Document BvdpLKY5Z2JD3ZJRqo1R1eXdE

186 CHAPTER 10 1949 Guide prudent engineer will design equipment .using these values as the upper limits of air contamination, and will incorporate a reasonable margin of safety in his estimates of ventilation capacity. Information on the properties and effects, with respect to health, of specific industrial air contaminants is available in publications listed at the end of this chapter. Table 5. Maximum Allowable Concentrations op Dusts* Substance Aluminum oxide abrasive..... Cement (Portland). .. ... ............. Coal (less than 5 per cent quartz) / Dusts containing less than 10 per cent free silica. ..................... Gvpsum (hvdrated calcium sulfate) Limestone (calcium carbonate).. Marble (calcium carbonate)........ Mica ... Nuisance dusts (non-toxic, non-silica) Quartz (silicon dioxide)............. Sand (silica, silicon dioxide)....... Sandstone.................................. .... ..... ............ . Silica (free or uncombined silicon dioxide) Silicates (combined silicon dioxide).......... Slate.. ................................ . `' Talc...................... ................................... Total (maximum concentration for mixed dtists)~_________ ___ Million Particles per Cubic Foot of Air, Daily Exposures'5 15-100 5 io-100 50-100 50-100 10-100 10- 25 .50-100 50-100 50-100 10-100 50-100 5 5 5 5 15-JOO 15-100 10- 50 50-100 a Recommendations of state and local industrial hygiene agencies compiled by'the American Conference of Governmental Industrial Hygienists; and other authoritative sources. b Includes only particles from 1 to 10 microns approximately, as determined by the light field microscope counting technic, using the 10X objective. Dark field counts (and the corresponding allowable concen trations) are anywhere from 2 to 100 times the light field counts for the same sample, according to the pro portion of dust smaller than 1 micron (See Industrial Dust, Chapter VII, by Drinker and Hatch, McGraw Hill Book Co.). - FLAMMABLE GASES AND VAPORS Adequate ventilation is a primary requirement for minimizing the hazard of fire or explosion due to gases and vapors.' The need for good ventila tion is not removed by the use of other precautions, such as the elimina tion of known ignition sources, segregation of hazardous operations, adop tion of safe building construction, and installation of automatic alarms. Some safety engineers regard overventilation of an operation employing flammable liquids as a legitimate operating charge for the privilege or necessity of using a dangerous process. However, it is not possible to apply a reasonable safety factor to the ventilation estimate without con sideration of the concentrations of gases or vapors that approach the danger point. Safety engineers prefer to limit the concentration to J or | of the 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 upper explosive limit, in the open areas of buildings or rooms--even though unoccupied--because the `Air Contaminants ' 187 r of temporary drop of gas concentration to a point within the ex plosive range is too great. . V Ability of a flammable liquid to form explosive mixtures is determined plv by its vapor pressure, volatility, or rate of evaporation. Flash 'nt is a convenient method of expressing this property in terms of the fioerature scale. It may be defined as the temperature to which a com- hstible liquid must be heated to produce a flash when a small flame is bu"j across the surface of the liquid. The higher the flash point, the we 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 information12 IS. Design of equipment for the control of combustible anesthetics is out lined in Chapter 13. Construction of equipment for. handling air contain 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 in per cent by volume, or volume per cent. 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 per cent = 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 per cent, while the maximum allowable concentrations for gases and vapors in Table 3 are below 1000 ppm or 0.1 per cent 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 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 opera- . tion. (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- y