Handbook of Pneumatic Conveying Engineeringnguyen.hong.hai.free.fr/EBOOKS/SCIENCE AND...

22Health and Safety Issues

1 INTRODUCTION

Dust generated from most bulk solids poses a potential health problem, and manymaterials that have to be conveyed are potentially toxic. Pneumatic conveying isoften chosen for hazardous materials because the system provides a totally en-closed environment for their transport. It is also considered that the majority ofconveyed materials are potentially explosive, and this certainly applies to mostfood products, fuels, chemicals and metal powders. Pneumatic conveying systemsare basically quite simple and are eminently suitable for the safe transport of pow-dered and granular materials in factory, site and plant situations.

The system requirements are a source of compressed gas, usually air, a feeddevice, a conveying pipeline, and a receiver to disengage the conveyed materialand carrier gas. The system is totally enclosed, and if it is required, the system canoperate entirely without moving parts coming into contact with the conveyed ma-terial. High, low or negative pressure air can be used to convey materials. For po-tentially 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 andarrangement of equipment, materials can be conveyed from a hopper or silo in onelocation, 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 bemade into a common line, and a single line can be discharged into a number ofreceiving hoppers. With vacuum systems, materials can be picked up from openstorage or stockpiles, and they are ideal for clearing dust accumulations and spill-ages.

Pipelines can run horizontally, as well as vertically up and down, and withbends in the pipeline any combination of orientations can be accommodated in asingle pipeline run. A very wide range of materials can be handled, and they aretotally enclosed by the system and pipeline. This means that potentially hazardousmaterials can be conveyed quite safely, with the correct choice of system andcomponents. There is minimal risk of dust generation, and so these systems gener-ally meet the requirements of any local Health and Safety legislation with little orno difficulty.

1.2 System Integration

Dust, mess and spillage that are often found surrounding bulk solids handlingplant are not generally caused by pneumatic conveying systems. Feeders forpneumatic conveying systems, for example, usually fit under hoppers, and these inturn are fed from above by other systems, such as chain and flight (en-masse) con-veyors.

Dust and mess in the area often comes from poor integration of the me-chanical conveyor with the hopper, and not with the pneumatic conveyor. In termsof plant safety, therefore, due consideration must be given to the interfacing ofdifferent systems, particularly if they are operating in series [I].

Pneumatic conveying systems provide a totally enclosed environmentthroughout for the transport of materials, and along the conveying route there areno moving parts at all, unless diverter valves are employed for multiple point off-loading. Some feeding devices, such as blow tanks, Venturis and vacuum nozzleshave no moving parts, apart from valves opening and closing at the start and endof the process. Although pneumatic conveying systems are capable of releasingdust 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 beconveyed it is essential that any dust associated with the material should remainwithin the conveying system throughout the entire transportation process. If anymaterial is deemed to be toxic to any degree there should be no possibility of anydust being released into the atmosphere. There is also a wide range of materials,which, in a finely divided state, dispersed in air, wi l l propagate a flame through thesuspension if ignited.


Health and Safety 625

These materials include foodstuffs such as sugar, flour and cocoa, syntheticmaterials such as plastics, chemical and pharmaceutical products, metal powders,and fuels such as coal and sawdust. If conveyed with air there is the possibility ofa dust explosion within the system. If the dust is released from the system there isthe possibility of a dust explosion external to the conveying system.

The potential magnitude of the problem can be illustrated by the fact thatduring the 17 years from 1962 to 1979 there were 474 recorded dust explosions inthe UK alone, resulting in 25 deaths [2]. This covers the whole area of bulk solidshandling, transport and storage and the number of explosions that could be attrib-uted directly to pneumatic conveying systems is not known.

In just two years (1976 and 1977), dust explosions in grain handling plant inthe United States claimed the lives of 87 workers and caused injuries to over 150more [3]. It is believed that most of these explosions were in bucket elevators andnot pneumatic conveying systems, but these statistics highlight the potential fordust explosions, regardless of the source.

2.1 Dust Emission

Excepting the potentially explosive materials, the most undesirable dusts are thosethat are so fine that they present a health hazard by remaining suspended in the airfor long periods of time. The terminal velocity of a 1 um particle of silica, for ex-ample, is about 1 mm in 30 seconds (0-006 ft/min), whereas that of a 100 urn par-ticle is about 100 mm/s (20 ft/min).

The terminal velocity of an object depends upon its density and size, and isapproximately proportional to the square of its size. Data on the terminal velocityof 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 Fig-ure 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. Prolongedexposure to such dusts can cause permanent damage to the lung tissues, symp-tomized by shortness of breath and increased susceptibility to respiratory infection.Legislation has been introduced in many countries, therefore, that specify maxi-mum exposure times to such dusts.

Prevention of the emission of these fine particles into the atmosphere is thusof paramount importance, regardless of source. Emissions of larger particles mayalso give rise to complaints, more often in a social context, created by the deposi-tion of the particles on neighboring properties or on vehicles belonging to a com-pany'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 to100 um in diameter, but it is the particles of 0'2 to 5 um diameter that are mostdangerous for the lungs, as mentioned above.


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Paint Pigments Pollen

104

Mean Particle Size - um

Figure 22.1 Approximate size range of some familiar types of airborne particulatematerials.

Thus the existence of visible dust gives only indirect evidence of danger, asfiner invisible particles will almost certainly be present as well. The fact that nodust can be seen is no reliable indication that dangerous dust may not be present inthe air. The large visible particles in a dust cloud will quickly fall to the floor, butit will take many hours for the fine dangerous particles to reach the ground.

Airborne dusts that may be encountered in industrial situations are generallyless than about 10 urn in size and can be taken into the body by ingestion, skinabsorption or inhalation. The former is rarely a serious problem, but diseases ofthe skin are of not infrequent occurrence. Allergic reactions are known to becaused by powders containing, for example, metals such as chrome, nickel andcobalt. It is, however, inhalation that presents the greatest hazards for workers in adusty environment.

Relatively large particles of dust that have been inhaled and become depos-ited in the respiratory system will usually be carried back to the mouth by cilliaryaction and be subsequently swallowed or expectorated. At the other extreme, ultra-fine particles (less than about 0-2 urn) which become deposited are likely to passrelatively quickly, generally into solution in the extra-cellular fluids of the lungtissues. Much of this is excreted by the kidneys, either unchanged or after detoxi-cation by the liver. This is the fate of many systemic poisons, such as lead, whichgain entry to the body via the lungs [5].



Inhaled particles with the approximate size range of 0-2 to 5 um can reachthe lower regions of the lungs where they will probably be retained. Prolongedexposure to such dusts can cause various diseases, most of them potentially seri-ous, and often resulting in permanent damage to the lung tissues. The best knownare probably the diseases collectively designated 'pneumoconiosis' and character-ized by chronic fibrosis of the lungs as a result of continuous inhalation of mineraldusts such as silica, asbestos and coal.

Generally the symptoms are chronic shortage of breath and increased sus-ceptibility to respiratory infection. Other dust-related diseases include pneumonitis(an acute inflammation of the lung tissue or bronchioles) and lung cancer. Therelative dangers of some common dusts are compared in Table 22.1 in which theminerals 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 1974Health and Safety at Work Act (UK), and other relevant statutory provisions, isthe concentration of airborne dust. The measured concentration is compared withvariously defined 'threshold limit values' (TLV's) which are also functions of theduration of exposure of personnel to the dust. The most commonly used defini-tions of Threshold Limit Value are [6]:

TLV-TWA Time-weighted average concentration for a normal eight hourworking day or forty hour week, to which most workers canbe repeatedly exposed day after day, without adverse effect.

TVL-STEL Short term exposure limit. This is the maximum concentra-tion to which workers can be exposed for a period of up to15 minutes, provided that no more than four excursions tothis value occur each day.

TVL-C Threshold limit ceiling. This is the concentration that shouldnot be exceeded, even instantaneously.

For further information on actual threshold limit values Reference 7 shouldbe consulted. Different countries, of course, will have their own regulations andguidelines on these issues. This data is included to highlight the fact that such leg-islation is in place in most countries.

2.1.3 Dust Suppression

Where a test for dustiness, or previous experience with a material, indicates thatthe generation of dust is likely to present a problem, serious consideration shouldbe given to methods of reducing the material's dustability. It may be appropriate tore-examine the manufacturing process to see if the proportion of fines could belessened. Agglomeration of the particles, for example by pelletizing, should have asignificant effect


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Table 22.1 Relative Dangers of Some Common Dusts

GROUP 1

Very Dangerous

Expert advice should always be sought.

BerylliumParticularly as the oxide.

Silica (SiO2) which has been heated.In these circumstances silica undergoesmodification into biologically activeforms.

Calcined kieselguhr (diatomaceous earth)is also dangerous on this account.

Crocidolite (blue asbestos).Evidence associates this variety of as-bestos with the development of malig-nant tumors of pleura and peritoneum.

GROUP 2

Dangerous

A visible haze of any of these dusts isintolerable and no possible source ofsuch should be ignored whether ornot there is a visible cloud.

Asbestos, other than crocidoliteThe two important varieties in commerceare amosite (brown asbestos) and chry-sotile (white asbestos).

Silicaeg as quartz, ganister, gritstone, etc.

Mixed DustsContaining 20% or more of free silica,such as pottery dust, granite dust andsteel foundry dust.

Fireclay DustWith a total silicate (as silica) content inexcess of 60%.

GROUP 3

Moderate Risk

Emission of any of these dusts to form adense local cloud should cause concern.

Mixed DustsContaining some free silica but arbitrarilyless than 20%. In this group are includedthe dusts of Fe and non-ferrous foundries.

Coal Dust GraphiteSynthetic Silicas TalcKaolin (china clay, fullers earth)Non-crystalline silica

including unheated kieselguhrCarbides of some metalsCotton Dust

and other dusts of vegetable originAluminous Fireclay Mica

GROUP 4

Minimal Risk

Visible concentrations of these dusts,although inexcusable on general grounds,probably represent more danger to welfarethan to health.

Alumina ('aloxite', corundum)Glass (including glass fiber)Mineral Wool and Slag WoolPearlite and dusts from other basic rocks.Silicates, other than those already

mentioned.Tin Ore and OxidesZirconium Silicate and OxideMagnesium OxideCarborundumCementEmeryFerrosilicon

Zinc OxideBariteLimestoneIron Oxide



If dust is generated during transport, it may be possible to change the trans-port routing or conveying parameters. Total enclosure of the processing and han-dling plant is probably the most desirable approach but, in addition to the highcost, there are obvious problems over accessibility.

A generally more satisfactory arrangement is to use some kind of partial en-closure or hood in conjunction with an exhaust system which wil l draw off thedusty 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 enclosuremust then be filtered, or otherwise cleaned, before it can be released into the at-mosphere.

2.2 Explosion Risks

Apart from choking lungs, irritating eyes and blocking pores, some seeminglyinnocuous dusts can ignite to cause fires. Many materials, in a dust cloud, can ig-nite and cause an explosion which could be capable of demolishing a factory. Acorn starch powder explosion at General Foods, Banbury in the UK did just this in1981 and nine men were burned. The company pneumatically conveyed cornstarch, used in custard powder production, from a transfer hopper to feed bins, viaa diverter valve.

An accumulation of corn starch on the operating cylinder caused a malfunc-tion of this diverter valve. When one hopper was full, the flow should have beendiverted to the next hopper. An already full hopper, therefore, was over-filledcausing powder to be dispersed into the atmosphere. The actual explosion oc-curred outside the processing plant where the dust cloud was ignited by electricalarcing from nearby electrical switchgear, burning nine men and blowing outbrickwork and windows on all four walls [8].

When an explosive dust cloud is ignited in the open air there is a flash firebut 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 leadto a build-up of pressure. The magnitude of this pressure depends upon the volumeof 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 haz-ard to exist.

At some point in a pneumatic conveying system, or time in the conveyingcycle, whether dilute or dense phase, positive or negative pressure, the materialwi l l be dispersed as a suspension. A typical point is at discharge into a receivingvessel and a common time is during a transient operation such as start-up or shut-down. Consideration, therefore, must be given to the possibility of an explosionand its effects on the plant, should a source of ignition be present.

Because of legal and Health and Safety Executive requirements it is advis-able for specialist advice to be sought on dust explosion risks. Authoritative litera-ture on the subject is widely available and there are many tests that can be carriedout 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 explo-sion 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 sufficientlyenergetic source of ignition must be provided and secondly the concentration ofthe material in the air must be favorable. Two sources of ignition frequently met inindustrial plant are a hot surface and a spark. Consequently, the minimum ignitiontemperature and the minimum ignition energy are the ignition characteristicscommonly measured in routine testing for explosibility.

Ignition temperature, however, is not constant for a given dust cloud, for itdepends upon the size and shape of the apparatus used to measure it. Minimumignition temperatures, therefore, are determined in a standardized form of appara-tus, which enables meaningful comparisons between materials to be made. Typicalvalues of minimum ignition temperature for sugar, coffee and cocoa are 660, 770and 790°F respectively [9].

The minimum energy relates to ignition by sparks, whether produced byelectricity, friction or hot cutting. A characteristic of any form of spark is that asmall particle or a small volume of gas at high temperature is produced for a shortperiod of time.

Since it is much easier for experimental purposes to measure the energy de-livered by an electric spark than by friction or thermal processes, the routine testfor determining this characteristic uses an electric spark ignition source. Typicalvalues of minimum ignition energy for titanium, polystyrene and coal are 10, 15and 60 mJ respectively [9],

2.2.2 Explosibility Limits

For a flame to propagate through a dust cloud the concentration of the material inair must fall within a range which is defined by the lower and upper explosibilitylimits. The lower explosibility limit, or minimum explosible concentration, may bedefined as the minimum concentration of material in a cloud or suspension neces-sary for sustained flame propagation. This is a fairly well defined quantity and canbe determined reliably in small scale tests. Values are usually expressed in termsof the mass of material per unit volume of air. Typical values for wood flour andgrain dust are 40 and 55 oz/103 ff respectively [9].

As the concentration of the material is increased above the lower explosibil-ity limit the vigor of the explosion increases. When the dust concentration is in-creased beyond the stoichiometric value, the dust has a quenching effect. Eventu-ally a concentration is reached at which flame propagation no longer occurs. Thisconcentration is the upper explosion limit. This limit is not as easy to determinebecause of the difficulty of achieving a uniform dispersion of the material.

From values that have been determined it would appear that for most com-mon materials the upper limit is probably in the range of 0-1 to 0-6 Ib/ff [4]. This



is equivalent to solids loading ratios of about \Vi to 8, which covers a significantpart of the dilute phase conveying range. It is reasonable to conclude from this thatshould 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 withrespect to explosions.

With truly dense phase systems, concentrations are well above the min imumexplosibility limit and so it is highly unlikely that an explosion wil l occur in thepipeline 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 acyclone or receiver. Consideration should also be given to the start-up and shut-down transients associated with dense phase systems, for with certain modes ofoperation dilute phase situations may exist.

2.2.3 Pressure Generation

If a dust explosion occurs in industrial plant spectacular destruction can result if itis initially confined in a system that is ultimately too weak to stand the ful l force ofthe explosion. Two other characteristics of dust explosions, therefore, are alsoderived by means of tests.

One is the maximum explosion pressure generated, which would be requiredif it was desired to contain the explosion within the system. The other characteris-tic is the maximum rate of pressure rise, which would be relevant to the needs ofsuppressing an explosion within the system.

Typical explosion characteristics of some well known materials are pre-sented in Table 22.2 [9]. The data in the last two columns serves to illustrate themagnitude 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 pressurerise 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 with-standing the full explosion pressure. If this is so, the resulting pressure wavewould pass along the pipeline and be relieved at the weakest point, which is usu-ally the collection hopper or cyclone. Because of their size these are generally onlycapable of withstanding pressures of 3 to 5 psig and, if exposed to higher internalpressures, may burst or disintegrate. Consequently, the collection unit is likely tobe 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 orin an uncontrolled expansion. It is the expansion effect, or the pressure rise if theexpansion is restricted, that presents one of the main hazards in dust explosions.The expansion effects arise principally because of the heat developed in the com-bustion and, in some cases, to gases being evolved from the dust because of thehigh temperature to which it has been exposed.


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Table 22.2 Explosion Characteristics of Some Well Known Materials

Minimum Minimum Minimum Maximum MaximumMaterial ignition explosible ignition explosion rate of

temperature coneentration energy pressure pres. rise°F oz/103ft3 mJ lbf/in2 alm/s

Metal PowdersAluminumMagnesiumZinc

PlasticsNylonPolyethylenePolystyrene

AgriculturalCoffeeGrain dustSugarWheat Flour

MiscellaneousCoalWood flour

1180970

1110

930730910

770810660720

610810

4020

480

302015

85553550

5540

1540

650

201015

160303050

6020

909550

958090

50959095

85110

10001000

120

270510480

17190340250

150370

The pressure wave resulting from an uncontrolled dust explosion in a build-ing usually shakes down more dust that has settled over a period of time onto pipe-lines, roof beams and supports, ledges, etc. This makes an ideal condition for thesecondary explosion that almost always follows. It is this secondary explosion thatcan demolish a factory and kill the operatives. It is essential, therefore, than anexplosion occurring in a pneumatic conveying system is not allowed to be dis-charged into a building, and that good housekeeping procedures are adopted tominimize 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 per-centage of oxygen in the conveying gas at which an explosion will occur for agiven material. If the oxygen level in air is reduced, a point will be reached atwhich a flame cannot be supported. If a material is considered to be highly explo-sive 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 withan open system. If the oxygen content needs to be reduced by only a smallamount, a proportion of nitrogen can be added to the air to keep the oxygen levelbelow the required concentration.



3 CONVEYING SYSTEMS

A wide range of pneumatic conveying systems are available to cater for an equallywide range of conveying applications. The majority of systems are generally con-ventional, continuously operating, open systems in a fixed location. To suit thematerial being conveyed or the process, however, innovatory, batch operating andclosed systems are commonly used, as well as mobile systems. To add to the com-plexity of selection, systems can be either positive or negative pressure in opera-tion, or a combination of the two [9].

3.1 Closed Systems

For the conveying of toxic or radioactive materials, where the air coming into con-tact with the material must not be released into the atmosphere, or must be veryclosely regulated, a closed system would be essential. A sketch of a typical systemis given in Figure 22.2. A closed system may also be chosen to convey a poten-tially explosive material, typically with an inert gas. In a closed system the gas canbe re-circulated and so the operating costs, in terms of inert gas, are significantlyreduced.

A null point needs to be established in the gas only part of the system, wherethe pressure is effectively atmospheric, and provision for make up or control of theconveying gas can be established here. If this null point is positioned after theblower the conveying system can operate entirely under vacuum. If the null pointis 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 Open Systems

Where strict environmental control is not necessary an open system is generallypreferred, since the capital cost of the plant will be less, the operational complexitywil l be reduced, and a much wider range of systems will be available. Most pneu-matic conveying pipeline and channel systems can ensure totally enclosed materialconveying, and so with suitable gas-solid separation and venting, the vast majorityof materials can be handled quite safely in an open system. Many potentially com-bustible materials are conveyed in open systems by incorporating necessary safetyfeatures.

3.2.1 Positive Pressure Systems

Although positive pressure conveying systems discharging to a reception point atatmospheric pressure are probably the most common of all pneumatic conveyingsystems, the feeding of a material into a pipeline in which there is air at a highpressure does present a number of problems. A wide range of material feedingdevices are available that can be used with this type of system, from Venturis androtary valves to screws and blow tanks. With each type of feeder, however, thereis the potential of air leaking from the system, and carrying dust with it, as a resultof the adverse pressure gradient.

3.2.2 Negative Pressure (Vacuum) Systems

Negative pressure systems are commonly used for drawing materials from multi-ple sources to a single point. There is no adverse pressure difference across thefeeding device in a negative pressure system and so multiple point feeding into acommon line presents few problems. A particular advantage of negative pressuresystems, whether open or closed, in terms of potentially hazardous materials, isthat should a pipeline coupling be inadvertently left un-tightened, or a bend in thepipeline fail, air will be drawn into a system maintained under vacuum. With apositive pressure system a considerable amount of dust could be released into theatmosphere before the plant could be shut down safely.

3.3 System Components

The selection of components for a pneumatic conveying system is as important asthe selection of the type of conveying system for a given duty. Air movers, pipe-line feeding devices and gas-solid separation systems all have to be carefully con-sidered 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 bloweror compressor is incorrectly specified, in terms of either pressure or volumetricflow rate, the pipeline is likely to block, and with a toxic material this wi l l createits own hazards since the pipeline will have to be unblocked by some means. A



minimum gas velocity must be maintained throughout the pipeline system to en-sure satisfactory conveying, and it must be remembered that all gases are com-pressible with respect to both pressure and temperature when it comes to evaluat-ing flow rates from specified velocities.

3.3.1.1 Oil Free AirOil free air is generally recommended for most pneumatic conveying systems, andnot just those where the material must not be contaminated, such as food products,Pharmaceuticals and chemicals. Lubricating oil, if used in an air compressor, canbe carried over with the air and can be trapped at bends in the pipeline or obstruc-tions.

Most lubricating oils eventually break down into more carbonaceous matterwhich is prone to spontaneous combustion, particularly in an oxygen rich envi-ronment, and where frictional heating may be generated by moving particulatematter. Although conventional coalescing after-filters can be fitted, that are highlyefficient at removing aerosol oil drops, oil in the super-heated phase will passstraight through them. Super-heated oil vapor will turn back to liquid further downthe pipeline if the air cools.

Ultimately precipitation may occur, followed by oil breakdown, and eventu-ally a compressed air fire. The only safe solution, where oil injected compressorsare used, is to use chemical after-filters such as the carbon absorber type which arecapable of removing oil in both liquid droplet and super-heated phases. The solu-tion, however, is very expensive and requires continuous maintenance, and re-placement of carbon filter cells.

3.3.2 Pipeline Feeding

There have been numerous developments in pipeline feeders to meet the demandsof different material characteristics, and ever increasing pressure capabilities forlong distance and dense phase conveying. Although the majority of systemsprobably operate with positive displacement blowers at a pressure below 15 psig,discharging to atmospheric pressure, there is an increasing demand for conveyingsystems to feed materials into chemical reactors and combustion systems that op-erate at a pressure of 300 psig or more.

With positive pressure systems the main problem is feeding the material intoa pipeline that contains air at pressure. Because of the adverse pressure it is almostimpossible to prevent air from leaking across the feeding device. This air will al-most certainly carry dust with it, and so if this air or dust must be controlled thensome means of containment must be incorporated into the conveying system.

3.3.2.1 Rotary ValvesThe rotary valve is probably the most commonly used device for feeding convey-ing pipelines. By virtue of the moving parts and a need to maintain clearancesbetween the rotor blades and the casing, air will leak across the feeder when thereis a pressure difference. Rotary valves are ideally suited to both positive pressureand vacuum conveying.


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The rotary valve is a positive displacement device and so feed rate can becontrolled fairly precisely by varying the speed of rotation. The situation with re-gard to screw feeders is very similar, as these are also positive displacement de-vices. Air leakage can be minimized by reducing blade tip clearances, increasingthe number of rotor blades, and providing seals on the rotor end plates, but it can-not be eliminated.

Air at pressure will always return with the empty pockets, apart from leak-ing past blade tip clearances. The air leaking across a rotary valve will often re-strict the flow of material into a rotary valve from the supply hopper above. Tominimize this influence it is usual to vent a rotary valve in some way. A commondevice is to provide a vent on the return side of the valve. Since the vented air willcontain some fine material, this is either directed back to the supply hopper, ductedto 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 regu-larly cleaned, otherwise it will rapidly block and cease to be effective. If the air isvented into the supply hopper above, or to a separate filter, the pipe connecting thevent to the filter unit must be designed and sized as if it were a miniature pneu-matic conveying system, in order to prevent it from getting blocked. With lowpressure conveying systems a venturi can be used to feed the dusty gas from thevent directly back into the pipeline.

If the material to be conveyed is potentially explosive, the use of rotaryvalves will have to be questioned. With metal blades and a metal housing, ashower of sparks would result if the two were to meet, and a single spark wouldprovide an adequate source of ignition for many materials. With positive pressureconveying systems rotary valve blade tip clearances need to be very small and sodifferential expansion, resulting from the handling of hot material, or bearingwear, could cause the two to meet. Bearing failure on a rotary valve could wellresult in a surface at a sufficiently high temperature to provide a necessary ignitionsource, both within and external to the conveying system. In a fault situation dustcan leak from a pressurized conveying system and so bearings external to the sys-tem are vulnerable.

3.3.2.2 Blow TanksThe use of blow tanks has increased considerably in recent years and there havebeen many developments with regard to type and configuration. A particular ad-vantage 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 thefeeder are largely eliminated.

Although continuous air leakage does not occur with blow tanks, as it doeswith rotary valves, consideration does have to be given to the venting of the blowtank at the end of the conveying cycle, as well as on filling. A similar situationexists with regard to gate valve feeders. Blow tanks generally form the basis ofmobile conveying systems, such as road and rail vehicles, and so special provisionmust be made for venting these during filling operations.



Blow tanks can be de-pressurized through the conveying pipeline, but this isa slow process. If the blow tank is de-pressurized separately it would be recom-mended that it should be vented to the top of the supply hopper, provided that ithas a suitable filter. When the next batch of material is loaded into the blow tank,the air in the blow tank wil l need to be vented. If it is not vented the displaced airwill impede the flow of material into the blow tank as it will have to flow againstthe incoming material. The vented air is likely to carry dust with it and so thisshould be directed to the top of the supply hopper where it can be filtered. Withcontinuously operating blow tank systems the lock hopper has to be de-pressurizedeach cycle and this should be similarly vented.

3.4 Conveying Operations

Consideration must be given to some conveying operations and the conveying ofcertain materials with regard to safety provisions. Mention has already been madeof start-up and shut-down transients, for example. In most dense phase conveyingsituations, the concentration of the material will be well above the value at whichan explosion would be possible.

During transient operation, however, and plant shut-down in particular, theconcentration of the material in the air cannot be guaranteed to be above the re-quired value while the system is being purged. Regardless of the conveying sys-tem and the mode of conveying, however, the material will generally be dis-charged into a receiving vessel, where there is every possibility of the materialbeing dispersed in a low concentration cloud.

Pneumatic conveying is an extremely aggressive means of conveying mate-rials, and particularly so in dilute phase conveying where high gas velocities arerequired. As a result, abrasive particles can cause severe wear of the conveyingplant and friable particles can suffer considerable degradation. The consequencesof these influences must be given every consideration.

3.4.1 Tramp Materials

High conveying air velocities also mean that tramp materials can be conveyedthrough the pipeline with the material being conveyed. It is possible for nuts, boltsand washers to find their way into the conveyed material, somewhere in the sys-tem, and these will be conveyed quite successfully through the pipeline, with thepotential of generating showers of sparks, as they will inevitably make numerouscontact 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 be-tween them. The amount of charge transfer depends upon the type of contactmade, as well as on the nature of the materials. Almost all bulk solids acquire anelectrostatic charge in conveying and handling operations. In a large number ofcases 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 elec-tric fields.

Very often these are just a nuisance, but occasionally they can attain hazard-ous levels. In all cases where dust clouds are present the build-up of an electro-static charge should be prevented. Pneumatic conveying systems are prolific gen-erators of static electricity. Frictional charging of the particles moving along thewalls 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 inthe receiving vessel, because of the difficulties of leakage through an insulatingmedium. In the case of conducting solids, electrostatic problems can still arisewhen the particles are suspended in air. In such a case the air prevents the electriccharge 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 aspark. Such a spark may have sufficient energy to provide the necessary source ofignition for the dust cloud in the vessel, and hence cause an explosion. A 'rule ofthumb' value of 25 mJ is often taken, and materials with ignition energies lessthan this may be regarded as being particularly prone to ignition by static electric-ity. In these cases special precautions should be taken.

3.4.2.1 EarthingFrom an electrostatic point of view, pneumatic conveying lines should be con-structed of metal and be securely bonded to earth. All flanged joints in the pipe-work should maintain electrical continuity across them, to reduce the chance ofarc-over within the pipe.

Particular attention should be given to areas where rubber or plastic is in-serted for anti-vibration purposes, and where sight glasses are positioned in pipe-lines. Regular routine checks of the integrity of the earthing of all metal parts ofthe system should be carried out. The use of well grounded facilities can help toreduce these potential hazards.

Although certainly safer than systems that have plastic sections, wherecharge 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 conveyedthrough them. The charge created on the pipe will flow instantly to earth, but thaton the particles may remain for long periods. The storage potential is particularlyimportant with regard to operations subsequent to conveying, for it is quite possi-ble for such a charge on a material to be transferred to operatives.

If this occurred in the presence of an appropriate concentration of the mate-rial, 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-staticclothing 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 insu-lated floor, such as is often found in modern buildings. The operatives shouldstand on an earthed metal grid or plate at the point of operation.



3.4.2.2 Humidity ControlStatic generation on a material increases as the relative humidity of the surround-ing air decreases, and since it is more difficult to generate and store charges undermore humid conditions, increasing the relative humidity of the conveying air to 60to 75% may also be used as a means of controlling the problem. The use of humid-ity for charge control is obviously not suitable for hygroscopic materials, and mustbe considered in relation to the possibility of condensation and freezing in anyapplication.

3.4.3 Particle A tlrition

Of those materials that are explosive, research has shown that it is only the fractionwith a particle size less than about 200 (im that poses the problem. If a size analy-sis of a material to be conveyed shows that there is no significant amount of mate-rial below this size, the possibility of an explosion occurring during its conveyingshould not be dismissed. Degradation caused by pneumatic conveying can resultin the generation of a considerable number of fines, particularly if the material isfriable. This point was illustrated in the previous chapter with Figure 21.2 thatshows the fractional size distribution of a material both before and after convey-ing. In terms of explosion risks the material after conveying could be a seriouscontender.

3.4.4 Erosive Wear

Many materials that require conveying are abrasive. These include some of thelarger bulk commodities such as cement, alumina, fly ash and silica sand. With aconveying air velocity of only 4000 ft/min silica sand is capable of wearing a holein a regular steel bend in a pipeline in less than two hours. Erosive wear can bereduced with wear resistant materials and special bends, but it cannot be elimi-nated. Even straight pipeline is prone to wear under some circumstances.

If an abrasive material has to be conveyed, therefore, consideration must begiven 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 convey-ing system. Bends are available that have detectors embedded into them so thatnotice 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 pos-sibility of material coming out of suspension in dilute phase conveying and depos-iting on the bottom of the pipeline. Accumulations of material such as pulverizedcoal in a pipeline could result in a fire, through spontaneous combustion, and pos-sibly an explosion. An increase in conveying air velocity will generally help toreduce the problem but this is not an ideal solution. A disturbance to the flow witha turbulence generator usually cures the problem.


640 Chapter 22

Food products, of course, will deteriorate if left in pipelines, and contamina-tion of subsequent material could result. Since it is unlikely to be known whethersuch deposition occurs or not, it is necessary to physically clean all lines periodi-cally. For food and pharmaceutical products, pipelines and all valves and compo-nents that could possibly come into contact with the material being conveyed arelikely to be made of stainless steel. A particular problem with carbon steel is that itis liable to rust, as a result of condensation in the pipeline, and so contaminate thematerial.

3.4.5.1 Pipeline PurgingIf 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 pipe-lines because they will trap the conveyed material and so provide protection to abend from abrasive particles, since the particles will impact against each otherrather than the bend wall. Material will require a much longer purging time to becompletely cleared from blind tees, however. If additional air is available for purg-ing, the process will be more effective. Air stored in a receiver will help here, par-ticularly if it is at pressure but care must be taken not to overload the filtrationplant during this operation.

In dense phase conveying, air velocities employed are very much lower thanthose required for dilute phase conveying. Pipeline purging can be a major prob-lem if additional air is not available. This point was considered in some detail inChapter 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 thepipeline to be stepped to a larger bore along its length once or twice in order toallow the air to expand and so prevent excessive velocities from occurring towardsthe end of the pipeline. This does, however, create problems if such a pipelineneeds to be purged clear, for the purging velocity will decrease at each step to alarger bore and so considerably more air would be needed for the purpose. Thispoint 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 atthe design stage so that back-up systems and preventative measures can be incor-porated at the time of installation. With a pneumatic conveying system the plantwill generally shut itself down safely on loss of power, but whether it can bestarted 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 thesystem will be to physically remove the material from where the pipeline isblocked, usually at the bottom of a vertical lift. If this is not an option then a stand-by power system must be available to take over. Alternatively an air receiver canbe built into the air supply system, and this will provide air to purge the lines suf-



ficiently clear of material so that the system can be restarted when power is rein-stated (see Figure 19.10).

If the possibility of the pipeline becoming blocked from any eventualitymust be avoided, consideration should be given to the use of an innovatory sys-tem. In 'conventional systems' the material is simply blown or sucked through thepipeline. In 'innovatory systems' the material is either conditioned as it is fed intothe pipeline, or along the length of the pipeline. There is no difference in any ofthe peripheral system components employed.

Air pulsing or trace air lines are generally employed. Parallel lines are usedeither to inject air into the pipeline, to give the material artificial air retention, or toallow the air within the pipeline to by-pass short sections of material, to give thematerial artificial permeability. Depending upon the properties of the material tobe conveyed, one or other of these innovatory systems will generally guaranteethat the pipeline can be restarted on full load.

4 EXPLOSION PROTECTION

Despite the fact that the potential for an explosion in a pneumatic conveying sys-tem is high, the demand for such systems remains high. This is partly due to thefact that the system totally encloses the material, such that dust generation externalto the system is virtually eliminated, and with a pipeline total flexibility in theconveying route is possible without material transfer or staging. There are also anumber of different means by which a pneumatic conveying system can quite eas-ily be protected.

Since the dispersion of powdered and granular materials in air is fundamen-tal to pneumatic conveying, it is evident that if a material is known or shown to beexplosive, then consideration should be given to the hazard that this presents at thedesign stage of a system, or when re-commissioning an existing system to conveya different material. Whilst it is equally obvious that the generation of sources ofignition should be minimized, unforeseen mechanical, electrical or human failuresmean that the complete elimination of ignition sources cannot be relied upon, par-ticularly where powered machinery is involved.

To avoid the potentially catastrophic effects of an explosion, therefore, reli-ance is normally placed on the adequate functioning of a means of protection forthe system. Such protection is normally based on one or more of the followingapproaches:

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 wil l depend upon a number of factors.These include the design of any associated plant or process, the running costs, the


642 Chapter 22

economics of alternative protection methods, the explosibility of the material, andthe extent to which an explosion and its consequences can be foreseen, togetherwith 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, asfar as possible, all potential sources of ignition. The minimum ignition tempera-ture is relevant to ignition by hot surfaces. Rotary valve bearings have alreadybeen mentioned in this context, as an example, and welding operations on any partof the system should be prohibited while the system is operating. The possibilityof 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 forconveying the material. Alternatively, nitrogen can be added to the air in order toreduce the percentage of oxygen present in the conveying air to a level at which aflame cannot be supported. The maximum oxygen concentration is one of themany standard tests that can be carried out with a material, as mentioned earlier.Since inert gases are rather expensive, these methods are generally used withclosed or semi-closed loop systems.

4.2 Containment

The combustion of a dust cloud will result in either a rapid build up of pressure orin an uncontrolled expansion. It is the expansion effect, or the pressure rise if theexpansion is restricted, that presents one of the main hazards in dust explosions.The expansion effects arise principally because of the heat generated in the com-bustion and, in some cases, to gases being evolved from the dust because of thehigh temperature to which it has been exposed.

If the presence of evolved gases is neglected, the situation can be modeledvery approximately with the thermodynamic relationship:

T-

(1)

If the explosion is confined, V '/ will equal V2 and so the resulting pressure,, will be given by:

T2Pi = Pi x - - - - - - - - - - - ( 2 )



Flame temperatures are typically a couple of thousand degrees and so it canbe seen that explosion pressures can reach 100 psig quite easily. The critical in-formation from Table 22.2, however, is that this pressure can be reached in milli-seconds.

When a dust explosion occurs in industrial plant spectacular destruction canresult if it is initially confined in a system that is ultimately too weak to withstandthe full force of the explosion. The literature on this subject generally includesnumerous photographs of the resulting destruction to plant in order to reinforce thefact that explosions do occur and that they can be catastrophic. Blow tanks androtary valves, however, can be obtained that wil l withstand these pressures and ina positive pressure system the compressor or blower can be protected by means ofnon-return valves in the air supply line. Most pipelines are also capable of with-standing this order of pressure.

If this is so, the resulting pressure wave would pass along the pipeline andbe relieved at the weakest point, which is usually the reception vessel. Due to theirsize these are generally only capable of withstanding very low pressures, as men-tioned 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 sys-tem. It is unlikely to be an economic proposition to design the reception vessel towithstand the explosion pressure. There are, however, alternative means of pro-tecting the receiving vessel.

4.3 Explosion Relief Venting

The usual solution to the problem in situations where the risk of an explosion isonly very slight, is to allow an explosion to take its full course, whilst employingsuitable precautions to ensure that it does so in a safe manner. As an alternative tocontainment, the reception vessel can be fitted with appropriate relief venting. Thismay take the form of bursting panels, displacement panels or hinged doors thatoperate once a predetermined pressure has been reached.

In venting explosions to atmosphere strict attention must be paid to the safedissipation of the explosion products. It is a characteristic that the volume of flamedischarged from vents can be very large, and obviously must be directed to a safeplace away from operatives and neighboring plant. If this is necessary it is nor-mally achieved by attaching a length of ducting to the vent, or by installing deflec-tor plates. The duct attached to the vent should be short, free from bends (if at allpossible) 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 isparticularly important. This is related to the maximum rate of pressure rise and themore vigorously explosive materials require larger areas of venting. The size ofvent is also dependent upon the volume of the receiving hopper or silo.

This can also be modeled very approximately from the above thermody-namic 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:


644 Chapter 22

T,V2 = V, x — . . . . . . . . . . (3)•N

The volumetric flow rate of the gases now leaving the reception vessel willbe about seven times higher than normal, and this does not take account of gasesgenerated as a result of the explosion. There is no possibility of the existing filterplant 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 aspossible, the duct will clearly have to be of a large section area. Since pressuredrop varies approximately with the square of velocity, the velocity of the gases inthe ducting will have to be very much lower than that of the incoming conveyingair. Combined with the seven fold increase in steady state flow rate, and the factthat this is a transient situation, duct sizing is a complex task and should only beassessed by an expert.

4.4 Detection and Suppression

If a system is inconveniently sited to allow for venting; a vent of the required sizecannot be fitted onto the existing hopper; or if the material is toxic, so that it can-not be freely discharged to atmosphere, the protection may be achieved by a detec-tion and suppression approach.

Although there may be only a few tens of milli-seconds between the ignitionof the material, to the build up of pressure to destructive proportions, this is suffi-cient for an automatic suppression system to operate effectively, as illustrated inFigure 22.3.

Maximum unsuppressedexplosion pressure

Suppressedexplosion / Unsuppressed

System / explosionactivationpressure

Maximum pressure that theplant can withstand

ssed explosion pressure

Time - ms

Figure 22.3 Comparison of pressure-time histories of unsuppressed and suppressedexplosions.



Commercial equipment is available that is capable of both detecting the on-set 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 offthe air supply and stop the feeding device in order to prevent the conveying of anyfurther material. A signal can also be sent to operate the automatic opening of aventing system. An automated opening has the advantage that vents are openedextremely rapidly and for very explosive materials this helps to reduce the maxi-mum 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 withinwhich the explosion is developing. The composition of the agent depends upon thematerial being conveyed, and is typically a halogenated hydrocarbon, or an inertgas or powder. The suppressant is contained in a sealed receptacle attached to theplant and is rapidly discharged into the system by means of an electrically fireddetonator or a controlled explosive charge. Thus, as soon as the existence of anexplosion is detected, the control mechanism fires the suppressant into the plantand the flame is extinguished.

Alternatively the explosion can be automatically vented. When the explo-sion is detected a vent closure is ruptured automatically, thus providing a rapidopening of the vent. The vented explosion then proceeds as for cases in which thevents are opened by the pressure of the explosion.

The automatic method has the advantage that vents are opened extremelyrapidly, and for highly explosive materials this helps to reduce the maximum ex-plosion pressure. Since it is obvious that once an explosion has been initiated, nomore material should be fed into the system, plant shut-down can also be rapidlyachieved with the detector approach.

Action Signal

Blower/Compressor

Shutdown

Vent to Atmosphere

Detector

Feeder

ActionSignals

Suppressant

gmtionSource

Figure 22.4 Basic scheme for detection and suppression.


646 Chapter 22

4.5 Secondary Explosions

With positive pressure conveying systems there is always the possibility of a fail-ure or defect in the system resulting in the discharge of a dust cloud into the at-mosphere. Abrasive materials wearing holes in pipeline bends and neglecting totighten pipeline couplings have already been mentioned. Filters can also representa weak link. A pressure surge from a blow tank, or supplementary air from an airreceiver on purging a pipeline, may result in the release of dust, or even the failureof a filter element. A flammable dust cloud can be produced quite accidentally inmany different circumstances.

There must, therefore, be no possible sources of ignition external to the sys-tem. One of the major sources of ignition in this situation comes from electricalequipment. If the material being conveyed is potentially explosive, therefore, it isessential that all the lighting, switches and switchgear, contacts and fuses, andelectrical equipment in the vicinity, or within the same building, should be of astandard or class that would not be able to provide a source of ignition, whether aspark or hot surface. This is standard practice in chemical plant where fumes andvapors are likely to be present, but tends to be overlooked with respect to the pos-sible release of dusts.

The release of a dust explosion from a conveying system into a building, orthe 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 de-velop from either of these sources of explosion within a large building, if short-lived.

The pressure wave generated, however, usually shakes down large quanti-ties of dust that has settled over a period of time onto pipe-work, roof beams andsupports, ledges, lighting, etc. This then makes an ideal condition for the secon-dary explosion that almost always follows. It is this secondary explosion that candemolish a factory and kill the operatives.

It is extremely important, therefore, that good housekeeping is maintained atall times within all areas within buildings, such that any dust release is not allowedto accumulate on any surfaces anywhere, and on lighting and electrical equipmentin particular.

4.6 Determination of Explosion Parameters

In most countries all tests concerned with assessing the explosibility or measure-ment of explosion characteristics of materials in suspension are methods agreed,typically with a National Factory Inspectorate, and are generally carried out in thesequence shown in Figure 22.5.

As a result of these established procedures, data regarding the explosioncharacteristics of many materials already exists. With a material that has not beenpreviously tested, the first step should be to determine whether it is potentiallyexplosive. The outcome of such a test will then indicate the necessity of incorpo-rating precautionary measures into the system design.



Material Sample Explosion Characteristics Relevant I lazard orMethod of Protection

Classification Tests

Group A

Explosive —>

Group BNon-Explosive

Minimum Ignition Temperature

Maximum PermissibleOxygen Concentration

Material Concentration Limits

Minimum Ignition Energy

Maximum Pressure andRate of Pressure Rise

I lot Surfaces

Use of Inert Gas

Type of System

Static Electricity

Containment andExplosion ReliefVenting

Figure 22.5 Basic scheme of explosion tests.

4.6.1 Test Apparatus

Test apparatus used to measure explosion parameters is often classified as shownin Table 22.3.

Table 22.3 Classification of Test Apparatus

Apparatus Direction ofDispersionof Material

Ignition Source Application

Vertical Tube VerticallyUpwards

HorizontalTube

Horizontal

Inflammator VerticallyDownwards

Electric Spark orElectrically HeatedWire Coil

Electrically HeatedCoil at 2370°F

Electrically HeatedWire Coil orElectric Spark

All types of dust

Carbonaceous materials,especially of smallparticle size

Carbonaceous and metaldusts, especially largeor fluffy particles


648 Chapter 22

In the vertical tube apparatus the dust is placed in a cup and dispersed up-wards over the ignition source by a controlled air blast. Observation of the flamepropagation can then be made. Modification of the electrodes allow this device tobe used for the determination of minimum ignition energy. The Hartmann bomb isa strong version of this apparatus that can also be used for the measurement ofexplosion pressure and rate of pressure rise [11].

The horizontal tube apparatus also involves the dispersion by air of a dustsample over an ignition source. Since the residence time of a dust near the coil isshort, any material that is observed to propagate a flame must be regarded as pre-senting a serious explosion hazard. The inflammator is essentially a verticallymounted glass tube. A sample of dust, held in a horizontal tube, is blown by airand is directed downwards by a deflector plate.

Although convenient for the testing of explosion characteristics, the Hart-mann bomb has been criticized on the grounds that test results do not reliably scaleup to correspond to industrial plant. This has led to the development of the so-called 20-litre sphere apparatus. This consists of a spherical stainless steel vesselfitted with a water jacket. A dust cloud is formed in the vessel as the dust entersfrom a pressurized chamber through a perforated dispersion ring. 60 millisecondsafter the dust is released into the sphere the detonator is fired and the resultingpressure rise is monitored [ 1 1 ) .

4.6.2 Material Classification

Depending on the outcome of such tests the material is simply classified with re-spect 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, itwould be a wise precaution, or even a legal requirement, to incorporate protectionmeasures into the system. The range of materials which fall into this group iswide, as indicated earlier. Sand, alumina and certain paint pigments are examplesof Group B materials. Some Group B materials, although not explosible, may nev-ertheless present a fire risk.

If a material is shown to be of the Group A type, further information on theextent of the explosion hazard may be required when considering suitable precau-tions for its safe handling. The following parameters can be determined by use ofthe 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.



Since the explosion characteristics, in terms of these parameters, of manymaterials are well documented it is not appropriate to include this informationhere. In order to illustrate the magnitude of the quantities involved, however, de-tails regarding a few well known materials are given in Table 22.2. A summary ofthe applications of the results of these various tests to practical conditions is in-cluded in Figure 22.5.

REFERENCES

1. D. Mills. Safety aspects of pneumatic conveying. Chem Eng. Vol 106, No 4, pp 84-91. April 1999.

2. P. Field. Dust explosions. Handbook of Powder Technology. Vol 4. Hlsevier. 1982.3. J. Cross and D. Fairer. Dust Explosions. Plenum Press. 1982.4. HM (UK) Factory inspectorate technical data note 14. Health: dust in industry.

IIMSO 1970.5. C. Schofield. Dust: the problems and approaches to solutions. Proc Solidex '82 Conf.

Harrogate. March/April 1982.6. Health and Safety Executive. Guidance Note EH 15/80. Threshold l imi t values.

HMSO 1980.7. Corn starch dust explosion at General Foods Ltd, Banbury, 18 November 1981.

Health and Safety Executive Report. HMSO. London. 1983.8. HM (UK) Factory Inspectorate. Dust explosions in factories. Health and Safety at

Work Booklet No 22. HMSO. London. 1976.9. D. Mills. Pneumatic conveying: cost effective design. Chemical Engineering, pp 70-

82. February 1990.10. 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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