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Contents
  1. 1How a dust cloud burns
  2. 2When dust can explode
  3. 3Ignition sources
  4. 4Explosion pressure and the deflagration index
  5. 5Secondary explosions
  6. 6Prevention and protection
  7. 7Test data and their limits
  8. 8See also
  9. 9Notes
  10. 10References

A dust explosion is an explosion of a cloud of combustible dust or powder suspended in air. Once the cloud is ignited, flame spreads through it, and in a confined space the hot combustion gases cannot expand freely, so the pressure rises rapidly.[1][2] Dusts known to explode include food powders such as sugar, corn starch and milk powder, as well as coal, plastics and metal powders such as aluminium.[3] In closed test vessels, twenty representative dusts reached maximum pressures of 6.7 to 11.2 bar above atmospheric pressure, about the same range as flammable gases.[4]

The cloud can be ignited by sparks, static electricity, hot surfaces or dust deposits that heat up by themselves.[5][6] A first explosion inside equipment can also lift settled dust into a new cloud, and the resulting secondary explosion in an occupied building can cause many casualties.[7] Protection relies on avoiding explosive clouds, removing ignition sources, inerting, and limiting the pressure by venting, suppression or an enclosure strong enough to resist it.[1][8]

How a dust cloud burns

A dust explosion first needs a cloud, and some disturbance or air movement is always needed to raise dust into the air.[9] Once the cloud is ignited, flame travels through the mixture of dust and air. In an enclosure the confinement stops the hot combustion products from expanding freely.[1] Ignition of a dust cloud in an unvented enclosure usually produces a deflagration, a flame that moves away from the ignition point slower than sound.[10] For dust as for gas, the pressure reached depends on how far the flame has spread and on the temperature and composition of the burned gas.[10] NUREG-1805, a fire hazard analysis guide of the US Nuclear Regulatory Commission, lists dust explosions among the combustion explosions, together with gas, mist and backdraft explosions.[11]

Air movement suspends dust, ignition propagates flame and confinement raises pressure.
A dust explosion requires dispersion, ignition and flame propagation. Based on Zalosh (2002), pp. 3-402–3-406.

For organic dusts, the particles have to give off flammable vapour first, and the flame then burns in the gas phase.[12][13] This need to turn the fuel into vapour is why the lower flammable limit of a dust is expected to be higher than that of a comparable gas.[12] For very small particles the limit no longer depends on particle size and comes close to that of the matching gas, provided all of the fuel turns into vapour.[12] Metal and graphite dusts are different, because for them burning on the surface of the particles plays an important part in spreading the flame.[13]

Turbulence has an even larger effect on the rate of pressure rise in dust explosions than in gas explosions, because some motion is always needed to raise the cloud.[9] Dust particles also take a finite time to burn, which gives a dust flame a finite thickness.[9] A mixture of a combustible gas with a combustible dust is called a hybrid mixture. Explosion venting is used for such mixtures as well as for gases, dusts and mists.[14][15] When fire spreads rapidly through a dust cloud without producing damaging pressure, the event is called a flash fire.[16]

When dust can explode

Not every dust cloud can explode. Combustible dusts and powders pose an explosion hazard when their typical particle size is below 100 to 400 micrometres, depending on how combustible the material is.[2] The concentration must also lie between the lower and upper explosive limits. For dusts finer than about 100 micrometres the lower limit is typically 30 to 60 g/m³, roughly the same as for many flammable gases in these units.[2] The upper limit is typically 2,000 to 6,000 g/m³, but it is difficult to measure and even more difficult to use as a means of prevention.[2] In Zalosh's table of representative dusts, mostly tested in a 1 m³ vessel, the minimum explosible concentration ranges from 15 g/m³ for naphthalene and phenolic resin to 250 g/m³ for zinc.[3]

Smaller particles generally lower the minimum explosible concentration, until a limiting size is reached below which it falls no further.[3] Above that size, burning appears to be limited by how fast the particles give off vapour, which depends on their surface area. Below it, the chemistry of the gas flame seems to set the limit.[3] For a given material and particle size, the minimum explosible concentration falls steadily as the temperature rises.[3] For bituminous coal the lower limit stays well above the 48 g/m³ typical of gaseous hydrocarbons, because not all of the coal turns into vapour. It is not expected to fall below 50 g/m³ even in pure oxygen.[12]

The most violent explosions do not occur at the lower limit. The concentration that gives the highest pressure is often about 500 g/m³. For many organic materials this is roughly twice the amount needed for complete burning and five to ten times the minimum explosible concentration.[4]

Concentrations in real plants are often unknown or deceptively low. In a plastic powder silo, for example, concentrations can become much higher than those measured during normal filling and emptying. This can happen when vibration or air blasts are used to free blocked powder.[3] A worked example compares two coal mills. The dust leaving one mill, at 0.27 kg/m³, is well inside the explosive range. The other, at 1.96 kg/m³, is close to the upper limit of 2 to 4 kg/m³ reported for coal.[5]

Ignition sources

The minimum ignition energy of a dust cloud is measured with an electric spark in a 1.2-litre tube, sometimes called a modified Hartmann tube.[5] For the dusts in Zalosh's table with measured values, it ranges from less than 1 mJ to more than 2,000 mJ.[5] NUREG-1805 states that a dust cloud may ignite with as little as 20 mJ, compared with 0.3 mJ for a flammable gas or vapour.[17] Several dusts in Zalosh's table need even less, such as sugar at 14 mJ and naphthalene at less than 1 mJ.[3] Longer sparks ignite dust more easily: adding inductance or resistance to the circuit can lower the minimum ignition energy tenfold. An electrostatic discharge therefore needs more energy to ignite a dust cloud than a spark from an electrical switch.[5]

Static electricity is a real hazard in powder handling. In a worked example, a poorly earthed silo would have to reach about 14 kV before its spark could ignite a powder needing 50 mJ from a capacitive discharge. Friction and impact of the powder during filling and emptying can produce such voltages.[5] Earthing the silo prevents charge from building up on the silo itself. Charge can still collect, however, on ungrounded objects inside it and possibly on powder with a high electrical resistance.[18]

Hot surfaces are a second group of ignition sources. The minimum ignition temperature of a dust cloud is measured by blowing dust through a heated vertical tube. Most dusts in Zalosh's table ignite between 440 °C and 750 °C.[3] A dust layer 5 mm thick on a hot plate often ignites at a significantly lower temperature, because a layer loses heat less easily than a cloud.[3][5] Siwek and Cesana's data suggest that a hot spot of at least 900 °C, for example from friction in a mill, can ignite a cloud with an ignition temperature below 600 °C.[19] Sparks from friction and impact, by contrast, only ignite dusts with a minimum ignition energy below 10 mJ and an ignition temperature below 500 °C.[19]

Dust can also ignite itself. Deposits on hot process surfaces can undergo accelerated self-heating and eventually ignite.[6] In a milk-drying plant, the air entering a spray dryer was heated to 200 °C, and surfaces near the inlet might reach this temperature too. Powder collecting on such hot surfaces could ignite spontaneously and then act as the ignition source for a dust explosion. Such explosions have occurred in milk-drying plants with devastating consequences.[20] Smouldering in grain silos is a related problem, and it must be completely extinguished before a silo is emptied, to avoid the possibility of a dust explosion.[21]

Explosion pressure and the deflagration index

In standard closed-vessel tests, the maximum pressures of the twenty dusts in Zalosh's table range from 6.7 bar for zinc to 11.2 bar for aluminium.[3] For three of the four dusts in one test series, the maximum pressure did not change as long as the median particle size was no more than 100 micrometres.[4] Polyvinyl chloride (PVC) dust, which is significantly less combustible than the other three, gave lower pressures above 20 micrometres and could not be ignited at all from 150 micrometres.[4]

The rate of pressure rise matters for protection, because the vent area needed depends on it as well as on the size of the enclosure.[15] The rate of pressure rise depends mainly on how fast the flame spreads, on the size of the vessel and on the flame temperature.[4] The maximum rate of pressure rise measured in a closed vessel, scaled by the size of the vessel, is expressed as the deflagration index, KSt, in bar·m/s.[9]

Some venting models use the index to represent how fast a dust burns.[15] In the vent sizing equation proposed for the 2002 edition of NFPA 68, the US venting guide of the National Fire Protection Association, the vent area increases in proportion to KSt.[22][23] In Zalosh's table, aluminium has the highest value at 515 bar·m/s. The other dusts lie between 28 and 209 bar·m/s, with corn starch at 128 and coal at 59.[3]

The turbulence in a test vessel is highest when the dust is first blown in and decays by the time the whole cloud has entered. The delay between dust injection and ignition therefore changes the measured pressures.[9] A standard test method was developed so that different vessels and laboratories obtain reproducible results.[9] In the United States, the American Society for Testing and Materials standard ASTM E1226 (1988 edition) is used for KSt in a 20-litre vessel.[24] ASTM E1515 (1998 edition) specifies a pyrotechnic igniter of 2,500 or 5,000 J in a 20-litre vessel for the minimum explosible concentration. Some materials are so sensitive to the igniter that tests in a 1 m³ vessel are needed.[3]

Secondary explosions

Dust explosions have a particular tendency to cause damaging secondary explosions.[9] A first explosion can burst a piece of process equipment that cannot withstand the full closed-vessel pressure, sending out a blast wave and strong air movement. This disturbance often lifts dust that has settled on walls and floors into a new cloud.[9] Even if this cloud fills only a small part of the building, the flame escaping from the broken equipment often ignites it.[9] In an occupied building, the secondary explosion can cause many casualties as well as structural damage.[7]

A primary vessel explosion lifts building dust into a cloud that ignites in a secondary explosion.
Secondary explosions can involve dust accumulated outside the original equipment. Based on Zalosh (2002), pp. 3-409–3-410.

A thin layer of dust can be enough. In a worked example, Zalosh considers bituminous coal dust lying on the floor at 1,000 g/m², a layer about 2 mm thick, in a room with a 4 m ceiling.[25] Suppose a first explosion lifts this dust into a worst-case cloud of 500 g/m³ in the lower part of the room. Without venting, the secondary explosion then raises the pressure by about 4.6 bar.[25] That is at least thirty times the 7 to 15 kPa at which unrestrained brick walls typically fail.[26] NUREG-1805 also lists secondary explosions leading to catastrophic damage among the hazards of a deflagration, together with equipment failure and injury.[6]

Prevention and protection

Dust explosion protection interrupts the chain of events at one of three points: preventing an explosive cloud, removing ignition sources, or limiting the pressure rise.[1] Where the dust concentration in equipment cannot be kept below the minimum explosible concentration, inerting adds an inert gas to lower the oxygen concentration.[3] With carbon dioxide, the limiting oxygen concentration for organic dusts lies between 11 and 15 per cent by volume, but the values for some metals are much lower.[3] Zirconium powder has a limit of zero, a sign of pyrophoric behaviour, that is, spontaneous combustion on exposure to air.[3][27] In the United States, NFPA 69 (1997 edition) states that oxygen should be kept at least 2 percentage points below the limit under continuous monitoring, and further below it otherwise.[2]

Ignition sources are controlled, for example, by earthing equipment to prevent electrostatic discharges.[1] In the coal mill example, a temperature monitor could stop the mill well before friction or smouldering coal heated its wall to 600 °C.[5] In the hazardous area classification summarised in NUREG-1805, a US guide, Division 1 covers places where flammable gases or combustible dust may be present at ignitable concentrations during normal operation.[28] NFPA 499 (1997 edition) is a US recommended practice for classifying combustible dusts and such locations for electrical installations in chemical process areas.[29]

Explosion venting releases the combustion gases through openings, usually covered by panels or rupture discs, so that the pressure stays below the level that would damage the enclosure.[15] Perhaps the most extensive large-scale venting tests were those of Bartknecht and colleagues in Basel, Switzerland, mostly with dusts in vessels of 2.4 to 250 m³.[24] They used energetic igniters and worst-case dust concentrations, and their data were correlated in the various versions of VDI 3673, the dust venting guideline of the Association of German Engineers (VDI).[24][30] For low-strength enclosures, NFPA 68 (1998 edition) groups dusts by their deflagration index: below 200, from 200 to 300, and above 300 bar·m/s.[22] In one worked example, a 400 m³ electrostatic precipitator in a coal-fired power plant needed about 2 m² of vent area.[31] Because KSt can vary widely even between coals, Zalosh stresses that the vent area should be based on samples from the actual site.[23]

Explosion suppression detects an explosion in its first moments, usually with pressure detectors for dust, and discharges an extinguishing agent before the pressure reaches the damage threshold.[23] Unlike venting it releases no flame or fuel, so it suits indoor equipment, but it is costly.[23] The remaining pressure is usually below 5 per cent of the unsuppressed maximum for organic fuels. For aluminium dust, the most demanding material tested, recent tests suggest that it can now be kept to 10 to 20 per cent.[32] Work with dried milk powders at an Irish agricultural research facility has given evidence that water mist with very small drops may quell a dust explosion.[33] Deflagration pressure containment instead designs a vessel and its fittings to withstand the full pressure of an internal deflagration.[8]

Five layers address cloud formation, ignition, oxygen, pressure relief and vessel strength.
Dust-explosion protection acts at different points in the event sequence. Based on Zalosh (2002), pp. 3-402–3-403 and pp. 3-414–3-416.

Test data and their limits

Explosibility data are measured in standard laboratory tests. Results depend on the ignition source, on the turbulence at the moment of ignition, and on the size and shape of the test vessel.[3][15] For a hazard evaluation, Barry lists dust explosion data for samples that reflect the process conditions among the material properties needed.[34] Two samples of the same material can behave very differently. In Zalosh's table, polyethylene with a median particle size below 10 micrometres has a KSt of 156 bar·m/s, against 54 bar·m/s for a sample of 150 micrometres.[3]

Dust explosions are more difficult to model than gas explosions.[15] Venting guidelines are based on tests and do not account for the turbulence and equipment obstructions of a particular installation. Most of them also ignore how much of the enclosure is filled with the combustible mixture.[22] The vent area calculated in the precipitator example also does not allow for the weight of the vent panels.[23]

Risk indexing methods for process plants take dust into account. The Dow Fire and Explosion Index adds a penalty for a dust explosion hazard. It counts this among its special process hazards, the items that increase the probability of a fire or explosion.[35] NUREG-1805 also lists several NFPA standards that deal with dust explosions. They include NFPA 654 (2000 edition) on combustible particulate solids, NFPA 61 (2002 edition) on agricultural and food products facilities, and NFPA 484 (2002 edition) on combustible metals and metal dusts.[29]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Zalosh (2002), p. 3-402.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Zalosh (2002), p. 3-403.
  3. ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16 Zalosh (2002), p. 3-404.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 Zalosh (2002), p. 3-407.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Zalosh (2002), p. 3-405.
  6. ↑ 6.0 6.1 6.2 Iqbal and Salley (2004), p. 15-3.
  7. ↑ 7.0 7.1 Zalosh (2002), pp. 3-409–3-410.
  8. ↑ 8.0 8.1 Iqbal and Salley (2004), p. F-20.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 Zalosh (2002), p. 3-409.
  10. ↑ 10.0 10.1 Zalosh (2002), p. 3-406.
  11. ↑ Iqbal and Salley (2004), p. 15-2.
  12. ↑ 12.0 12.1 12.2 12.3 Beyler (2002b), p. 2-182.
  13. ↑ 13.0 13.1 Beyler (2002b), p. 2-183.
  14. ↑ Iqbal and Salley (2004), p. F-37.
  15. ↑ 15.0 15.1 15.2 15.3 15.4 15.5 Zalosh (2002), p. 3-414.
  16. ↑ Iqbal and Salley (2004), p. F-32.
  17. ↑ Iqbal and Salley (2004), p. C-1.
  18. ↑ Zalosh (2002), pp. 3-405–3-406.
  19. ↑ 19.0 19.1 Siwek and Cesana (1995), as cited in Zalosh (2002), p. 3-405.
  20. ↑ Beever (1988), as cited in Gray (2002), p. 2-226.
  21. ↑ Ohlemiller (2002), p. 2-207.
  22. ↑ 22.0 22.1 22.2 Zalosh (2002), p. 3-416.
  23. ↑ 23.0 23.1 23.2 23.3 23.4 Zalosh (2002), p. 3-417.
  24. ↑ 24.0 24.1 24.2 Zalosh (2002), p. 3-415.
  25. ↑ 25.0 25.1 Zalosh (2002), p. 3-410.
  26. ↑ Harris (1983), as cited in Iqbal and Salley (2004), p. 15-13.
  27. ↑ Iqbal and Salley (2004), p. F-47.
  28. ↑ Iqbal and Salley (2004), p. B-24.
  29. ↑ 29.0 29.1 Iqbal and Salley (2004), pp. E-3, E-8.
  30. ↑ Zalosh (2002), p. 3-421.
  31. ↑ Zalosh (2002), pp. 3-416–3-417.
  32. ↑ Zalosh (2002), p. 3-418.
  33. ↑ Mawhinney and Back (2002), p. 4-330.
  34. ↑ Barry (2002), p. 5-178.
  35. ↑ Watts (2002), pp. 5-129–5-130.

References

  • Barry, T. F. (2002). Quantitative risk assessment in chemical process industries. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-176–5-213). National Fire Protection Association; Society of Fire Protection Engineers.
  • Beyler, C. L. (2002b). Flammability limits of premixed and diffusion flames. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-172–2-187). National Fire Protection Association; Society of Fire Protection Engineers.
  • Gray, B. (2002). Spontaneous combustion and self-heating. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-211–2-228). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Mawhinney, J. R., & Back, G. G., III. (2002). Water mist fire suppression systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-311–4-337). National Fire Protection Association; Society of Fire Protection Engineers.
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-200–2-210). National Fire Protection Association; Society of Fire Protection Engineers.
  • Watts, J. M., Jr. (2002). Fire risk indexing. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-125–5-142). National Fire Protection Association; Society of Fire Protection Engineers.
  • Zalosh, R. (2002). Explosion protection. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-402–3-421). National Fire Protection Association; Society of Fire Protection Engineers.
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