Australasian Mining Review
Issue link: http://ebook.aprs.com.au/i/37922
128 Understanding environmental surveillance Battery-powered, direct-reading instruments are classified as two groups - single-gas instruments or multiple-gas instruments - typically monitoring one or a combination of the following atmospheric conditions: Figure 1 1. oxygen deficiency or enrichment; 2. the presence of combustible gas; and 3. the presence of certain toxic gases. Depending on the capabilities of the instrument, monitoring can be conducted simultaneously for oxygen and combustible gas, or for oxygen, combustible gas and toxic gases. These devices are commonly referred to as 2-in-1, 3-in-1, 4-in-1 or 5-in-1 alarms. Figure 2 No matter which type of instrument is used to check environmental gas concentrations, regular monitoring should be performed because a contaminant’s level of combustibility or toxicity might increase even if it initially appears to be low or non-existent. In addition, oxygen deficiency can occur unexpectedly. Figure 3 Atmospheric composition To determine the composition of an atmosphere, reliable instruments should be used to draw air samples. If possible, do not open the entry portal to the confined space before this step has been completed. Sudden changes in atmospheric composition within the confined space could cause violent reactions, or dilute the contaminants in the confined space, giving a false low initial gas concentration. When testing permit-required spaces for acceptable entry conditions, always test in the following order: 1. oxygen content 2. flammable gases and vapors 3. potential toxic air contaminants Comprehensive testing should be conducted in various locations within the work area. Some gases are heavier than air, and tend to collect at the bottom of a confi ned space. Others are lighter, and are usually in higher concentrations near the top of the confi ned space. Still others are the same molecular weight as air, so they can be found in varying concentrations throughout the space. This is why test samples should be drawn at the top, middle and bottom of the space to pinpoint varying concentrations of gases or vapors (see Figure 1). The results of the atmospheric testing will have a direct impact on the selection of protective equipment necessary for the tasks in the area. It may also dictate the duration of worker exposure to the environment of the space, or whether an entry will be made at all. Substance-specifi c detectors should be used whenever actual contaminants have been identifi ed. Combustible gases In order for combustion to occur, there must be three elements: 1. fuel 2. oxygen to support combustion 3. heat or a source of ignition This is known as the fire triangle, but if you remove any one of the legs, combustion will not occur (see Figure 2). The percentage of combustible gas in the air is important, too. For example, a manhole filled with fresh air is gradually filled by a leak of combustible gas such as methane or natural gas, mixing with the fresh air. As the ratio of gas to air changes, the sample passes through three ranges: lean, explosive and rich (see Figure 3). In the lean range, there isn’t enough gas in the air to burn. On the other hand, the rich range has too much gas and not enough air. However, the explosive range has just the right combination of gas and air to form an explosive mixture. Care must be taken, however, when a mixture is too rich, because dilution with fresh air could bring the mixture into the flammable or explosive range. An analogy is the automobile that won’t start on a cold morning (a lean atmosphere because the liquid gasoline has not vaporised sufficiently), but can be flooded with too much gasoline (a rich atmosphere with too much vaporisation). Eventually, when the right mixture of gas and air finally exists (explosive), the car starts. How combustible gas monitors work To understand how portable combustible gas detection instruments work, it is fi rst important to understand what is meant by the Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL). When certain proportions of combustible vapors are mixed with air and a source of ignition is present, an explosion can occur. The range of concentrations over which this reaction can occur is [Gas Detection] _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _