Combustible Dusts

When did the HSE first produce best practice dust explosion guidance?

A dust that can be burned can also explode if the concentration in air is just right for the conditions to support an explosion.

Rowan House has been closely involved in the UK with writing the guidelines for dust explosions in drying applications in the Water Industry as well as being involved in the revised HSE guidance "Safe Handling of Combustible Dusts".

Rowan House can support your company by minimising your exposure to dust explosions risks through counter measures, process design, operations, advice, risk assessments and hazardous area zoning.

Rowan House Limited has extensive experience in assisting clients with the prevention of dust explosions and the choice and installation of systems for the prevention of such dust explosions. Also, sadly, Rowan House also has considerable experience of investigating dust explosions and identifying how they could and should have been prevented.

Explosion protection falls into two categories: prevention and mitigation.

For a dust explosion to take place, three separate components are required simultaneously. These are:

• A cloud of dust
• A supply of oxygen
• A source of ignition

For an explosion to occur, the cloud of dust has to have a density of at least 30 g/m3 and probably at least 60 g/m3. Such a dense dust cloud will generally appear to be opaque.

The supply of oxygen has to have a minimum oxygen concentration above the minimum required to sustain an explosion what the particular dust. This minimum oxygen concentration for the dust is called the Limiting Oxygen Concentration (LOC) and is typically in the range of 8% to 11% oxygen (where atmospheric air has an oxygen concentration around 21%). If the oxygen concentration in the air can be reduced below LOC by the addition of additional inert components to the 'air', then an explosion can be prevented. The LOC must be experimentally determined for the dust concerned.

For an explosion to occur, the source of ignition must have sufficient ignition energy and/or a surface at a hot enough temperature to ignite the dust cloud. Sources of ignition in the vicinity of dust clouds are normally controlled by the classification of hazardous areas according to the recommendations in BS EN 50281-3: 2002 Electrical apparatus for use in the presence of combustible dust. Part 3: Classification of areas where combustible dusts are or may be present. Hazardous areas due to clouds of combustible dust are classified as zones 20, 21 or 22 depending generally on whether the release of dust is always present, present periodically during normal operations or not present during normal operations but present at other times. These sources of ignition may be electrical or non-electrical.

Examples of methods of preventing dust explosions are as follows:

• Inerting where inert gases are added to the 'air' in the system to ensure that the oxygen level is always maintained below the LOC.
• Control of Sources of Ignition where all potential sources of ignition in areas where dust clouds might form are controlled such that ignition energies and surface temperatures are always inadequate to ignite the dust cloud. Typically this control is exercised by the definition and classification of appropriate hazardous areas.
• Limiting generation or release of dust clouds: If the process can be designed so that dust clouds are not generated or, if they are generated, are not released into areas where the oxygen level is above the LOC and/or sources of ignition are present, then explosions should be avoided.
• Suppression where the presence of either a spark (sensed by infra-red detectors) or a developing explosion (sensed by a pressure sensor) is sensed and extinguished by the very rapid injection of a quenching material (which might decompose to generate an inert gas) to quench the explosion before it has the opportunity to cause damage.

Methods of mitigating the effect of dust explosions a include:

• Relief Panels where one area of the equipment is made deliberately weaker than the remainder and which preferentially bursts to relieve the pressure of the explosion before it damages the remainder of the plant. Rowan House is very experienced in the use of both UK (IchemE) and German (VDI) codes for relief panel sizing. Particular areas of concern are obstruction of the relief panel area by elements within the vessel (such as the filter bags in a bag filter) and retention of the burning solid particles so that secondary fires are not started. In both cases the sizing of the relief panel has to allow for these effects.
• Containment where the plant is made strong enough to withstand the explosion without suffering serious damage.

A dust that can be burned can also explode if the concentration in air is just right for the conditions to support an explosion.

Rowan House has been closely involved in the UK with writing the guidelines for dust explosions in drying applications in the Water Industry as well as being involved in the revised HSE guidance "Safe Handling of Combustible Dusts" and HSE847/9 for the water industry and sludge drying.

Rowan House can support your company by minimising your exposure to dust explosions risks through counter measures, process design, operations, advice, risk assessments and hazardous area zoning.

The correct zoning for dusts is an important aspect of protection against dust explosions and their ignition sources: HAZARDOUS.html

For a summary of the basic points about dust explosions download this briefing note HERE in pdf format.