Document 71D58Leoz63Xj4a4y16j4Og0j

574 IRVING HARTMANN Under special conditions the products of combustion during an explosio cannot be released immediately to the outside atmosphere but may have to K diverted into adjoining spaces of larger but finite dimensions. In such circu stances the pressure in the explosion space does not necessarily decrease with i" crease in vent area (see Figure 9) because part of the dust mixture is ejected ihrg and explodes in the larger chamber. The proper areas of vents and the suitability of a vent closure depend an several factors, which include: (a) strength of structure or enclosure; (b) tn VENT RATIO, SQ. FT. PER 100 CU. FT. OF GALLERY VOLUME Figure 9. Venting coal dust explosions from 1 cubic foot gallery to atmosphere and to cubL chambers of various sizes. (Courtesy Bureau of Mines, U. S. Department of the Interior!! maximum explosion pressure and the average rate of pressure rise of the explosiy^ dust; (c) the position of the vents with respect to the origin of the explosion;if bursting strength of the vent closure, or the minimum pressure required to 6,, the vent closure if movable as a whole; (e) the inertia of the vent cloSfim Recommended (unrestricted) venting areas range from 1 square foot for eachf! to 30 cubic feet of enclosure for small equipment and light construction, .tjplf. square, foot per 80 c.ublcffeet for large.,rooms and buildings,jwitli;heavy,. concrete .6d'fiSt^i|tjcai. Additional recommendations bn tfiis subject are giwn the NFPA Guide for Explosion Venting. 6. Dust collectors should preferably be located outside of buildings or lii], tachedtfooaiB-providedLW-ith-adequateLexplosion-vents'rthis-applies-paFticaii dry collectors. For certain dusts, wet collectors, preferably of liquid precipitsifl type, are recommended; these should have positive ventilation of the sump|? dividual dust-collection units, installed close to the dust source, should be us^ Ducts leading to and from collectors should be as short and straight as possible HAZARDS OP COMBUSTIBLE DUST 575 .lowers for drawing dust-laden air into collectors should be on the clean-air sides ^collectors. 6. Grinding, conveying, and other equipment can frequently be protected by _ troducing a continuous flow of inert gas into the equipment, thereby reducing jfre normal oxygen content to a point at which the dust will not explode.21 The -4ert gas for this purpose can be obtained by purifying furnace or boiler flue gases; (By hydrocarbon combustion, ammonia combustion, or air fractionation; dilution fair with carbon dioxide or with nitrogen from high-pressure cylinders; or by Je of argon or helium. Various methods of producing and using inert gases are scribed in the NFPA Standard for Fire and Explosion Prevention by Inerting. TABLE 16 . Maximum Permissible Oxygen Content to Prevent Ignition of Combustible Metal Powders Type of dust Limiting percentage of oxygen, spark ignition" Air-COj Air-Nj Air-He Air-A mixture mixture mixture mixture luminum, atomized timony fn, hydrogen reduced bn carbonyl agnesium, atomized agnesium, milled and stamped langanese ieon----:----------------------------- ladium line ipwmetal fotitanium, low carbon Hfrosilicon (88% Si) 'agnesiurn-aluminum (50-60) h'orium fjrium hydride (ThHj) (fanium 7 9 10 -- 16 -- -- -- 13 -- -- -- 10 -- -- -- <3 2 3 -- t -- ---- 16 -- -- -- 13 -- -- -- 16 -- -- -- 13 -- -- -- 10 10 ---- s3 3-- 13 -- ---- / 19 -- b6 6 b 2.6 6 2 66 b6 64 74 itanium hydride (TiHj) ranium iium.hydridei.UH)......... ... irconium tfconium hydride (ZrHj) # 13.6 b 0.6 b 11 10 1.6 .2.6 4 8 8,6 S 2.5 2 42 54 8.6 6 I In the furnace test, dust clouds of Zr, Th, U, and UHj also ignited in ,01. Curing heating Several minutes undispersed layers of samples of the following metal powders ignited (glowed) l3t:-Btamped^lp-Mg^:ZnrAlgxAl,J3awmetaI^JIig.TiHi^r,.JZtHlijhi;ThHB;3Ki'-Mi.d--UHi,. iJiBle, burning of dust layers was also observed in N> with powders of Mg, 8n, Mg-Al, Dowmetal, ||riH,, Zr, Th, ThHj, U, and UHj. i b Dust clouds of these powders ignited by electric spark in an atmosphere of commercial 1H. R. Brown, U. S. Dept. Agr. Tech. Bull. No. 74, 1928. ,