Mists, defined

Short Term Exposure Limits - Defined as the parts of vapor (gas) per million parts of contaminated air by volume at 25 C (77 °F) and atmospheric pressure. The limits are given in milligrams per cubic meter for chemicals that can form a fine mist or dust. The values are the maximum permissible average exposures for the time periods specified. The term Short Term Exposure Limit, or STEL, is also used and is considered interchangeable with Short Term Inhalation Limit. The STEL designation is derived from the OSHA standards. 

The corrosion rates and lives of zinc coatings in UK atmospheres are given in Table 13.8. These are based on practical experience as well as exposure trials. The figures should be taken only as a guide because of the difficulty of defining atmospheres in a word or two (indeed there is now a tendency for research workers to define the corrosivity of an atmosphere in terms of the corrosion rate of zinc) because of unpredictable local variations from place to place and time to time. For example, moorland which is frequently covered with acid-laden mist can be very corrosive. 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

In the 1998 Draft for Public Comment, an MRL of 0.0001 mg chromium(VI)/m3 had been derived for both intermediate and chronic exposures as chromium trioxide mist and other dissolved hexavalent chromium aerosols and mists. This MRL was also based on the study Lindberg and Hedenstiema (1983), but an exposure level of 0.001 mg chromium(VI)/m3 had been considered a NOAEL, and there had been no adjustment from intermittent to continuous exposure. Further evaluation of this study indicated that a NOAEL could not be clearly defined therefore, the LOAEL of 0.002 mg chromium(VI)/m3 was selected and adjusted for continuous exposure for the concern that the nasal lesions could accumulate at a greater rate than the repair mechanisms. The MRL of 0.000005 mg/m3 no longer applies for chronic exposure because concern that carcinogenicity associated with chronic exposure to hexavalent chromium compounds takes precedence. 

This secondary type of explosion protection will be the main object throughout all the following chapters. It covers burnable substances like gases, vapours, mists as well as dusts, and usually refers to atmospheric air as the second component forming a hazardous atmosphere. Atmospheric conditions are defined as total pressures from 8 104Pa (0.8 bar) to 1.1 105Pa... 

Following the historical development of electrical engineering and explosion protection, zone classification was the objective of national standards and installation rules. Most of the leading industrial countries established an installation practice for chemical plants and the oil and gas industry with two or three zones for areas hazardous due to gas- or vapour-air mixtures and two zones for areas with hazardous dust-air mixtures. Apart from this philosophy, the coal mining industry in most countries tends to avoid an area classification and defines only one category of explosion protection ( firedamp-proof ). More recent standards or directives present a three-zone concept for areas endangered by combustible gas- (vapour-, mist-) air mixtures and dust-air mixtures in industrial plants (other than coal mines). 

TGL 30042 has defined a four-zone concept for areas hazardous due to combustible gases, vapours and mists EG 1, comparable with zone 0 EG 2 and EG 3, covering zone 1 EG 4, comparable with zone 2. In contradiction to the majority of standards covering the zone 1 requirements with all types of protection such as flameproof enclosure, increased safety, pressurization, oil immersion, intrinsic safety, powder filling, EG 2 excludes the general application of, e.g., increased safety e, which has been the domain of EG 3. 

Steam produced in a boiler is frequently wet — thai is, it is a mist composed of saturated water vapor and entrained liquid droplets. The quality of a wet steam is defined as the fraction of the mixture by mass that is vapor. 

The terms dust , mist and vapour are defined as follows ... 

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