1、Area Classification inHazardous (Classified) Dust LocationsApproved 29 January 1988ISA12.101988FormerlyISAS12.101988STANDARDISA The Instrumentation,Systems, andAutomation Society TMCopyright g2271988 by the Instrument Society of America. All rights reserved. Printed in the UnitedStates of America. N
2、o part of this publication may be reproduced, stored in a retrieval system, ortransmitted in any form or by any means (electronic, mechanical, photocopying, recording, orotherwise), without the prior written permission of the publisher.ISA67 Alexander DriveP.O. Box 12277Research Triangle Park, North
3、 Carolina 27709ISA-12.10-1988Area Classification in Hazardous (Classified) Dust LocationsISBN 1-55617-095-5ISA-12.10-1988 3PrefaceThis preface is included for informational purposes and is not part of ISA-12.10-1988.This standard has been prepared as part of the service of ISA toward a goal of unifo
4、rmity in the field of instrumentation. To be of real value, this document should not be static, but should be subject to periodic review. Toward this end, the Society welcomes all comments and criticisms, and asks that they be addressed to the Secretary, Standards and Practices Board, ISA, 67 Alexan
5、der Drive, P.O. Box 12277, Research Triangle Park, NC 27709, Telephone (919) 549-8411, e-mail: standardsisa.org.The ISA Standards and Practices Department is aware of the growing need for attention to the metric system of units in general, and the International System of Units (SI) in particular, in
6、 the preparation of instrumentation standards. The Department is further aware of the benefits to U.S.A. users of ISA standards of incorporating suitable references to the SI (and the metric system) in their business and professional dealings with other countries. Toward this end, this Department wi
7、ll endeavor to introduce SI-acceptable metric units in all new and revised standards to the greatest extent possible. The Metric Practice Guide, which has been published by the Institute of Electrical and Electronics Engineers as ANSI/IEEE Std. 268-1982, and future revisions will be the reference gu
8、ide for definitions, symbols, abbreviations, and conversion factors.It is the policy of ISA to encourage and welcome the participation of all concerned individuals and interests in the development of ISA standards. Participation in the ISA standards-making process by an individual in no way constitu
9、tes endorsement by the employer of that individual, of ISA, or of any of the standards that ISA develops.The information contained in the preface, footnotes, and appendixes is included for information only and is not a part of the standard.The following people served as members of ISA Subcommittee S
10、P12.10:Original SP12.10 SubcommitteeNAME COMPANYT. Moodie, Chairman The Pillsbury Co.J.H. Anderson International Multifoods, Inc.C.R. Backes General Mills, Inc.W. Carlson CPC International, Inc.Current SP12.10 SubcommitteeNAME COMPANYR.J. Buschart, Chairman Monsanto CompanyU. Dugar Ashland Petroleum
11、 CompanyJ.H. Kuczka Killark Electric Mfg. Company4 ISA-12.10-1988The following people served as members of ISA Committee SP12:NAME COMPANY*E.M. Nesvig, Chairman ERDCO Engineering CorporationA.B. Anselmo R. Stahl Inc.A.A. Bartkus Underwriters Laboratories Inc.T. Bartoffy/W.W. Shao* Canadian Standards
12、 AssociationD.N. Bishop Chevron U.S.A. Inc. J.A. Bossert Energy Mines and Resources CanadaR.J. Buschart Monsanto CompanyK.M. Collins ConsultantH.G. Conner ConsultantJ.D. Cospolich Waldemar S. Nelson it also contains references to, and data on, the explosivity of common dusts, as well as references t
13、o laboratory equipment and test procedures for evaluating the explosivity of dusts.This standard is intended for use by persons trained in the design and installation of instrument systems and by inspection authorities in the approval of such installations.NOTE: This standard is not intended to addr
14、ess hazards created by the emission of Class I combustible gases from combustible dusts, e.g., the emission of methane from freshly crushed coal.3 DefinitionsCombustible dusts: Dusts which (when mixed with air in certain proportions) can be ignited and will propagate flame.Combustible dust layer: An
15、y surface accumulation of combustible dust that is large enough to propagate flame or will degrade and ignite.Dust: Any finely divided solid material 420g109m or smaller in diameter (material passing a U.S. No. 40 Standard Sieve).NOTE: Larger-sized particles can also cause explosions see References
16、4 and 15.Minimum cloud ignition temperature: The minimum temperature at which a combustible dust atmosphere will autoignite and propagate an explosion.*National Electrical Codesand NECare registered trademarks of the National Fire Protection Asso-ciation, Inc., Quincy, Mass.10 ISA-12.10-1988Minimum
17、dust layer ignition temperature: The minimum temperature of a surface that will ignite dust lying on it after a long time (theoretically, until infinity) (see Reference 4). In most dusts, free moisture has been vaporized before ignition.Minimum explosion concentration: The minimum concentration of a
18、 dust cloud that, when ignited, will propagate flame away from the source of ignition.NOTE 1: The measurable combustible properties of dusts depend not only on the chemical structure of the dust, but on test conditions, dust particle size, weight, density, and other particle characteristics.NOTE 2:
19、See Appendix A.1 for NEC definitions.4 Dust explosion parameters and variables4.1 The nature of a dust explosion. A dust explosion is the rapid burning of a cloud of dispersed dust accompanied by the release of thermal energy in a pressure wave. The flame can travel rapidly through the dust cloud, i
20、gniting other adjacent combustibles.An initial explosion often dislodges settled dust from building structures and machinery. This dislodged settled dust may be ignited by glowing residue from the initial explosion, and a secondary explosion may follow. The quantity of dust thus redispersed may caus
21、e more extensive injury and property damage than the initial dust cloud. Good housekeeping in an area made hazardous (classified) by dust is vitally important, and will keep accumulations on structures and machinery to a minimum.Even a puff of air in the presence of an ignition source or a small fla
22、me can initiate a dust explosion. While dust accumulated as a layer will not explode ordinarily, it may melt, char, or burn. This burning can generate heat and air turbulence that may disperse further amounts of dust, which may then explode.4.2 Dust dispersion mechanics and control. Dust often dispe
23、rses horizontally from its source. The extent of dispersion depends on the initial horizontal air velocity, the release height, and the particle settling time. For spherical particles the settling time can be estimated from Stokess Law, as given in Equation 1:(Eq. 1)whereVs= still air settling veloc
24、ity, in centimeters per secondh = release height, in centimeterst = settling time, in secondsg = gravitational constant, 980 cm/s2g114 = particle density, in grams per cubic centimeter (1.44 g/cm3for dry flour)D = particle size (diameter), in micrometersg109 = air viscosity, in poises (180 X 10-6P X
25、 101= Pas at normal temperature and pressure)Vsht-gg114D210818g109-=ISA-12.10-1988 11Equation 1 is applicable for particles from 1 to 100 g109m in diameter (in free fall), with approximately a 10-percent error at low concentrations. The larger particles will fall nearby; the smaller particles will f
26、all some distance from the release point.Well-designed process equipment and buildings have surfaces which are smooth and easily cleaned, and lack ledges or inaccessible pockets where dust could accumulate. Process dust should be mechanically sealed in process piping, vessels, and ducts when practic
27、al. Where dusty materials must be handled in the open, dust should be collected. Frequent cleanup of settled dust will substantially reduce the likelihood of combustible dust accumulations in a location and substantially reduce the likelihood of a major secondary dust explosion.4.3 Dust cloud igniti
28、on. The factors usually considered in dust cloud ignition are the following:1) Minimum explosion concentration2) Minimum cloud ignition temperature3) Minimum ignition energy4) Maximum explosion pressure5) Maximum rate of explosion pressure rise6) Minimum oxygen concentrationThese terms are defined a
29、nd discussed in References 3,4, 69, 11, and 12.The preceding factors are influenced by1) Composition2) Particle size3) Moisture content4) Particle shape5) Particle surface area6) Oxygen concentrationTable 1 compares the relative ignition properties of Class I and Class II materials. Laboratory explo
30、sion pressures are approximately the same for both classes, but detonation and pressure piling can cause that pressure to increase significantly. Whether this will happen or not depends on the shape of the enclosure. The lower explosive limit (LEL) is a measure of Class I explosive concentration. Th
31、e values of explosion pressure, time to peak pressure, and explosion pressure rate of rise depend on the volume and geometry of the test apparatus. The minimum explosion concentration is a similar measure for Class II materials. Unlike Class I materials, the surface ignition temperature for Class II
32、 materials (dusts) must be expressed both for layer and cloud form. The two usually are not equal.4.4 Dust layer ignition. The two factors to be considered in combustible dust layer ignition are the layer ignition temperature and the combustible dust layer thickness.The layer ignition temperature of
33、 most materials is lower than their cloud ignition temperature. (A few comparisons are made in Table 1.) A prolonged elevated temperature will decrease the minimum layer ignition temperature. For cornstarch, the layer ignition temperature changes from 500C for 0.3 hours (h) to 150C for 70 h.12 ISA-1
34、2.10-1988The combustible dust layer thickness is the minimum thickness that will allow a fire or glow to propagate in a settled dust. A method for determining the minimum combustible dust layer thickness is given in Appendix A.5.5 Procedures for the classification of various areas of existing plants
35、(Measurement procedures and examples are given in Appendix A.)5.1 Division 1 Locations. Locations should be classified as Division 1 if any of the following conditions exist:1) The dust cloud exists under normal conditions at concentrations above the minimum explosion concentration.NOTE: Combustible
36、 dust clouds may be seen. An explosive dust cloud concentration (see A.3) must usually reach the point where visibility is limited to a few feet. Division 1 locations usually exist inside and around process equipment, e.g., at filling and dispensing points, and inside and around leak points where ac
37、cumulations of dust exist.2) “Large“ dust accumulations (refer to Appendix A.5 and Figure 1) may exist for extended periods of time (usually due to inadequate housekeeping or dust removal systems).NOTE: Good housekeeping and/or the installation of dust collection systems can substantially reduce the
38、 accumulation of dust.Table 1 Comparison of relative ignition properties of Class I and Class II materialsClass I: Gases and2) Where the combustible dust layer exists infrequently.Examples are:a) Where excellent housekeeping is provided to areas adjacent to a Division 2 location.b) Where the dust la
39、yer has diminished to less than the combustible dust layer thickness.The means of defining the extent of the areas to be classified as “nonhazardous“ (unclassified) are1) Measurement refer to the appendixes.2) Appraisal use judgment based on experience with the measurement of dusty areas.Probability
40、 definitions of Divisions 1 and 2 have been mentioned in international standards and are discussed in Reference 2; they are included in Appendix A as reference material.14 ISA-12.10-1988Figure 1 Dust cloud concentration vs. dust layer depth for height of 3 m (10 ft) assuming uniform dust distributio
41、nISA-12.10-1988 156 Procedures for the classification of various portions of new process areas during the design stageSet classification goals for each location in the design stage of new process areas by a team comprised of persons responsible for1) Process and electrical design2) Corporate insuran
42、ce3) Concurrence with fire and electrical regulationsThis team should prepare confirming documents delineating the agreed limits of Division 1, Division 2, and the nonhazardous areas.6.1 This team should establish the classification of each location on the basis of1) Experience and data from similar
43、 installations2) The types of material being processed and their physical properties3) Layout of the facilities4) Ventilation of the location and local exhaust at emission points5) Housekeeping standardsNOTE: After the classifications have been assigned, criteria are specified to ensure construction
44、 of a facility meeting the requirements for the classified areas.6.2 The minimum layer and dust cloud ignition temperature of the materials to be handled or processed should be determined. If the temperature is lower than that designated in Article 500 of the NEC, the classification documentation sh
45、ould include temperature requirements for the specific application.Knowledgeable and responsible individuals should maintain proper housekeeping practices to ensure that the established classifications are not violated.7 References1) Bartknecht, W. Explosions. Berlin/Heidelberg/New York: Springer-Ve
46、rlag; 1981.2) Buschart, R. J. “An Analytical Approach to Electrical Area Classification: Flammable Vapors and Gases.“ ISA Paper 75763. Advances in Instrumentation. 30(3); 1975. p. 763.3) Dorsett, Henry, et al. “Dust Explosibility of Chemicals, Drugs, Dyes, and Pesticides.“ RI 7132. Washington, D.C.:
47、 U.S. Bureau of Mines; 1968.4) . “Laboratory Equipment and Test Procedures for Evaluation of Explosibility of Dusts.“ RI No. 5624. Washington, D.C.: U.S. Bureau of Mines; 1960.16 ISA-12.10-19885) Eckhoff, Rolf K. “Towards Absolute Minimum Ignition Energies for Dust Clouds?“ Combustion and Flame. 24:
48、5364; 1975.6) Hartmann, Irving, et al. “Recent Studies on the Explosibility of Cornstarch.“ RI No. 4725. Washington, D.C.: U.S. Bureau of Mines; 1950.7) Jacobson, Murray, et al. “Explosibility of Agricultural Dusts.“ RI No. 5753. Washington, D.C.: U. S. Bureau of Mines; 1961.8) . “Explosibility of D
49、usts Used in the Plastics Industry.“ RI No. 5971. Washington, D.C.: U.S. Bureau of Mines; 1962.9) . “Explosibility of Metal Powders.“ RI No. 6516. Washington, D.C.: U. S. Bureau of Mines; 1964.10) Magison, E.C. Electrical Instruments in Hazardous Locations. Research Triangle Park, N.C.: ISA; 1978.11) Nagy, John, et al. “Explosibility of Carbonaceous Dusts.“ RI No. 6597. Washington, D.C.: U.S. Bureau of Mines; 1965.12) . “Explosibility of Miscellaneous Dusts.“ RI No. 7208. Washington, D.C.: U.S. Bureau of Mines;
