Preventing dust explosions in the food industry

This guidance explains how to control fire and explosion hazards in the food industry from dusty and powdered substances, such as flour.

What the law says

Under the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) all workplaces where substances that could cause fire or explosion are handled or stored must be fully assessed and protected.

Other hazards 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 should be assessed by a competent person and appropriate measures applied if you are to comply with the law.

Relevant standards

You can find the guidance you need about relevant standards on the British Standards Institution website.

Dust explosion hazards

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

Examples of explosible dusts in the food industries include flour, custard powder, instant coffee, sugar, dried milk, potato powder and soup powder.

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 cause an explosion are low (a proportion of 75 to 1000 cubic grammes of air). 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.

Ignition energies vary with different substances and for similar substances with differing moisture content and particle size. They may be as low as the static discharge when taking off a synthetic fibre jumper, or as high as that from a ‘fixed flame’ such as a gas-fired boiler.

Common causes of fire and explosion

Common processes generating explosible dusts in the food industry include:

flour and provender milling
sugar grinding
spray drying of milk and instant coffee
moving or storing whole grains and finely divided materials

Some food industry activities may also cause fire and explosion through the use of:

finely sprayed oils, mixing with potable flammable solvents (ethanol)
sterilisation techniques such as high temperature drying or spraying with hydrogen peroxide solutions

Self-heating and contact with hot surfaces (causing spontaneous ignition) can create additional hazards.

Precautions against dust explosion hazards

Applying the precautions described on this page will help you operate your process plant safely and meet the requirements of DSEAR.

They are suitable for whole grains such as:

maize, barley, wheat, oats, rye, soya beans, sorghum (milo)
explosible meals and flours

Storage and handling

Locate plant in the open air or in a strong, steel-framed building with lightweight panel walls, so the roof and wall cladding panels can lift and act as explosion relief if necessary.

Building design

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 one square metre per 24 cubic metres 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 should be:

moved to a safe place, or
adequately tied (for example by cables)

Seal joints and leakage points around powder handling systems to prevent escape and accumulation of dust in the building and onto surrounding plant items.

Cleaning and dust extraction

Maintain scrupulous cleanliness by using a fully earthed, centralised piped vacuum cleaning system. Avoid sweeping brushes and compressed air except for non-dusty cleaning activities.

Maintain slight negative pressure on storage vessels such as bins and silos by using extraction systems.

Provide adequate arrangements for separating powder from its transporting air (such as cyclones and bag filters), when pneumatic conveyor systems are used.

Fit silos or bins with explosion relief and vent to an unoccupied place of safety, preferably outside the building. Ensure 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.

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.

Totally enclose open bag (unenclosed sock) dust collecting filter units and fit with explosion relief.

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) has information on the spacing of intermediate explosion vents on single and twin-leg elevators. Preferably, fit bucket elevators with underspeed switches and alignment monitors.

Ignition sources

Exclude obvious ignition sources. Use electrical equipment dust protected to IP5X or IP6X (under part 14 of European Standard BS EN 61241), 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 (with the specified type of battery) secured against accidental dropping, or tripods with fixed lamps placed over inspection hatches.

Use an effective permit-to-work system to control hot work, welding etc.

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.

Pneumatic conveying systems

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

Vessels supplied by pneumatic conveying systems must have adequate air stripping capacity and be fitted with level gauges to prevent overfilling.

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.

Chokes

Explosion isolation devices should be type tested following the procedures set out in the European Standard BS EN 15089: Explosion isolation systems.

Screw conveyors can be modified to act as effective chokes by removing 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.

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.

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 (such as sodium bicarbonate) rapidly into the ducting of the pipework connecting the 2 vessels, helping to diminish the explosive event.

Sizing of explosion relief vents

There are various approaches to sizing of explosion relief vents. The Institution of Chemical Engineers (IChemE) has guidance on dust explosion prevention and protection.

The accepted method of sizing is by using the nomographs in that IChemE guidance. It is also 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.

IChemE guidance and the European Standard BS EN14491: ‘Dust Explosion Venting Protective Systems’ give details of vent calculation techniques for isolated enclosures.

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 the HSE guidance on explosion relief for small bins and silos (in the National Archives).

There is also a relevant British Standard BS EN 14797: ‘Explosion venting devices’.

Ducting explosion relief vents to open air

As a general rule, explosion relief vents should be ducted directly to open air using a strong straight duct not much longer than 3m. The vent should terminate in a safe, unoccupied area.

Take the effects of any ductwork (such as back pressure) into account to ensure the protected equipment does not experience forces that may cause it to rupture:

longer ducts involving bends may still be effective provided they are within the detailed parameters described the Institution of Chemical Engineers guidance on dust explosion prevention and protection
straight ducts should be designed to BS EN14491

Vacuum cleaners for flammable dust

This section of the guidance does not authorise the use of unsuitably rated electrical equipment in defined hazardous areas, but may help employers who cannot justify the acquisition of ATEX-rated cleaning equipment. It explains how they should be designed, used, located and maintained.

Selecting vacuum cleaners

The ideal vacuum cleaner for picking up flammable dust deposits does not allow the air flow to pass over the motor – usually only larger industrial vacuum cleaners with 3-phase induction motors. These are too large or expensive for most premises, so commercial vacuum cleaners can be used.

However, such a cleaner (where the exhaust air passes through the motor) should have at least 2 filters in series to minimise the risk of dust passing through to the motor. You should also consider other aspects such as:

avoiding the use of aluminium tools
earthing metallic tools
care with spiral wound reinforcing wires

Do not use domestic vacuum cleaners, as they are unlikely to be robust enough to deal with daily cleaning regimes in commercial premises.

The European Standard BS EN 50281-2-1:1: ‘Electrical apparatus for use in the presence of combustible dust’ is used to determine the dust cloud ignition temperature and temperature of ignition of dust layers. As dust and small fragments of naturally occurring plant growth materials can form explosible dust clouds, take care when using a vacuum cleaner which does not present a risk of ignition of the substance when it is picked up.

This HSE guidance assumes the vacuum cleaner is only used in an area classified as non-hazardous – not defined as a Zone 20, Zone 21 or Zone 22 area according to European Standard BS EN 61241-10: ‘Electrical apparatus for use in the presence of combustible dust’.

If the area in which the vacuum cleaner is to be used is defined as a hazardous area, then an ATEX-certified vacuum cleaner will be required. In a non-hazardous area, the vacuum cleaner will have to be constructed to minimise the risk of ignition of the flour when it is vacuumed up.

Domestic vacuum cleaners

These vacuum cleaners:

are fitted with wheels and an integral carpet beater belt driven from the motor (upright models), while others are simply fitted with suction hoses and shaped attachments (cylinder models)
have a motor driving a small fan (1-2kW) at high speed
use a filtration system which is typically a single disposable paper bag and secondary fine polymer mesh filter in case the paper bag leaks or tears
are sometimes fitted with a cyclone to deposit most of the dust into a transparent receptacle, followed by one or more disposable or washable filters
can be additionally fitted with a high-efficiency particulate air (HEPA) filter as an additional stage of filtration for harmful, irritant or allergenic substances.
have a 240v single-phase series wound motor fitted with a commutator and brushes, and can rotate at up to about 8,000rpm

Commercial vacuum cleaners

Commercial vacuum cleaners are often used in offices, laboratories and small businesses, but are sometimes used in a domestic situation. They typically consist of:

a robust squat vertical cylindrical metal or plastic container (or upright drum) fitted with wheels and castors, having an integral motor driving a fan
a long suction hose, typically 38mm in diameter, with several interchangeable tools for various cleaning duties
a filter, usually a disposable paper bag or a cleanable cloth bag to contain the dust, with a secondary cartridge filter (usually pleated paper) to act as a secondary filter in case the first filter bursts
the filtered air flows through and over the motor before discharge to atmosphere
a 240v single-phase series wound motor fitted with a commutator and brushes, which can rotate at up to 8,000rpm

Industrial vacuum cleaners

Industrial vacuum cleaners typically:

have a large metal dust container fitted with a dust collection bag, and a separate adjacent fan unit, with the fan belt-driven by a separate 3-phase 415v induction motor
use a motor and starter suitable for a hazardous area
are usually quite heavy and mounted on a wheeled trolley
use a large, 50mm diameter hose connected to the suction
have filtration which is usually a cyclone with a dust-containing bag, followed by a disposable paper filter, possibly followed by a HEPA filter if toxic materials are to be cleaned up

Potential ignition sources

The main potential ignition sources in most small, portable units are:

the electric motor, which can spark or become hot during use
electrostatic sparks and thermite sparks

Electrostatic charge is generated wherever there is relative motion between non-conductors, such as flour dust and plastic hoses. Providing the charge does not accumulate to a level which is sufficient to ignite the dust, it can be tolerated.

Thermite sparks are produced from the reaction between rusty steel and aluminium, magnesium and their light alloys. As the tools on many vacuum cleaners are made from aluminium for lightness, it is important to avoid the potential for impact onto rusty steel.

Although most equipment used in food production is stainless steel that cannot form thermite sparks, the structural steelwork of many buildings is made of carbon steel which may have rust beneath an apparently sound painted surface. An aluminium tool is likely to break this sort of paint film (and allow impact and smearing of aluminium) onto the steel below. It is therefore recommended that only stainless steel tools are used in conjunction with the vacuum cleaners.

Dust clouds

When removed with a vacuum cleaner, dust lying on a solid surface will form a well-mixed cloud passing up the suction hose and depositing in the collection bag or chamber.

When in use, a dust cloud will therefore almost always be present inside the equipment at some point. If the collection bag bursts or becomes detached, the sudden increase in air flow is likely to disturb the dust and it will form an explosive cloud inside the vacuum cleaner.

Where small vacuum cleaners are used, with relatively small collection chambers, there is a separation device which is usually one of the following:

cotton fabric collection bag
synthetic fabric collection bag
disposable paper collection bag

After these, there is usually a secondary filter, such as a pleated paper one. This prevents any fines passing through the first collection bag to be retained before the air passes over the motor and is released into the atmosphere. Alternatively, a cyclone followed by one or more stages of filtration is used.

The motor and fan

The loss of filtration caused by a split collection bag in a domestic vacuum cleaner will often let dust in significant quantities pass directly through the machine and over the motor. As the motors for this smaller type of vacuum cleaner are of the ‘universal’ type, running at high speed (up to 8,000rpm), they are fitted with commutators and brushes that are prone to sparking, even when new.

Dust cloud passing through the motor

Passing a dust cloud through a sparking motor is therefore likely to ignite the dust cloud and an internal explosion could occur. A small vacuum cleaner (with a volume of around 20 litres) would be vented by both the suction hose and the exhaust vent. However, any internal explosion would almost certainly cause some injury and damage as the equipment is not designed for any significant internal pressure.

Where air passes over the motor, inevitably some fine dust passes through the filters and deposits on it. Typically, motors on such simple vacuum cleaners are open frame ones. When the fan casing is opened up, the windings are exposed, so the dust deposits sit within the motor.

On purely dry non-flammable dusts, a small deposit such as this is not particularly harmful unless it becomes heavy enough to impede the heat dissipation from the windings. However, with flour, sugar or other organic-based materials, the motor’s heat will often cause the deposits to become thermally unstable and potentially to smoulder.

This can be a particular problem after the motor has been switched off, as the heat is still there despite stopping the flow of cooling air. If the hot motor is then switched on again, a fire could result within the motor housing.

The previous section on selecting vacuum cleaners explains the safest ones to use, depending on the workplace.

There is no advantage in using a 110v motor unless equipment is used in wet conditions, where a 55-0-55v supply can be beneficial in reducing the risk of electric shock. However, where wet conditions are encountered, ensure the filters will not disintegrate if wet.

Electrostatic and other problems

Where dust passes over an insulating surface, electrostatic charges are generated. Providing these do not accumulate to a level where a discharge could ignite any dust cloud, there is little problem. As the typical equivalent energy of a discharge from a plastic is typically 4 milli joules, then a discharge from an insulating plastic tool or hose would not be a problem as the minimum ignition energy of flour is over 10 milli joules.

However, where a stainless steel tool is fitted to the end of a plastic hose, the tool becomes an isolated conductor, and can accumulate enough electrostatic charge to cause an incendive discharge capable of igniting a dust cloud of flour. To avoid this, any metallic tools should be earthed.

Reinforcing for hoses

Some hoses are fitted with a helical reinforcing wire to prevent them collapsing. If the wire is either embedded in the plastic of the hose, or is wound round the outside of it, the hose can become highly charged with the charge residing on its inside surface.

The wire increases the hose’s electrical capacitance, so it can store far more charge and hence more energy. When the stored charge eventually discharges to earth, it does so with a very high-energy spark that can result in a brief, but severe electric shock to any operator in contact with the hose, and potentially ignite any dust cloud.

This problem can be prevented by using an unreinforced hose, or if a wire-reinforced hose has to be used, using one where the wire is actually in the bore of the hose and in contact with the dust passing through. The wire should always be electrically continuous and connected to the metal tool, and a fixed earthing point.

The earthing path should not depend on the fixed electrical wiring routes, as these may become live through earthing faults on unassociated equipment. Ideally, establish and use a separate earthing point (for example to the plant girder supports).

Wet material

Most commercial vacuum cleaners use air passing through the cleaner to cool the motor. Smaller and less expensive than industs, they are more frequently used to pick up flour. Users should therefore be aware of potentially picking up wet material when the cleaner has paper bags or filters in it.

Any wet material would initially cake up on the filter, but the water would eventually weaken the paper until the filter bursts and lets the dust through. This may not happen immediately the wet material is picked up, but the bag may soften while the cleaner is idle.

The next time the cleaner is switched on, the weakened bag will split, distributing the dust throughout the machine, and through the motor.

Smouldering materials

Another potential ignition source often overlooked is the picking up of smouldering materials. While smoking materials may not be present in a bakery, if the same vacuum cleaner used to pick up split flour in the bakery is then taken into the shop to pick up dirt from the public area, a discarded cigarette may still be smouldering and could inflame when sucked up.

A similar situation would occur if woodworking machinery jammed or rubbed during cutting, due to resin content of the wood, and localised smouldering material dropped into the wood waste. Therefore take care to ensure such smouldering materials are not drawn into the cleaner.

Recommendations for safe use

The following recommendations will help you use vacuum cleaners safely and prevent dust explosions.

Tools and hoses

Do not use aluminium tools
If metallic tools are to be used, then ensure that they are earthed
Avoid the use of wire-reinforced hoses. If a wire-reinforced hose has to be used, then make sure that the wire is earthed and is on the inside surface of the bore of the hose

Materials picked up

Ensure wet materials are not picked up using the vacuum cleaner
Ensure smouldering, burning or hot materials are not picked up

Dust bags, filters and cleaning

Empty the dust collecting bag or container frequently to avoid overloading the vacuum cleaner
Change any disposable filters regularly. It is best to replace these at fixed intervals rather than rely on the discretion of the operator, as often they will not change the filter until it has burst or the vacuum cleaner fails to pick up properly
Clean any non-disposable filters regularly. It is best to clean these at fixed intervals rather than rely on the discretion of the operator, as often they will not clean the filter until it has choked, burst or the vacuum cleaner fails to pick up properly
Ensure people who clean or replace the filters are properly trained to refit the filters correctly and seal them effectively, as the filters are the only devices preventing the flour contacting the sparking internals of the motor
Make sure the vacuum cleaner is not operated unless all its filters are fitted properly
Where oily materials or materials liable to oxidise are to be picked up, ensure the vacuum cleaner is properly cleaned out afterwards to avoid thermal degradation of the material occurring inside
Periodically open the vacuum cleaner to inspect the motor for deposits of dust, and thoroughly clean the motor. Only a competent person should undertake such dismantling, cleaning and reassembly

Solids processing

The following guidance will help you process solids such as feedstock and bulk deliveries of grain safely.

Milling and grinding

Feedstock is commonly treated by screening, destoning, pneumatic separation and magnets to remove foreign bodies and prevent impact sparks from milling operations.

Hammer mills are often engineered to be strong enough to contain a dust explosion, but sparks or smouldering 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

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

Where fabric topped silos are used for storing 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.

Fabric silos

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.

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 30 minutes’ 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 European Standard BS EN 61241 Part 14.

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

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, for example using sloping surfaces to minimise dust accumulation.

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.

Measurements of dust levels during grain handling have shown that explosible concentrations are unlikely to be reached unless substantial deposits of fine dust are disturbed.

Grain throwers pose a risk of generating dust and should not be used.

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.

Grain heaps should be aerated to prevent self-heating and may be monitored to detect any temperature rise.