RWkW Safety considerations in conveying of bulk solids and powders Stanley S. Grossel HofSmann-La Roche Inc., Corporate Engineering, 340 Kingsland Street, Building 105, Nutley, NJ 07110, USA This review covers a wide range of safety factors which need to be considered when handling bulk solids and powders. The physical dangers of the mechanical equipment are well covered by published standards and codes, as are noise levels and the use of electrical equipment. Dust explosions caused both by static electricity and other ignition sources, are more complex. The plant requires careful investigation to ensure that explosion hazards are kept to a minimum and suitable protective measures installed. Different types of conveyers, e.g. belt, pneumatic and bucket elevators, each pose their own hazard and the system chosen should be considered carefully in the light of the material to be handled. (Keywords: bulk solids handling: safety factors; dust explosions) There are a wide range of safety factors to be considered in the handling of bulk solids, from the safety of the machinery used to the toxicity and explosibility of fine powders. This review looks at these different safety aspects. Conveyor personnel protection considerations Machinery guarding All bulk solids/powders conveyors have a large number of moving parts, including power transmission machin- ery and equipment (shafts, pulleys, couplings, speed reducers, etc). In accordance with OSHA and ANSI requirements guards must be provided to protect per- sonnel and state and local codes and regulations must also be satisfied. The OSHA and ANSI standards listed below should be consulted for details. 0 OSHA Safety and Health Standards (29CFR 1910): General Industry, Paragraph 1910.219 (1981) - Mechanical Power - Transmission Apparatus. 0 ANSI B15.1 (1972) - Safety Standard for Mechan- ical Power Transmission Apparatus. 0 ANSI/ASME B20.1 (1984) - Safety Standard for Conveyors and Related Equipment. Noise exposure Equipment and machinery noise levels must be kept below the values given in OSHA Safety and Health Standards (29CFR 1910), Paragraph 1910.95 - Occupa- tional Noise Exposure (see Table I). If equipment/ Received 19 January 1988 OSSO-4230/%%/020062-13S3.00 0 1988 Butterworth & Co. (Publishers) Ltd 62 J. Loss Prev. Process Jnd., 1988, Vol 1, April Table 1 Permissible noise exposures Sound level dBA slow Duration Per day fhl ~l3SpO”Sl3 8 90 6 92 4 95 3 97 2 100 1: 102 1 105 : 110 : or less 115 When the daily noise exposure is two or more periods of noise exposure of different levels their combined effect should be considered, rather than the individual effect of each. If the sum of the following fractions: Cl/T, + G/T2 G/T, exceeds unity, then, the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and Tn indicates the total time of exposure permitted at that level. Exposure to impulsive or impact noise should not exceed 140 d6 peak sound pressure level. machinery cannot be obtained with noise levels comply- ing with OSHA standards, then engineering controls, such as sound mufllers, must be installed. If this does not bring the sound levels within the limits listed in Table I, then suitable protective equipment must be provided for the operators. Electrical equipment All electrical equipment (e.g. motors, switchgear, wiring,) must be designed and installed to comply with the latest edition of the National Electrical Code (NEC), and paragraphs 1910.301 to .399 of OSHA Safety and Health Standards (29CFR 1910).
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RWkW
Safety considerations in conveying of bulk solids and powders
Stanley S. Grossel HofSmann-La Roche Inc., Corporate Engineering, 340 Kingsland Street, Building 105, Nutley, NJ 07110, USA
This review covers a wide range of safety factors which need to be considered when handling bulk solids and powders. The physical dangers of the mechanical equipment are well covered by published standards and codes, as are noise levels and the use of electrical equipment. Dust explosions caused both by static electricity and other ignition sources, are more complex. The plant requires careful investigation to ensure that explosion hazards are kept to a minimum and suitable protective measures installed. Different types of conveyers, e.g. belt, pneumatic and bucket elevators, each pose their own hazard and the system chosen should be considered carefully in the light of the material to be handled.
There are a wide range of safety factors to be considered in the handling of bulk solids, from the safety of the machinery used to the toxicity and explosibility of fine powders. This review looks at these different safety aspects.
Conveyor personnel protection considerations
Machinery guarding All bulk solids/powders conveyors have a large number of moving parts, including power transmission machin- ery and equipment (shafts, pulleys, couplings, speed reducers, etc). In accordance with OSHA and ANSI requirements guards must be provided to protect per- sonnel and state and local codes and regulations must also be satisfied.
The OSHA and ANSI standards listed below should be consulted for details.
0 OSHA Safety and Health Standards (29CFR 1910): General Industry, Paragraph 1910.219 (1981) - Mechanical Power - Transmission Apparatus.
0 ANSI B15.1 (1972) - Safety Standard for Mechan- ical Power Transmission Apparatus.
0 ANSI/ASME B20.1 (1984) - Safety Standard for Conveyors and Related Equipment.
Noise exposure Equipment and machinery noise levels must be kept below the values given in OSHA Safety and Health Standards (29CFR 1910), Paragraph 1910.95 - Occupa- tional Noise Exposure (see Table I). If equipment/
When the daily noise exposure is two or more periods of noise exposure of different levels their combined effect should be considered, rather than the individual effect of each. If the sum of the following fractions: Cl/T, + G/T2 G/T, exceeds unity, then, the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and Tn indicates the total time of exposure permitted at that level. Exposure to impulsive or impact noise should not exceed 140 d6 peak sound pressure level.
machinery cannot be obtained with noise levels comply- ing with OSHA standards, then engineering controls, such as sound mufllers, must be installed. If this does not bring the sound levels within the limits listed in Table I, then suitable protective equipment must be provided for the operators.
Electrical equipment All electrical equipment (e.g. motors, switchgear, wiring,) must be designed and installed to comply with the latest edition of the National Electrical Code (NEC), and paragraphs 1910.301 to .399 of OSHA Safety and Health Standards (29CFR 1910).
Safety considerations in conveying bulk solids and powders: S. Grossel
When electrical equipment needs to be repaired or maintained, it must be locked out and tagged to prevent injury to personnel. There is currently no OSHA stand- ard, but legislation has been proposed and submitted for review, entitled ‘Control of Hazardous Energy Sources’ (Lockout/Tagout). There is also an ANSI consensus standard 2244.1 (1982).
Static electricity and dust explosion hazards
Static electricity hazards
When conveying bulk solids and powders, especially organic ones, static electrical charges can develop. These charges arise from contacts made between sur- faces during the movement of the particles. The charge on a powder particle is governed by three factors: the charge production rate; the charge leakage rate when the particle is in contact with a ground; and the electrical breakdown of air initiated by the high field around the charged particle.
There are five fundamental quantities in an under- standing of electrostatics. The most basic is the electric charge that is transferred to a material, usually by friction. When an object is charged it exerts a force on any other charged object, and is then said to have an electric potential or voltage, k’. The rate of change of voltage with distance or potential gradient is the electric held, E. The potential reached by an object having a charge q depends on its electrical capacity, C. The higher the capacity the more charge is needed to achieve a given potential.
The rate at which charge dissipates depends primarily on the electrical resistance R between the stored charge and ground. An electrostatic spark occurs when an isolated charged object is suddenly grounded. The accumulation of static electricity on an object produces an electric field around it and a spark will occur if the field strength exceeds the breakdown value of the sur- rounding atmosphere. For air, this is approximately 3000 kV/m.
Electrostatic sparks can cause dust explosions if they achieve a minimum ignition energy and the dust cloud in the air is within the explosive concentration range. The minimum ignition energy is the energy which just ignites the most easily ignited mixture and is usually measured with a capacitor discharge by varying the charge quan- tity, the capacitance and the electrode separation at standard temperature and pressure conditions (1 bar, 2O”C), or when applicable, in saturated vapour.
The minimum ignition energy of a dust-air mixture (with the exception of explosives and other reactive materials is lo-100 mJ and is therefore 50 to 1000 times greater than those of gas-air or vapour-air mixtures. Determining the minimum ignition energy is, with other data, an important aid in quantifying the potential hazard of electrostatic charges.
Cross and Farrer ’ present an excellent discussion of static electricity phenomena and measurement tech- niques for minimum ignition energy. Also, the book by Haase’ is a good source of information on electrostatic
hazards. Palmer3 lists minimum ignition energies for many bulk solids and powders. These values are listed in Table 2.
Dust explosion hazards The subject of dust explosions is too large and com- plicated to cover in depth in this review, but certain aspects of it will be discussed below to present some fundamentals and background materials. For further reading on the subject, the following books are recom- mended: Cross and Farrer’, Palmer’, Bartknecht4, Field’, and Nagy and Verakis6.
A dust explosion results when finely divided combus- tible matter is dispersed in an atmosphere containing sufficient oxygen to permit combustion and a source of ignition of appropriate energy is present. Dust explosions have certain similarities to gas explosions, especially with regard to the chemical processes involved and in cases where the particle size of the dust is less than 5 pm. However, there are significant differences which make the study of dust explosions extremely difficult.
For a dust explosion to occur there must be a degree of turbulence, if only to disperse the dust into a suspension. Gas explosions can occur when the gas is in a quiescent state, the mixture being homogeneous and consisting of molecular-size particles. The suspensions of dusts encountered in dust explosions are, however, unlikely to be homogeneous, and would normally con- tain a range of concentrations of particles which are many orders of magnitude larger and heavier than gas molecules and which settle out of suspension due to gravity.
A dust explosion involves such a high rate of combus- tion that individual particles and agglomerates are either consumed or oxidized. The combustion of carbon in organic material produces gaseous products which in themselves take up more space than the solids of the parent material. An expanding flame front will also result from the ignition of flammable gases produced by the decomposition of the dust. A dust explosion there- fore requires more space because of the expansion of the hot gaseous products.
In industrial plant, the heat released during a dust explosion is likely to exceed the natural rate of cooling and consequently an explosion would be accompanied by significant, and, in some cases, uncontrolled expan- sion effects. In an unconfined situation, there would be mainly localized flames and pressure effects. However, in the confined situations commonly found in plant handling particulate matter, the expansion effects are likely to be sufficient to burst through the confines of the plant equipment and/or piping.
The following conditions must exist for a dust explosion to occur:
l The dust must be combustible. l The dust must be in suspension in the atmosphere
which must contain sufficient oxygen to support combustion.
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Safety considerations in come ying bulk solids and powders: S. Grossel
Table 2 Dust explosion parameters
Dust
Maximum
oxygen Minimum co”ce”-
ignition Minimum Maximum tration temperature explosible Minimum Maximum rate of
f°C) to prevent
CO”CB”- ignition explosion pressure ignition tration energy pressure rise 1% by
The dust must have a particle size distribution that will propagate flame. The dust concentration in the suspension must be within the explosible range. The dust suspension must be in contact with an ignition source of sufficient energy.
Under these conditions the hazard from a dust explosion depends upon the explosibility of the dust, the
70 J. Loss Prev. Process Ind., 1988, Vol 7, April
volume and characteristics of the vessel or chamber containing the dust suspension, the dispersion and concentration of the dust suspension and the degree of turbulence in the vessel.
The explosibility of a dust can be determined by tests which are described by Field’. The tests specific to dust explosion venting are described by Field’ and Schofield ‘.
Generally speaking, the explosibility of a combustible
Safety considerations in conveying bulk solids and powders: S. Grossel
dust is greater the smaller the particle size. The minimum ignition energy is reduced and the maximum explosion pressure and rate of pressure rise are increased with a decrease in particle size. In addition, fine particles more readily stay in suspension increasing the probability of producing an explosible concen- tration. Particles greater than 2500 pm diameter are unlikely to cause dust explosions, although the possi- bility of coarser materials producing fine dust by attrition during handling must be anticipated.
Minimum explosible concentrations in air are typ- ically in the range lo-500 gme3 and some values are given in Table 2 together with other explosion param- eters. Explosible concentrations are much higher than those associated with toxic hazards or nuisance prob- lems (which might range from = 1 to 10 mgme3) and such explosible concentrations would most likely occur very close to a dust source or within an enclosed space where the dust cloud cannot spread. Indeed, enclosure and confinement of dust sources to solve occupational hygiene or nuisance problems may well concentrate the dust cloud increasing the risk of explosion. This danger is often overlooked by environmental protection engi- neers.
Ignition sources of sufficient energy to cause a dust explosion are many but the main ones are listed below:
Flames Hot surfaces Incandescent material Spontaneous heating Welding or cutting operations Friction heating or sparks Impact sparks Electrical sparks Electrostatic discharge sparks
Precautions against dust explosions may be for preven- tion or protection and are summarized in Table 3.
The following general approach to dust explosion precautions is recommended:
Where possible select less dusty alternatives for materials and minimize attrition. Minimize handling of dusty materials and design the handling system to minimize dust generation and the size of dust clouds. Avoid the accumulation of dust (which can be dis- turbed to form a dust cloud) by the detailed design of equipment, building and working practices. Anticipate possible ignition sources and eliminate them, as far as is reasonably practicable, by appro- priate equipment design, grounding, maintenance and working practices. Take appropriate additional precautions, where practicable, such as inerting, containment, venting or suppression. Isolate vulnerable plant where appropriate.
Prevention and protective measures wi!l be described in detail below.
Pneumatic conveying systems
Pneumatic conveying systems cause the highest risk of dust explosions and fires for the following reasons:
Static electricity is generated by contact between particles themselves and between particles and the pipewall. Dust concentrations within the explosible range can arise at the delivery point where the dust is separated from the air (silos, cyclones, baghouses). Heated particles which are created during grinding or drying may be carried in a pneumatic transport system and fanned to a glow by the high air velocity. These particles can then cause an ignition in the storage or collection system at the end of the pneumatic transport. Tramp metal in pneumatic systems may also cause frictional heating or sparks as it passes through the system.
Table 3 Summary of dust explosion precautions
Method Comments
Prevention (of an explosion occurring) Exclusion of dust cloud Material can be rendered less dusty and handling system designed to minimize dust. Impossible to
guarantee total dust free environment short of changing to wet process. Exclusion of ignition sources All practical measures must be taken to exclude ignition sources but because sources are often
unknown It is difficult to guarantee so other precautions usually taken. Exclusion of oxygen (inerting) Reduces oxygen content below minimum necessary to support combustion (typically c6-15%). using Nz, CO2 or other suitable Requires continuous monitoring of oxygen content. Usually requires closed system to conserve gas. inert gas. Expensive. Diluent dust addition - to Non-combustible diluent, well mixed with dust, acts as heat sink thus reducing explosibility of reduce explosibility of dust dust. Limited application because of contamination and expense. (Typically the amount of inert dust
exceeds 50%). Containment Vessel and associated pipework etc built sufficiently strong to withstand the maximum explosion
pressure. Expensive in all but the smallest systems. Venting Vents provided in walls of vessel to allow escape of dust and combustion products to limit pressure
nse to an acceptable level. Widely used. Suppression Start of explosion detected by instruments which trigger release of fire suppressants. Useful where
venting is unacceptable or impracticable e.g. when the dust is toxic.
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Safety considerations in conveying bulk solids and powders: S. Grosset
Design considerations to minimize hazards Pneumatic conveying systems should possess the following general characteristics:
They should be air tight, to prevent the escape of dust from the system where it might present a fire, explosion or health hazard. If operating under a negative pressure the system should be air tight to prevent pulling in air or other contaminants. They should be strong enough to remain intact and air tight under normal operating conditions, includ- ing vibration; and, in some cases, to withstand or contain explosive pressures. Static electrical charges should be minimized by grounding, including bonding across joints where necessary. Eletrically isolated metallic objects within the system may accumulate dangerous static charges. Wire braid within rubber-covered transfer hose may act as a static accumulator.
Electrically conductive bags may be used in bag filters handling extremely static-sensitive materials. Care must be taken to ensure that the conductive bags do not become ungrounded.
0
0
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An electrical installation must meet the electrical classification imposed by the conveyed materials as well as the surrounding environment. Suitable construction materials compatible with the materials handled and the surrounding environment should be used. Screens, magnets and metal detectors should be installed for the detection or removal of any foreign material which might create hazards in the system. Where appropriate special materials should be used, such as non-ferrous metals, to minimize mechanical sparking in the event of misalignment or failure of moving parts with the process stream. An adequate programme of maintenance and inspec- tion must be instigated to ensure proper alignment of drives, proper clearances, dust tightness, electrical bonding and grounding and control of ignition sour- ces. Joints and openings should be suitably located to facilitate cleaning or unplugging. High velocities (3000 to 4000 fpm) will minimize settling and there- fore reduce frequency of cleaning.
Fire and explosion protective measures All potentially dangerous pneumatic conveying systems which transport bulk solids should be designed to contain an explosion or provided with explosion vents. Two alternative protective measures are: to use an inert gas, usually nitrogen, to transport the solids; and to provide explosion suppression systems.
The sizing and design of explosion vents is covered in detail in NFPA 68’, and this document should be consulted when explosion venting is to be provided for the conveying piping and/or downstream collection or storage facility (cyclone, baghouse, silo). With respect to explosion containment, design of the conveying
piping and even cyclones for this is feasible. Usually, large-volume baghouses are not designed to contain explosions, for economic reasons. In recent years, how- ever, several baghouse vendors have been offering small-to-medium volume units designed for up to 150 psig, which can contain dust explosions.
Explosion suppression is a technique which has been in use for many years and is highly regarded. The shock wave is detected by pressure transducers, located strate- gically in and on the plant item where an explosion is anticipated. The pressure wave is subsequently attacked by a very fast injection of a high concentration of Halon or other suppressant. The vaporization of this Halon breaks down the flame front and leaves behind an inert atmosphere, which prevents a secondary fire being started by any residual glowing particles within the vessel. Plant vibrations and pressure .fluctuations may disturb this sort of system. It is also quite expensive, especially when spurious trips occur and the container has to be refilled. A good discussion of explosion suppression systems is presented in NFPA6gY.
Another protection system has recently been developed which can detect glowing particles at temp- eratures down to 400°C (radiation detector operating in the wavelength 1.5-3.0 cm). The detector activates a fire water injection system which extinguishes the glow- ing particles lo.
Several other good discussions of fire and explosion protection of pneumatic conveying systems and their components have been published3V”-‘3.
Mechanical conveyors
Screw conveyors Screw conveyors have minimal free volumes so that dust suspensions cannot form and thus dust explosions are not usually a problem. However, screw conveyors may occasionally act as a source of ignition by generating friction, which can lead to subsequent overheating and ignition of the conveyed material. This hazard can be minimized by installing an overload trip on the motor driving the screw.
Belt conveyors A belt conveyor system presents two principal fire hazards: first, those of the belt itself, and second, those of the material being conveyed. Belts made of rubber or synthetic products are combustible. Their combus- tibility and the extent to which the heat is released from the belt can cause additional damage to housing struc- tures covering the conveyors.
Some belts are less easily ignited than others, but they are still essentially combustible. Those made of poly- vinyl chloride are one example. Those meeting the fire retardancy standards of the US Bureau of Mines or the British and Canadian equivalents are specific examples. Tests in the Factory Mutual material testing calorimeter indicate extremely high heat release rates once ignited. Several of the more promising types of ‘fire retardant’
72 J. Loss Prev. Process Ind., 7988, Vol I, April
Safety considerations in conveying bulk solids and powders: S. Grosset
belting, meeting the specifications of the US Bureau of Mines, etc., were ignited in tests conducted at FM Research Corporation and, once ignited, burned to completion with high heat release rate. However, use of these will reduce fire frequency because they are more difficult to ignite.
Belt conveyors can also overheat either because of a jammed idler roller or, if the belt jams, as a result of drive rollers continuing to run. The belt can also generate static electricity, and should therefore be of anti-static material.
Belt conveyors used for dusty systems should be enclosed. The free volume within the enclosure is likely to be much greater than with a drag link conveyor and the explosion hazard is thereby increased. The drag link type should be used in preference. If a belt conveyor is chosen, then full protection against explosion should be provided in accordance with NFPA 68.
Factory Mutual recommends a number of protective measures for belt conveyors’4: Automatic sprinklers or water spray protection should be installed for all com- bustible conveyor belts. The belt drive should be inter- locked to shut down on sprinkler water flow.
0 The system should be hydraulically designed for the operation of ten automatic sprinklers and two small hose streams (sprinkler spacing 100 ft’ per head). The system should be designed for a flow pressure of 10 psi on the end sprinkler. (This will result in about 200 gpm for sprinklers and the same for small hose.) In a conveyor enclosure less than 15 ft wide, this can usually be accomplished by installing a single con- tinuous supply line with an unlimited number of sprinklers.
0 Water supplies should be adequate for one hour of use.
l High initial water pressures should be used to over- come friction and elevation loss. A constant dis- charge pressure system would be one method of providing such initial high pressure.
A small hose or equivalent should be provided at suitable intervals with sufficient FM-approved li in hose at all locations to reach any part of the conveyor system. Small hose connections can be made to the sprinkler piping. A suitable alternative to this is the provision of hydrants and easy access to the conveyor by fire fighting equipment. All weeds, brush and trees should be cleared from underneath and at least 25 ft from both sides of conveyor supports. Unprotected combustible buildings and similar exposures should be removed. Each con- veyor belt system should be provided with tamperproof devices arranged to automatically shut off driving power in the event of greater than 20% belt slowdown or misalignment of belts. In addition, interlocking devices should be arranged to shut off power on all contributing conveyors. Where conveyor belts are critical to plant operations, spare belts in quantity consistent with expected fire damage (with recom- mended protection in service) should be kept on hand.
Some damage will occur even with recommended pro- tection in service.
Bucket elevators Bucket elevators are often subject to explosions and fires, and numerous examples have occurred in the past, particularly with vegetable dusts, for which these elevators have customarily been used3. The design of these elevators lead to dust clouds being continuously present during working, particularly at the head and the boot of the elevator. The buckets are also regularly subject to impact and the belt supporting the buckets can slip on the pulleys and generate frictional heat. As a result, a source of ignition and a dust suspension can be present simultaneously, causing explosion or fire. Modern high capacity elevators, with separate delivery and return legs, have a reduced risk because of the reduced volume per unit weight of dust conveyed. In the general case, with explosible dusts, other types of elevator are preferable, and are particularly necessary for dusts of more severe explosibility, e.g., those giving maximum rates of pressure rise in excess of 5000 lb in-’ s (35000 kN m-‘s) in small scale test appa- ratus. Use of elevators should also be avoided for dusts known to be readily ignited by friction, e.g., sulphur.
Where the use of bucket elevators is unavoidable, their positioning should be carefully considered and regular maintenance is essential. The elevators should be mounted outside the building, e.g., supported by an outside wall, and the intake and delivery points should preferably be isolated from the rest of the dust handling plant by means of chokes. The elevator casing should be a fire resistant construction, sufficient to retain a fire, dust-tight and of sufficient strength not to rupture in the event of an explosion. To meet the strength require- ments, the casing should be provided either with auto- matic suppression or with explosion relief at the head and the boot, with vent areas calculated per NFPA 68. Long elevators, say more than 6 m (20 ft), may require additional relief at intervals along the casing to ensure that no point is too remote from a vent.
Particular care should be taken to ensure that flame burning dust, etc., discharged from the vents during an explosion cannot injure operators or damage nearby plant. Provision of ducting or deflectors over the vents may be required. Where it is unavoidable to site a bucket elevator inside a building it is desirable for the internal pressure to be slightly below atmospheric to minimize leakage of dust. Discharge of combustion products from vents to the outside of the building is essential, and the design requirements of ducting from vents should be in accordance with NFPA 68. The need for dust tightness and adequate casing strength and fire resistance should be rigorously met.
Steps should also be taken when designing bucket elevators to minimize the generation of ignition sources. These steps may include the provision of strong fixing for the buckets and strong bearings for all shafts, external to the casing, provided with detectors for
J. Loss Prev. Process lnd., 1988, Vol I, April 73
Safety considerations in conveying bulk solids and powders: S. Grossel
overheating. The main drive to the elevator should be external to the casing. Belt slip within the casing can be detected by belt speed meters, and anit-runback devices can also be installed. Development of friction within the casing can thereby be reduced.
Exhaust ventilation can be applied to the casing of bucket elevators which assists the removal of suspension of dust in air. These suspensions contain the dust fractions of smaller particle size, and ventilation would also give a slight negative pressure relative to atmos- phere, within the casing. The ventilation system should be conducted to a dust collection unit in a safe area, and should be provided with explosion protection on a similar basis to that in the elevator casing itself. Care should be taken to ensure that the air flow is sufficient to prevent deposition of dust in the ducting of the collec- tion unit, and also sufficient to permit the collector to function efficiently. Dust collection systems are con- sidered in more detail in the literature3*“.
Automatic explosion suppression can be used to protect bucket elevators when explosion relief is not practicable, often because of the situation of the el- evator being located within a building. The design of the suppression system should be done by a qualified vendor.
Gillis and Fishlock conducted experimental investiga- tions on bucket elevator explosions to determine ways to limit their effects, or to extinguish the explosion’5. The purpose of these tests was twofold: first, to obtain data and information on how bucket elevators may be vented to eliminate physical damage and to minimize explosion propagation; and second, to demonstrate the effect- iveness of an explosion suppression system in detecting and extinguishing a dust explosion within the bucket elevator.
Halon suppression systems were used successfully to isolate flames from propagating in the head and boot section during explosion vent tests. A specially designed flame retarding distributor, supplied by Union Iron Works, was tested and found effective in limiting flame propagation to a single path when all but one outlet was sealed. Various types of optical detectors were tested as means of detecting the initiation of an explosion and compared with a pressure detector. Results from this
research should shed new knowledge on how to vent duct-like volumes containing obstructions and shows that supression systems can be effectively used to stop explosions.
En Masse Conveyors
The explosion hazard situation in en masse conveyors is similar to that in bucket elevators but more complex especially when operated vertically. In the elevating leg the channel will be full of material, probably above the upper explosive limit, but during start-up, or when the inlet is starved, an explosible concentration could occur. It is impracticable to vent the elevating leg so this should be strengthened and vented at the top.
Exposion vents should be provided on the empty return leg in the same way as for bucket elevators.
References
I Cross, J. and Farrer, D. ‘Dust Explosions’, Plenum Press, New York. 1982
2 Haase, H., ‘Electrostatic Hazards: Their Evaluation and Control’, Verlag Chemie, Weinheim, West Germany and New York, 1977, English translation by M. Wald
3 Palmer, K. N., ‘Dust Explosions and Fires’, Chapman and Hall, London. UK. 1973
4 Bartknecht, W., ‘Explosions-Course. Prevention, Protection’. Springer Verlag, Berlin, Germany, and New York, 1981, English translation
5 Field, P., ‘Dust Explosion’, Handbook of Powder Technology, Volume 4, Elsevier, Amsterdam, The Netherlands, 1982
6 Nagy, J. and Verakis, H. C., ‘Development and Control of Dust Explosions’, Marcel Dekker, Inc., New York, 1983
7 Schofield, C., ‘Guide to Dust Explosion Prevention and Protec- tion, Part 1 - Venting’, IChemE. Rugby, UK, 1984
8 NFPA 68 ‘Guide for Venting of Deflagrations’, National Fire Protection Association, Quincy, MA, USA, 1988
9 NFPA 69, Standard on Explosion Prevention Systems’, National Fire Protection Association. Ouincv. MA. USA. 1986
IO Forsyth, V. G., ‘Dust Explosi& Prc%ection in Pneumatic Convey- ing Processes’, Fire Prevention, No. 135, March 1980, pp. 25-30 Bennett, N., ‘Explosion Protection for Fabric Dust Collectors’, Specifying Engineer, September 1982, pp. 81-83 soo. s. L.. J. PiDPlimY 1981. I. 57
II
12 13
14
15
NFPA 650. Pneimatic Conveying Systems for Handling Com- bustible Materials’, National Fire Protection Association, Quincy, MA, USA, 1984 Factory Mutual Loss Prevention Data Sheet 7-1 I, ‘Fire Protection for Belt Conveyors’, August 1972 Gillis, J. P. and Fishlock, F. H., J. Powder ond Bulk Solids Technology 1982, 6(2), 5