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A '''Dust explosion''' is the rapid combustion of a cloud of combustible particles suspended in air, producing heat and a potentially damaging pressure rise. The hazard arises when particle size and concentration support flame propagation and an ignition source is present. Confinement can prevent combustion products from expanding freely and greatly increase the consequences.<ref name="Zalosh-3-402">Zalosh (2002), pp. 3-402–3-404.</ref><ref name="NUREG-F22">U.S. Nuclear Regulatory Commission (2005), p. F-22.</ref>
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.<ref name="Z402">Zalosh (2002), p. 3-402.</ref><ref name="Z403">Zalosh (2002), p. 3-403.</ref> 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.<ref name="Z404">Zalosh (2002), p. 3-404.</ref> 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.<ref name="Z407">Zalosh (2002), p. 3-407.</ref>


Dust explosions can occur in process equipment, silos, rooms and industrial buildings. Settled dust can also be lifted by an earlier event and form a new cloud, creating a secondary explosion. Evaluation therefore needs to address dispersed dust, accumulated layers, ignition sources and the strength and geometry of the enclosure.<ref name="Zalosh-3-410">Zalosh (2002), pp. 3-409–3-411.</ref>
The cloud can be ignited by sparks, static electricity, hot surfaces or dust deposits that heat up by themselves.<ref name="Z405">Zalosh (2002), p. 3-405.</ref><ref name="IS-15-3">Iqbal and Salley (2004), p. 15-3.</ref> 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.<ref name="Z409-410">Zalosh (2002), pp. 3-409–3-410.</ref> 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.<ref name="Z402" /><ref name="IS-F20">Iqbal and Salley (2004), p. F-20.</ref>


Explosion protection combines prevention of explosive clouds, control of ignition, inerting and pressure-limiting measures. No single measure is suitable for every dust or installation. Test results must retain their particle, concentration, ignition and turbulence conditions.<ref name="Zalosh-3-403" /><ref name="Zalosh-3-414">Zalosh (2002), pp. 3-414–3-416.</ref>
== How a dust cloud burns ==


== Formation of an explosive dust cloud ==
A dust explosion first needs a cloud, and some disturbance or air movement is always needed to raise dust into the air.<ref name="Z409">Zalosh (2002), p. 3-409.</ref> 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.<ref name="Z402" /> 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.<ref name="Z406">Zalosh (2002), p. 3-406.</ref> 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.<ref name="Z406" /> 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.<ref name="IS-15-2">Iqbal and Salley (2004), p. 15-2.</ref>
 
Zalosh describes combustible dusts and powders as potentially explosive when their characteristic particles are small enough and suspended between lower and upper explosive concentrations. The cited particle-size range is 100 to 400 micrometres depending on the material. Those values are guidance from the 2002 handbook, not a universal threshold for every dust.<ref name="Zalosh-3-403" />
 
For dusts with characteristic particles below about 100 micrometres, typical lower explosive concentrations are reported as 30 to 60 grams per cubic metre. Typical upper values of 2,000 to 6,000 grams per cubic metre are difficult to measure and apply in practice. These ranges must not replace material-specific testing.<ref name="Zalosh-3-403" />
 
A cloud forms when particles become suspended by pouring, air movement, vibration or another disturbance. Normal operation may not produce the highest concentration, while upset conditions can dislodge accumulated material. The relevant assessment therefore includes credible abnormal as well as steady operation.<ref name="Zalosh-3-404">Zalosh (2002), pp. 3-404–3-405.</ref>
[[File:fse-dust-explosion-v1.svg|thumb|upright=1.5|alt=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.]]
[[File:fse-dust-explosion-v1.svg|thumb|upright=1.5|alt=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.]]


An [[Explosion]] follows if the cloud ignites and flame propagates through it. The fuel and oxidant are already distributed within the cloud, and the combustion products heat and expand. In a closed or partly confined volume, pressure then develops according to flame propagation, burned-gas conditions and heat losses.<ref name="Zalosh-3-406">Zalosh (2002), pp. 3-406–3-408.</ref>
For organic dusts, the particles have to give off flammable vapour first, and the flame then burns in the gas phase.<ref name="Beyler-182">Beyler (2002b), p. 2-182.</ref><ref name="Beyler-183">Beyler (2002b), p. 2-183.</ref> 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.<ref name="Beyler-182" /> 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.<ref name="Beyler-182" /> Metal and graphite dusts are different, because for them burning on the surface of the particles plays an important part in spreading the flame.<ref name="Beyler-183" />
 
Uniform cloud assumptions can be misleading. Industrial dust clouds may be stratified or localised, and concentration can change during dispersion and settling. Measurement and modelling must represent where the cloud is and how it changes over time.<ref name="Zalosh-3-403" /><ref name="Zalosh-3-404" />


== Particle properties and concentration ==
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.<ref name="Z409" /> Dust particles also take a finite time to burn, which gives a dust flame a finite thickness.<ref name="Z409" /> 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.<ref name="IS-F37">Iqbal and Salley (2004), p. F-37.</ref><ref name="Z414">Zalosh (2002), p. 3-414.</ref> When fire spreads rapidly through a dust cloud without producing damaging pressure, the event is called a flash fire.<ref name="IS-F32">Iqbal and Salley (2004), p. F-32.</ref>


Particle size affects the surface available for devolatilisation and heat transfer. Smaller particles generally reduce the minimum explosive concentration until a limiting size is reached. Below that limiting size, other combustion processes can control the result, so the trend does not continue indefinitely.<ref name="Zalosh-3-404" />
== When dust can explode ==


Particle size also affects maximum pressure and rate of pressure rise. Zalosh gives examples where pressure changes little below 100 micrometres, but a less combustible material can become difficult or impossible to ignite above a particular size. Those observations are dust-specific rather than universal.<ref name="Zalosh-3-407">Zalosh (2002), pp. 3-407–3-408.</ref>
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.<ref name="Z403" /> 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.<ref name="Z403" /> 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.<ref name="Z403" /> 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.<ref name="Z404" />


Concentration influences both ignition and pressure. The maximum pressure often occurs around 500 grams per cubic metre in the cited organic-dust tests, several times the minimum explosive concentration. Using the minimum explosive concentration as the design worst case would therefore miss the concentration associated with greatest pressure.<ref name="Zalosh-3-407" />
Smaller particles generally lower the minimum explosible concentration, until a limiting size is reached below which it falls no further.<ref name="Z404" /> 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.<ref name="Z404" /> For a given material and particle size, the minimum explosible concentration falls steadily as the temperature rises.<ref name="Z404" /> 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.<ref name="Beyler-182" />


Dust chemistry and moisture, temperature and turbulence also affect behaviour. The chapter reports decreasing minimum explosive concentration with increasing temperature for a given material and particle size. These dependencies must be established with representative material and test conditions.<ref name="Zalosh-3-404" />
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.<ref name="Z407" />


Measured industrial concentration is itself uncertain. A sample at one point and time may not represent another region of the vessel or a later upset. A safety case based on a normal low reading can be challenged by local deposits and later resuspension.<ref name="Zalosh-3-404" />
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.<ref name="Z404" /> 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.<ref name="Z405" />


== Ignition energy and temperature ==
== Ignition sources ==


[[Minimum ignition energy]] is the smallest spark energy that can ignite a combustible cloud under specified test conditions. The cited method uses a modified Hartmann tube and supports a range of spark energies. Results depend on discharge type and duration as well as the dust.<ref name="Zalosh-3-405">Zalosh (2002), pp. 3-405–3-406.</ref>
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.<ref name="Z405" /> For the dusts in Zalosh's table with measured values, it ranges from less than 1 mJ to more than 2,000 mJ.<ref name="Z405" /> 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.<ref name="IS-C1">Iqbal and Salley (2004), p. C-1.</ref> Several dusts in Zalosh's table need even less, such as sugar at 14 mJ and naphthalene at less than 1 mJ.<ref name="Z404" /> 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.<ref name="Z405" />


Capacitive and inductive discharges should not be treated as equivalent. Zalosh reports that long-duration inductive conditions can reduce the energy required by an order of magnitude in the cited comparison. The test circuit and spark duration must therefore accompany any minimum-energy value.<ref name="Zalosh-3-405" />
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.<ref name="Z405" /> 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.<ref name="Z405-406">Zalosh (2002), pp. 3-405–3-406.</ref>


Minimum ignition energy also varies with concentration, particle size, temperature and dust-air velocity. The table values in the handbook span less than 1 to more than 2,000 millijoules for the listed samples and conditions. A generic spark-energy threshold cannot be assigned to every dust cloud.<ref name="Zalosh-3-405" />
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.<ref name="Z404" /> 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.<ref name="Z404" /><ref name="Z405" /> 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.<ref name="SC">Siwek and Cesana (1995), as cited in Zalosh (2002), p. 3-405.</ref> 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.<ref name="SC" />


Cloud ignition temperature is usually measured by injecting dust through a heated furnace tube. Layer ignition temperature is instead measured using a dust layer on a controlled hot plate. These are different thermal conditions and can produce different results.<ref name="Zalosh-3-404" /><ref name="Zalosh-3-405" />
Dust can also ignite itself. Deposits on hot process surfaces can undergo accelerated self-heating and eventually ignite.<ref name="IS-15-3" /> 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.<ref name="Beever">Beever (1988), as cited in Gray (2002), p. 2-226.</ref> 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.<ref name="Ohlemiller">Ohlemiller (2002), p. 2-207.</ref>


Layer temperatures are often lower than cloud ignition temperatures because the layer dissipates heat differently. Local hot spots and nonuniform wall temperatures further complicate the comparison. A mean equipment-surface temperature may not reveal the small region capable of ignition.<ref name="Zalosh-3-405" />
== Explosion pressure and the deflagration index ==


== Deflagration pressure and KSt ==
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.<ref name="Z404" /> 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.<ref name="Z407" /> 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.<ref name="Z407" />


A dust deflagration is flame propagation at subsonic speed through the combustible cloud. It can produce high enclosure pressures even though it is not a detonation. Zalosh reports worst-case pressures for listed dusts of 6.7 to 11.2 bar gauge under their test conditions.<ref name="Zalosh-3-406" /><ref name="Zalosh-3-407" />
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.<ref name="Z414" /> The rate of pressure rise depends mainly on how fast the flame spreads, on the size of the vessel and on the flame temperature.<ref name="Z407" /> 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.<ref name="Z409" />


[[Deflagration index (KSt)]] is a size-normalised measure derived from the maximum rate of pressure rise and test-vessel volume. It is used to compare dust reactivity in explosion-venting methods. KSt must be linked to the standard test method and conditions rather than treated as a material constant independent of testing.<ref name="Zalosh-3-410" /><ref name="Zalosh-3-415">Zalosh (2002), p. 3-415.</ref>
Some venting models use the index to represent how fast a dust burns.<ref name="Z414" /> 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.<ref name="Z416">Zalosh (2002), p. 3-416.</ref><ref name="Z417">Zalosh (2002), p. 3-417.</ref> 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.<ref name="Z404" />


Rate of pressure rise depends on flame propagation, flame temperature and vessel size. Turbulence has a particularly strong effect in dust explosions because particle dispersion itself creates turbulent motion. The delay between dust injection and ignition changes the turbulence at ignition.<ref name="Zalosh-3-409">Zalosh (2002), pp. 3-409–3-410.</ref>
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.<ref name="Z409" /> A standard test method was developed so that different vessels and laboratories obtain reproducible results.<ref name="Z409" /> 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.<ref name="Z415">Zalosh (2002), p. 3-415.</ref> 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.<ref name="Z404" />


Zalosh reports measured maximum pressures for many flammable gases and dusts in similar ranges under their respective tests. This does not mean their ignition difficulty, flame propagation or protection design is identical. Dust dispersion and finite particle burning introduce additional processes.<ref name="Zalosh-3-407" /><ref name="Zalosh-3-410" />
== Secondary explosions ==


Closed-vessel calculations assume particular conditions, including flame propagation through the mixture and burned-gas behaviour. Near flammability limits, incomplete combustion and heat losses can reduce measured pressure below theoretical values. Those limits matter when interpreting pressure estimates.<ref name="Zalosh-3-406" /><ref name="Zalosh-3-407" />
Dust explosions have a particular tendency to cause damaging secondary explosions.<ref name="Z409" /> 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.<ref name="Z409" /> Even if this cloud fills only a small part of the building, the flame escaping from the broken equipment often ignites it.<ref name="Z409" /> In an occupied building, the secondary explosion can cause many casualties as well as structural damage.<ref name="Z409-410" />
[[File:fse-dust-explosion-v2.svg|thumb|upright=1.5|alt=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.]]


== Secondary explosions and accumulated dust ==
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.<ref name="Z410">Zalosh (2002), p. 3-410.</ref> 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.<ref name="Z410" /> That is at least thirty times the 7 to 15 kPa at which unrestrained brick walls typically fail.<ref name="Harris">Harris (1983), as cited in Iqbal and Salley (2004), p. 15-13.</ref> NUREG-1805 also lists secondary explosions leading to catastrophic damage among the hazards of a deflagration, together with equipment failure and injury.<ref name="IS-15-3" />


A primary explosion inside equipment can breach its enclosure and create a blast wave and strong air movement. That disturbance can lift dust deposited on floors, walls and equipment. The resulting cloud may then ignite and produce a secondary explosion in the occupied building.<ref name="Zalosh-3-410" />
== Prevention and protection ==
[[File:fse-dust-explosion-v2.svg|thumb|upright=1.5|alt=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. 3-410–3-411.]]


Secondary explosions can be more damaging than the original event. They involve larger occupied spaces and can cause casualties and structural damage. The assessment must therefore include accumulated dust outside the process vessel, not only the material normally inside it.<ref name="Zalosh-3-410" />
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.<ref name="Z402" /> Where the dust concentration in equipment cannot be kept below the minimum explosible concentration, [[inerting]] adds an inert gas to lower the oxygen concentration.<ref name="Z404" /> 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.<ref name="Z404" /> Zirconium powder has a limit of zero, a sign of pyrophoric behaviour, that is, spontaneous combustion on exposure to air.<ref name="Z404" /><ref name="IS-F47">Iqbal and Salley (2004), p. F-47.</ref> 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.<ref name="Z403" />


Even a cloud occupying a fraction of a building can produce severe consequences. Zalosh emphasises that dispersed layers can supply enough fuel for a secondary event. The quantity, location and ease of resuspension of those layers belong in the hazard review.<ref name="Zalosh-3-410" />
Ignition sources are controlled, for example, by earthing equipment to prevent electrostatic discharges.<ref name="Z402" /> In the coal mill example, a temperature monitor could stop the mill well before friction or smouldering coal heated its wall to 600 °C.<ref name="Z405" /> 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.<ref name="IS-B24">Iqbal and Salley (2004), p. B-24.</ref> NFPA 499 (1997 edition) is a US recommended practice for classifying combustible dusts and such locations for electrical installations in chemical process areas.<ref name="IS-E">Iqbal and Salley (2004), pp. E-3, E-8.</ref>


Layer and cloud hazards are linked but different. A layer can ignite through a hot surface and later act as a source for a cloud, while a cloud requires suspension and a suitable concentration. Controls must address both the initial fuel state and the transition between them.<ref name="Zalosh-3-405" /><ref name="Zalosh-3-410" />
[[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.<ref name="Z414" /> 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.<ref name="Z415" /> They used energetic igniters and worst-case dust concentrations, and their data were correlated in the various versions of the German dust venting guideline VDI 3673.<ref name="Z415" /><ref name="Z421">Zalosh (2002), p. 3-421.</ref> 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.<ref name="Z416" /> In one worked example, a 400 m³ electrostatic precipitator in a coal-fired power plant needed about 2 m² of vent area.<ref name="Z416-417">Zalosh (2002), pp. 3-416–3-417.</ref> Because KSt can vary widely even between coals, Zalosh stresses that the vent area should be based on samples from the actual site.<ref name="Z417" />


The time sequence matters for protection. Preventing the primary explosion or limiting its pressure can reduce the disturbance that creates a secondary cloud. Housekeeping and ignition control reduce the fuel and initiation opportunities, while venting addresses pressure in the primary enclosure.<ref name="Zalosh-3-410" /><ref name="Zalosh-3-414" />
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.<ref name="Z417" /> Unlike venting it releases no flame or fuel, so it suits indoor equipment, but it is costly.<ref name="Z417" /> 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.<ref name="Z418">Zalosh (2002), p. 3-418.</ref> 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.<ref name="MB">Mawhinney and Back (2002), p. 4-330.</ref> Deflagration pressure containment instead designs a vessel and its fittings to withstand the full pressure of an internal deflagration.<ref name="IS-F20" />
 
== Prevention and inerting ==
 
Prevention starts by avoiding explosive concentrations and controlling ignition sources. Process ventilation, limiting resuspension and reducing accumulated layers can address cloud formation. Grounding equipment can prevent electrostatic charge from reaching ignition conditions where those conditions are understood.<ref name="Zalosh-3-402" /><ref name="Zalosh-3-406" />
 
[[Inerting]] adds an inert gas to reduce oxidant concentration below the level needed for combustion. Zalosh defines the limiting oxygen concentration as the maximum safe oxygen concentration for prevention by inerting. Its value depends on the dust and the inert gas.<ref name="Zalosh-3-403" />
 
For the organic dusts listed in the cited table, carbon-dioxide-inerting values are around 11 to 15 volume percent oxygen. Some metals have much lower values, including pyrophoric behaviour for the zirconium example. Those examples show why inerting cannot use one universal oxygen setpoint.<ref name="Zalosh-3-403" />
 
Monitoring accuracy, response time and nonuniform mixtures affect inerting reliability. The cited NFPA 69 guidance calls for a margin below the limiting oxygen concentration when continuously monitored, with a larger margin when not monitored. The applicable edition and process conditions must be checked.<ref name="Zalosh-3-403" />
 
[[Hazardous area classification]] can help organise ignition-control measures by where explosive atmospheres may occur. The supplied documents do not provide a detailed zone-classification method for dusts, so no zone numbers or dimensions are stated here. That absence must not be filled with an invented layout.<ref name="Zalosh-3-402" /><ref name="Zalosh-3-404" />
 
== Venting and other protective measures ==
 
[[Explosion venting]] discharges combustion gases during a deflagration to keep pressure below the enclosure damage threshold. Vents commonly use panels, rupture discs or engineered devices that open after initiation. Their purpose is pressure limitation, not prevention of ignition.<ref name="Zalosh-3-414" />
 
Early deployment and large, unrestricted vent area improve the cited venting arrangements. Required area depends on enclosure volume, mixture reactivity, turbulence, geometry and vent characteristics. A vent-area value from a test or guide cannot be transferred without checking those conditions.<ref name="Zalosh-3-414" /><ref name="Zalosh-3-415" />
 
Explosion venting is not suitable for detonations. A detonation produces a local shock and maximum pressure almost instantaneously when it reaches a point, leaving no useful period for vent deployment. Deflagration and detonation must therefore be distinguished before selecting protection.<ref name="Zalosh-3-411">Zalosh (2002), pp. 3-411–3-412.</ref>
 
Deflagration suppression detects and arrests combustion during an incipient stage. It is another pressure-limiting approach distinct from inerting and venting. Its performance depends on timely detection and distribution of the suppressing action.<ref name="NUREG-F20">U.S. Nuclear Regulatory Commission (2005), p. F-20.</ref><ref name="Zalosh-3-402" />
[[File:fse-dust-explosion-v3.svg|thumb|upright=1.5|alt=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.]]
[[File:fse-dust-explosion-v3.svg|thumb|upright=1.5|alt=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.]]


Pressure containment instead designs the vessel and its appurtenances to withstand expected deflagration pressure. This addresses the enclosure's resistance rather than preventing combustion. The relevant pressure and structural response must be demonstrated for the actual arrangement.<ref name="NUREG-F20" /><ref name="Zalosh-3-407" />
== Test data and their limits ==
 
== Testing, interpretation and uncertainty ==
 
Explosibility tests use defined vessels, dust dispersion systems and ignition sources. Zalosh describes near-spherical vessels of 20 to 30 litres for common tests and larger vessels where ignition-energy effects require them. Results can change with vessel size and turbulence.<ref name="Zalosh-3-404" />
 
Minimum explosive concentration is sensitive to igniter energy and turbulence. The 20-litre method uses specified ignition devices, while a 1-cubic-metre vessel may reduce ignition-energy influence. A measured result must therefore identify the apparatus and ignition method.<ref name="Zalosh-3-404" />
 
KSt and maximum pressure results also depend on dispersion delay and ignition timing. Standardised injection and ignition procedures improve reproducibility between laboratories. Applying the values to an installation still requires an assessment of its actual turbulence and geometry.<ref name="Zalosh-3-410" /><ref name="Zalosh-3-415" />
 
Test data should be checked against the page image and units when used quantitatively. The extracted source text shows that some particle sizes and ignition temperatures need particular care because symbols can be lost. No values in this article should be used as a design table without that verification.<ref name="Zalosh-3-405" />
 
A full protection assessment maps the sequence from dust formation to ignition, pressure development and enclosure response. It identifies which measures interrupt each stage and what happens if one fails. This reveals secondary explosions and common dependencies that a simple combustibility label would miss.<ref name="Zalosh-3-402" /><ref name="Zalosh-3-410" />


Material-specific assessment should use dust representative of the installation. A table entry with the same generic material name can have different particle size, processing history and ignition behaviour. Selection should therefore explain how the tested sample compares with the dust actually produced and accumulated.<ref name="Zalosh-3-404" /><ref name="Zalosh-3-405" />
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.<ref name="Z404" /><ref name="Z414" /> For a hazard evaluation, Barry lists dust explosion data for samples that reflect the process conditions among the material properties needed.<ref name="Barry">Barry (2002), p. 5-178.</ref> 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.<ref name="Z404" />


Protection layers should be evaluated for shared dependencies. An inerting system that fails can leave a vessel exposed to the same ignition sources that the vent is intended to address. Monitoring, maintenance and upset conditions belong in the evaluation of each protective layer.<ref name="Zalosh-3-403" /><ref name="Zalosh-3-415" />
Dust explosions are more difficult to model than gas explosions.<ref name="Z414" /> 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.<ref name="Z416" /> The vent area calculated in the precipitator example also does not allow for the weight of the vent panels.<ref name="Z417" />


The assessment record should distinguish cloud, layer and secondary-explosion evidence. It should identify the test methods used for ignition and reactivity, the expected dust dispersion paths and the enclosure's damage criterion. Those details make clear which parts of the protection strategy address each stage. They also help identify whether the available evidence describes the whole installation or only one test vessel and set of conditions.<ref name="Zalosh-3-405" /><ref name="Zalosh-3-410" />
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.<ref name="Watts">Watts (2002), pp. 5-129–5-130.</ref> 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.<ref name="IS-E" />


== See also ==
== See also ==


* [[Minimum ignition energy]]
* [[Self-heating]]
* [[Self-heating]]
* [[Explosion venting]]
* [[Deflagration]]
* [[Detonation]]
* [[Vapour cloud explosion]]


== Notes ==
== Notes ==


<ref name="Zalosh-3-403">Zalosh (2002), pp. 3-403–3-404.</ref>
<references />
<references />


== References ==
== References ==


* U.S. Nuclear Regulatory Commission. (2005). ''Fire dynamics tools: Quantitative fire hazard analysis methods for the U.S. nuclear regulatory commission fire protection inspection program'' (NUREG-1805). Author.
* 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.
* Zalosh, R. (2002). Explosion protection. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* 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.


[[Category:Explosions & Industrial Fires]]
[[Category:Explosions & Industrial Fires]]

Revision as of 19:08, 2 November 2026

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 the German dust venting guideline VDI 3673.[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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