Test Equipment for Use in Determining Classifications of Combustible Dusts (1979) / Chapter Skim
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Chapter 2: Layer Ignition Temperature
Chapter 2: Layer Ignition Temperature
Pages 5-18
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From page 5... ...
Available test data indicated the various hot plate methods produce results that essentially are reproducible from one hot plate Method to another, between different laboratories using the same test method, and between repeated tests on the same equipment. It was also noted that the difference in results between the Bureau of Mines' hot plate method and furnace method is about +20 °C.
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From page 6... ...
also have been made by the panel to reduce the ~ ikelihood of differences between layer ignition temperatures obtained us ing the test method recorrunended and layer ignition temperatures reported by the Bureau of Mines. The panel ' s recorurnendation that the men ting temperature be cons idered the ignition temperature is based on the behavior of mod ten material, which is different from the behavior of material in the form of dust.
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From page 7... ...
b. \Ignition Temperature of a Dust Layer -- The lowest temperature of a hot surface, rounded down to the nearest integral multiple of 10 °C, at which ignition occurs in a dust Payer of given thickness on the hot surface when the procedure in this recommendation is followed.
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From page 8... ...
! I i '~1 , ~ \~W \ \ A — Heated Plate B — Ski rt C — Heater D — Heater Base E — Heater Connection to Power Supply and Controller T 200 mm - ~ F — Ring for Dust Layer G — Plate Thermocouple to Controller H — Plate Thermocouple to Recorder I — Dust Layer Thermocouple to Recorder J — Screw Adjustment for Thermocouple Height K — Coil Spring FIGURE 1 General arrangement of hot pl ate Argot to seal e)
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From page 9... ...
, (4) The temperature control shall be such that the recorded plate temperature will not change by more than +5 °C during the placing of the dust layer and will be restored to within 2 °C of the previous value within 5 minutes of placing the dust layer; (5)
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From page 10... ...
f. Dens' ty of Dust Layer The dust layer shall not be compressed unduly (i.e., the dust shall be put into the ring with a spatula and distributed mainly with sideways movement of the spatula until the ring is slightly over-fi1led; the layer shall then be leveled by drawing a straight edge across the top of the ring and the excess dust shall be swept away)
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From page 11... ...
for forming dust layers. The scoop with the concave edge s upports the ring and co]
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From page 12... ...
Slots 6.25+0.1 mm In' l=9 1 12 ~ - 2+0.2mm 1 1 ~ 1 +0.2 mm _ 1 1 . 100 + 2.0 mm i/D 12.5 + 0.1 mm 6.25 + 0.1 mm _ i~ ~ 1 FIGURE 3 Ring f or f arming dus t 1 aye rs
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From page 13... ...
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 bill 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 ~ steady value below the temperature of the hot surface.
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From page 14... ...
Ignition FIGURE 4 Typica 1 temperature-time curves f or igni tion of dust layer on hot surface
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From page 15... ...
~ Time to ignition, or time to the maximum temperature in the case of no ignition, shall be measured to the nearest-2 minutes from the time of placing the dust layer on the hot surface and shall be reported. If melting occurs, this fact and the melting temperature shall be recorded and the test shall be discontinued.
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From page 16... ...
. Trial s in which the hot surf ace dif fered Joy more than +2 0 ° C from the recorded ignition temperature need not be reported unless unusual observations were noted at temperatures higher than the recorded ignition temperature.
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From page 17... ...
More elaborate treatment based on thermal explosion theory will permit estimates of the ignition temperatures of layers in other configurations (e.g., layers on curved surfaces) , however, if accurate predictions of ' ignition temperature under widely different conditions of exposure (in particular, exposure to a symmetrical high-temperature environment rather than to an unsymmetrical environment like that on a hot plated are desired, it is preferable to use results obtained for an experimental procedure matching the different environment more closely (e.g., ignition in an oven)
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From page 18... ...
In the case of dusts, the proposed layer ignition temperature test method represents the conditions to which dust is exposed in the vicinity of electrical equipment. Further, there will be surface temperature differences and ignition temperature differences for different dust layer thicknesses.
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