Appendix 1 - Guidance on the selection of vacuum cleaners for low combustibility organic granules and dusts (e.g. flour)

Introduction

1.1 This appendix has been prepared to give some guidance on the selection and use of vacuum cleaners for the removal and collection of flour. It may also be considered as outline guidance for other low combustibility solids. It does not give recommendations as to a particular make, model or manufacturer of vacuum cleaner, but merely gives guidance on the features commonly available on domestic, commercial and industrial vacuum cleaners, and the effect that these may have on selection.

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

1.3 There is a European Standard Test¹ to determine the dust cloud ignition temperature and the temperature of ignition of layers of dust. As dust and small fragments of naturally occurring plant growth materials can form explosible dust clouds, care will be required to use a vacuum cleaner which does not present a risk of ignition of the substance when it is picked up.

1.4 In this guidance, it is assumed that the vacuum cleaner is to be used solely in an area that is classified as non-hazardous². If the area in which the vacuum cleaner is to be used is defined as a hazardous area (i.e. Zone 20, Zone 21 or Zone 22), then an ATEX certified vacuum cleaner will be required. In a non-hazardous area, the vacuum cleaner will have to be constructed so as to minimise the risk of ignition of the flour when it is vacuumed up.

1.5 In this note, there are descriptions of three main types of vacuum cleaner - domestic, commercial and industrial. These are characterised as follows:

Domestic: Typically used for cleaning carpets etc in the home. Some are fitted with wheels and an integral carpet beater belt driven from the motor (upright models), whilst others are simply fitted with suction hoses and shaped attachments (cylinder models). The motor drives a small fan (1-2kW) at high speed, and the filtration system consists typically of a single disposable paper bag and secondary fine polymer mesh filter in case the paper bag leaks or tears. Alternative design vacuum cleaners are fitted with a cyclone to deposit the majority of the dust into a transparent receptacle, followed by one or more disposable or washable filters. Some are additionally fitted with a High Efficiency Particulate Air (HEPA) filter as an additional stage of filtration for harmful, irritant or allergenic substances. The motor is a 240 volt single-phase series wound motor fitted with a commutator and brushes, and can rotate at up to about 8000 r.p.m.

Commercial: Typically consists 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 38 mm in diameter, is provided with several interchangeable tools for various cleaning duties. The filter is 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. The motor is a 240 volt single-phase series wound motor fitted with a commutator and brushes and can rotate at up to 8,000 r.p.m. This type is often used in offices, laboratories and small businesses, although they are sometimes used in a domestic situation.

Industrial: Typically consists of 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 three-phase 415 volt induction motor. The motor and starter may be suitable for use in a hazardous area. The whole unit is usually quite heavy and is mounted on a wheeled trolley, and uses a large, 50mm diameter hose connected to the suction. The filtration 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

2.1 There are several potential ignition sources involved in vacuum cleaners. The principal potential ignition source in most small potable units is the electric motor, which can spark or become hot during use. This is discussed later. Other potential ignition sources are electrostatic sparks and thermite sparks.

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

2.3 Thermite sparks are produced from the reaction between rusty steel and aluminium, magnesium and their light alloys. Since the tools on many vacuum cleaners are made from aluminium for lightness, it is necessary to avoid the potential for impact onto rusty steel. Although a majority of the equipment used in the food production workplace will be stainless steel that cannot form thermite sparks, it should be remembered that the structural steelwork of many buildings is made of carbon steel which may rust. The rust may be beneath an apparently sound painted surface, but impact of an aluminium tool on such a paint film is liable to break the film (and allow impact and smearing of aluminium) onto the steel below. Therefore it is recommended that only stainless steel tools are used in conjunction with the vacuum cleaners.

Dust clouds

3.1 Dust lying on a solid surface is not in the form of a dust cloud, but when removed with a vacuum cleaner, the dust will form a well-mixed dust cloud passing up the suction hose and depositing in the collection bag or chamber. Therefore when in use, a dust cloud will almost always be present inside the equipment at some point. If the collection bag were to burst or become detached, then the sudden increase in air flow is likely to disturb the dust such that it will form an explosive dust cloud inside the vacuum cleaner.

3.2 Where small vacuum cleaners are used, with relatively small collection chambers, there is a separation device which is usually a:

cotton fabric collection bag;
synthetic fabric collection bag; or
disposable paper collection bag.

After these, there is usually a secondary filter, such as a pleated paper filter (rather like those fitted as the air cleaner for cars) which prevents any fines passing through the first collection bag to be retained before the air passes over the motor and is exhausted to atmosphere. Alternatively, a cyclone followed by one or more stages of filtration is used.

The motor and fan

4.1 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 8,000 r.p.m.), they are fitted with commutators and brushes that are prone to sparking, even when new. Thus passing a dust cloud through a sparking motor is liable to ignite the dust cloud and an internal explosion could occur. Although a small vacuum cleaner with a volume of say 20 litres would be vented by both the suction hose and the exhaust vent, any internal explosion would almost certainly cause some injury and damage as the equipment is not designed for any significant internal pressure.

4.2 A secondary problem with vacuum cleaners where the air passes over the motor is that inevitably some fine dust does pass through the filters and deposits on the motor. Typically the motors on such simple vacuum cleaners are open frame motors, in that when the fan casing is opened up to reveal the motor, the windings are exposed. Consequently 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, in the case of flour, sugar or other organic-based materials, the heat of the motor will often cause the deposits to become thermally unstable and could cause them to smolder. This can be a particular problem after the motor has been switched off, as the heat is still there yet the flow of cooling air has ceased. If the hot motor is then switched on again, a fire could result within the motor housing.

4.3 Consequently, the ideal vacuum cleaner is one where the air flow does not pass through the motor housing and over the windings. However, this type of construction is usually only available in the larger industrial scale vacuum cleaners which have large three-phase induction motors and large fans operating at 2-pole speeds (2850 r.p.m.) to achieve the high volume flow and adequate suction pressure. The smaller domestic or office style vacuum cleaner usually has an integral 240 volt single phase motor and hence run at higher speed to achieve a similar suction with a smaller diameter fan.

4.4 There is no particular advantage in the use of a 110 volt motor unless then equipment is to be used in wet conditions, where a 55-0-55 volt supply can be beneficial in reducing the risk of electric shock. However, where wet conditions are encountered, care should be taken to ensure that the filters will not disintegrate if wet.

Electrostatic problems

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

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

5.3 There is a secondary problem with the hoses if they are fitted with a helical reinforcing wire to prevent the hose collapsing. If the wire is either embedded in the plastic of the hose, or is wound round the outside of the hose, then the hose can become highly charged with the charge residing on the inside surface of the hose.

The wire increases the electrical capacitance of the hose, so that 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 (or jolt) 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, a separate earthing point (say to the plant girder supports) should be established and used.

Conclusions

7.1 The ideal type of vacuum cleaner for picking up flammable dust deposits is one where the air flow does not pass over the motor. However, recognising that this type may be too large (or expensive) for a majority of premises, then a commercial type can be used. However, it is advisable that such a cleaner (where the exhaust air passes through the motor) is a type with at least two filters in series to minimise the risk of dust passing through to the motor. Other aspects such as avoiding the use of aluminium tools, earthing metallic tools and care with spiral wound reinforcing wires also need to be considered.

Recommendations

Selection of vacuum cleaner

The best choice of vacuum cleaner is one where the motor and electrical equipment is not in the path of the air drawn through the vacuum cleaner (this is usually only found on larger industrial vacuum cleaners with three-phase induction motors).
Where a small commercial vacuum cleaner is to be used, then one with at least two filters and preferably three filters in series should be used.
Do not use domestic vacuum cleaners, as these are unlikely to be robust enough to deal with the rigors of the daily cleaning regimes in commercial premises.

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 that wet materials are not picked up using the vacuum cleaner.
Ensure that 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 that the personnel who clean or replace the filters in the vacuum cleaners 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 that 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 that 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.

Footnotes

BS EN 50281-2-1:1998 Electrical apparatus for use in the presence of combustible dust - Part 2-1: Test methods - Methods of determining minimum ignition temperatures.Back to reference of footnote 1
A non-hazardous area is one which is not defined as a Zone 20, Zone 21 or Zone 22 area according to BS EN 61241 - 10:2004. Back to reference of footnote 2