Exploding issues – Explosive dust control

Author : Christopher Birch, technical sales engineer, Hanningfield Process Systems

29 November 2010

The control of hazardous and explosive environments and substances is a duty of practically every organisation from manufacturing to education. The dangers associated with flammable, toxic and explosive substances such as wood, solvents or fuels are universally well understood and controls are carefully applied. The management and control of dust is perhaps less well understood despite the critical and catastrophic potential hazard.

Between the years 1980 and 2008, there have been 350 serious dust explosions in US factories killing 133 people and injuring many more.

About 2,000 dust explosions occur in Europe every year.

Dust build up is progressive and often occurs out of plain sight in ceiling voids, on top of machinery or in dust collection systems and their filters. Materials that are not normally considered flammable are often lethal as a dust, this includes most metals and other none flammable solids. In the Pharmaceutical industry the problem is compounded further by a general trend towards NANO scale high potency Active Pharmaceutical Ingredients (API’s) which are extremely toxic and because of their small size can be even more explosive.

Why is dust an explosive hazard?
Dusts have a very large surface area compared to their mass. Burning of solids or liquids can only occur at the surface, where it reacts with oxygen, dust particles are very small but have a high surface area. For example, a 1kg sphere of a material with a density of 1g/cm3 would be about 27 cm across and have a surface area of 0.23 m². However, if that sphere was broken down into spherical dust particles of 50µm in diameter (about the size of flour particles) it would have a surface area of 60 m²! This greatly increased surface area allows the material to burn much faster, and the extremely small mass of each particle allows it to catch on fire with much less energy than the bulk material as there is no heat loss to conduction within the material. When a dust and air mixture explodes, especially in a confined space such as a silo or warehouse, a significant increase in pressure is created, often more than sufficient to demolish the structure.

There are four necessary conditions for a dust explosion or deflagration:
1. Presence of a combustible dust.
2. The dust is suspended in the air at a high concentration (LEL see below).
3. There is an oxidant (typically atmospheric oxygen).
4. There is an ignition source.

Sources of ignition.
 Electrostatic discharge (common),
 Friction,
 Arcing from machinery or other equipment,
 Hot surfaces, overheated bearings,
 Fire

Mechanism of dust explosions
Dust explosions are classified as being either primary or secondary in nature. Primary dust explosions occur inside process plant or similar enclosures and are generally controlled by pressure relief through purpose-built ducting to atmosphere. Secondary dust explosions are the result of dust accumulation inside the factory being disturbed and ignited by the primary explosion, resulting in a much more dangerous uncontrolled explosion inside the workplace. Historically, fatalities from dust explosions have largely been the result of secondary dust explosions.

What is special about dust explosions?
There are a few aspects of dust explosions that make such explosions special. When a flammable dust cloud is created, the dust particles are suspended in air, either because it falls from a height or because a blast of air raises the dust from a surface. Since dust is heavier than air, this will only last as long as the force that created the cloud is present. Some finer particles may remain suspended in air while the larger particles settle. Once settled the dust cloud can be raised again to form another flammable dust cloud.

This is different from gases and vapours that, once dispersed to below the lower flammable limit, cannot form an explosive mixture again.

This capacity to be re-suspended has its most serious consequences when a small explosion whirls up the dust deposits in the plant and then ignites the cloud. These secondary dust explosions are the reason why some plants have been destroyed virtually completely.
Another special aspect of dust explosions is that dust deposits can be ignited, for example, due to self-heating, or because the dust sits on a hot surface. A smouldering or burning dust deposit can be an ignition source for any dust cloud that is generated. Alternatively, the deposit may burn, causing fire damage.

Below a certain value, the Lower Explosive Limit (LEL), there is simply insufficient dust to support the combustion at the rate required for an explosion. A figure 20% lower than the LEL is considered safe. Similarly, if the fuel/air ratio increases above the upper explosive limit there is insufficient oxidant to permit combustion to continue at the necessary rate.

Classifying hazardous areas into zones – HSE Guidance.
Once an area has been identified as hazardous it should be classified into zones based on the frequency and persistence of the potentially explosive atmosphere. This then determines the controls needed on potential sources of ignition in that area. These controls apply to fixed and mobile equipment, electrostatic discharges and other sources of ignition that maybe brought into the area, such as matches or lighters.

The international standard, BS EN 61241/3, published in 2002 explains the basic principles of area classification for dust hazards. This standard forms a suitable basis for assessing the extent and type of zone, and can be used as a guide to complying with the requirements of DSEAR. Site specific factors should always be taken into account.

Hazardous areas for explosive dust atmospheres are classified into three zones.

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

On 6 April 2008, the Corporate Manslaughter and Corporate Homicide Act 2007 came into force in the UK, which means companies could face severe fines or even imprisonment should a dust explosion occur.

Marking of equipment.
A standardised scheme is used to identify equipment suitable for a specific area. Equipment will carry the explosion protection symbol “Ex” in a hexagon, the equipment category number (1, 2 or 3), the letter G and or D depending on whether it’s intended for a gas or dust atmosphere and other essential safety information. In many cases this will include a temperature rating expressed as a “T” marking and sometimes a gas group. These indicate limitations to safe use.

Protection and mitigation from dust explosions
Much research has been carried out in Europe and elsewhere to understand how to control these dangers, but explosions still occur. Increased awareness of the problem and the requirement to perform systematic risk assessments of all operations will lead to identification of hazardous situations and many should be eliminated before an explosion can occur. Good housekeeping practices and operating procedures should be used to prevent the build up of dust.

The alternatives for making processes and plants safer depend on the industry. Some industries exclude air from dust-raising processes, known as "Inerting". Typically this uses nitrogen, carbon dioxide or argon, which are incombustible gases and thereby inhibit combustion. The same method is also used in large storage tanks where inflammable vapours can accumulate. Usage of oxygen-free gases, however, carries a risk of asphyxiation of the operator. Other traditional methods are used to accommodate primary dust explosions in process machinery. These include 10 barg explosion arrest valves and sealed, positively pressurised control panels. Increasingly dust extraction is used to protect workers from exposure. Dust filters not only collect all the fine dust from the plant, they also create dust clouds during their cleaning or maintenance, so in effect the problem is simply moved from one location to another. In the pharmaceutical industry this issue of cleaning and emptying dust collection systems is made more hazardous when a high potency API is present.

Cyclones that remove over 95% of the air entrained dust before it reaches the filter are being used as an effective way to control and capture potentially dangerous excess dust. This method of dust control also helps pharmaceutical companies reconcile lost material as is required by cGMP standards.

Unlike solvents or explosive gasses, dust does not automatically disperse in the natural environment. Unless removed from areas of build up the risk of dust explosion progressively increases.

Avoidance of dust explosions and their catastrophic effect can be achieved by careful adherence to HSE guidelines and ATEX and DSEAR requirements. Good housekeeping practices enforced by well adopted operating procedures should be used to eliminate the build-up of deposits of combustible dust. Organisations should regularly inspect areas where dust can build up unseen.

Extraction and then capture of excess dust, by cyclone, before that dust can build up in dust filtration systems is an effective control measure in line with cGMP standards.