Document e1Mq5DaVE5K069bdBx6KZQkyq

160 apply a reasonable safety factor to the ventilation estimate without con-; sideration of the concentrations of gases of, vapors that approach the danger / point.' . Safety engineers prefer to limit the concentration to { or j of the v lower explosive limit, and this fact should be given full weight in determin- : .ing the capacity and design of ventilating equipment. Rarely should consideration be given to operation above the vpper explosive limit in the[( open areas of buildings or rooms--even though unoccupied--because the ; danger of temporary drop of gas concentration to a point within the ex- plosive range is too great. : . Ability of a flammable liquid to form explosive mixtures is determined . largely by its vapor pressure, volatility, or rate of evaporation. Flash l Table 4. Limits fob -Toxic Dusts, Fumes and Mists Substance Antimony.......................................... Arsenic............................................... Barium................................. ....... C..a.d.m...iu..m.................................................................... Chromic acid & chromates as CrO*. Cyanide as CN................................... Dinitrotoluene.................................... O-Dinitrocresol........................................ Fluorides............................................. , Iron Oxide fume................................................... Lead.............. ....................................................... Magnesium oxide fume....................................... Manganese............................................................ Mercury.................................................. Pentachloroanphthalene Pentachlorophenol......... Phosphorus (yellow).... Phosphorus pent&chloride. Phosphorus pentasulfide... Selenium as Se.-........ Sulfuric acid................. Tellurium................ Tetryl T`rriicchhlo( ronaphthafene.. Trinitrotoluene............ Zinc oxide fumes......... A.S.A. Standards. M.A.C. mg/cu m .0.1 (W) 0.1 0.15 6 0.1 Threshold Limit Values. A.C.OJ.H. 1952 mg/cu m 0.5 0.5 0.5 0.1 1 0.1 5 1.5 0.2 2.5 15 0.15 165 0.1 0.5 0.5 01 .1 1 0.1 1 0.1 1.5 5 point is a convenient method of expressing this property in terms of the 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 is 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 Design of equipment for the control of combustible anesthetics is out lined in Chapter 7. Construction of equipment for handling air contain-, ing flammable substances, or operating in atmospheres so contaminated, is discussed in Chapter 46. Air Contaminants 161 It is customary to report the concentrations of flammable gases or vapors in percent by volume, or volume percent.' Comparison with concentra tions on the part-per million scale used in chemical, medical or industrial 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 words, cubic feet of contaminant per million cubic feet of air). It .will be noted in Table 6 that nearly all of the substances listed have lower explosive limits above 1.0 percent, while the maximum allowable concentrations for gases and vapors in Table 3 are below 1000 ppm or 0.1 percent in most cases. Therefore, control of toxic or injurious vapors to levels below their maximum allowable concentrations for health usually requires much more effective ventilation than for the prevention of a fire hazard. 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 Table 5. Limits fob Minebal Dusts Substance Threshold Limit Values A.C.OJ.B. 1952 mppcf* Dust (nuisance, no free silica)...............r...................................................... * mppcf--million particles per cubic foot of air. standard light field count. 50 5 50 50 20 . 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 prevent accumulation of highly flammable dust at points where it could ignite inside the equipment; and (2) to so design process ventilation as to 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 dust cloud. Investigations on the explosibility of dusts require determina tion of the maximum pressure developed during explosion of a known air concentration, as well as determination of the rate of pressure rise. In vestigators frequently experience difficulty in obtaining dust suspensions