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Hazardous area classification for explosible dusts in the rubber and plastics industries

SIM 03/2009/08

Summary

This SIM replaces SIM 03/2003/67 but there are no changes other than a new serial number.  It introduces the requirement for hazardous area classification for explosible dusts under the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). It does not contain a comprehensive description of the precautions to be applied when handling combustible dusts. These are set out in other sources and a list of references is included at paragraph 25. The SIM is limited in scope to assessing the dust explosion risk and introducing the concept of zoning (hazardous area classification), giving examples of typical classifications for various areas in which explosible dust clouds might be expected to occur in the rubber and plastics industries. It also explains how area classification should be used as the basis for selecting fixed electrical equipment, specifying limitations on the use of portable or mobile equipment and where rules over the control of ignition sources should apply. Further information is contained in OC 284/7 Dangerous Substances and Explosive Atmospheres Regulations 2002 SI 2002 No 2776.

Background

1. DSEAR came into force on 9 December 2002. The Regulations update existing safety legislation on the use and storage of flammable materials and bring to dust handling activities the procedures and terminology that have long been widely used where flammable liquids and gases are handled. The Regulations include a requirement for the duty-holder to assess where flammable dust clouds do, or could, form and so create an explosion risk.

2. If fine combustible dusts are present in the process the duty-holder must identify those parts of the plant and areas where dust clouds or layers may form either in normal operation or as a result of some unintended or infrequent event. Once the hazardous areas are identified they form the basis for setting the controls over ignition sources. Possible ignition sources include fixed and moveable electrical equipment, portable tools that create hot surfaces and sparks (e.g. grinders, power saws), equipment that has a high surface temperature in normal operation (e.g. ovens, space heaters), tools with open flames (e.g. welding and brazing equipment) and other naked flames (e.g. matches, cigarettes).

General principles of assessment and hazardous area classification

3. The risk depends on the probability of an explosible dust cloud or layer of combustible dust being present. Where an explosible dust cloud is expected to be present for long periods of time the area concerned is classified as Zone 20. Where it occurs regularly during normal activities and operations but is not constantly present the area is classified as zone 21. Where an explosible dust cloud is likely to occur only rarely but is still possible the area is classified as zone 22.

4. Where layers or clouds of combustible dust are found inside items of process plant the inside of the plant concerned will be classified as Zone 20 or 21 depending on the length of time that a dust cloud is present. For example, the inside of a dust collection cyclone may contain an explosible dust cloud all the time the plant is running. A silo, on the other hand, might contain an explosible dust cloud only while it is being filled and for a few minutes afterwards.

5. Explosible dust clouds may also be formed when combustible dust escapes from an item of plant, for example at a bag tipping point, when filter elements are replaced, from ill-fitting connections in transfer lines, from inspection hatches or where samples of materials are being taken.

6. Layers of combustible dust may form even when there is no single release big enough to form a visible dust cloud. Fine dust can spread widely around the building on air currents and settle out on the surfaces of the building, fixtures, and fittings and on items of equipment. Occasionally dust layers may be lifted into clouds, by the rapid movement of air e.g. through an open door or window or from a fan or compressed air line. In this way a layer of dust may give rise to a cloud of dust that can explode in the presence of an ignition source even though the original dust layer may have been hard to ignite.

7. Dust clouds dense enough to explode are too thick to see through and are unlikely to form in open workrooms under normal circumstances except very locally. Handling techniques and dust collection systems should be designed to prevent dust escaping in sufficient quantity to give rise to an explosible dust cloud. If dust releases are controlled effectively for occupational hygiene reasons and it has been shown that personal respiratory protective equipment is not required to prevent people breathing in dust, the extent of any zone 21 area will be minimal or non-existent. A small zone 21 may exist if extraction equipment is ineffective, around powder transfer points on open conveyor systems or where sacks or larger containers are filled. Direct observation of the process is likely to be the best way of deciding the extent of the zone.

8. To assess the extent of a zone 22 area it is necessary to determine where dust regularly collects and to assess where dust could be released for short periods during normal operation or as a result of some fault in the plant. Simple examples would include manual transfer of product to a different container, accidental overfilling of a weigh hopper, tearing a sack or fabric IBC, failure of an explosion vent panel or perished seals on any part of a plant processing combustible dust. Any event that would lead to a release of less than 10Kg of dust at any one time will normally be regarded as insignificant from the point of view of the dust explosion risk.

9. When assessing where dust regularly collects particular attention should be paid to high-level ledges, beams and light fittings. Where dust can be dislodged from high level ledges within rooms to create a large dust cloud the whole room should be classified as zone 22 unless the dust is of a type that readily absorbs water such that it cannot be easily raised into a cloud after it has settled as a layer, in which case the area need not be treated as zone 22.

10. Fine dust can travel a long way around a building on convection currents. To avoid the need for extensive zoning, internal doors can often be made close fitting and self-closing.

Equipment located outdoors

11. Not much process equipment is located in the open air in the rubber or plastics processing industries. Sometimes large silos and cyclones or other dust collectors are located outdoors. Unless the plant item is sited where it is very sheltered from the wind it is difficult to envisage a dense dust cloud of any size existing for more than a few moments externally to the plant item (the space inside the plant item will constitute a hazardous zone as explained in paragraph 4 above). Hazardous zones in the open air will therefore be minimal except for equipment that can be submerged in dust such as at a tanker discharge point in which case closer consideration will be needed.

Materials encountered in the rubber and plastics industries that can explode when in the form of a cloud of finely divided dust.

12. Many materials used in the rubber and plastics industries can give rise to a serious explosion hazard when they are in the form of a cloud of finely divided dust. A dust explosion in January 2003 at a plant in the USA making rubber goods for pharmaceutical applications killed 6 people and injured many more. One month later, again in the USA, an explosion involving phenolic resin dust at a factory making acoustic insulation components for the motor industry killed 7 people and injured 37. A dust explosion in 1997 at a plant in Taiwan manufacturing ABS wrecked a large number of silos.

13. Although considerable progress has been made in the rubber and plastics industries in using dust-reduced forms of ingredients (e.g. predispersed forms consisting of rubber additives mixed with wax or polymer to produce master batches as slabs, rods or pellets; pastilles and lozenges; pellets and granules; and dust reduced powders) there are still a number of processes and activities that can generate explosible dust clouds. A systematic assessment of the risks is required.

14. Duty-holders should start by reviewing the powders that they handle and the dusts that are generated by the process to decide if any create a risk of explosion. It is not possible to list here all the explosible powders and dusts that might be encountered in the rubber and plastics industries. The following is a guide and duty-holders will need to obtain advice from their material suppliers. The following materials are expected to give rise to a dust explosion risk if they are in the form of a cloud of finely divided particles (i.e. where a significant proportion of the particles are smaller than 200 microns in size).

MaterialExplosion Risk Expected
Natural rubber Yes
Synthetic rubbers Yes
Sulphur Yes (A special hazard)
Other rubber-curing agents and additives Depends on the product
Most basic plastics/polymers Yes
Polyethylene Yes
Polypropylene Yes
PVC Marginal
Inorganic pigments Depends on the product
Organic pigments Yes
Inorganic fillers No
Carbon black Yes, likely
Wood flour Yes
Dust from FRP/GRP products Yes
Flame-retardants No

Safe handling of combustible dusts

15. The duty-holder should eliminate or reduce the risk of flammable dust clouds forming in the first place and control any residual risks by applying the precautions set out in HSG 103 "Safe handling of combustible dusts: Precautions against explosions".

16. RUBIAC publication "Dust control in powder handling and weighing: A revised COSHH guide " focuses on dust control from the point of view of the health risks. Nevertheless it contains useful advice on preventing the generation of dust clouds by using dust-reduced forms of raw materials (predispersed forms, pastilles or lozenges, pellets and granules etc) and by the application of appropriate engineering methods. The guidance in these publications is not repeated in this SIM, which is limited in scope to risk assessment and classification of hazardous zones.

Zone classifications for typical plant items and process areas in the rubber and plastics industries.

17. Explosible dust clouds are likely to form inside many types of process equipment, particularly blenders and also filters and associated dust extraction equipment. The insides of these plant items should be classified as zone 20, or, if the dust cloud inside the equipment last for only a short part of the operating period, as zone 21.

18. Outside of the powder and dust handling systems the aim should be to minimise the release of dust. This will reduce the size of any classified zones. Local exhaust ventilation may be required at particular points. However, a totally enclosed system is always better.

19. Dust clouds dense enough to explode are likely to be very localised. They may however, form around places where powders are tipped from bags or sacks, where dust filter elements are replaced or in the immediate vicinity of grinding, buffing or similar operations where those operations are not fully enclosed e.g. buffing rubber tyres in the tyre retread industry or trimming GRP fabrications using a grinding tool.

20. In deciding the extent of any zone 22 areas the duty-holder should look at how dust could be released suddenly from the process and at the thickness of any dust layers around the premises. Typical causes of a sudden release include tearing a sack of powder during routine handling, accidentally overfilling a hopper, failure of a dust filter and failure of a flexible coupling connecting parts of the plant.

21. The following table gives typical examples of hazardous zones in the rubber and plastics industries.

Examples of typical zones

DescriptionCommentsSuggested zone
Inside dry powder blender, conveying system, cyclone, grinding plant Providing the dust is fine enough zone 20
Inside silos, hoppers, other equipment filled intermittently with dusty product Filling lasts more than 6 hours/day
Filling takes less than 6 hours/day
zone 20

zone 21
Dirty side of dust filters, heavy dust burden on inlet stream Dust clouds will be present during each regeneration cycle zone 20 if filter regenerates frequently
zone 21 if filter regenerates infrequently
Dirty side of dust filter with light dust burden e.g. downstream of a cyclone   zone 21
Clean side of dust filter Allows for the case of displaced/torn filter element zone 22
Around places where dust is regularly released Local exhaust ventilation will normally be needed
Visual inspection of operation needed
Small area of zone 21 if dense dust clouds visible, should rarely extend more than 1m from the source
Otherwise zone 22
Conveying plant running at pressures below atmospheric No leaks likely No need to classify surrounding area unless there are other sources of release
Conveying plant running above atmospheric pressure Small leaks are common, unless maintenance standards are very high Likely to create dust layers and an area of zone 22
Work rooms Visual inspection needed, look at high level Any regular deposits, zone 22 whole room
Offices, control rooms, and other rooms adjacent to classified rooms, containing no source of dust release Internal self -closing doors should reduce the spread of dust Unclassified if permanently clean, and door is kept closed
Zone 22 otherwise
Locations close to hot surfaces e.g. space heater or water boiler Need to ensure tight controls prevent dust releases to keep it a safe (i.e. unclassified) zone

Use of hazardous area classification to selecting fixed electrical equipment etc

22. Once the duty-holder has completed the hazardous area classification he/she should use it as the basis for:

The hazardous area classification should be documented, usually in the form of simple plans or drawings.

23. New electrical equipment installed within an area classified as zone 22, for example, should bear the explosion protection symbol (illustrated below) and meet the requirements of ATEX (Atmosphere Explosive) category 3D. Some non-electrical equipment that creates a risk of ignition (e.g. because it has a heated outside surface) will also be marked as ATEX category 3D. In an area classified as zone 21 category 2D equipment would be suitable.

Explosion Protection Symbol

24. ATEX equipment will also have a surface temperature rating, expressed as a T class (e.g. T2). The rated surface temperature of the equipment should be below the ignition temperature of the dusts present in the area. Information about particular dusts should be obtained from the supplier of the powder, from published sources or by having sampling and testing carried out by a recognised laboratory.

References

25. The following references contain helpful guidance. Inspectors may obtain further advice from their local SGs.

1.Dust Explosion Prevention and Protection: A Practical Guide. Institute of Chemical Engineers, Rugby ISBN 0 85295 410 7 (aimed at plant designers).

2. BS EN 50281-1-2 1999. Electrical apparatus for use in the presence of combustible dust: selection, installation and maintenance.

3. BS EN 50281-3 2002. Classification of areas where combustible dusts are, or may be, present.

4. HSG 103. Safe handling of combustible dusts. HSE (new edition expected June 2003)

5.RUBIAC publication "Dust control in powder mixing and weighing: a revised COSHH guide".

6. OC 284/7.Dangerous Substances and Explosive Atmospheres Regulations 2002 SI 2002 No 2776.

Cancellation of instructions

26 SIM 03/2003/67 - cancel and destroy.

Updated 2009-06-18