Dr. Andy Fowler, technical and operations director, Sigma-HSE Ltd

Many common powders and dusts (i.e. flour and sugar) are not specifically classified as dangerous substances (there is no hazard pictogram indicating their flammability characteristics on the containers), but they have been directly involved in a large number of significant fires and explosions worldwide.

Any solid materials that are not fully oxidized are potentially capable of bursting into flames or exploding, and the propensity of the hazard is primarily associated with the particle size distribution or fineness of the powder. Dusts can be dispersed in air readily creating a flammable atmosphere. Such flammable atmospheres are easily ignited either by sparks (electrostatic or mechanical) or hot surfaces.

When powders are stored in closed packages such as raw material or finished goods warehouses, static storages within silos or hoppers, dispersion in air is not present. When the powder is moved during filling or emptying operations of silos and hoppers, or if leakages occur, dust dispersion in air can occur readily, creating the flammable atmospheres.

Is Your Dust or Powder Combustible?

First, you need to know whether the powders you store, and use, are in fact capable of bursting into flames or exploding. This cannot be determined from calculations, but must be determined by the physical testing of substances against recognized testing standards. Regardless of how experienced you are with combustible dust safety or how long you have had a dust safety policy in place, strong testing data is essential for developing strategies to prevent or mitigate the risk of fire and explosive occurrences.

It is recommended that you obtain flammability and explosivity data on your powders for your basis of safety for each unit operation on site. Ideally, this should be completed before system designs are finalized, or to ensure that existing protection systems are suitable. Fire and explosive characteristics for your potentially combustible dusts must be tested in accordance with relevant standards using correct laboratory equipment.

Are Your Storage Areas Suitably Classified?

Special safety measures can be put in place via the designation of an explosive atmosphere as a hazardous zone.

When necessary, process equipment should be designed, operated, and located in such a way that even when it is malfunctioning, the extent of any hazardous areas or zones are minimized.

Hazardous zones may occur when moving powders in and out of storage, accidental powder droppage, or storage splitting. These examples may create a flammable atmosphere and should be zoned as Zone 22.

Zone 22 pinpoints the area where explosive atmospheres are unlikely to occur normally, but if they do, they will only last for a short time. As a result, suitable ATEX-certified equipment is needed. Although a silo or other storage unit may have an ATEX rating, it must be remembered that the controls, instrumentation, and other ancillary systems and components must also be ATEX certified.

Do You Fully Understand the Standards and Guidance?

Typical standards and guidance for dust and powder storages are:

BS EN 60079-10-2

Although they cannot cover all situations, correct interpretations of these standard can be helpful. Examples of standards in relation to dust and powder storage are:

Silos

• Zone 20 within a silo

• Zone 21 for pneumatic transfers, rotary valves, and screws

• Zone 22 within clean side of silo filter, fan, and exhaust

Bulk Bags

• Zone 21 within a bulk bag and beneath the grill of a tipping station

• Zone 22 for 3m around the bulk bag

• Zone 22 for 1m around tipping station if LEV, if not then Zone 21

• All dust zones down to impervious floor level because dust clouds will fall to the ground

If it is not possible to achieve the end goal of the standards, it does not mean that the area should be redesigned, as it may be possible to “interpret” what the requirements of that distance are. This interpretation may aid in achieving the overall aim by another “equally acceptable” means.

Dr. Andy Fowler is technical and operations director, Sigma-HSE Ltd (Hampshire, UK) For more information, call +44 (0)1962 835998 or visit www.sigma-hse.com.