This article discusses the hazards of handling powders and how dust explosions can arise in the bakery industry.

Who would have thought bakeries can be dangerous? Ingredients like sugar and flour are important to the baker, but they can also be dangerous. They are essential for breads, pastries, cakes and other confectionery.

We use them, process them and eat them. But flour, sugar and similar powdered ingredients have been responsible for dozens of dust explosions over the past three decades in the EU.

Explosion hazards in bakeries

Most organic powders in the food industry are combustible. If a powder is finely ground it will ignite more readily. If any combustible substance is mixed or suspended in air at the correct concentrations and comes into contact with an ignition source, then an explosion can result.

Explosions can occur (and may propagate) within a range of concentrations between values known as the lower and upper explosion limits. The figure below illustrates the five constituent elements for an explosion to occur.

Figure 1: Requirements for explosions

Static electricity is probably the most common ignition source when handling bulk powders. Common processes generating explosible dust mixtures in bakeries include: 

• Pneumatic conveying of flour
• Powder sifting
• Ingredient weighing and sieving
• Dry mixing
• Drying in ovens
• Collection of dust in bag filters 

Characterisation of powders

Particle size greatly influences the explosion properties of powders; the lower the particle size, the more likely a powder will exhibit explosion properties.  Generally powders finer than 500 µm can display explosion properties.

The Minimum Ignition Energy (MIE) of a dust/ air mixture defines the lowest energy capable of igniting a sample under test conditions. The lower the MIE, the higher is the risk of explosion as a very small energy input can trigger a dust cloud explosion. 

MIE values < 10 mJ indicate the powder is sensitive to ignition but low values like this are more commonly found in the pharmaceutical sector. In the food sector the MIE of powders typically range from 25 to 1000 mJ. The Lower Explosion Level (LEL) is the minimum concentration of the powder in air, which can represent an explosive atmosphere.

The explosion severity test generates the Kst and Pmax values which are the explosive properties measured in the laboratory to quantify the severity of a dust explosion.

The KSt value determines the normalized rate of pressure rise of a combustible dust and the Pmax value measures the maximum explosion overpressure generated in the test chamber. More simply put, these data values inform us how much pressure an explosion will generate and how fast the explosion will travel.

The Minimum Ignition Temperature (MIT) is the lowest temperature at which a dust cloud will ignite when in transient contact with a hot surface in a closed temperature field. 

Figure 2: Wheat flour 

Explosion protection strategies 

Best practice for explosion protection is to adopt the following measures in order of priority:

1. Prevention:  As a first step, one should endeavour to prevent explosive atmospheres. Measures include designing the process to fully contain powders, using sealed equipment and maintaining a clean plant. In bakeries the conveying of flour and ingredient powders in hygienic equipment is important. Flexible connections between equipment can be vulnerable points. These must be robust and of leak-proof design.
2. The pneumatic conveying of powders can result in overpressures being generated in hoppers and silos. It is important therefore that vent lines, vent socks and aspiration filters are suitably incorporated into the design as necessary.
3. Avoidance of ignition sources: The designer should try to avoid installing equipment in hazardous areas. If this is not possible, then one must ensure that suitably rated ATEX certified equipment is installed in such areas. Hazardous areas need to be zoned in accordance with the ATEX Directive 1999/92/EC. 

For combustible dusts the following zones apply:

• Zone 20 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently. 
• Zone 21 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally.  
• Zone 22 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only. 

Only ATEX rated equipment suitable for the relevant zones (e.g. Equipment Protection Level 1D, 2D or 3D) should be installed. This applies to electrical equipment and to non-electrical equipment.

Mitigation: finally, if after implementing the foregoing measures, there is still potential for an explosion, then explosion mitigation measures may be necessary. 

Mitigation measures may include explosion venting, explosion suppression and explosion resistant design. In bakeries, most bulk flour silos are equipped with explosion vents. Explosion vents can also be installed on flour feed hoppers at production lines and in bag filters. 

Figure 3: Finished bread rolls in a bakery  

Specific technical issues

A number of technical issues frequently arise in bakeries surrounding explosive powders. These include:

1. Leakages at sifters. Large in-line vibratory sifters are commonly used to screen bulk flour. The continuous motion and vibration can give rise to wear and tear and result in leakages. To address this, frequent preventive maintenance is required to inspect and replace worn flexible connectors, seals and joints;
2. Cleaning. While good housekeeping is critical for any food facility, the method of cleaning is equally important. Compressed air should never be used as it simply raises the very fine dust which settles elsewhere. In a bakery, dust can settle in remote areas such as on ledges at high level including over the hot ovens. This can represent a serious fire risk and a secondary explosion hazard. Raised dust can also enter the production hall ventilation ducting and impair its performance.
3. Earthing of unloading trucks. Delivery trucks must be earthed prior to unloading bulk materials to silos. Preference should be given to interlocked systems where unloading cannot commence until earthing is electrically confirmed. Manual earthing systems can be ineffective, when operators through complacency or human error, do not implement earthing properly.
4. Operator exposure. Personnel exposed to flour dust are at risk of health effects such as asthma, respiratory sensitization and allergy to flour dust. Exposure to flour dust can be a significant hazard, particularly at sieving, weighing, bag emptying and cleaning activities. The OEL for flour dust is currently 1mg/m3. This is a factor of 60,000 less than the LEL of flour!  Risk assessments must therefore be carried out.  Protection measures such as local extract ventilation or containment booths could be considered to address exposure.
5. Flame-failure devices in ovens. Flame failure can happen in ovens. If there is no ignition, the gas supply must be automatically cut off.  Similarly, if the oven flame is extinguished during operation, this should be automatically detected and the supply of gas shut off.

Conclusions

Explosions and fires within the bakery industry are a known hazard and they can have devastating and irreversible effects. Explosion hazards should be designed out as far as possible at the design stage. 

This can be achieved by specifying dust-tight equipment, providing adequate venting for hoppers and silos, installing robust earthing systems and ensuring leak-proof connectors. Vacuum cleaning and local extract ventilation should be provided at vulnerable areas.

Author: Richard Coffey CEng MIEI is a senior consultant in the Environmental Health & Safety Department of PM Group. PM Group is an engineering and construction management company with clients from the biopharma, food, energy and healthcare sectors. The group is headquartered in Ireland and has operations internationally including the UK, Poland, Singapore, India and the US.