Classification of Dusts Relative to Electrical Equipment in Class II Hazardous Locations (1982) / Chapter Skim
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Appendix B: Layer Ignition Temperature
Appendix B: Layer Ignition Temperature
Pages 29-42
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From page 29... ...
The proposed IT test method was chosen by the panel as the foundation of its recommended method because there i s a relatively wide data base for this method that indicates that it produced results as reproducible as any method studied. The method recommended by the panel for determining the layer ignition temperature of combustible dusts is described below, and the differences between it and the IEC test method are discussed.
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From page 30... ...
There are no reported data correlating the layer ignition temperatures using dust samples identical to each other except for the particle size. Although most experts believe that the difference in layer ignition tempera~ ture between dusts passing through 0.2mmm and 0.074mmm sieves is minimal, the panel decided to recommend use of the 0.074-mm sieve because of the lack of reported correlation data and the mass of information already available on layer ignition temperatures using dust passing through a 0.074-mm sieve.
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From page 31... ...
The heated surface should consist of a circular stainless steel plate 200 mm in diameter and not less than 20 mm thick. The plate should be heated by an electrical heating element and its temperature should be controlled by a device for which the sensing element is a thermocouple mounted in the plate at the center and with its junction in contact with the plate and within 1 +0.5 mm of the upper surface.
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From page 32... ...
_L,L~_ A — Heated Plato B — Ski rt C — Heater D — Heater Base E — Heater Connection to Power Supply and Controller F — Ring for Dust Layer G — Plate Thermocoupic to Controller H — Plate Thermocouple to Recorder J I — Dust Layer Thermocouple to Recorder Screw Adjustment for Thermocouple Height K — Coil Spring FIGURE B-1 General arrangement of hot plate (not to scale)
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From page 33... ...
Ambient Temperature. The ambient temperature should be measured by a thermometer placed in a convenient position within 1 m of the hot plate but shielded from heat convection and radiation from the plate.
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From page 34... ...
The dust layer should not be compressed unduly ( i . e , =e muse ~ beheld oe put into the ring with a spatula and distributed mainly with sideways movement of the spatula until the ring is slightly over-filled; the layer then should be leveled by drawing a straight edge acro ss the top of the ring and the excess dust should be swept away ~ .
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From page 35... ...
If flaming or glowing is not seen, the behavior of the dust layer is to be observed by means of a fine wire thermocouple supported within it and connected to a temperature recorder. It usually will be found that, provided the temperature of the plate is high enough, the temperature in the layer will increase slowly to a maximum value that may be in excess of the temperature of the hot surface and then fall slowly to a steady value below the temperature of the hot surface.
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From page 36... ...
Ignition FIGURE B-4 Typical temperature-time curves for ignition of dust layer on hot surf ace .
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From page 37... ...
With dust layers composed of certain divided metals, ignition may be characterized by the relatively sudden appearance of highly incandescent smoldering combustion progressing rapidly through the layer. In determining the ignition temperature for a layer of given thickness, repeated trials should be carried out using a fresh layer of dust each time and adjusting the temperature of the hot plate until a temperature is found that is high enough to cause ignition in the layer but that is no more than 10°C higher than a temperature which fails to cause ignition.
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From page 38... ...
The test report should include a brief description of the nature of the combustion following ignition, noting especially behavior such as unusually rapid combustion or violent decomposition. Factors likely to affect the significance of the results also should be reported; these include difficulties in the preparation of layers, distortion of layers during heating, decrepitation, and melting.
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From page 39... ...
TABLE B-1. Typical Table of Test Results Depth of Set Surface Time to Ignition or Layer, mm Temperature, °C Result of Trial Maximum Temperature' min 12.5 180 Ignition 16 170 Ignition 26 160 - No Ignition 30 160 No Ignition 30 160 No Ignition 30 150 No Ignition 30 Application of Results The values of minimum ignition temperature determined in accordance with this recommended test method apply to layers having the thickness used in the tests.
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From page 40... ...
Aluminum and ordinary steel are not recommended for the heated surface because of the potential for corrosion problems and the possibility that an aluminum surface could be destroyed when metal powders are being tested. There are two ways of achieving a sufficiently uniform temperature distribution across the heated plate, the choice of which depends primarily on the heating device available.
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From page 41... ...
41 / _ i' 1 ~5 o cr: ~ ' _ ~ _ o ~ ~ 4 - ~ ~ ~ An, J In ~ _ ·_ ~ O SO a' C)
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From page 42... ...
International E1 ectrotechnical Cocci s sign, An Apparatus and Method f or Determining the Set f-lgnition Temperature of a Dust Layer, Report IEC/31 H/WG2, Geneva, Switzerland, 1973.
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