22 Health and Safety Issues
1
INTRODUCTION
Dust generated from most bulk solids poses a potential health problem, and many materials that have to be conveyed are potentially toxic. Pneumatic conveying is often chosen for hazardous materials because the system provides a totally enclosed environment for their transport. It is also considered that the majority of conveyed materials are potentially explosive, and this certainly applies to most food products, fuels, chemicals and metal powders. Pneumatic conveying systems are basically quite simple and are eminently suitable for the safe transport of powdered and granular materials in factory, site and plant situations. The system requirements are a source of compressed gas, usually air, a feed device, a conveying pipeline, and a receiver to disengage the conveyed material and carrier gas. The system is totally enclosed, and if it is required, the system can operate entirely without moving parts coming into contact with the conveyed material. High, low or negative pressure air can be used to convey materials. For potentially explosive materials, an inert gas such as nitrogen can be employed. 1.1
System Flexibility
Pneumatic conveying systems are particularly versatile. With a suitable choice and arrangement of equipment, materials can be conveyed from a hopper or silo in one location, to another location some distance away. Considerable flexibility in both
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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plant layout and operation are possible, such that multiple point feeding can be made into a common line, and a single line can be discharged into a number of receiving hoppers. With vacuum systems, materials can be picked up from open storage or stockpiles, and they are ideal for clearing dust accumulations and spillages. Pipelines can run horizontally, as well as vertically up and down, and with bends in the pipeline any combination of orientations can be accommodated in a single pipeline run. A very wide range of materials can be handled, and they are totally enclosed by the system and pipeline. This means that potentially hazardous materials can be conveyed quite safely, with the correct choice of system and components. There is minimal risk of dust generation, and so these systems generally meet the requirements of any local Health and Safety legislation with little or no difficulty. 1.2
System Integration
Dust, mess and spillage that are often found surrounding bulk solids handling plant are not generally caused by pneumatic conveying systems. Feeders for pneumatic conveying systems, for example, usually fit under hoppers, and these in turn are fed from above by other systems, such as chain and flight (en-masse) conveyors. Dust and mess in the area often comes from poor integration of the mechanical conveyor with the hopper, and not with the pneumatic conveyor. In terms of plant safety, therefore, due consideration must be given to the interfacing of different systems, particularly if they are operating in series [I]. Pneumatic conveying systems provide a totally enclosed environment throughout for the transport of materials, and along the conveying route there are no moving parts at all, unless diverter valves are employed for multiple point offloading. Some feeding devices, such as blow tanks, Venturis and vacuum nozzles have no moving parts, apart from valves opening and closing at the start and end of the process. Although pneumatic conveying systems are capable of releasing dust into the atmosphere, it generally occurs only as a result of a fault situation, and is not an endemic problem with the conveying system. 2
DUST RISKS
Many dusts represent a very significant health hazard. If these materials are to be conveyed it is essential that any dust associated with the material should remain within the conveying system throughout the entire transportation process. If any material is deemed to be toxic to any degree there should be no possibility of any dust being released into the atmosphere. There is also a wide range of materials, which, in a finely divided state, dispersed in air, will propagate a flame through the suspension if ignited.
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These materials include foodstuffs such as sugar, flour and cocoa, synthetic materials such as plastics, chemical and pharmaceutical products, metal powders, and fuels such as coal and sawdust. If conveyed with air there is the possibility of a dust explosion within the system. If the dust is released from the system there is the possibility of a dust explosion external to the conveying system. The potential magnitude of the problem can be illustrated by the fact that during the 17 years from 1962 to 1979 there were 474 recorded dust explosions in the UK alone, resulting in 25 deaths [2]. This covers the whole area of bulk solids handling, transport and storage and the number of explosions that could be attributed directly to pneumatic conveying systems is not known. In just two years (1976 and 1977), dust explosions in grain handling plant in the United States claimed the lives of 87 workers and caused injuries to over 150 more [3]. It is believed that most of these explosions were in bucket elevators and not pneumatic conveying systems, but these statistics highlight the potential for dust explosions, regardless of the source. 2.1
Dust Emission
Excepting the potentially explosive materials, the most undesirable dusts are those that are so fine that they present a health hazard by remaining suspended in the air for long periods of time. The terminal velocity of a 1 um particle of silica, for example, is about 1 mm in 30 seconds (0-006 ft/min), whereas that of a 100 urn particle is about 100 mm/s (20 ft/min). The terminal velocity of an object depends upon its density and size, and is approximately proportional to the square of its size. Data on the terminal velocity of particles with respect to particle size and density was presented in Figure 18.16. Comparative size ranges of some familiar airborne particles are illustrated in Figure 22.1 [1]. Particles falling in the size range of approximately 0-5 to 5 um, if inhaled, can reach the lower regions of the lungs where they may be retained. Prolonged exposure to such dusts can cause permanent damage to the lung tissues, symptomized by shortness of breath and increased susceptibility to respiratory infection. Legislation has been introduced in many countries, therefore, that specify maximum exposure times to such dusts. Prevention of the emission of these fine particles into the atmosphere is thus of paramount importance, regardless of source. Emissions of larger particles may also give rise to complaints, more often in a social context, created by the deposition of the particles on neighboring properties or on vehicles belonging to a company's own employees [4J. 2.1.1 Dust as a Health Hazard When suspended in air the smallest particle visible to the naked eye is about 50 to 100 um in diameter, but it is the particles of 0'2 to 5 um diameter that are most dangerous for the lungs, as mentioned above.
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Paint Pigments
Pollen
104
Mean Particle Size - um Figure 22.1 materials.
Approximate size range of some familiar types of airborne particulate
Thus the existence of visible dust gives only indirect evidence of danger, as finer invisible particles will almost certainly be present as well. The fact that no dust can be seen is no reliable indication that dangerous dust may not be present in the air. The large visible particles in a dust cloud will quickly fall to the floor, but it will take many hours for the fine dangerous particles to reach the ground. Airborne dusts that may be encountered in industrial situations are generally less than about 10 urn in size and can be taken into the body by ingestion, skin absorption or inhalation. The former is rarely a serious problem, but diseases of the skin are of not infrequent occurrence. Allergic reactions are known to be caused by powders containing, for example, metals such as chrome, nickel and cobalt. It is, however, inhalation that presents the greatest hazards for workers in a dusty environment. Relatively large particles of dust that have been inhaled and become deposited in the respiratory system will usually be carried back to the mouth by cilliary action and be subsequently swallowed or expectorated. At the other extreme, ultrafine particles (less than about 0-2 urn) which become deposited are likely to pass relatively quickly, generally into solution in the extra-cellular fluids of the lung tissues. Much of this is excreted by the kidneys, either unchanged or after detoxication by the liver. This is the fate of many systemic poisons, such as lead, which gain entry to the body via the lungs [5].
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Inhaled particles with the approximate size range of 0-2 to 5 um can reach the lower regions of the lungs where they will probably be retained. Prolonged exposure to such dusts can cause various diseases, most of them potentially serious, and often resulting in permanent damage to the lung tissues. The best known are probably the diseases collectively designated 'pneumoconiosis' and characterized by chronic fibrosis of the lungs as a result of continuous inhalation of mineral dusts such as silica, asbestos and coal. Generally the symptoms are chronic shortage of breath and increased susceptibility to respiratory infection. Other dust-related diseases include pneumonitis (an acute inflammation of the lung tissue or bronchioles) and lung cancer. The relative dangers of some common dusts are compared in Table 22.1 in which the minerals are conveniently classified in Groups I to IV [5]. 2.1.2 Dust Concentration Limits One of the criteria used in monitoring the compliance of companies with the 1974 Health and Safety at Work Act (UK), and other relevant statutory provisions, is the concentration of airborne dust. The measured concentration is compared with variously defined 'threshold limit values' (TLV's) which are also functions of the duration of exposure of personnel to the dust. The most commonly used definitions of Threshold Limit Value are [6]: TLV-TWA
Time-weighted average concentration for a normal eight hour working day or forty hour week, to which most workers can be repeatedly exposed day after day, without adverse effect.
TVL-STEL
Short term exposure limit. This is the maximum concentration to which workers can be exposed for a period of up to 15 minutes, provided that no more than four excursions to this value occur each day.
TVL-C
Threshold limit ceiling. This is the concentration that should not be exceeded, even instantaneously.
For further information on actual threshold limit values Reference 7 should be consulted. Different countries, of course, will have their own regulations and guidelines on these issues. This data is included to highlight the fact that such legislation is in place in most countries. 2.1.3 Dust Suppression Where a test for dustiness, or previous experience with a material, indicates that the generation of dust is likely to present a problem, serious consideration should be given to methods of reducing the material's dustability. It may be appropriate to re-examine the manufacturing process to see if the proportion of fines could be lessened. Agglomeration of the particles, for example by pelletizing, should have a significant effect
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Table 22.1
Relative Dangers of Some Common Dusts GROUP 1
GROUP 2
Very Dangerous
Dangerous
Expert advice should always be sought.
A visible haze of any of these dusts is intolerable and no possible source of such should be ignored whether or not there is a visible cloud.
Beryllium Particularly as the oxide. Silica (SiO 2 ) which has been heated. In these circumstances silica undergoes modification into biologically active forms. Calcined kieselguhr (diatomaceous earth) is also dangerous on this account. Crocidolite (blue asbestos). Evidence associates this variety of asbestos with the development of malignant tumors of pleura and peritoneum.
Asbestos, other than crocidolite The two important varieties in commerce are amosite (brown asbestos) and chrysotile (white asbestos). Silica eg as quartz, ganister, gritstone, etc. Mixed Dusts Containing 20% or more of free silica, such as pottery dust, granite dust and steel foundry dust. Fireclay Dust With a total silicate (as silica) content in excess of 60%.
GROUP 3
GROUP 4
Moderate Risk
Minimal Risk
Emission of any of these dusts to form a dense local cloud should cause concern.
Visible concentrations of these dusts, although inexcusable on general grounds, probably represent more danger to welfare than to health.
Mixed Dusts Containing some free silica but arbitrarily less than 20%. In this group are included the dusts of Fe and non-ferrous foundries. Coal Dust Graphite Synthetic Silicas Talc Kaolin (china clay, fullers earth) Non-crystalline silica including unheated kieselguhr Carbides of some metals Cotton Dust and other dusts of vegetable origin Aluminous Fireclay Mica
Alumina ('aloxite', corundum) Glass (including glass fiber) Mineral Wool and Slag Wool Pearlite and dusts from other basic rocks. Silicates, other than those already mentioned. Tin Ore and Oxides Zirconium Silicate and Oxide Magnesium Oxide Carborundum Zinc Oxide Cement Barite Emery Limestone Ferrosilicon Iron Oxide
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If dust is generated during transport, it may be possible to change the transport routing or conveying parameters. Total enclosure of the processing and handling plant is probably the most desirable approach but, in addition to the high cost, there are obvious problems over accessibility. A generally more satisfactory arrangement is to use some kind of partial enclosure or hood in conjunction with an exhaust system which will draw off the dusty air and so minimize the dispersion of solid particles into the atmosphere. Dusty air collected from a booth, hood or other type of partial or total enclosure must then be filtered, or otherwise cleaned, before it can be released into the atmosphere. 2.2
Explosion Risks
Apart from choking lungs, irritating eyes and blocking pores, some seemingly innocuous dusts can ignite to cause fires. Many materials, in a dust cloud, can ignite and cause an explosion which could be capable of demolishing a factory. A corn starch powder explosion at General Foods, Banbury in the UK did just this in 1981 and nine men were burned. The company pneumatically conveyed corn starch, used in custard powder production, from a transfer hopper to feed bins, via a diverter valve. An accumulation of corn starch on the operating cylinder caused a malfunction of this diverter valve. When one hopper was full, the flow should have been diverted to the next hopper. An already full hopper, therefore, was over-filled causing powder to be dispersed into the atmosphere. The actual explosion occurred outside the processing plant where the dust cloud was ignited by electrical arcing from nearby electrical switchgear, burning nine men and blowing out brickwork and windows on all four walls [8]. When an explosive dust cloud is ignited in the open air there is a flash fire but little hazardous pressure develops. If the dust cloud is in a confined situation, however, such as a conveyor or storage vessel, then ignition of the cloud will lead to a build-up of pressure. The magnitude of this pressure depends upon the volume of the suspension, the nature of the material, and the rate of relief to atmosphere. Research has shown that the particle size must be below about 200 urn for a hazard to exist. At some point in a pneumatic conveying system, or time in the conveying cycle, whether dilute or dense phase, positive or negative pressure, the material will be dispersed as a suspension. A typical point is at discharge into a receiving vessel and a common time is during a transient operation such as start-up or shutdown. Consideration, therefore, must be given to the possibility of an explosion and its effects on the plant, should a source of ignition be present. Because of legal and Health and Safety Executive requirements it is advisable for specialist advice to be sought on dust explosion risks. Authoritative literature on the subject is widely available and there are many tests that can be carried out to determine the seriousness of the problem. It is strongly recommended that a
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specialist in this field is consulted if there is any doubt about the potential explosion risk connected with the pneumatic conveying of any material. 2.2.7 Ignition Sources For an explosion to occur two conditions must be satisfied. Firstly, a sufficiently energetic source of ignition must be provided and secondly the concentration of the material in the air must be favorable. Two sources of ignition frequently met in industrial plant are a hot surface and a spark. Consequently, the minimum ignition temperature and the minimum ignition energy are the ignition characteristics commonly measured in routine testing for explosibility. Ignition temperature, however, is not constant for a given dust cloud, for it depends upon the size and shape of the apparatus used to measure it. Minimum ignition temperatures, therefore, are determined in a standardized form of apparatus, which enables meaningful comparisons between materials to be made. Typical values of minimum ignition temperature for sugar, coffee and cocoa are 660, 770 and 790°F respectively [9]. The minimum energy relates to ignition by sparks, whether produced by electricity, friction or hot cutting. A characteristic of any form of spark is that a small particle or a small volume of gas at high temperature is produced for a short period of time. Since it is much easier for experimental purposes to measure the energy delivered by an electric spark than by friction or thermal processes, the routine test for determining this characteristic uses an electric spark ignition source. Typical values of minimum ignition energy for titanium, polystyrene and coal are 10, 15 and 60 mJ respectively [9], 2.2.2 Explosibility Limits For a flame to propagate through a dust cloud the concentration of the material in air must fall within a range which is defined by the lower and upper explosibility limits. The lower explosibility limit, or minimum explosible concentration, may be defined as the minimum concentration of material in a cloud or suspension necessary for sustained flame propagation. This is a fairly well defined quantity and can be determined reliably in small scale tests. Values are usually expressed in terms of the mass of material per unit volume of air. Typical values for wood flour and grain dust are 40 and 55 oz/103 ff respectively [9]. As the concentration of the material is increased above the lower explosibility limit the vigor of the explosion increases. When the dust concentration is increased beyond the stoichiometric value, the dust has a quenching effect. Eventually a concentration is reached at which flame propagation no longer occurs. This concentration is the upper explosion limit. This limit is not as easy to determine because of the difficulty of achieving a uniform dispersion of the material. From values that have been determined it would appear that for most common materials the upper limit is probably in the range of 0-1 to 0-6 Ib/ff [4]. This
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is equivalent to solids loading ratios of about \Vi to 8, which covers a significant part of the dilute phase conveying range. It is reasonable to conclude from this that should a favorable source of ignition be present in a pneumatic conveying system, then dilute phase systems are more of a problem than dense phase systems with respect to explosions. With truly dense phase systems, concentrations are well above the minimum explosibility limit and so it is highly unlikely that an explosion will occur in the pipeline of such a system. Care should still be exercised with such installations, however, since it is possible for an explosive concentration to exist at entry to a cyclone or receiver. Consideration should also be given to the start-up and shutdown transients associated with dense phase systems, for with certain modes of operation dilute phase situations may exist. 2.2.3 Pressure Generation If a dust explosion occurs in industrial plant spectacular destruction can result if it is initially confined in a system that is ultimately too weak to stand the full force of the explosion. Two other characteristics of dust explosions, therefore, are also derived by means of tests. One is the maximum explosion pressure generated, which would be required if it was desired to contain the explosion within the system. The other characteristic is the maximum rate of pressure rise, which would be relevant to the needs of suppressing an explosion within the system. Typical explosion characteristics of some well known materials are presented in Table 22.2 [9]. The data in the last two columns serves to illustrate the magnitude and rapidity of the sequence of events that follows such an explosion. Explosion pressures may be as high as 100 psig and the maximum rate of pressure rise may be in excess of 1000 atmospheres/second, or 15,000 Ibf/irf per second, which means that it could take less than 0-01 seconds to reach maximum pressure. If ignition occurs within a pipeline, the pipeline may be capable of withstanding the full explosion pressure. If this is so, the resulting pressure wave would pass along the pipeline and be relieved at the weakest point, which is usually the collection hopper or cyclone. Because of their size these are generally only capable of withstanding pressures of 3 to 5 psig and, if exposed to higher internal pressures, may burst or disintegrate. Consequently, the collection unit is likely to be the most vulnerable part of the system. 2.2.4 Expansion Effects The combustion of a dust cloud will result in either a rapid build up of pressure or in an uncontrolled expansion. It is the expansion effect, or the pressure rise if the expansion is restricted, that presents one of the main hazards in dust explosions. The expansion effects arise principally because of the heat developed in the combustion and, in some cases, to gases being evolved from the dust because of the high temperature to which it has been exposed.
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Table 22.2
Explosion Characteristics of Some Well Known Materials
Material
Metal Powders Aluminum Magnesium Zinc Plastics Nylon Polyethylene Polystyrene Agricultural Coffee Grain dust Sugar Wheat Flour Miscellaneous Coal Wood flour
Minimum Minimum Minimum ignition explosible ignition temperature coneentration energy °F oz/10 3 ft 3 mJ
Maximum explosion pressure lbf/in 2
Maximum rate of pres. rise alm/s
1180 970 1110
40 20 480
15 40 650
90 95 50
1000 1000 120
930 730 910
30 20 15
20 10 15
95 80 90
270 510 480
770 810 660 720
85 55 35 50
160 30 30 50
50 95 90 95
17 190 340 250
610 810
55 40
60 20
85 110
150 370
The pressure wave resulting from an uncontrolled dust explosion in a building usually shakes down more dust that has settled over a period of time onto pipelines, roof beams and supports, ledges, etc. This makes an ideal condition for the secondary explosion that almost always follows. It is this secondary explosion that can demolish a factory and kill the operatives. It is essential, therefore, than an explosion occurring in a pneumatic conveying system is not allowed to be discharged into a building, and that good housekeeping procedures are adopted to minimize the build up of potentially explosive dusts on surfaces in such buildings. 2.2.5 Oxygen Concentration Another characteristic of dust explosions, that can also be measured, is the percentage of oxygen in the conveying gas at which an explosion will occur for a given material. If the oxygen level in air is reduced, a point will be reached at which a flame cannot be supported. If a material is considered to be highly explosive it would generally be conveyed with an inert gas such as nitrogen, and not air. For many materials, however, such an extreme measure is not necessary. The use of nitrogen will add significantly to the operating costs, particularly with an open system. If the oxygen content needs to be reduced by only a small amount, a proportion of nitrogen can be added to the air to keep the oxygen level below the required concentration.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Health and Safety
3
633
CONVEYING SYSTEMS
A wide range of pneumatic conveying systems are available to cater for an equally wide range of conveying applications. The majority of systems are generally conventional, continuously operating, open systems in a fixed location. To suit the material being conveyed or the process, however, innovatory, batch operating and closed systems are commonly used, as well as mobile systems. To add to the complexity of selection, systems can be either positive or negative pressure in operation, or a combination of the two [9]. 3.1
Closed Systems
For the conveying of toxic or radioactive materials, where the air coming into contact with the material must not be released into the atmosphere, or must be very closely regulated, a closed system would be essential. A sketch of a typical system is given in Figure 22.2. A closed system may also be chosen to convey a potentially explosive material, typically with an inert gas. In a closed system the gas can be re-circulated and so the operating costs, in terms of inert gas, are significantly reduced. A null point needs to be established in the gas only part of the system, where the pressure is effectively atmospheric, and provision for make up or control of the conveying gas can be established here. If this null point is positioned after the blower the conveying system can operate entirely under vacuum. If the null point is located before the blower it will operate as a positive pressure system.
Figure 22.2
Closed loop pneumatic conveying system.
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3.2
Chapter 22
Open Systems
Where strict environmental control is not necessary an open system is generally preferred, since the capital cost of the plant will be less, the operational complexity will be reduced, and a much wider range of systems will be available. Most pneumatic conveying pipeline and channel systems can ensure totally enclosed material conveying, and so with suitable gas-solid separation and venting, the vast majority of materials can be handled quite safely in an open system. Many potentially combustible materials are conveyed in open systems by incorporating necessary safety features. 3.2.1 Positive Pressure Systems Although positive pressure conveying systems discharging to a reception point at atmospheric pressure are probably the most common of all pneumatic conveying systems, the feeding of a material into a pipeline in which there is air at a high pressure does present a number of problems. A wide range of material feeding devices are available that can be used with this type of system, from Venturis and rotary valves to screws and blow tanks. With each type of feeder, however, there is the potential of air leaking from the system, and carrying dust with it, as a result of the adverse pressure gradient. 3.2.2 Negative Pressure (Vacuum) Systems Negative pressure systems are commonly used for drawing materials from multiple sources to a single point. There is no adverse pressure difference across the feeding device in a negative pressure system and so multiple point feeding into a common line presents few problems. A particular advantage of negative pressure systems, whether open or closed, in terms of potentially hazardous materials, is that should a pipeline coupling be inadvertently left un-tightened, or a bend in the pipeline fail, air will be drawn into a system maintained under vacuum. With a positive pressure system a considerable amount of dust could be released into the atmosphere before the plant could be shut down safely. 3.3
System Components
The selection of components for a pneumatic conveying system is as important as the selection of the type of conveying system for a given duty. Air movers, pipeline feeding devices and gas-solid separation systems all have to be carefully considered and there are multiple choices for each. 3.3.1 Blowers and Compressors With air movers a positive displacement machine is generally required. If a blower or compressor is incorrectly specified, in terms of either pressure or volumetric flow rate, the pipeline is likely to block, and with a toxic material this will create its own hazards since the pipeline will have to be unblocked by some means. A
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minimum gas velocity must be maintained throughout the pipeline system to ensure satisfactory conveying, and it must be remembered that all gases are compressible with respect to both pressure and temperature when it comes to evaluating flow rates from specified velocities. 3.3.1.1 Oil Free Air Oil free air is generally recommended for most pneumatic conveying systems, and not just those where the material must not be contaminated, such as food products, Pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, can be carried over with the air and can be trapped at bends in the pipeline or obstructions. Most lubricating oils eventually break down into more carbonaceous matter which is prone to spontaneous combustion, particularly in an oxygen rich environment, and where frictional heating may be generated by moving particulate matter. Although conventional coalescing after-filters can be fitted, that are highly efficient at removing aerosol oil drops, oil in the super-heated phase will pass straight through them. Super-heated oil vapor will turn back to liquid further down the pipeline if the air cools. Ultimately precipitation may occur, followed by oil breakdown, and eventually a compressed air fire. The only safe solution, where oil injected compressors are used, is to use chemical after-filters such as the carbon absorber type which are capable of removing oil in both liquid droplet and super-heated phases. The solution, however, is very expensive and requires continuous maintenance, and replacement of carbon filter cells. 3.3.2 Pipeline Feeding There have been numerous developments in pipeline feeders to meet the demands of different material characteristics, and ever increasing pressure capabilities for long distance and dense phase conveying. Although the majority of systems probably operate with positive displacement blowers at a pressure below 15 psig, discharging to atmospheric pressure, there is an increasing demand for conveying systems to feed materials into chemical reactors and combustion systems that operate at a pressure of 300 psig or more. With positive pressure systems the main problem is feeding the material into a pipeline that contains air at pressure. Because of the adverse pressure it is almost impossible to prevent air from leaking across the feeding device. This air will almost certainly carry dust with it, and so if this air or dust must be controlled then some means of containment must be incorporated into the conveying system. 3.3.2.1 Rotary Valves The rotary valve is probably the most commonly used device for feeding conveying pipelines. By virtue of the moving parts and a need to maintain clearances between the rotor blades and the casing, air will leak across the feeder when there is a pressure difference. Rotary valves are ideally suited to both positive pressure and vacuum conveying.
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The rotary valve is a positive displacement device and so feed rate can be controlled fairly precisely by varying the speed of rotation. The situation with regard to screw feeders is very similar, as these are also positive displacement devices. Air leakage can be minimized by reducing blade tip clearances, increasing the number of rotor blades, and providing seals on the rotor end plates, but it cannot be eliminated. Air at pressure will always return with the empty pockets, apart from leaking past blade tip clearances. The air leaking across a rotary valve will often restrict the flow of material into a rotary valve from the supply hopper above. To minimize this influence it is usual to vent a rotary valve in some way. A common device is to provide a vent on the return side of the valve. Since the vented air will contain some fine material, this is either directed back to the supply hopper, ducted to a separate filter unit, or re-introduced back into the conveying pipeline. Because there will be a carry-over of material any filter used must be regularly cleaned, otherwise it will rapidly block and cease to be effective. If the air is vented into the supply hopper above, or to a separate filter, the pipe connecting the vent to the filter unit must be designed and sized as if it were a miniature pneumatic conveying system, in order to prevent it from getting blocked. With low pressure conveying systems a venturi can be used to feed the dusty gas from the vent directly back into the pipeline. If the material to be conveyed is potentially explosive, the use of rotary valves will have to be questioned. With metal blades and a metal housing, a shower of sparks would result if the two were to meet, and a single spark would provide an adequate source of ignition for many materials. With positive pressure conveying systems rotary valve blade tip clearances need to be very small and so differential expansion, resulting from the handling of hot material, or bearing wear, could cause the two to meet. Bearing failure on a rotary valve could well result in a surface at a sufficiently high temperature to provide a necessary ignition source, both within and external to the conveying system. In a fault situation dust can leak from a pressurized conveying system and so bearings external to the system are vulnerable. 3.3.2.2 Blow Tanks The use of blow tanks has increased considerably in recent years and there have been many developments with regard to type and configuration. A particular advantage with these systems is that the blow tank also serves as the feeding device, and so many of the problems associated with pressure differentials across the feeder are largely eliminated. Although continuous air leakage does not occur with blow tanks, as it does with rotary valves, consideration does have to be given to the venting of the blow tank at the end of the conveying cycle, as well as on filling. A similar situation exists with regard to gate valve feeders. Blow tanks generally form the basis of mobile conveying systems, such as road and rail vehicles, and so special provision must be made for venting these during filling operations.
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Blow tanks can be de-pressurized through the conveying pipeline, but this is a slow process. If the blow tank is de-pressurized separately it would be recommended that it should be vented to the top of the supply hopper, provided that it has a suitable filter. When the next batch of material is loaded into the blow tank, the air in the blow tank will need to be vented. If it is not vented the displaced air will impede the flow of material into the blow tank as it will have to flow against the incoming material. The vented air is likely to carry dust with it and so this should be directed to the top of the supply hopper where it can be filtered. With continuously operating blow tank systems the lock hopper has to be de-pressurized each cycle and this should be similarly vented. 3.4
Conveying Operations
Consideration must be given to some conveying operations and the conveying of certain materials with regard to safety provisions. Mention has already been made of start-up and shut-down transients, for example. In most dense phase conveying situations, the concentration of the material will be well above the value at which an explosion would be possible. During transient operation, however, and plant shut-down in particular, the concentration of the material in the air cannot be guaranteed to be above the required value while the system is being purged. Regardless of the conveying system and the mode of conveying, however, the material will generally be discharged into a receiving vessel, where there is every possibility of the material being dispersed in a low concentration cloud. Pneumatic conveying is an extremely aggressive means of conveying materials, and particularly so in dilute phase conveying where high gas velocities are required. As a result, abrasive particles can cause severe wear of the conveying plant and friable particles can suffer considerable degradation. The consequences of these influences must be given every consideration. 3.4.1 Tramp Materials High conveying air velocities also mean that tramp materials can be conveyed through the pipeline with the material being conveyed. It is possible for nuts, bolts and washers to find their way into the conveyed material, somewhere in the system, and these will be conveyed quite successfully through the pipeline, with the potential of generating showers of sparks, as they will inevitably make numerous contact with the bends and pipeline walls in their passage through the pipeline. 3.4.2 Static Electricity Whenever two dissimilar materials come into contact, a charge is transferred between them. The amount of charge transfer depends upon the type of contact made, as well as on the nature of the materials. Almost all bulk solids acquire an electrostatic charge in conveying and handling operations. In a large number of cases the amount of charge generated is too small to have any noticeable effect,
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but in many cases appreciable charge generation can occur, resulting in high electric fields. Very often these are just a nuisance, but occasionally they can attain hazardous levels. In all cases where dust clouds are present the build-up of an electrostatic charge should be prevented. Pneumatic conveying systems are prolific generators of static electricity. Frictional charging of the particles moving along the walls of a pipeline can lead to a carry-over of net charge into the receiving hopper. In the case of non-conducting materials a build-up of charge might occur in the receiving vessel, because of the difficulties of leakage through an insulating medium. In the case of conducting solids, electrostatic problems can still arise when the particles are suspended in air. In such a case the air prevents the electric charge on each individual particle from leaking away. It is possible, therefore, for high electric fields to exist in receiving hoppers. In many cases the charge may reach the breakdown level for air and produce a spark. Such a spark may have sufficient energy to provide the necessary source of ignition for the dust cloud in the vessel, and hence cause an explosion. A 'rule of thumb' value of 25 mJ is often taken, and materials with ignition energies less than this may be regarded as being particularly prone to ignition by static electricity. In these cases special precautions should be taken. 3.4.2.1 Earthing From an electrostatic point of view, pneumatic conveying lines should be constructed of metal and be securely bonded to earth. All flanged joints in the pipework should maintain electrical continuity across them, to reduce the chance of arc-over within the pipe. Particular attention should be given to areas where rubber or plastic is inserted for anti-vibration purposes, and where sight glasses are positioned in pipelines. Regular routine checks of the integrity of the earthing of all metal parts of the system should be carried out. The use of well grounded facilities can help to reduce these potential hazards. Although certainly safer than systems that have plastic sections, where charge can build up, earthed metal systems will not ensure that the system is safe. Metal pipes provide a very effective source of charge for particles conveyed through them. The charge created on the pipe will flow instantly to earth, but that on the particles may remain for long periods. The storage potential is particularly important with regard to operations subsequent to conveying, for it is quite possible for such a charge on a material to be transferred to operatives. If this occurred in the presence of an appropriate concentration of the material, the spark could provide the necessary ignition energy to cause an explosion. In this case special precautions should be taken, including the use of anti-static clothing and conducting footwear by all people in direct contact with a dust cloud. These, however, would be quite useless if they were to be used on a highly insulated floor, such as is often found in modern buildings. The operatives should stand on an earthed metal grid or plate at the point of operation.
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3.4.2.2 Humidity Control Static generation on a material increases as the relative humidity of the surrounding air decreases, and since it is more difficult to generate and store charges under more humid conditions, increasing the relative humidity of the conveying air to 60 to 75% may also be used as a means of controlling the problem. The use of humidity for charge control is obviously not suitable for hygroscopic materials, and must be considered in relation to the possibility of condensation and freezing in any application. 3.4.3 Particle A tlrition Of those materials that are explosive, research has shown that it is only the fraction with a particle size less than about 200 (im that poses the problem. If a size analysis of a material to be conveyed shows that there is no significant amount of material below this size, the possibility of an explosion occurring during its conveying should not be dismissed. Degradation caused by pneumatic conveying can result in the generation of a considerable number of fines, particularly if the material is friable. This point was illustrated in the previous chapter with Figure 21.2 that shows the fractional size distribution of a material both before and after conveying. In terms of explosion risks the material after conveying could be a serious contender. 3.4.4 Erosive Wear Many materials that require conveying are abrasive. These include some of the larger bulk commodities such as cement, alumina, fly ash and silica sand. With a conveying air velocity of only 4000 ft/min silica sand is capable of wearing a hole in a regular steel bend in a pipeline in less than two hours. Erosive wear can be reduced with wear resistant materials and special bends, but it cannot be eliminated. Even straight pipeline is prone to wear under some circumstances. If an abrasive material has to be conveyed, therefore, consideration must be given to the possibility of a bend or some other component in the system failing, with the consequent release of dust, particularly with a positive pressure conveying system. Bends are available that have detectors embedded into them so that notice can be given in advance of an impending failure. 3.4.5 Material Deposition In long straight horizontal pipe runs, and large diameter pipelines, there is the possibility of material coming out of suspension in dilute phase conveying and depositing on the bottom of the pipeline. Accumulations of material such as pulverized coal in a pipeline could result in a fire, through spontaneous combustion, and possibly an explosion. An increase in conveying air velocity will generally help to reduce the problem but this is not an ideal solution. A disturbance to the flow with a turbulence generator usually cures the problem.
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Food products, of course, will deteriorate if left in pipelines, and contamination of subsequent material could result. Since it is unlikely to be known whether such deposition occurs or not, it is necessary to physically clean all lines periodically. For food and pharmaceutical products, pipelines and all valves and components that could possibly come into contact with the material being conveyed are likely to be made of stainless steel. A particular problem with carbon steel is that it is liable to rust, as a result of condensation in the pipeline, and so contaminate the material. 3.4.5.1 Pipeline Purging If a pipeline is to be purged with the conveying air, in order to clear it of material, radiused bends should be used rather than blind tees. Blind tees are used in pipelines because they will trap the conveyed material and so provide protection to a bend from abrasive particles, since the particles will impact against each other rather than the bend wall. Material will require a much longer purging time to be completely cleared from blind tees, however. If additional air is available for purging, the process will be more effective. Air stored in a receiver will help here, particularly if it is at pressure but care must be taken not to overload the filtration plant during this operation. In dense phase conveying, air velocities employed are very much lower than those required for dilute phase conveying. Pipeline purging can be a major problem if additional air is not available. This point was considered in some detail in Chapter 9 with data on fly ash with Figure 9.9 and cement in Figure 9.10. If high pressure air is used for conveying a material it is common for the pipeline to be stepped to a larger bore along its length once or twice in order to allow the air to expand and so prevent excessive velocities from occurring towards the end of the pipeline. This does, however, create problems if such a pipeline needs to be purged clear, for the purging velocity will decrease at each step to a larger bore and so considerably more air would be needed for the purpose. This point was considered in Chapter 9 with Figure 9.8. 3.4.6 Power Failure The consequences of a power failure on system operation need to be considered at the design stage so that back-up systems and preventative measures can be incorporated at the time of installation. With a pneumatic conveying system the plant will generally shut itself down safely on loss of power, but whether it can be started up again will depend upon the type of conveying system, pipeline routing, mode of conveying and material properties. In many cases the pipeline will block and the only method of restarting the system will be to physically remove the material from where the pipeline is blocked, usually at the bottom of a vertical lift. If this is not an option then a standby power system must be available to take over. Alternatively an air receiver can be built into the air supply system, and this will provide air to purge the lines suf-
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ficiently clear of material so that the system can be restarted when power is reinstated (see Figure 19.10). If the possibility of the pipeline becoming blocked from any eventuality must be avoided, consideration should be given to the use of an innovatory system. In 'conventional systems' the material is simply blown or sucked through the pipeline. In 'innovatory systems' the material is either conditioned as it is fed into the pipeline, or along the length of the pipeline. There is no difference in any of the peripheral system components employed. Air pulsing or trace air lines are generally employed. Parallel lines are used either to inject air into the pipeline, to give the material artificial air retention, or to allow the air within the pipeline to by-pass short sections of material, to give the material artificial permeability. Depending upon the properties of the material to be conveyed, one or other of these innovatory systems will generally guarantee that the pipeline can be restarted on full load. 4
EXPLOSION PROTECTION
Despite the fact that the potential for an explosion in a pneumatic conveying system is high, the demand for such systems remains high. This is partly due to the fact that the system totally encloses the material, such that dust generation external to the system is virtually eliminated, and with a pipeline total flexibility in the conveying route is possible without material transfer or staging. There are also a number of different means by which a pneumatic conveying system can quite easily be protected. Since the dispersion of powdered and granular materials in air is fundamental to pneumatic conveying, it is evident that if a material is known or shown to be explosive, then consideration should be given to the hazard that this presents at the design stage of a system, or when re-commissioning an existing system to convey a different material. Whilst it is equally obvious that the generation of sources of ignition should be minimized, unforeseen mechanical, electrical or human failures mean that the complete elimination of ignition sources cannot be relied upon, particularly where powered machinery is involved. To avoid the potentially catastrophic effects of an explosion, therefore, reliance is normally placed on the adequate functioning of a means of protection for the system. Such protection is normally based on one or more of the following approaches: j Minimizing sources of ignition and prevention of ignition. _: Allowing the explosion to take its full course but ensuring that it does so safely by either containment or explosion relief venting. n Detection and Suppression. The method of protection selected will depend upon a number of factors. These include the design of any associated plant or process, the running costs, the
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economics of alternative protection methods, the explosibility of the material, and the extent to which an explosion and its consequences can be foreseen, together with the requirements of any local regulatory authorities concerned. 4.1
Minimizing Sources and Prevention of Ignition
The first step in any explosion protection program is to minimize or eliminate, as far as possible, all potential sources of ignition. The minimum ignition temperature is relevant to ignition by hot surfaces. Rotary valve bearings have already been mentioned in this context, as an example, and welding operations on any part of the system should be prohibited while the system is operating. The possibility of sparks must also be reviewed, with due consideration given to valve operations, friction with conveyed materials, and electrostatic generation. 4.1.1 Inert ing Prevention of ignition can be guaranteed by using an inert gas such as nitrogen for conveying the material. Alternatively, nitrogen can be added to the air in order to reduce the percentage of oxygen present in the conveying air to a level at which a flame cannot be supported. The maximum oxygen concentration is one of the many standard tests that can be carried out with a material, as mentioned earlier. Since inert gases are rather expensive, these methods are generally used with closed or semi-closed loop systems. 4.2
Containment
The combustion of a dust cloud will result in either a rapid build up of pressure or in an uncontrolled expansion. It is the expansion effect, or the pressure rise if the expansion is restricted, that presents one of the main hazards in dust explosions. The expansion effects arise principally because of the heat generated in the combustion and, in some cases, to gases being evolved from the dust because of the high temperature to which it has been exposed. If the presence of evolved gases is neglected, the situation can be modeled very approximately with the thermodynamic relationship:
(1)
T -
If the explosion is confined, V'/ will equal V2 and so the resulting pressure, , will be given by: T
Pi
=
Pi
x
2
- - - - - - - - - - -
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(2)
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Flame temperatures are typically a couple of thousand degrees and so it can be seen that explosion pressures can reach 100 psig quite easily. The critical information from Table 22.2, however, is that this pressure can be reached in milliseconds. When a dust explosion occurs in industrial plant spectacular destruction can result if it is initially confined in a system that is ultimately too weak to withstand the full force of the explosion. The literature on this subject generally includes numerous photographs of the resulting destruction to plant in order to reinforce the fact that explosions do occur and that they can be catastrophic. Blow tanks and rotary valves, however, can be obtained that will withstand these pressures and in a positive pressure system the compressor or blower can be protected by means of non-return valves in the air supply line. Most pipelines are also capable of withstanding this order of pressure. If this is so, the resulting pressure wave would pass along the pipeline and be relieved at the weakest point, which is usually the reception vessel. Due to their size these are generally only capable of withstanding very low pressures, as mentioned earlier, and so if exposed to higher internal pressure, would rupture or burst. Consequently the collection unit is likely to be the most vulnerable part of the system. It is unlikely to be an economic proposition to design the reception vessel to withstand the explosion pressure. There are, however, alternative means of protecting the receiving vessel. 4.3
Explosion Relief Venting
The usual solution to the problem in situations where the risk of an explosion is only very slight, is to allow an explosion to take its full course, whilst employing suitable precautions to ensure that it does so in a safe manner. As an alternative to containment, the reception vessel can be fitted with appropriate relief venting. This may take the form of bursting panels, displacement panels or hinged doors that operate once a predetermined pressure has been reached. In venting explosions to atmosphere strict attention must be paid to the safe dissipation of the explosion products. It is a characteristic that the volume of flame discharged from vents can be very large, and obviously must be directed to a safe place away from operatives and neighboring plant. If this is necessary it is normally achieved by attaching a length of ducting to the vent, or by installing deflector plates. The duct attached to the vent should be short, free from bends (if at all possible) and other restrictions to flow, and be kept clear of dust at all times. The size of duct, in terms of flow cross-section area, for explosion venting is particularly important. This is related to the maximum rate of pressure rise and the more vigorously explosive materials require larger areas of venting. The size of vent is also dependent upon the volume of the receiving hopper or silo. This can also be modeled very approximately from the above thermodynamic relationship. If the explosion is to be vented to prevent a pressure rise, p, will now equal p2, but V, will no longer equal V2. Thus:
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T, V 2 = V, x —
. . . . . . . . . .
(3)
•N
The volumetric flow rate of the gases now leaving the reception vessel will be about seven times higher than normal, and this does not take account of gases generated as a result of the explosion. There is no possibility of the existing filter plant being able to cope with this increased flow rate and so venting is essential. To keep the pressure drop in the explosion relief ducting to as low a value as possible, the duct will clearly have to be of a large section area. Since pressure drop varies approximately with the square of velocity, the velocity of the gases in the ducting will have to be very much lower than that of the incoming conveying air. Combined with the seven fold increase in steady state flow rate, and the fact that this is a transient situation, duct sizing is a complex task and should only be assessed by an expert. 4.4
Detection and Suppression
If a system is inconveniently sited to allow for venting; a vent of the required size cannot be fitted onto the existing hopper; or if the material is toxic, so that it cannot be freely discharged to atmosphere, the protection may be achieved by a detection and suppression approach. Although there may be only a few tens of milli-seconds between the ignition of the material, to the build up of pressure to destructive proportions, this is sufficient for an automatic suppression system to operate effectively, as illustrated in Figure 22.3. Maximum unsuppressed explosion pressure
Suppressed explosion System activation pressure
/ /
Unsuppressed explosion
Maximum pressure that the plant can withstand ssed explosion pressure
Time - ms Figure 22.3 explosions.
Comparison of pressure-time histories of unsuppressed and suppressed
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Commercial equipment is available that is capable of both detecting the onset of an explosion and of suppressing the explosion before it is able to develop. The sensing device, on detecting a rise in pressure, can send signals to switch off the air supply and stop the feeding device in order to prevent the conveying of any further material. A signal can also be sent to operate the automatic opening of a venting system. An automated opening has the advantage that vents are opened extremely rapidly and for very explosive materials this helps to reduce the maximum explosion pressure. Alternatively a suppressant system can be triggered. Such equipment operates as illustrated in Figure 22.4. Suppression involves the discharge of a suitable agent into the system within which the explosion is developing. The composition of the agent depends upon the material being conveyed, and is typically a halogenated hydrocarbon, or an inert gas or powder. The suppressant is contained in a sealed receptacle attached to the plant and is rapidly discharged into the system by means of an electrically fired detonator or a controlled explosive charge. Thus, as soon as the existence of an explosion is detected, the control mechanism fires the suppressant into the plant and the flame is extinguished. Alternatively the explosion can be automatically vented. When the explosion is detected a vent closure is ruptured automatically, thus providing a rapid opening of the vent. The vented explosion then proceeds as for cases in which the vents are opened by the pressure of the explosion. The automatic method has the advantage that vents are opened extremely rapidly, and for highly explosive materials this helps to reduce the maximum explosion pressure. Since it is obvious that once an explosion has been initiated, no more material should be fed into the system, plant shut-down can also be rapidly achieved with the detector approach. Action Signals
Action Signal Shutdown Vent to Atmosphere Detector Blower/ Compressor
Suppressant Feeder
Figure 22.4
Basic scheme for detection and suppression.
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4.5
Chapter 22
Secondary Explosions
With positive pressure conveying systems there is always the possibility of a failure or defect in the system resulting in the discharge of a dust cloud into the atmosphere. Abrasive materials wearing holes in pipeline bends and neglecting to tighten pipeline couplings have already been mentioned. Filters can also represent a weak link. A pressure surge from a blow tank, or supplementary air from an air receiver on purging a pipeline, may result in the release of dust, or even the failure of a filter element. A flammable dust cloud can be produced quite accidentally in many different circumstances. There must, therefore, be no possible sources of ignition external to the system. One of the major sources of ignition in this situation comes from electrical equipment. If the material being conveyed is potentially explosive, therefore, it is essential that all the lighting, switches and switchgear, contacts and fuses, and electrical equipment in the vicinity, or within the same building, should be of a standard or class that would not be able to provide a source of ignition, whether a spark or hot surface. This is standard practice in chemical plant where fumes and vapors are likely to be present, but tends to be overlooked with respect to the possible release of dusts. The release of a dust explosion from a conveying system into a building, or the explosion of a dust cloud released from a conveying system inside a building, are both clearly very serious situations. Little hazardous pressure is likely to develop from either of these sources of explosion within a large building, if shortlived. The pressure wave generated, however, usually shakes down large quantities of dust that has settled over a period of time onto pipe-work, roof beams and supports, ledges, lighting, etc. This then makes an ideal condition for the secondary explosion that almost always follows. It is this secondary explosion that can demolish a factory and kill the operatives. It is extremely important, therefore, that good housekeeping is maintained at all times within all areas within buildings, such that any dust release is not allowed to accumulate on any surfaces anywhere, and on lighting and electrical equipment in particular. 4.6
Determination of Explosion Parameters
In most countries all tests concerned with assessing the explosibility or measurement of explosion characteristics of materials in suspension are methods agreed, typically with a National Factory Inspectorate, and are generally carried out in the sequence shown in Figure 22.5. As a result of these established procedures, data regarding the explosion characteristics of many materials already exists. With a material that has not been previously tested, the first step should be to determine whether it is potentially explosive. The outcome of such a test will then indicate the necessity of incorporating precautionary measures into the system design.
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647
Health and Safety Explosion Characteristics
Material Sample
Classification Tests
Group A Explosive —>
Relevant I lazard or Method of Protection
Minimum Ignition Temperature
I lot Surfaces
Maximum Permissible Oxygen Concentration
Use of Inert Gas
Material Concentration Limits
Type of System
Minimum Ignition Energy
Static Electricity
Maximum Pressure and Rate of Pressure Rise
Containment and Explosion Relief Venting
Group B Non-Explosive
Figure 22.5 4.6.1
Basic scheme of explosion tests.
Test Apparatus
Test apparatus used to measure explosion parameters is often classified as shown in Table 22.3. Table 22.3
Classification of Test Apparatus
Apparatus
Direction of Dispersion of Material
Ignition Source
Application
Vertical Tube
Vertically Upwards
Electric Spark or Electrically Heated Wire Coil
All types of dust
Horizontal Tube
Horizontal
Electrically Heated Coil at 2370°F
Carbonaceous materials, especially of small particle size
Inflammator
Vertically Downwards
Electrically Heated Wire Coil or Electric Spark
Carbonaceous and metal dusts, especially large or fluffy particles
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In the vertical tube apparatus the dust is placed in a cup and dispersed upwards over the ignition source by a controlled air blast. Observation of the flame propagation can then be made. Modification of the electrodes allow this device to be used for the determination of minimum ignition energy. The Hartmann bomb is a strong version of this apparatus that can also be used for the measurement of explosion pressure and rate of pressure rise [11]. The horizontal tube apparatus also involves the dispersion by air of a dust sample over an ignition source. Since the residence time of a dust near the coil is short, any material that is observed to propagate a flame must be regarded as presenting a serious explosion hazard. The inflammator is essentially a vertically mounted glass tube. A sample of dust, held in a horizontal tube, is blown by air and is directed downwards by a deflector plate. Although convenient for the testing of explosion characteristics, the Hartmann bomb has been criticized on the grounds that test results do not reliably scale up to correspond to industrial plant. This has led to the development of the socalled 20-litre sphere apparatus. This consists of a spherical stainless steel vessel fitted with a water jacket. A dust cloud is formed in the vessel as the dust enters from a pressurized chamber through a perforated dispersion ring. 60 milliseconds after the dust is released into the sphere the detonator is fired and the resulting pressure rise is monitored [11). 4.6.2 Material Classification Depending on the outcome of such tests the material is simply classified with respect to explosibility as follows: Group A - Materials that ignited and propagated a flame in the apparatus. Group B - Materials that did not propagate a flame in the test apparatus. Group A materials clearly represent a direct explosion risk and, as such, it would be a wise precaution, or even a legal requirement, to incorporate protection measures into the system. The range of materials which fall into this group is wide, as indicated earlier. Sand, alumina and certain paint pigments are examples of Group B materials. Some Group B materials, although not explosible, may nevertheless present a fire risk. If a material is shown to be of the Group A type, further information on the extent of the explosion hazard may be required when considering suitable precautions for its safe handling. The following parameters can be determined by use of the test methods described above: _• Minimum ignition temperature. 1 Maximum permissible oxygen concentration to prevent ignition. ".. Minimum explosible concentration. Minimum ignition energy. Maximum explosion pressure and rate of pressure rise.
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Since the explosion characteristics, in terms of these parameters, of many materials are well documented it is not appropriate to include this information here. In order to illustrate the magnitude of the quantities involved, however, details regarding a few well known materials are given in Table 22.2. A summary of the applications of the results of these various tests to practical conditions is included in Figure 22.5.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
D. Mills. Safety aspects of pneumatic conveying. Chem Eng. Vol 106, No 4, pp 8491. April 1999. P. Field. Dust explosions. Handbook of Powder Technology. Vol 4. Hlsevier. 1982. J. Cross and D. Fairer. Dust Explosions. Plenum Press. 1982. HM (UK) Factory inspectorate technical data note 14. Health: dust in industry. IIMSO 1970. C. Schofield. Dust: the problems and approaches to solutions. Proc Solidex '82 Conf. Harrogate. March/April 1982. Health and Safety Executive. Guidance Note EH 15/80. Threshold limit values. HMSO 1980. Corn starch dust explosion at General Foods Ltd, Banbury, 18 November 1981. Health and Safety Executive Report. HMSO. London. 1983. HM (UK) Factory Inspectorate. Dust explosions in factories. Health and Safety at Work Booklet No 22. HMSO. London. 1976. D. Mills. Pneumatic conveying: cost effective design. Chemical Engineering, pp 7082. February 1990. C.R. Woodcock and J.S. Mason. Bulk Solids Handling: an introduction to the practice and technology. Chapman and Hall. 1987.
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