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Air Contaminants 16b CHAPTER 8 1955 Guides 161 I apply a reasonable safety factor to the ventilation estimate without con--; sideration of the concentrations of gases or vapors that approach the danger ^ ing flammable substances, or operating in atmospheres so contaminated, is discussed in Chapter 46. ' '. It is customary to report the concentrations of flammable gases or vapors point. Safety engineers prefer to limit the concentration to | or of the J,. in percent by volume, or volume percent. Comparison with concentra lower explosive limit, and this fact should be'given full weight in determin- tions on the part per million scale used in chemical, medical or industrial ing the capacity and design of ventilating equipment. Rarely should^ consideration be given to operation above ihe upper explosive limit in the hygiene literature is readily made by the conversion: 1 percent -- 10,000 ppm (parts of contaminant per million parts of air, by volume, or in other BBSfi open areas of buildings or rooms--even though unoccupied---because the' p1 words, cubic feet of contaminant per million cubic feet of air). It will be danger of temporary drop of gas concentration to a point within the ex-.|i| noted in Table 6 that nearly all of the substances listed have lower explosive plosive range is too great. `j^*| limits above 1.0 percent, while the maximum allowable concentrations for Ability of a flammable liquid to form explosive mixtures is determined'j|| gases and vapors in Table 3 are below 1000 ppm or 0.1 percent in most largely by its vapor pressure, volatility, or rate of evaporation. Flash if1 cases. Therefore, control of toxic or injurious vapors to levels below their maximum allowable concentrations for health usually requires much more Table 4. Limits fob Toxic Dusts, Fumes and Mists effective ventilation than for the prevention of a fire hazard. Substance A.S.A. Standards, M-A.C. Threshold Limit - Values, A.C.GJ.8. 1954 mg/cu m COMBUSTIBLE DUSTS A dust explosion is essentially a sudden pressure rise caused by the very rapid burning of airborne dust. The primary explosion often originates Antimony............................................... Arsenic.................................................... Barium.................................................... Cadmium............................................... Cbiorodiphenyl............ ....................... Chromic acid <k chromates as CrOtCyanide as CN.................................... Dinitrotoluene..................................... O-Dinitrocresol...................................... Fluorides............................................... Iron Oxide fume......................... Lead.............................................. Magnesium oxide fume............. Mngnew*................................................................................................................... Mercury........................................................................................ Parathion (O.O-diethyl-O-p-nitropbenyl thiophosphate).. Pentachloronaphthalene............. .............................................. Pentachlorophenol...................................................................... Phosphorus (yellow)................................ *.......... ..................... Phosphorus pentachloride......................................................... Phosphorus pentasulfide........................................................... Selenium as Se............................................................................. Sulfuric acid................................................................................. Tellurium..................................................................................... Tetryl......... ........... ..................................................................... Tricbloronaphthalene................................................................. Trinitrotoluene............................................................................ Cranium (soluble compounds)................................................. Uranium (insoluble compounds). Zinc oxide fumes............................. 0.5 .0.5 0.5 01 .1 0.1 5 1.5 0.2 2.5 15 0.15 15 6 0.1 0.1 0.5 0.5 01 .1 1 01 .1 0.1 1.5 5 1.5 0.05 0.25 15 ;> _ Jf point is a convenient method of expressing this property in terms of thej. Table 5. Limits for Mineral Dusts Substance Threshold Limit Values A.C.G.l.H. 1954 mppcf* Alundum............................................. Asbestos.................................................... Carborundum...................................... Cement............ .................................. Dust (nuisance, no free silica)........... Mica (below 5% free silica)................ Silica-*-higb (above 50% free SiOi) . , Silica--medium (5 to 50% free SiOi). Silica--low (below 5% free SiOi) .... Slate (below 5% free SiOi)............. Soapstone (below 5% free SiOi)........ Tele........................................................ Total dust (below 5% free SiOi)....... mppcf--million particles j lie foot of air, standard light field count. 50 5 50 50 50 5 20 50 50 20 20 50 from a small amount of dust in suspension exposed to a source of ignition and the pressure and vibration it creates may be sufficient to dislodge large accumulations of dust on horizontal ledges or surfaces of the building and equipment, thereby creating a secondary explosion of great force. Thus the air conditioning engineer is involved for two reasons: (1) to obtain a movement of dust-laden air into exhaust hoods or openings, and through ventilating or pneumatic conveying ducts, in a manner that will temperature scale. It may be defined as the temperature to which a com-*: bustible liquid must be heated to produce a flash when a small flame % prevent accumulation of highly flammable dust at points where it could !gnite inside the equipment; and (2) to so design process ventilation as to passed across the surface of the liquid. The higher the flash point, the# more safely can the liquid be handled. Liquids with flash points under- 70 F should be regarded as highly flammable. Upper and lower limits of flammability of gases and vapors, and the flash points of the corresponding liquids are given in Table 6. Methods for estimating the flammable limits of mixtures of gases or vapors must be applied.with caution; the reader is referred to other publi-! cations for this information.12,13 prevent the escape of dust which might settle on horizontal surfaces and become a potential source of disaster at some distance from the dusty operation. (See Chapter 46). * Intensity of a dust explosion depends upon: chemical and thermal Properties of the dust; particle size and shape; concentration in air; propor tion of inert dust in the air; moisture content and composition of the air; size and temperature of the ignition source; and degree of dispersion of the oust cloud. Investigations on the explosibility of dusts require determination of the maximum pressure developed during explosion of a known air Design of equipment for the control of combustible anesthetics is out;, concentration, as well as determination of the rate of pressure rise. In- lined in Chapter 7. Construction of equipment for handling air contain-^ L