are not required to withstand high pressures (or ultra high pressures). Acetylene is so unstable that it cannot be stored by itself under pressures exceeding 15 psi. In fact, should the liquid be allowed to “slosh around” on its own, an explosion would occur due to its high instability. Acetylene bottles are manufactured by drawing steel through a set of dies and forming it into a cylinder with a bottom being welded into place. (Figure 2 depicts the two general tank configurations most commonly used. The most common cylinders range in size from 10 to 390 cu.ft.) The cylinder is filled with a porous material consisting of asbestos, monolithic filler and balsa wood. This sponge like material (designed in part to absorb heat of decomposition) is then filled with acetone (dimethyl ketone, CH₃COCH₃). Acetylene is readily absorbed into acetone at a rate approaching 25 times its own weight at 1 atmosphere and 300 volumes at 12 atmospheres). Cylinder pressures should not exceed 250 psi at 70 degrees F. (The acetylene can withstand this pressure because it has been dissolved in acetone.) A fuse plug is installed at the top of the tank, and in some cases an additional one will be located at the bottom. These fuse plugs typically melt at 212 degrees F. Although this may appear to be a rather low fuse temperature, consider the instability of the product. Cylinders are required to be placed in an upright position. This keeps the acetone from leaking into the valve, which will not only cause corrosion, but allow acetone to be burned off with the acetylene.

When cylinders are involved in fire with venting product, acetylene’s flammable range, as well as its flammable limits play a direct role in the tactical decision making process. Its wide range (97.5 by most accounts) limits our ability to effect our suppression approach. Its lel of 2.5 percent should be an obvious indicator that its ability to burn at such a lean mix makes even distant approaches difficult at best. Unlike most flammable gas emergencies we encounter, acetylene’s uel is not at all on the low end. In fact, most sources claim that acetylene will burn at a 100 percent “mix.” (This is because mixtures above 81 percent can explode when under pressure.) And unlike most flammable gas emergencies we encounter, its exceptionally rich burning characteristics precludes us from getting “up close and personal.” Flashback into the breached or fused cylinder is not only possible, but also likely. A common misconception regarding burning cylinders is the operation of the fuse plug. Unless the plug itself is heated to its fuse temperature of 212 degrees F., it will not vent properly and its container may be compromised. Figures 3 and 4 show an acetylene bottle that exploded due to direct flame impingement near the bottom of the tank. Note that the cylinder failed without ever melting the fuse plug. (The author recalls responding one evening to the bottom deck of the Oakland-San Francisco Bay Bridge for a vehicle fire. Though the vehicle fire seemed to be very much “routine,” it involved an acetylene cylinder that by all appearances should not have failed. The fuse plug did not function and the cylinder was projected straight upward only to be stopped by the bridge's upper deck). Note in Figure 3 that this is also the same type of reaction that would result from flashback into the cylinder.


Figure 3 — This tank ruptured due to flame impingement on the sidewall.	Figure 4 — Note the fuse plug has remained intact despite the ruptured cylinder sidewall.

Cylinders under fire conditions and venting properly will exhibit flame lengths dependent upon the actual heat of the cylinders. A rich yellow flame is evidence of the acetylene burning, not the acetone (although some of this product will naturally be involved). Though acetone possesses a vapor density of 2.0, the heated product will take on the characteristics of a burning gas. And containing oxygen in its structure, it burns very clean (if burning by itself). Once again, the primary hazard is one of flashback (of acetylene) into the container. Allowing the flame to completely burn off the escaping fuel, as we do with most properly venting vessels, may prove to be disastrous. (Remember that this is not a pressure-relief valve that vents under pressure and reseats itself as the pressure drops). This is especially true as the product burns off and flame lengths shorten, allowing fire to propagate back into the cylinder. Ideally, burning product should not be extinguished until the flame length is at its shortest. This becomes a calculated decision based on each and every dynamic situation. Fire suppression must be a primary concern. This action automatically triggers another hazard, that of creating a flammable vapor cloud. Allowing unburned product into an enclosed area is equally as disastrous. It is imperative that those areas subject to vapor incursion be evacuated and monitored. Fog streams will assist in dispersion of vapors, but remember acetylene is not water soluble (actually only 2% soluble). In addition, dispersion of vapor production must result in a “product” with an lel below 2.5%

The author was involved in a fire at an acetylene and oxygen manufacturing/storage plant that involved the acetylene tank farm. (See figures 5 and 6.) It was found that many of the cylinders involved were venting through the fuse plug. By allowing the fire to burn until flame lengths were noticeably shortened and pressure diminished, the fires were extinguished. Because this was outside and the wind in our favor, any remaining fuel was dispersed with master fog streams and three-inch hand lines. The success in halting further explosions was dramatic.


Figure 5 — This fire in west Oakland, which began in the acetylene tank farm, eventually destroyed a large portion of the storage facility and damaged the adjoining office structure. Photo by Oakland Police Dept.	Figure 6 — Cylinders rupture with ensuing fireball. Photo by Oakland Police Dept.

For those CAMEO (Computer Aided Management of Emergency Operations) users, you will not be able to generate a “foot print” or “area where chemical concentrations in air may become high enough to pose a hazard to people” (CAMEO). The reason is not so obvious or simple. Although the Response Information Data System (RIDS) within the CAMEO program lists the boiling point of acetylene as –119 degrees F., it does not carry over into ALOHA (Aerial Locations of Hazardous Atmospheres). ALOHA does not list a boiling point (required for a dispersion model) and uses its information through DIPPR (Design Institute for Physical Properties Data). This is the “default” data, which cannot be modified. In fact, ALOHA’s freezing point value for acetylene is actually higher than the RIDS value for acetylene’s boiling point. This is because the boiling point was measured at one atmosphere, while the melting point was measured at a higher pressure. You can generate a “Source Strength” by configuring a direct source release. This is only as accurate as the information you introduce. This means you will have to have an educated guess as to the release duration and amount, thus giving the rate.

In conclusion, it is imperative we understand and appreciate that, not only does acetylene differ from most flammable gas emergencies, but its cylinder is not typical of most compressed gas containers. One must not assume cylinders will always remain in an upright position. Loose or projected containers may result in flame impingement on other cylinders. Unless the fuse plug itself is brought to its fuse temperature, it will not function. Expect flame propagation back into the cylinders — acetylene by nature lends itself to this phenomenon. Consider the concept of risk vs. gain. The Orlando (Florida) Fire Department once experienced a major fire in a compressed and liquefied gas facility in which exploding cylinders directly threatened another major storage facility. Because of the potential life hazard, the district chief correctly decided to stand and fight this fire. Gain outweighed the risk fortunately resulting in only a few minor injuries.