Dust Explosions in the Process Industries Second Edition phần 2 pdf

Dust explosions: an overview 57 1.4 MEANS FOR PREVENTING AND MITIGATING DUST EXPLOSIONS 1.4.1 THE MEANS AVAILABLE: AN OVERVIEW The literature on the subject is substantial. Many authors have published short, general surveys on means of preventing and mitigating dust explosions in the process industry. A few fairly recent examples are Gibson (1978), Scholl, Fischer and Donat (1979), Kiihnen and Zehr (1980), Field (1982a), Woodcock and Reed (1983), Siwek (1986, 1987), Field (1987), Swift (1987, 1987a) and Bartknecht (1988). The books mentioned in Section 1.1.1.5 also contain valuable information. Table 1.9 gives an overview of the various means that are presently known and in use. They can be divided in two main groups, namely means for preventing explosions and means for their mitigation. The preventive means can again be split in the two categories prevention of ignition sources and prevention of explosible/combustible cloud. One central issue is whether only preventing ignition sources can give sufficient safety, or whether it is also necessary in general to employ additional means of prevention and/or mitigation. In the following sections the means listed in Table 1. 9 will be discussed separately. Table 1.9 Means of preventing and mitigating dust explosions: a schematic overview PRNEliTlON MITIGATION 1.4.2 PREVENTING IGNITION SOURCES 1.4.2.1 Introduction The characteristics of various ignition sources are discussed in 1.1.4, and some special aspects are elucidated more extensively in Chapter 5. Test methods used for assessing the ignitability of dust clouds and layers, when exposed to various ignition sources are discussed in Chapter 7. 58 Dust Explosions in the Process Industries Several authors have published survey papers on the prevention of ignition sources in process plant. Kiihnen (1978) discussed the important question of whether preventing ignition sources can be relied upon as the only means of protection against dust explosions. His conclusion was that this may be possible in certain cases, but not in general. Adequate knowledge about the ignition sensitivity of the dust, both in cloud and layer form, under the actual process conditions, and proper understanding of the process, are definite pre-conditions. Schafer (1978) concluded that relying on preventing ignition sources is impossible if the minimum electric spark ignition energy of the dust is in the region of vapours and gases (< 10 mJ). However, for dusts of higher MIE he specified several types of process plants that he considered could be satisfactorily protected against dust explosions solely by eliminating ignition sources. In a more recent survey, Scholl (1989) concluded that the increased knowledge about ignition of dust layers and clouds permits the use of prevention of ignition sources as the sole means of protection against dust explosions, provided adequate ignition sensitivity tests have shown that the required ignition potential, as identified in standardized ignition sensitivity tests, is unlikely to occur in the process of concern. Scholl distinguished between organizational and operational ignition sources. The first group, which can largely be prevented by enforcing adequate working routines, includes: 0 Smoking. 0 Openflames. Open light (bulbs). 0 Welding (gadelectric). 0 Cutting (gashotating disc). 0 Grinding. The second group arises within the process itself and includes: 0 Open flames. Hot surfaces. Self-heating and smouldering nests. 0 Exothermic decomposition. 0 Heat from mechanical impact between solid bodies (metal sparks/hot-spots). Exothermic decomposition of dust via mechanical impact. 0 Electric sparkdarcs, electrostatic discharges. 1.4.2.2 Self-heating, smouldering and burning of large dust deposits The tendency to self-heating in powdeddust deposits is dependent on the properties of the material. Therefore, the potential of self-heating should be known or assessed for any material before admitting it to storage silos or other part of the plant where conditions are favourable for self-heating and subsequent further temperature rise up to smouldering and burning. 0 Control of temperature, moisture content and other important powder/ dust properties Possible means of preventing self-heating include: before admitting powder/dust to e.g. storage silos. Dust explosions: an overview 59 Adjustment of powdeddust properties to acceptable levels by cooling, drying etc. , whenever required. 0 Ensuring that heated solid bodies (e.g. a steel bolt heated and loosened by repeated impacts) do not become embedded in the powdeddust mass. Continuous monitoring of temperature in powder mass at several points by thermo- meter chains. Monitoring of possible development of gaseous decomposition/oxidation products for early detection of self-heating. 0 Rolling of bulk material from one silo to another, whenever onset of self-heating is detected, or as a routine after certain periods of storage, depending on the dust type. 0 Inerting of bulk material in silo by suitable inert gas, e. g. nitrogen. Thermometer chains in large silos can be unreliable because self-heating and smould- ering may occur outside the limited regions covered by the thermometers. Inerting by adding nitrogen or other inert gas may offer an effective solution to the self-heating problem. However, it introduces a risk of personnel being suffocated when entering areas that have been made inert. In the case of nitrogen inerting. negative effects of lack of oxygen in the breathing atmosphere become significant in humans when the oxygen content drops to 15 vol% (air 21 vol%). If inerting is adopted, it is important to take into account that the maximum permissible oxygen concentration for ensuring inert conditions in the dust deposit may be considerably lower than the maximum concentration for preventing explosions in clouds of the same dust. Walther (1989) conducted a comparative study with three different dusts. using a 20 litre closed spherical bomb for the dust cloud experiments and the Grewer furnace (see Chapter 7) for the experiments with dust deposits. In the case of the dust clouds, oxidizability was quantified in terms of the maximum explosion pressure at constant volume, whereas for the dust deposits it was expressed in terms of the maximum temperature difference between the test sample and a reference sample of inert dust, exposed to the same heating procedure. The results are shown in Figure 1. 67. In the case of the pea flour it is seen that self-heating took place in the dust deposit right down to 5 vol% oxygen or even less, whereas propagation of flames in dust clouds was practically impossible below 15 vol% oxygen. Also for the coals there were appreciable differences. Extinction of smouldering combustion inside large dust deposits e.g. in silos is a dual problem. The first part is to stop the exothermic reaction. The second, and perhaps most difficult part, is to cool down the dust mass. In general the use of water should be avoided in large volumes. Limited amounts of water may enhance the self-heating process rather than quench it. Excessive quantities may increase the stress exerted by the powdeddust mass on the walls of the structure in which it is contained, and failure may result. Generally, addition of water to a powder mass will, up to the point of saturation, reduce the flowability of the powder and make discharge more difficult (see Chapter 3). Particular care must be taken in the case of metal dust fires where the use of water should be definitely excluded. Possible development of toxic combustion products must also be taken into account. The use of inert gases such as nitrogen and carbon dioxide has proven to be successful both for quenching of the oxidation reaction and the subsequent cooling of smouldering combustion in silos. However, large quantities of inert gas are required, of the order of 60 Dust Explosions in the Process Industries Figure 1.67 Comparison of the influence of oxygen content in the gas on the oxidizability of dust clouds and dust deposits (From Walther, 1989) 10 tonnes or more for a fair size silo. In the case of fine-grained products as wheat flour or maize starch, the permeability of the inert gas may be too low for efficient inerting of large bulk volumes. Further details concerning extinction of powder and dust fires are given by Palmer (1973) and Verein deutscher Ingenieure (1986). The use of inert gas for extinction of smouldering fires in silos was specifically discussed by Dinglinger (1981) and Zockoll and Nobis (1981). Chapter 2 gives some examples of extinction of smouldering fires in practice. Some synthetic organic chemicals, in particular cyclic compounds, can decompose exothermally and become ignited by a hot surface, a smouldering nest, frictional heat or other ignition source. Such decomposition does not require oxygen, and therefore inerting has no effect. Zwahlen (1989) gave an excellent account of this special problem. He pointed out that this type of exothermic decomposition can only be avoided by eliminating all potential ignition sources. However, by taking other processing routes one can eliminate or reduce the problem. Zwahlen suggested the following possibilities: 0 The hazardous powder is processed in the wet state, as a slurry or suspension. 0 If wet processing is impossible, one should avoid processes involving internal moving mechanical parts that can give rise to ignition. If this is not possible, strict control to prevent foreign bodies from entering the process must be exercised. Furthermore, detectors for observing early temperature and Dust explosions: an overview 6 1 pressure rise, and sprinkler systems must be provided. Adiabatic exothermal decompo- sition of bulk powder at constant volume can, due to the very high powder concentra- tion, generate much higher pressures than a dust explosion in air. 0 Generally the processed batches of the powder should be kept as small as feasible. Use of additives that suppress the decomposition tendency may be helpful in some cases. 1.4.2.3 Open flamedhot gases Most potential ignition sources of the open flame type can be avoided by enforcing adequate organizational procedures and routines. This in particular applies to prohibition of smoking and other use of lighters and matches, and to enforcement of strict rules for performing hot work. Hot work must not be carried out unless the entire area that can come in contact with the heat from the work, indirectly as well as directly, is free of dust, and hazardous connections through which the explosion may transmit to other areas, have been blocked. Gas cutting torches are particularly hazardous because they work with excess oxygen. This gives rise to ignition and primary explosion development where explosions in air would be unlikely. In certain situations in the process industry, hot gaseous reaction products may entrain combustible dust and initiate dust explosions. Each such case has to be investigated separately and the required set of precautions tailored to serve the purpose in question. Factory inspectorates in most industrialized countries have issued detailed regulations for hot work in factories containing combustible powders or dusts. 1.4.2.4 Hot surfaces As pointed out by Verein deutscher Ingenieure (1986), hot surfaces may occur in industrial plants both intentionally and unintentionally. The first category includes external surfaces of hot process equipment, heaters, dryers, steam pipes and electrical equipment. The equipment where hot surfaces may be generated unintentionally include engines, blowers and fans, mechanical conveyors, mills, mixers, bearings and unprotected light bulbs. A further category of hot surfaces arises from hot work. One possibility is illustrated in Figure 1 .lo. During grinding and disc-cutting, glowing hot surfaces are often generated, which may be even more effective as initiators of dust explosions than the luminous spark showers typical of these operations. This aspect has been discussed by Muller (1989). A hot surface may ignite an explosible dust cloud directly, or via ignition of a dust layer that subsequently ignites the dust cloud. Parts of glowing or burning dust layers may loosen and be conveyed to other parts of the process where they may initiate explosions. It is important to realize that the hot surface temperature in the presence of a dust layer can, due to thermal insulation by the dust, be significantly higher than it would normally be without dust. This both increases the ignition hazard and may cause failure of equipment due to increased working temperature. The measures taken to prevent ignition by hot surfaces must cover both modes of ignition. The measures include: 62 Dust Explosions in the Process Industries Removal of all combustible dust before performing hot work. Preventionhemoval of dust accumulations on hot surfaces. 0 Isolation or shielding of hot surfaces. 0 Use of electrical apparatus approved for use in the presence of combustible dust. 0 Use of equipment with minimal risk of overheating. Inspection and maintenance procedures that minimize the risk of overheating. 1.4.2.5 Smouldering nests Pinkwasser (1985, 1986) studied the possibility of dust explosions being initiated by smouldering lumps (‘nests’) of powdered material that is conveyed through a process system. The object of the first investigation (1985) was to disclose the conditions under which smouldering material that had entered a pneumatic conveying line would be extinguished, i.e. cooled to a temperature range in which the risk of ignition in the downstream equipment was no longer present. In the case of > 1 kg/m3 pneumatic transport of screenings, low-grade flour and C3 patent flour, it was impossible to transmit a 10 g smouldering nest through the conveying line any significant distance. After only a few metres, the temperature of the smouldering lump had dropped to a safe level. In the case of lower dust concentrations, between 0.1 and 0.9 kg/m3, Le. within the most explosible range, the smouldering nest could be conveyed for an appreciable distance as shown in Figure 1.68, but no ignition was ever observed in the conveying line. In the second investigation Pinkwasser (1986) allowed smouldering nests of 700°C to fall freely through a 1 m tall column containing dust clouds of 100-1OOO g/m3 of wheat flour or wheat starch in air. Ignition was never observed during free fall. However, in some tests Figure 1.68 Distance travelled in pneumatic tran- sport pipe by smouldering nest before becoming extinguished, as a function of dust concentration in the pipe. Air velocity in pipe 20 m/s (From Pink- wasser, 1985) Dust explosions: an overview 63 with nests of at least 25 mm diameter and weight at least 15 g, ignition occurred immediately after the nest had come to rest at the bottom of the test column. This may indicate the possibility that a smouldering nest falling freely through a dust cloud in a silo without disintegrating during the fall, has a higher probability of igniting the dust cloud at the bottom of the silo than during the fall. Jaeger (1989) conducted a comprehensive laboratory-scale investigation on formation of smouldering nests and their capability of igniting dust clouds. He found that only materials of flammability class larger than 3 (see the Appendix) were able to generate smouldering nests. Under the experimental conditions adopted it was found that a minimum smouldering nest surface area of about 75 cm2 and a minimum surface temperature of 900°C was required for igniting dust clouds of minimum ignition temperatures S 600°C. Zockoll (1989) studied the incendivity of smouldering nests of milk powder, and concluded that such nests would not necessarily ignite clouds of milk powder in air. One condition for ignition by a moving smouldering nest was that the hottest parts of the surface of the nest were at least 1200°C. However, if the nest was at rest, and a milk powder dust cloud was settling on to it, inflammation of the cloud occurred even at nest surface temperatures of about 850°C. Zockoll suggested that in the case of milk powder, the minimum size of the smouldering nest required for igniting a dust cloud is so large that carbon monoxide generation in the plant would be adequate for detecting formation of smouldering nests before the nests have reached hazardous sizes. Alfert, Eckhoff and Fuhre (1989) studied the ignition of dust clouds by falling smouldering nests in a 22 m tall silo of diameter 3.7 m. It was found that nests of low mechanical strength disintegrated during the fall and generated a large fire ball that ignited the dust cloud. Such mechanically weak nests cannot be transported any significant distance in e.g. pneumatic transport pipes before disintegrating. It was further found that mechanically stable nests ignited the dust cloud either some time after having come to rest at the silo bottom, or when being broken during the impact with the silo bottom. However, as soon as the nest had come to rest at the silo bottom, it could also become covered with dust before ignition of the dust cloud got under way. Infrared radiation detection and subsequent extinction of smouldering nests and their fragments during pneumatic transport, e.g. in dust extraction ducts, has proven to be an effective means of preventing fire and explosions in downstream equipment, for example dust filters. One such system, described by Kleinschmidt (1983), is illustrated in Figure 1.69. Normally the transport velocity in the duct is known, and this allows effective extinction by precise injection of a small amount of extinguishing agent at a convenient distance just when the smoulderinghurning nest or fragment passes the nozzles. Water is the most commonly used extinguishing agent, and it is applied as a fine mist. Such systems are mostly used in the wood industries, but also to some extent in the food and feed and some other industries. The field of application is not only smouldering nests, but also glowing or burning fragments from e.g. sawing machines and mills. 1.4.2.6 Heat from accidental mechanical impact Mechanical impacts produce two different kinds of potential ignition sources, namely small flying fragments of solid material and a pair of hot-spots where the impacting bodies 64 Dust Explosions in the Process Industries Figure 1.69 Illustration of automatic system for detection and extinction of smouldering nests and their fragments, applied to a multiduct dust filter system (From Kleinschmidt, 1983) touch. Sometimes, e.g. in rotating machinery, impacts may occur repeatedly at the same points on one or both of the impacting bodies, and this may give rise to hot-spots of appreciable size and temperature. The hazardous source of ignition will then be a hot surface, and what has been said in 1.4.2.4 applies. When it comes to single accidental impacts, there has been considerable confusion. However, research during the last decade has revealed that in general the ignition hazard associated with single accidental impacts is considerably smaller than often believed by many in the past. This in particular applies to dusts of natural organic materials such as grain and feedstuffs, when exposed to accidental sparking from impacts between steel hand tools like spades or scrapers, and other steel objects or concrete. In such cases the ignition hazard is probably non-existent, as indicated by Pedersen and Eckhoff (1987). The undue significance that has often been assigned to ‘friction sparks’ as initiators of dust explosions in the past, was also stressed by Ritter (1984) and Muller (1989). However, if more sophisticated metals are involved, such as titanium or some aluminium alloys, energetic spark showers can be generated, and in the presence of rust, luminous, incendiary thermite flashes can result. Thermite flashes may also result if a rusty steel surface covered with aluminium paint or a thin smear of aluminium, is struck with a hammer or another hard object. However, impact of ordinary soft unalloyed aluminium on rust seldom results in thermite flashes, but just in a smear of aluminium on the rust. For a given combination of impacting materials, the incendivity of the resulting sparks or flash depend on the sliding velocity and contact pressure between the colliding bodies. See Chapter 5. Although the risk of initiation of dust explosions by accidental single impacts is probably smaller than believed by many in the past, there are special situations where the ignition hazard is real. It would in any case seem to be good engineering practice to: 0 Remove foreign objects from the process stream as early as possible. 0 Avoid construction materials that can give incendiary metal sparks or thermite flashes. 0 Inspect process and remove cause of impact immediately in a safe way whenever unusual noise indicating accidental impact(s) in process stream is observed. Dust explosions: an overview 65 Figures 1.70 and 1.71 show two examples of how various categories of foreign objects can be removed from the process stream before they reach the mills. Figure 1.70 A permanent magnetic separator fitted in the feed chute of a grinding mill to remove magnetic tramp metal (From DEP, 1970) Figure 1.71 A pneumatic separator can be used to remove most foreign bodies from the feed stock: the air current induced by the mill is adjusted to convey the feed stock and to reject heavier foreign bodies (From DEP, 1970) 1.4.2.7 Electric sparks and arcs: electrostatic discharges The various types of electric sparks and arcs and electrostatic discharges are described in Section 1.1.4.6. Sparks between two conducting electrodes are discussed in more detail in Chapter 5. Sparks or arcs due to breakage of live circuits can occur when fuses blow, in rotating electric machinery and when live leads are accidentally broken. The main rule for minimizing the risk of dust explosions due to such sparks and arcs is to Obey regulations for electrical installations in areas containing combustible dust (see Section 1.5.11). 66 Dust Explosions in the Process Industries The electrostatic hazard is more complex and it has not always been straightforward to specify clearly defined design guidelines. However, Glor (1988) has contributed substan- tially to developing a unified approach. As a general guideline he recommends the following measures: Use of conductive materials or materials of low dielectric strength, including coatings, (breakdown voltage across dielectric layer or wall < 4 kV) for all plant items that may accumulate very high charge densities (pneumatic transport pipes, dust deflector plates, and walls of large containers that may become charged due to ionization during gravitational compaction of powders). This prevents propagating brush discharges. Earth all conductive parts of equipment that may become charged. This prevents capacitive spark discharges from equipment. 0 Earth personnel if powders of minimum ignition energies (MIE) < 100 mJ are handled. This prevents capacitive spark discharges from humans. 0 Earth electrically conductive powders (metals etc.) by using earthed conductive equipment without non-conductive coatings. This prevents capacitive discharges from conductive powder. If highly insulating material (resistivity of powder in bulk > lo1' Rm) in the form of coarse particles (particle diameter > 1 mm) is accumulated in large volumes in silos, containers, hoppers, etc., electrostatic discharges from the material in bulk may occur. These discharges can be hazardous when a fine combustible dust fraction of minimum ignition energy < 10-100 mJ is present simultaneously. So far, no reliable measure is known to avoid this type of discharge in all cases, but an earthed metallic rod introduced into the bulk powder will most probably drain away the charges safely. It is, however, not yet clear whether this measure will always be successful. Therefore the use of explosion venting, suppression or inerting should be considered under these circumstances. 0 If highly insulating, fine powders (resistivity of powder in bulk > lo1' Rm) with a minimum ignition energy d 10 mJ as determined with a low-inductance capacitive discharge circuit, is accumulated in large volumes in silos, containers, hoppers, etc. , measures of explosion protection should be considered. There is no experimental evidence that fine powders without any coarse particles will generate discharges from powder heaps, but several explosions have been reported with such powders in situations where all possible ignition sources, with the exception of electrostatics have been effectively eliminated. 0 If combustible powders are handled or processed in the presence of a flammable gas or vapour (hybrid mixtures), the use of electrically conductive and earthed equipment is absolutely essential. Insulating coatings on earthed metallic surfaces may be tolerated provided that the thickness is less than 2 mm, the breakdown voltage less than 4 kV at locations where high surface charge densities have to be expected, and conductive powder cannot become isolated from earth by the coating. If the powder is non- conducting (resistivity of the powder in bulk > lo6 am), measures of explosion prevention (e.g. inert gas blanketing) are strongly recommended. If the resistivity of the powder in bulk is less than lo6 Rm, brush discharges, which would be incendiary for flammable gases or vapours, can also be excluded. Glor pointed out, however, that experience has shown that even in the case of powders of resistivities in bulk < lo6 Rm it is very difficult in practice to exclude all kinds of [...]... of secondary explosions 74 Dust Explosions in the Process Industries 1.4.3.3 Adding inert dust This principle is used in coal mines, by providing sufficient quantities of stone dust either as a layer on the mine gallery floor, or on shelves, etc The blast that will always precede the flame in a dust explosion will then entrain the stone dust and coal dust simultaneously and form a mixture that is incombustible... inert gas used in the flushing process, and the process volume flushed Leaks in the process equipment may cause air to enter the inerted zone Air may also be introduced when powders are charged into the process It is important therefore to control the oxygen content in the inerted region, at given intervals or sporadically, depending on the size and complexity of the plant The supply of inert gas must... however, this is difficult in most cases, because the dust concentration inside process equipment most often varies in unpredictable and uncontrollable ways Dust explosions: an overview 71 On the other hand, maintaining the powdeddust in the settled state by avoiding entrainment or fluidization in the air is one way of ensuring that the dust concentration is either zero or well above the upper explosible... certain cases The Appendix gives some data for the maximum permissible oxygen concentration in the gas for inerting clouds of various dusts The design of gas inerting systems depends on whether the process is continuous or of the batch type, the strength of the process equipment and type and source of inert gas Two main principles are used for establishing the desired atmosphere in the process: 0 Pressure... incombustible in air, and the flame, when arriving, will become quenched In other industries than mining, adding inert dust is seldom applicable due to contamination and other problems It is nevertheless interesting to note the special war-time application for protecting flour mills against dust explosions initiated by high-explosive bombs, suggested by Burgoyne and Rashbash (1948) The Appendix contains some... a point where the flow direction is changed by 180" Due to the inertia of the fast flow caused by the explosion, the flow will 78 Dust Explosions in the Process Industries tend to maintain its direction rather than making a 180"turn However, the boundaries for the applicability of the principle have not been fully explored Parameters that may influence performance include explosion properties of dusts,... (1983) 86 Dust Explosions in the Process Industries 1.4.6 EXPLOSION VENTING 1.4.6.1 What is explosion venting? The basic principle is illustrated in Figure 1.94 The maximum explosion pressure in the vented explosion, Pred, is a result of two competing processes: 0 0 Burning of the dust cloud, which develops heat and increases the pressure Flow of unburnt, burning and burnt dust cloud through the vent,... quenching explosions in ducts The methods included automatic, very rapid injection of extinguishing agent in the duct ahead of the flame front, and various kinds of fast response mechanical valves Scholl, Fischer and Donat (1979) also included the concept of passive flame propagation interruption in ducts by providing a vented 180" bend system (see Figure 1. 82) Furthermore, they 76 Dust Explosions in the Process. .. clouds in air of wheat grain dust containing 10% moisture; length of light path 150 mm: optical density D,, defined as 'Ogl0 Incident light intensity Light intensity after 150 mm ( (From Eckhoff and Fuhre, 7 975) ) starch in a large-scale (23 6 m3) silo The compressed air for flushing the glass windows of the probe was introduced via the metal tubing constituting the main probe structure However, in the. .. interrupting dust (and gas) explosions in ducts is the Ventex valve described by Rickenbach (1983) and illustrated in Figure 1.85 Figure 1.84 Arrangement for interruptinghitigating dust explosions in ducts by venting at 90" bends in corners of buildings Figure 1.85 Ventex valve for passive interruption of dust explosions in ducts (From Ricienbach, 1983) In normal operation the dust cloud being conveyed in the . Dust Explosions in the Process Industries 1.4.3.3 Adding inert dust This principle is used in coal mines, by providing sufficient quantities of stone dust either as a layer on the mine. silo. The compressed air for flushing the glass windows of the probe was introduced via the metal tubing constituting the main probe structure. However, in the case of dust explosions in the. 7 1 On the other hand, maintaining the powdeddust in the settled state by avoiding entrainment or fluidization in the air is one way of ensuring that the dust concentration is either zero