How to Avoid Hazards in Combustible Gases and Liquids
Explosive Limits of Gases Vary Greatly and Must Be Determined for Each—Volatility of Liquids Is of Great Importance
THE many and varied uses of industrial gases makes it important that the characteristics of the numerous combustible gases be studied with regard to their flammability or explosiveness. Of even more widespread use are the volatile solvents used in paints, lacquers, metal and clothes cleaning, anti-freeze liquids, and many other purposes. The solvents vaporize into the air and become gases, so that the study of the flammability of gases gives the fundamental information on the entire system of gases and volatile liquids.
An explosive is a substance in which an oxygen-rich compound is mixed with a compound which is readily oxidized and large volumes of gaseous products of reaction result. The explosive force results from the extremely rapid liberation of the gases at a point of application. The explosive limits (or limits of flammability) of combustible gases is a very important piece of information regarding the particular gas. When a source of ignition is applied to a mixture of combustible gas and air, the entire mixture may burn slowly, or burn so rapidly that the result is an explosion, or may not burn after the source of ignition is removed. What determines which of these effects occurs? The amount of combustible gas mixed with the air is the determining factor. A flame spreads through such a mixture when enough heat is given off from the burning layer of gas surrounding the point of ignition to cause the ignition of the adjacent layer of gas and thus the flame is propagated throughout the gas mixture. The smallest percentage of combustible gas in air that will allow the flame to propagate itself is the lower explosive limit. An increase in the percentage of combustible gas will cause a greater liberation of heat for a given volume, the flame will propagate more rapidly and greater explosive violence will result. If the percentage of combustible gas continues to increase, it finally becomes so great that the amount of oxygen in the mixture is insufficient to cause enough combustion to liberate the heat necessary for continued ignition and the flame will not propagate. This is the upper explosive limit.
It must be kept in mind that the gas-air mixtures outside the explosive limits are combustible when a source of ignition is constantly maintained, but the combustion of the mixture will not support itself or propagate the flame when the source of ignition is removed.
The explosive limits must be experimentally determined for each combustible gas. They vary widely, depending upon the amount of heat liberated in the combustion of the mixture and its temperature of ignition. Table I gives the explosive limits for a number of the common gases, and some common solvents and refrigerants.
When an inert gas such as carbon dioxide (CO2) or nitrogen (N2) is mixed with the combustible gas, the explosive limits are changed.
Graph I shows the effect of mixing inert gases with carbon monoxide or with methane. Carbon monoxide is one of the important constituent of illuminating gas, blast furnace gas and products of incomplete combustion. Methane is the principal constituent of most natural gases. The explosive limits of the pure flammable gas mixed with air are shown on the left hand scale, thus for carbon monoxide the upper limit is 74.0 per cent and the lower limit 12.5 per cent, for methane the upper limit is 14.0 per cent and the lower 5.3 per cent. As the percentage of inert gas is increased, the lower limit remains about the same, but the upper limit decreases rapidly due to the replacement of oxygen of the air by the inert gas. Carbon dioxide is more effective in decreasing the explosive limits because more heat is required to raise its temperature than is required for nitrogen. A knowledge of these factors is of value as it permits the calculation of correct mixtures of gases that may be used as combustible gases, but that are not explosive.
The Pittsburgh gas holder explosion on November 14, 1927, is a horrible illustration of the destructive power of the explosive mixture of natural gas and air. Twenty-eight lives were lost and property damage of $10,000,000 resulted. A gas holder was being repaired and the gas was swept out with an insufficient amount of air, so that an explosive mixture remained in the holder. When an oxy-acetylene torch was applied to the holder, the gas mixture ignited and thirteen men on the holder were instantly killed and few traces of them ever found. Two nearby gas holders were demolished and the gas burned. Proper precautions concerning the explosive limits would have prevented this catastrophe.
The propagation of the flame depends upon the successive ignition of layers of the combustible mixture. Thus the flame moves through the mixture at a certain rate. This rate of flame propagation is characteristic of the particular combustible gas and also the percentage of it in the gasair mixture. Hydrogen burns more rapidly than any of the other gases. At its lower explosive limit the flame moves about 17 inches per second. The rate rapidly increases as the amount of hydrogen in the mixture increases, reaching a maximum rate of 197 inches per second at 38 per cent hydrogen in the mixture. The rate then decreases to about 20 inches per second at the upper explosive limit. The hydrocarbon gases, as methane, ethane, etc., all burn at about 8 inches per second at the lower and upper limits with a maximum rate of about 28 inches per second for the midpoint of the explosive range. The maximum explosive effect is found when the flame travels most rapidly.
The many new organic solvents being introduced into the paint and lacquer field as well as the widespread use of petroleum solvents make important the fire hazard caused by their use. The Underwriters laboratories have divided the liquids into four classes depending upon the most important of the hazardous properties of the liquid. Pure ether is rated as 100.
The volatility of the flammable liquid is of greatest importance as it is a measure of the ability of the liquid to vaporize and mix with the surrounding air. This gives a certain concentration of combustible which will eventually exceed the lower explosive limit and he a source of extreme danger. Also, the vapors of all these liquids are heavier than air and tend to sink to the lowest level and build up in a dangerous concentration.
The danger of this condition is apparent from the records showing the numerous and disastrous fires resulting from home dry cleaning where the explosive gas mixture had moved a considerable distance to a source of ignition. Unfortunately, the lower explosive limit is much lower for these vapors than for gases, so that a small concentration (only 1.0 to 1.5 per cent) is sufficient to give an explosive mixture.
Tragedies from Home Cleaning
No city has been free from some human tragedy due to home cleaning accidents. The frightful burns that can result from ignited cleaning naphtha are well known to all. The terrific explosive force of gasolineair mixtures are illustrated by the complete demolition of a nine room home in Toronto in which curtains were being cleaned in gasoline in a washing machine in the basement. The agitation in the washing machine generated large quantities of gasolineair vapors, which were finally exploded by an electric spark.
In order to reduce the fire hazard from volatile dry cleaning solvents, common practice is to add carbon tetrachloride until the vapors are no longer flammable. This is exactly the same as adding an inert gas to the combustible gas to change the explosive limits. Carbon tetrachloride is usually more volatile than the hydrocarbons in the solvent so a small proportion of it in the liquid gives a much greater proportion in the vapor phase.
No explosion hazard exists at ordinary temperatures with oils in the kerosene and paraffin oil groups, as the volatility of the oil is too low to give sufficient vapors to exceed the lower explosive limit. If the temperature is increased, then volatility also increases and the explosive limit can be exceeded. An excellent example is the repair of a tank on a fuel oil truck which exploded after an oxy-acetylene torch was used to repair leaks. A small amount of volatile fraction in the heavier fraction may slowly evaporate and give an explosive mixture, so that whenever a closed vessel has contained a petroleum fraction precautions should be taken to insure that no flammable vapors remain in the vessel when a flame is to be applied.
Ignition of the Flammable Mixture
The flammable mixture of gas or liquid vapors and air may be ignited in a number of ways. An open flame is always sufficient. This is employed in the safety lamp where the bulk of the explosive mixture is separated from the open flame by a fine wire netting. The gas-air mixture inside the screen ignites and gives warning of the presence of an explosive mixture. Various patented apparatus are also on the market to show the presence of combustible gases.
The temperature of ignition is an important characteristic of gases and liquids. Table II is taken from some results of the Underwriters Laboratories on compounds in common use as refrigerants. If the gas or liquid were discharged against a hot surface, ignition would occur if the surface were at a sufficiently high temperature.
The temperatures given are only relative as there may be wide variations among the results of different workers due to the use of different test methods, different hot surfaces and other factors.
Static electricity is a very common cause of ignition, especially in home dry cleaning fires. Whenever two dissimilar materials are in contact, there is a difference in electrical potential and if the surfaces are separated, the potential may become great enough so that a spark will pass between them. When a liquid flows in a pipe, static electricity is generated. All such sources of static electricity where flammable gases or liquids are present, should be grounded so that static charge can escape to the ground. Rooms in which machinery operates with flammable liquids should have a high humidity, as this prevents the accumulation of static charges. The introduction of tools that will not produce sparks when used on steel surfaces is very recent. The use of copper containing a small percentage of beryllium gives an alloy almost as strong and hard as steel, so that chisels can be made to cut off
rivet heads in an explosive atmosphere without fear of sparks from the tools.
Whenever combustible gases or volatile organic solvents can mix with air, the mixture may enter the explosive region. A source of ignition will cause the self-sustained combustion of the mixture, the rate of combustion varying from a relatively slow movement of flame to a violent explosion. The various factors influencing the explosive limits and means of ignition have been discussed.
(From a paper read before the annual Northwest Fire College.)