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911 Title: Terrorism in Dallas? Or failed practices in Acetylene cylinder filling? This event was not unlike what happened in St. Louis in 2005. Here then is a brief synopsis of what transpired: The fire started on a loading dock on the back side of the company's complex, said Lt. Joel Lavender, a spokesman for Dallas Fire-Rescue. It spread into the building, where tanks of oxygen, helium and acetylene began blowing up. Fire officials said two employees were filling canisters with acetylene using a series of connecting tubes in what is known as "pigtailing." A connection malfunctioned, apparently creating enough pressure for one of the canisters to ignite. For safety, pressure in acetylene tanks must be kept below a certain point. The canisters exploded in a chain reaction, burning two men on their upper bodies. For more than an hour after the incident began about 9:30 a.m., fireballs shot from the company's site at 538 Industrial Blvd., and a column of thick, black smoke rose on a slight northward wind, headed mostly across other business and industrial areas. A good read on how Acetylene is handled by some European contries: http://www.ha-research.gov.uk/projects/projectdocuments.php?method=download&ID=82 Here is an account and analysis of what happened in ST. Louis, take special note of Figure 3 (picture of a cylinder with a flame-plume coming out of it) in the above document. St. Louis was experiencing a heat wave with bright sunlight and temperatures reaching 97 deg. F (36 deg. C) on June 24, 2005. Praxair operations proceeded normally during the morning and early afternoon; however, about 3:20 pm, a technician retrieving cylinders from an outside storage area saw a ten-foot high flame coming from a cylinder (Figure 3) and activated the fire alarm. Security camera video from the facility shows the release and ignition of gas from a cylinder in the propylene return area. As workers and customers evacuated, the fire spread to adjacent cylinders. Security camera video shows nearby cylinders igniting in the first minute. At 2 minutes, cylinders begin exploding, flying into other areas of the facility, and spreading the fire After 4 minutes, the fire covers most of the facility's flammable gas cylinder area and explosions are frequent. Here is how Acetylene 'bottles' are normally filled; note that this process by itslef generates heat (makes the cylinders warm) and is a cause for going slow during said filling process: As is well known, acetylene gas is relatively unstable at high pressures and cannot be transported safely in such open-chambered cylinders as are used to transport other industrial gases. For safety reasons, acetylene is usually transported in elongate steel cylinders of a specialized type, each containing a porous mass within which a solvent for acetylene is absorbed. The porous mass normally fills the cylinder body, and typically comprises a very porous, concrete-like substance such as calcium metasilicate having an admixture of a suitable fibrous material, for example asbestos, to increase its mechanical strength. The solvent is typically acetone or N,N-dimethylformamide. From the point of view of safety it is important that the porous mass which fills the body of an acetylene cylinder leave no large cavities within the body wherein acetylene gas can collect and be compressed, causing an explosive decomposition of acetylene to take place. Decomposition of acetylene into its elements can be prevented by minimizing the size of the spaces defined (1) within the porous mass, and (2) between the porous mass and the surrounding internal wall surfaces of the cylinder. The filling of acetylene cylinders involves problems beyond those normally encountered in filling cylinders with gases other than acetylene. When an acetylene cylinder is returned from a customer, it contains an unknown quantity of residual acetylene gas, and an unknown quantity of solvent. The quantity of solvent remaining in the cylinder is almost always less than the desired nominal amount due to discharge of evaporated solvent from the cylinder as acetylene gas is used by the customer. The only information which can be ascertained about a returned cylinder by physically inspecting it is whether or not the cylinder appears to be damaged, and such cylinder data as is stamped or otherwise inscribed on the outside of the cylinder or on a cap affixed to the cylinder. The cylinder data carried on the outside of an acetylene gas cylinder normally includes a cylinder type designation (which defines the cylinder's internal volume), and the cylinder's "tare weight," i.e., weight of the cylinder together with the weight of the porous mass and the weight of a proper nominal charge of acetone solvent. By weighing a returned cylinder, it is possible to determine the total weight of the cylinder and such solvent and residual gas as remain in the cylinder. By measuring the pressure of the contents of the cylinder, and by taking into account the temperature of the cylinder, the exact quantities of residual gas and solvent which are present in the cylinder can be calculated readily. In a conventional acetylene cylinder filling process, each cylinder is first physically inspected for possible damage. The contents of the cylinder are then checked with reference to the cylinder's tare weight, pressure and temperature, to determine the quantities of solvent and gas which remain in the cylinder. A calculation is then made to determine the quantities of solvent and gas which should be supplied to refill the cylinder. In accordance with conventional filling procedures, an acetylene cylinder which has been checked in the manner described above is refilled first by charging it with the requisite amount of missing solvent, and then by introducing the requisite quantities of acetylene gas. The gas is fed into the cylinder at a relatively low pressure during a period of time which extends for many hours. While cylinders are charged on an individual basis to supply them with the requisite quantities of solvent, a plurality of solvent-replenished cylinders are connected together or "ganged" for simulateous filling with acetylene. While a cylinder is being filled with acetylene, the temperature of the cylinder is caused to rise due to the high heat of solution of acetylene. Stated in another way, as acetylene gas is dissolved by a cylinder-carried solvent such as acetone, a substantial amount of heat energy is released, causing a marked elevation in the temperature of the cylinder and its contents. As the temperature of the cylinder's contents rises, so too does the pressure within the cylinder. When the pressure within the ganged cylinders reaches approximately 25 bar (about 360 psia), filling is halted because, from a safety point of view, this is considered to be about the highest pressure to which acetylene should normally be compressed. The partially filled cylinders are then left standing to cool. After a sufficient period of cooling time, the cylinders stabilize in temperature, and can then be further charged with acetylene to complete the filling procedure. The filled cylinders are then disconnected from the filling apparatus and individually weighed to make certain that they contain, within certain tolerances, the prescribed quantity of acetylene. Any cylinders showing excess weight are slightly emptied. Any cylinders which have been insufficiently filled are given an additional filling. In temperate climates, about seven hours is a normal time for the initial filling of a solvent-charged cylinder with acetylene gas, followed by about a twelve hour pause for cooling, whereafter a final filling with acetylene usually requires about an additional two hours. In hotter climates these filling and cooling times are considerably longer. In order to diminish filling time, it has been proposed to cool acetylene cylinders during filling by spraying their outer walls with a liquid coolant. The coolant is discharged onto the cylinders from overhead nozzles, and typically comprises either cold water, or a cold antifreeze solution such as a mixture of water and alcohol. The overhead arrangement of spray nozzles causes coolant to flow along the full lengths of the outer walls of the cylinders. Cooling the cylinders during filling not only serves to reduce cylinder filling time, but also enables larger numbers of cylinders to be processed through a filling station, and minimizes the need for extensive banks of filling equipment. Moreover, the cost of labor per filled cylinder is reduced. Where cylinders being filled are cooled along their entire length, the temperature of the contents of the cylinder is substantially uniform, and acetylene tends to be disolved substantially uniformly throughout the cylinder by the acetone carried in the cylinder. From a safety standpoint, it is undesirable to have a high concentration of acetylene near an upper end region of the cylinder because, in the event that the upper end region of the cylinder is exposed to a sudden increase in pressure, as occurs in a "backfire situation," the high concentration of acetylene in the upper end region of the cylinder may begin to decompose. Stated in another way, the previously proposed approach of cooling cylinders along their full lengths during filling with acetylene does not provide an optimum type of cooling, from a safety point of view, for it tends to provide an unduly high concentration of acetylene in the upper end regions of cylinders being filled, and does nothing to discourage this undesireable condition. Post Comment Private Reply Ignore Thread |
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