Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Chapter 2 LAYER IGNITION TEMPERATURE A. Background The panel considered five methods of determining the layer ignition temperature of combustible dusts: l. The hot plate method of the U.S. Department of the Interior, Bureau of Mines.l0 2. The modified Godbert-Greenwald furnace method of the U.S. Department of the Interior, Bureau of Mines.ll 3. The standard method of test for ignition properties of plastics of the American Society for Testing and Materials (ASTM Designation D l929-68). 4. The hot plate method developed by the U.K. Fire Research Station.-1-2 5. The hot plate method proposed as the International Electrotechnical Commission (IEC) test method by IEC Subcom- mittee 3lH, Working Group 2.^ It was the consensus of the panel that the test method to be recommended should be a hot plate method because of the simplicity and availability of the equipment involved. Avail- able 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. The proposed IEC test method was cho- sen by the panel as the foundation of its recommended method because there is a wider data base for this method indicating that it produced results as reproducible as any method stud- ied. The method recommended by the panel for determining the
6 layer ignition temperature of combustible dusts is given in section B of this chapter, and the differences between it and the IEC test method are discussed below. Most of the data readily available in the United States on the ignition temperature of combustible dust layers is the result of tests by the U.S. Bureau of Mines using dust passing through a 0.074-mm (200-mesh) sieve. The test method recommended by the IEC requires that the dust pass through a 0.2-mm (70-mesh) sieve, with an exception if it is necessary to test coarser dust. 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 temper- ature between dusts passing through 0.2-mm and 0.074-mm 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. (See section B, paragraph 3.) Other relatively minor changes from the IEC recommended test method (e.g., thickness of dust layer) also have been made by the panel to reduce the likelihood of differences between layer ignition temperatures obtained using the test method recommended and layer ignition temperatures reported by the Bureau of Mines. The panel's recommendation that the melting temperature be considered the ignition temperature is based on the behav- ior of molten material, which is different from the behavior of material in the form of dust. Electrical equipment eval- uated and found acceptable for use in the presence of dust may not be acceptable when exposed to molten material. B. Recommended Test Method for Ignition Temperatures of Dust Layers l. Scope This test method is intended to determine the minimum temperature of a hot surface that will result in the igni- tion of a layer of particulate solid, or dust, of specified thickness deposited upon it. The test is not suitable for use with substances having explosive properties. 2. Definitions For the purpose of this recommendation the following definitions apply:
a. IgnitionâThe initiation of combustion in the mate- rial under test. Ignition shall be considered to have taken place at the minimum hot plate temperature at which: (l) There is visible evidence of combustion such as red glow or flame; or (2) The slope of the temperature-time curve for a thermocouple in the center of the dust layer reverses sign; or (3) A 50 °C temperature rise in the dust occurs. b. Ignition Temperature of a Dust LayerâThe lowest temperature of a hot surface, rounded down to the near- est integral multiple of l0 °C, at which ignition oc- curs in a dust layer of given thickness on the hot sur- face when the procedure in this recommendation is fol- lowed . 3. Preparation of Dust Sample The dust shall be able to pass a 0.074 mm (200-mesh) sieve. If necessary, any dust passing a 0.84 mm (20-mesh) but not a 0.074-mm (200-mesh) sieve shall be ground to reduce the particle size until all of the dust passes a 0.074-mm (200-mesh) sieve. The ground and unground (fine dust) samples then shall be mixed. The sample must be re- presentative of the dust received and the dust used in the test should be well mixed. Any changes caused in the pro- perties of the dust samples (e.g., by sieving or grinding) should be reported. 4. Apparatus The apparatus is shown schematically in Figure l. Es- sential details and performance requirements are described below. Methods of construction that will enable these re- quirements to be met are presented in Appendix A. a. Heated Surface The heated surface shall consist of a circular stain- less steel plate 200 mm in diameter and not less than 20 mm thick. The plate shall be heated by an electrical heating element and its temperature shall 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 l mm ±0.5 nun of the upper surface.
A - Heated Plate B - Skirt C - Heater D - Heater Base E â Heater Connection to Power Supply and Controller F â Ring for Dust Layer G - Plate Thermocouple to Controller H - Plate Thermocouple to Recorder I â Dust Layer Thermocouple to Recorder J â Screw Adjustment for Thermocouple Height K - Coil Spring FIGURE l General arrangement of hot plate (not to scale)
The same thermocouple shall be connected to a temperature recorder for recording the temperature of the plate during a test. The heated plate and its controller shall satisfy the following performance requirements: (l) The plate shall be capable of attaining a maximum temperature of 400 °C without a dust layer in position; (2) The temperature of the plate must be constant to within ±5 °C throughout the period of the test; (3) When the temperature of the plate has reached a constant value, the temperature across the plate shall be uniform to within ±5 °C when measured across two diameters at right angles using the procedure described in Appendix B (this requirement shall be satisfied at plate temperatures of 200 ±5 °C and 350 ±5 °C measured at the center of the plate) ; (4) The temperature control shall be such that the re- corded 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) Temperature control and measurement devices shall be calibrated and shall be correct to within ±3 °C. b. Test Thermocouple A fine thermocouple (0.20 to 0.25 mm in diameter) shall be located so that the junction is at the geometric center of the dust sample. The thermocouple shall be stretched across the heated plate paralell to the surface. This thermocouple shall be connected to a temperature re- corder for observing the behavior of a dust layer during tests. c. Temperature Measurement Temperature measurements with the thermocouple shall be made either relative to a fixed reference junction or with automatic cold junction compensation. In either case, calibration shall satisfy requirement 4a(5), above. d. Ambient Temperature The ambient temperature shall be measured by a thermo- meter placed in a convenient position within 1 mm of the hot plate but shielded from heat convection and radiation from the plate. The ambient temperature shall be within the range from 10 to 30 °C.
10 e. Dust Layers Dust layers shall be prepared by filling the cavity formed when a stainless steel ring is placed on the surface of the hot plate and by leveling the layer to the top of the ring (Figure 2). Tha ring shall be l00 mm in diam- eter and shall have slots at opposite ends of the diameter to clear the test thermocouple (Figure 3). The ring shall be left in place during a test. A given dust shall be tes- ted in a layer that is l2.5 iom deep. A ring of the appro- priate depth will be required. f. Density 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 dis- tributed mainly with sideways movement of the spatula until the ring is slightly over-filled; 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). To minimize spillage, it is convenient to form a pan around one half of the ring and then to draw the straight edge towards the pan. A layer of each dust shall be formed in the above man- ner on a tared sheet of paper and weighed. The apparent density shall be calculated from the weight of the dust and the filled volume of the ring, and shall be reported. 5. Procedure a. General Basis Ignition in particulate or porous solids exposed to elevated temperatures generally is preceded by a more or less protracted period of self-heating (usually due to at- mospheric oxidation). Depending on the temperature of ex- posure, self-heating may result in no more than a tran- sient, although sometimes substantial, rise in temperature within the solid that does not lead to the propagation of combustion. Further, the "induction period" for ignition at temperatures near the minimum required for ignition is usually many times greater than for ignition in dust clouds or in gases and vapors. For these reasons, recog- nition of the minimum ignition temperature for layers is less straightforward than for dust clouds or for gases and vapors, and it is necessary to be certain that failure to ignite at a given temperature is not merely a result of premature termination of a test. The occurrence of ignition in a layer of dust on a surface at a given temperature depends critically on the balance between the rate of heat generation (self-heating)
ll FIGURE 2 Scoops recommended for forming dust layers. The scoop with the concave edge supports the ring and collects excess dust swept toward it in leveling the layer formed inside the ring.
12 6.25 ± 0.1 mm Slots 12.5 ±0.1 mm 6.25 ±0.1 mm i » _L I I mm 1 ± 0.2 mmâ .~. FIGURE 3 -Ring for forming dust layers
l3 in the layer and the rate of heat lost to the surroundings. The temperature at which ignition of a given material occurs therefore depends on the thickness of the layer. Values determined for two or more thicknesses of a given dust may be used for predictive purposes (see section 8, page l7). If the dust is seen to flame or glow, this is suffi- cient evidence of ignition. 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. This behavior is evidence of self-heating in the dust layer and may often be accompanied by a discoloration of the dust but not by active and visible combustion of the layer. Discoloration shall not be considered to be ignition. If the temperature of the hot surface is slightly higher, it will be found that the temperature measured in the dust layer will con- tinue to rise (instead of passing through a maximum) and to lead to ignition. Simple temperature-time curves illus- trating this behavior are shown in Figure 4. If there is no ignition within 30 minutes, the test can be consid- ered concluded. With organic dusts, combustion usually will take the form of charring followed by smoldering and glowing that will progress through the layer and leave a residue of ash. Sugars, starches, and some other dusts turn dark, melt, expand, foam, and sometimes char with or without ignition. For these dusts, visible observations and notes should be included with the temperature-time curve. With dust layers composed of certain divided met- als, ignition may be characterized by the relatively sud- den appearance of highly incandescent smoldering combus- tion progressing rapidly through the layer. In the determination of the ignition temperature for a layer of given thickness, repeated trials shall be car- ried out using a fresh layer of dust each time and adjust- ing 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 l0 °C higher than a temperature which fails to cause ignition. Temperatures at which igni- tion fails to occur must be confirmed by continuing a test long enough to establish that any self-heating is definite- ly decreasing in rate (i.e., the temperature at the point of measurement in the layer is decreasing to a steady value lower than the temperature of the plate).
l4 LU oc Dust Layer TIME A. Self-heating without ignition LU Dust Layer cc oc LU Q. TIME B. Ignition FIGURE 4 Typical temperature-time curves for ignition of dust layer on hot surface
l5 b. Method The apparatus shall be set up in a position free from drafts and under a hood capable of extracting smoke and fumes. An angled mirror some distance above the test sam- ple or equivalent means for visual observation of the dust during the test shall be provided. The temperature of the hot plate shall be adjusted to a desired value and shall be allowed to become steady within the prescribed limits (see requirement 4a(2)). A ring of the required height shall be placed centrally on the surface of the plate and shall be filled with the dusts to be tested and leveled off within a period of 2 minutes. The test thermocouple recorder then shall be started. The surface of the heated plate shall be cleaned after each test. c. Results Tests shall be repeated with fresh layers of dust until an ignition temperature has been determined. This shall be the lowest hot plate temperature, rounded to the nearest in- tegral multiple of l0 °C, at which ignition occurs in a layer of a given thickness. At least two observations of ignition at temperatures differing by no more than 20 °C shall be re- corded. The temperature at which ignition does not occur also shall be recorded. This temperature shall not be more than l0 °C lower than the temperature at which ignition does occur and shall be confirmed by at least two more tests. The test shall be discontinued if ignition of a dust layer does not occur below a hot surface temperature of 400 °C. This fact and the maximum duration of the test shall be reported. Time to ignition, or time to the maximum temperature in the case of no ignition, shall be measured to the near- est 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. The melting temperature shall be considered to be the ignition temperature.* If flames appear above the surface of the dust but the dust itself does not ignite, the temperature at which flames appear above the dust should be considered to be the igni- tion temperature.** If foaming of the dust layer occurs, this fact shall be recorded and the testing shall be continued until ignition, flaming, or melting is observed.*** *Some materials, such as sulfur dusts, melt prior to ignition. **This phenomenon may occur with some hydrides, for example. ***Some dusts, such as starch dusts, may foam when heated.
l6 6. Validity of Test Results a. Repeatability Duplicate results obtained by the same operator on dif- ferent days shall be considered unsatisfactory if they differ by more than 20 °C, and the tests shall be repeated. b. Reproducib ili ty Results obtained in different laboratories shall be considered to be unsatisfactory if they differ by more than 20 °C, and the tests shall be repeated. c. Difficult Materials Repeatability and reproducibility sometimes may be very poor for reasons associated with the physical nature of the dusts and the behavior of layers during the testing. When this occurs, it shall be reported (see section 7, "Reporting of Results") and all results shall be accepted as equally valid. The test report shall include a brief description of the nature of the combustion following ignition, noting es- pecially behavior such as unusually rapid combustion or violent decomposition. Factors likely to affect the signi- ficance of the results also shall be reported; these include difficulties in the preparation of layers, distortion of lay- ers during heating, decrepitation, and melting. 7. Reporting of Results The test report shall include the name, source, and description (if not implicit in the name) of the material tested; the date and serial number of the test; the room temperature; and the apparent density of the material as tested (see section 4f, page ll). The report shall state that the determination of ignition temperature has been car- ried out in accordance with this recommended method. The ignition tests shall be reported in the manner shown in Table l (showing results in descending order of temperature rather than in the order in which tests were performed). All test data shall be reported, e.g., the ig- nition temperature for the l2.5 mm layer described in Table l would be recorded as l70 °C). Trials in which the hot sur- face differed by more than ±20 °C from the recorded ignition temperature need not be reported unless unusual observations were noted at temperatures higher than the recorded ignition temperature.
l7 Table l. Typical Table of Test Results Time to Ignition Depth of Set Surface or Maximum Temper Layer , mm Temperature, °C Result of Trial ature, Minutes l2.5 l80 Ignition l6 l70 Ignition 26 l60 No Ignition 30 l60 No Ignition 30 l60 No Ignition 30 l50 No Ignition 30 8. Application of Results The values of minimum ignition temperature determined in accordance with this recommended test method apply to layers having the thicknesses used in the tests. It is pos- sible to estimate minimum hot surface temperatures for the ignition of layers of a given dust of lesser, intermediate, or greater thickness by linear interpolation or extrapolation of the experimental results plotted as the logarithm of the thickness versus the reciprocal of the ignition tem- perature in degrees Kelvin. This is the simplest predictive procedure that has some theoretical justification. 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); how- ever, if accurate predictions of ignition temperature under widely different conditions of exposure (in particular, ex- posure to a symmetrical high-temperature environment rather than to an unsymmetrical environment like that on a hot plate) are desired, it is preferable to use results obtained for an experimental procedure matching the different environ- ment more closely (e.g., ignition in an oven). When extensive prediction is intended, it is recommended that ignition tem- peratures be determined for at least three layer thicknesses and that thicker layers be emphasized. C. Recommendation for Installation codes The temperature of the heated surface of electrical equipment (except, perhaps, electrically heated surfaces of large ovens) can be considerably higher than the ignition temperature of gases and vapors recorded using various rec- ognized test methods; yet, ignition will not occur. Gener- ally this is so because the ignition temperature of a gas or vapor is determined by slowly heating the flammable gas- air or vapor-air mixture in a small closed container. When
l8 the same flammable gas-air or vapor-air mixture contacts the heated surface of electrical equipment (e.g., lighting fixtures or motors), the convection flow that develops normally keeps the flammable mixture from reaching the ig- nition temperature, even if the surface is somewhat above that temperature. 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. The panel therefore recommends that instal- lation codes require the maximum surface temperature of any piece of equipment to be lower than the dust ignition temperature by some differential value. A differential of 25°C is recommended; however, in special cases a dif- ferent differential, based on experience or other con- siderations, may be appropriate. It also is recommended that an upper limit to the surface temperature of the equip- ment be established.
