Prevention of dust explosions in the food industry
1.1 Explosions and fires within the food industry from combustible materials are a known hazard and they can have devastating and irreversible effects. This information sheet concentrates solely on the fire and explosion hazards encountered in the food industries from dusty and powdered substances.
1.2 General guidance on the prevention of fires and explosions from dusts is contained in HSE publication HSG103 'Safe handling of combustible dusts – precautions against explosions'. This note should be read in conjunction with that guidance.
1.3 There are other hazards that may also need to be controlled, such as those from flammable gas (fuels for ovens), flammable liquids and vapours (spirit based flavourings and cooking/coating oils). The risks from these hazards will need to be assessed by a competent person and appropriate measures applied if you are to comply with the law. The control of these risks is beyond the scope of this document.
1.4 Legislation introduced in 2002 - the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) - rationalised workplace assessment for fire and explosion hazard identification, and methods of prevention or mitigation. DSEAR requires all workplaces where substances that could give rise to fire or explosion are handled or stored must be fully assessed and protected.
1.5 Industrial fire and explosion hazards are controlled through the application of DSEAR 2002, enforced primarily by HSE and Local Authorities.
2. Dust explosion hazards
2.1 A dust cloud of any combustible material will explode where:
- the concentration of dust in air falls within the explosion limits; and
- a source of ignition of the required energy is present.
Containment is not always required for people to be injured and property damaged.
Substances capable of explosion
2.2 Examples of explosible dusts in the food industries include materials such as: flour, custard powder, instant coffee, sugar, dried milk, potato powder and soup powder.
2.3 If a solid substance is finely ground it may ignite more readily or at a lower energy. If any combustible substance is mixed or suspended in air at the correct concentrations and contained in a vessel or building when ignition occurs, then a violent explosion can result. If it is uncontained then a fireball may occur. The typical concentration ranges that can give rise to an explosion are low (75- >1000 g/m3 of air). As a guide, at these lower concentrations it is difficult for an observer to distinguish solid shapes at distances of 60cm or less.
Explosions can occur (and may propagate) within a range of concentrations between values known as the lower and upper explosion limits.
2.4 Ignition energies vary with different substances and for similar substances with differing moisture content and particle size, but may be as low as the static discharge experienced when taking off a synthetic fibre jumper, or as high as that from a 'fixed flame' such as a gas fired boiler.
2.5 Common processes generating explosible dusts in the food industry include flour and provender milling, sugar grinding, spray drying of milk and instant coffee and conveyance/storage of whole grains and finely divided materials.
2.6 Some food industry activities may also be at risk of fire and explosion through the use of finely sprayed oils, mixing with potable flammable solvents (ie ethanol), or sterilisation techniques such as high temperature drying or spraying with hydrogen peroxide solutions.
Self-heating and contact with hot surfaces (giving rise to spontaneous ignition) can give rise to additional hazards. Many of these topics are already addressed in a number of HSE and industry trade body publications.
3. Precautions against dust explosion hazards
3.1 Application of the precautions described below will assist you to operate your process plant safely and to meet the requirements of DSEAR. They are considered suitable for whole grains such as: maize, barley, wheat, oats, rye, soya beans, sorghum (milo) and also explosible meals and flours.
Storage and handling
3.2 Locate plant in the open air or in a strong steel framed building with lightweight panel walls, so that the roof and wall cladding panels can if necessary lift and act as explosion relief. Within older brick/stone built premises, provide the maximum possible area of explosion relief so far as is reasonably practicable. Aim for a minimum of 1 m2 per 24 m3 of building volume. For a more energetic dust this 'vent ratio' may not be adequate and a greater venting area may be required. To prevent injury from flying debris relief panels need to be displaced to a safe place or adequately tied (eg by cables).
3.3 Seal joints and leakage points around powder handling systems to prevent escape and accumulation of dust in the building and onto surrounding plant items.
Maintain scrupulous cleanliness by the use of a fully earthed, centralised piped vacuum cleaning system. Avoid the use of sweeping brushes and compressed air except for non-dusty cleaning activities.
3.4 Maintain slight negative pressure on storage vessels such as bins and silos by use of extraction systems.
3.5 Provide adequate arrangements for separating powder from its transporting air (eg cyclones & bag filters), when pneumatic conveyor systems are used.
3.6 Fit silos or bins with explosion relief and vent to an unoccupied place of safety, preferably outside the building. Ensure that the venting system is designed to an acceptable standard. Vent openings should be unrestricted to allow the free venting of an explosion. Protective systems such as explosion relief panels need to be ATEX certified.
3.7 Equip dust-collecting silos with appropriately designed explosion relief and a rotary valve at the base to act as an explosion choke. If the explosion relief is located above the vortex finder it is essential that the strength of the vortex finder ('thimble') is adequate to withstand an explosion within the cyclone.
3.8 Totally enclose open bag (unenclosed sock) dust collecting filter units and fit with explosion relief.
3.9 Equip bucket elevators (unless wooden) with explosion relief at the head of the elevator and as close as practicable to the boot. Fit each leg of the elevator with explosion relief equal in area to the cross sectional area of the leg. Intermediate explosion vents may also be required. The Institution of Chemical Engineers (IChemE) guide (2002) gives further information on the spacing of intermediate explosion vents on single and twin leg elevators.
3.10 Preferably, fit bucket elevators with underspeed switches and alignment monitors.
3.11 Exclude obvious ignition sources. Use electrical equipment dust protected to IP5X or IP6X (see BS EN 61241 part 14), depending on dust levels. Surface temperatures should be controlled to a maximum of 200°C (lower for milk powder, some fish meals and other products with unsaturated (such as linseed or rapeseed) oils in them). Prohibit the use of inspection lamps with flexible cables. To check levels in bins use an explosion-proof battery powered hand lamps (note that they require the specified type of battery) secured against accidental dropping, or tripods with fixed lamps placed over inspection hatches.
3.12 Use an effective permit-to-work system to control hot work, welding etc.
3.13 Equip all explosion relief vents with index switches to close down the plant in the event of explosion relief being activated to prevent the onward transmission of burning material.
Selection and use of vacuum cleaners
3.14 Guidance on the selection and use of vacuum cleaners for low combustibility organic granules and dusts is set out in Appendix 1.
Pneumatic conveying systems
3.15 These systems are often equipped with under and over pressure sensors to close down the system. An under pressure would occur downstream of any substantial leak which developed. An over pressure would be caused if someone tried to overfill a receiving vessel.
3.16 Vessels supplied by pneumatic conveying systems must have adequate air stripping capacity and be fitted with level gauges to prevent overfilling.
3.17 To prevent discharges due to static electricity all metalwork of powder handling systems, including delivery tankers, should be electrically bonded together and earthed. The resistance to earth should not exceed 10 ohms. Ducting and conveying pipework should be electrically conductive.
3.18 Explosion isolation devices should be type tested following the procedures detailed in the European Standard prEN15089 'Explosion isolation systems' (when it becomes an adopted standard in Europe and generally available to all).
3.19 Screw conveyors can be modified to act as effective chokes by the removal of a section of the screw helix. A baffle plate is then attached to the casing to provide a plug to prevent explosion transmission. Such a modification should be examined using the standard above before being brought into use.
3.20 Only rotary valves that have been type tested and ATEX-certified should be used as explosion chokes; they will often feature rigid blades and small clearances from the valve casing. Rotary valves with rubber edged blades or with excessive gaps between vanes and casing do not act as an effective choke.
3.21 Where space does not allow fitting of a rotary valve, chemical flame suppressant barriers may be used. In such arrangements explosion detectors are located in both connected vessels. Triggering of either of these releases suppressant material (eg sodium bicarbonate) rapidly into the ducting of the pipework connecting the two vessels, helping to diminish the explosive event.
Sizing of explosion relief vents
3.22 A variety of approaches to sizing of explosion relief vents exist and these are described in the IChemE Guide 'Dust explosion prevention and protection: A practical guide' (2002). The accepted current method of sizing is by using the nomographs in that guide, but it is also considered acceptable to continue to use equipment that is protected with relief vents originally sized according to the 'vent ratio method', as that method tended to overestimate the required explosion relief.
3.23 The IChemE Guide 'Dust explosion prevention and protection: A practical guide' (2002) and the European Standard BS EN14491 'Dust Explosion Venting Protective Systems' give details of the current vent calculation techniques for isolated enclosures.
3.24 There is no established size of vessel below which explosion relief vents are not required. In each case the consequences of an explosion need to be considered. The factors to take into account are set out in Appendix 2, Figure 1.
Ducting explosion relief vents to open air
3.25 As a general rule explosion relief vents should be ducted directly to open air by means of a strong straight duct not much longer than 3m. The vent should terminate in a safe, unoccupied area.
3.26 The effects of any ductwork (eg back-pressure) need to be taken into account to ensure the protected equipment does not experience forces that may cause it to rupture.
3.27 Longer ducts involving bends may still be effective provided they are within the detailed parameters described in the IChemE guide (2002). Straight ducts should be designed to EN14491; ducts involving a bend should follow the method described in the IChemE guide (2002).
4. Solids processing
Milling and grinding
4.1 Feed stock is now very commonly treated by screening, de-stoning, pneumatic separation and magnets to remove foreign bodies in order to prevent impact sparks from milling operations. Hammer mills are often engineered to be strong enough to contain a dust explosion, but sparks or smoldering particles may spread from the mill to other more vulnerable equipment. Associated cyclone/dust collector units are not as strongly constructed, and should be equipped with explosion relief and a rotary valve at the discharge.
Bulk tank deliveries
4.2 Some fires and minor explosions have occurred involving the pneumatic blower unit on the discharging vehicle. Product may enter the blower fan causing frictional heat or blinding of the clean air intake filter or both, resulting in ignition of filter material. Non-return valves downstream of the blower may be ineffective, particularly if the vehicle driver switches off the blower and relies on residual pressure within the bulk tank to discharge the last of the product. This may cause product to enter the blower. Drivers should be warned of the danger and instructed to keep blowers going until the discharge is fully complete.
Fabric topped silos
4.3 Fabric topped silos are occasionally used for the storage of flour. The top of the silo acts as an explosion relief and should, where reasonably practicable be ducted directly to open air. If not, the silo should be treated as for fabric silos.
4.4 The main fire and explosion risk is involvement in an external fire resulting in melting and burning of the fabric and the release of large quantities of flour which could form an explosible dust cloud.
4.5 Where open-air location is not practicable, preferably site the silo in a room that has adequate fire separation from the remaining premises and is itself fitted with explosion relief. If sitting the silo in a process area is unavoidable, it should be partitioned by an enclosure of at least half hour fire resistance. Space may be needed around the silo for observation during filling, inspection, maintenance and cleaning. Preferably there should be no electrical equipment within the enclosure: if there is it should conform with the requirements of BS EN 61241 Part 14.
4.6 Aspirated air from the enclosure should preferably be vented outside the building but where this is not reasonably practicable, a fully or partially open top may be permissible.
Flat floor grain storage
4.7 Dedicated storage buildings are normally used with no processing plant installed or other work activities carried on in them. Eliminate high-level horizontal surfaces where possible, eg by use of sloping surfaces to minimise dust accumulation.
4.8 Fluorescent tubes with enclosures to IP5X at roof level would appear to be satisfactory light fittings, but this depends on the level of contamination and housekeeping.
4.9 Measurements of dust levels during grain handling have shown that explosible concentrations are not likely to be reached unless substantial deposits of fine dust are disturbed.
4.10 Grain throwers pose a risk of generating dust and should not be used.
4.11 Dust levels in the building are likely to be a health hazard. Roof or side wall extraction ventilation fans with electrical enclosure to IP6X standard may need to be installed.
4.12 Grain heaps should be aerated to prevent self-heating and may be monitored to detect any temperature rise.
5. Sources of further information
1. Safe handling of combustible dusts: Precautions against explosions HSG103, HSE Books, ISBN 0 7176 0725 9
2. The DSEAR ACOP and guidance series documents (all HSE Books publications):
L134 Design of plant, equipment and workplaces ISBN 0 7176 2199 5
L135 Storage of dangerous substances ISBN 0 7176 2200 2
L136 Control and mitigation measures ISBN 0 7176 2201 0
L137 Safe maintenance, repair and cleaning procedures ISBN 0 7176 2202 9
L138 DSEAR Approved code of practice and guidance ISBN 0 7176 2203 7
3. Dust explosion and prevention, Institution of Chemical Engineers, 2002, ISBN 0852954107
4. BS EN 14491:2006: Dust explosion prevention systems
5. BS EN 61241 Part 10: Electrical equipment for use in combustible dust atmospheres
6. BSEN 14797 Explosion venting devices
7. BSEN 14373 Explosion suppression devices and systems
10. SIM 01/2003/54 Hazardous area classification for dust handling plant in the food industry