The Ex Files

By Ratcliff, Ian

Ian Ratcliff discusses the risk of explosion in the workplace, looking at the science behind such events and at how companies can manage their activities responsibly to avoid them.

IF ASKED WHICH INDUSTRIES INVOLVE A high risk of explosion, most people would probably say chemical or petrochemical sites – particularly after the catastrophic blast at the Buncefield fuel depot a year ago. However, the risk is not confined to such obvious candidates; industries such as food and flavours, contract logistics, ship or boat building, automotive manufacture, and even the production of Christmas wrapping paper all involve areas or processes where an explosion could occur. Personally, I would prefer to work in a flammable chemical processing area, where the risk is managed correctly, than in a small manufacturing company where there is no sign of explosion risk management.

An explosion is a violent release of energy, which, in industry, will usually result in destruction of varying degrees. The fire triangle, or ignition triangle as it is sometimes known (figure 1), illustrates the three elements that have to come together for an explosion to occur: a fuel source, an oxidiser (air), and an ignition source. An explosion cannot occur if one of these elements is taken away. Fuel, on industrial sites, can be present in many forms, including flammable liquids (vapour), gases, powders and fibres.

The stolchiometric mix

Relating the fire triangle and explosions to the workplace is, of course, rather more complicated than this basic diagram suggests. Each substance used in industry has a very different set of characteristics, which will, in turn, affect the way that it is managed. Firstly, for an explosion to occur the fuel-to-air ratio has to be right in order to support an ignition. It can’t be too ‘lean’, i.e. not enough fuel, or too ‘rich’, i.e. too much fuel. The perfect mixture is known as the stoichiometric mix, which has two levels – a lower one, at which point it is flammable, and an upper one, at which point there is too much fuel. These are known as the Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL).

The flashpoint

Starting with flammable liquids they have what is known as a flashpoint, which is the lowest temperature at which sufficient vapour is generated to support ignition. So, if a liquid has a low flashpoint – for example, the chemical intermediate/cleaning solvent acetone, at 19C – the vapour it gives off could, assuming the stoichiometric mix is right, very likely cause an explosion if an ignition source, such as a spark, gets near it, even at room temperature.

The ignition temperature

The vapour from flammable liquids (and gases) may also ignite if it comes into contact with a hot surface. This is defined by the ignition temperature, which is the lowest temperature at which the gas or vapour will ignite. Acetone has a very high ignition temperature (535C), but something like ethyl methyl ether (used in medicine) is low, at 190C, and also has a relatively low flashpoint of 37C. This means that if there is flammable gas or vapour in the atmosphere and it comes into contact with a hot surface – on a piece of equipment, say – it could ignite.

In the case of powder, dust, or fibres there are two ignition temperatures: layer ignition, where the substance has settled on a surface; or cloud ignition, where it is in the air. Even basic foodstuffs may be flammable; cocoa powder, for instance, will ignite at 200C as a layer, or at 300C as a cloud. Flour, coffee, milk, cornstarch, custard, sugar, and paper fibres all have flammable characteristics. Chemical and metal dusts are also flammable, e.g. soot, carbon black, sulphur, petroleum coke, polyvinyl acetate, aluminium, magnesium, bronze, zinc, and iron. The flammable characteristics vary by substance, according to particle size and shape, moisture content, and conductivity (in the case of metal dusts), so analysis of the dust is the first step in managing the risk.

Energy ignition

Depending on the characteristics of the flammable material it can also ignite if sufficient energy is present – for example, from electrostatic discharge generated by fabrics, or sparks released by friction in mechanical equipment, or operation of electrical equipment. The amount of energy required for ignition to occur again depends on the substance – hydrogen requires little energy whereas propane requires a lot.

Managing the risk

So, flammable materials are present in most industrial sites and workplaces but the risk they pose depends on how they are managed. As with most risks, there is a hierarchy of control measures. The first step is to replace the flammable material required for the operation with something non-flammable. Where this is not possible or practicable, the primary aim is to prevent the substance from reaching the atmosphere, where it could become a hazard. Sealing the substance in drums, IBCs, tanks, silos, pipelines, aerosols, tins, etc. is one way to do this. However, such methods do bring their own risks, such as a spillage, or crushing of an aerosol tin while moving a pallet, or a leak from a seal in a pipe, all of which can lead to the substance escaping into the air.

In operations where the flammable material reaches the atmosphere as part of the process – for example, where substances are blended in vats – the release should be kept to a minimum by such measures as putting lids on the vats, or improving the ventilation.

If the operation or process cannot be changed or made safer by any of the above methods, and the substance is, one way or another, going to be present in the atmosphere, preventing ignition is the next step. Selecting equipment for the process area that will not be an ignition source is essential to break the fire triangle. Lighting, switches, torches, etc. are all available as standard explosion-proof versions, but it is also possible to engineer solutions for all types of bespoke processing equipment. Ignition sources exist on all types of equipment and machinery, such as hot surfaces, arcing and sparking components, or overspeed on diesel engines. The risk of ignition can be eliminated by applying a combination of explosion protection techniques, such as intrinsic safety, encapsulation, or explosion-proof enclosures. (see case studies above for practical examples of how various firms have used some of these methods to manage the risk of explosion in their work sites.)

DSEAR/ATEX

Legislation-wise, the duty to manage risks to safety from explosions is imposed by the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). These Regulations put into effect the requirements of European Directive 99/92/EC – commonly known as the ATEX

Directive.1 Under reg.7 of DSEAR dutyholders must classify areas in the workplace where an explosive atmosphere may occur into hazardous or non-hazardous places. The effect area classification decisions have on a business can be tremendous, with significant capital investment in explosionproof equipment, etc. Other responsibilities under DSEAR include making arrangements in the event of an accident or emergency, training for employees, provision of clearly visible safety signage, and regular and efficient maintenance of work processes and equipment. (see panel on p47 for an overview of requirements.)

There are some work situations that fall outside of DSEAR/ATEX yet there may still be a risk of ignition. In such cases it is important to take an overall view of the site and investment and consider managing the risk with alternative methods. The airline industry, for example, applies gas detection and shutdown systems to its mobile access platforms used in low-risk situations.

Summary

It is possible to effectively manage flammable materials in the workplace so that they do not cause an explosion – not just by addressing how they are physically handled but also by managing the sources of ignition and activities in the surrounding areas so that the three elements of the fire triangle do not come together. Various practical steps can be taken to prevent an ignition and legislation and guidance is widely available to help duty-holders ‘stick to the rules’ and protect not only their investment but also their staff and their local community.

EXPLOSION LEGISLATION

To comply with the requirements of ATEX/DSEAR companies are required to:

* Prevent the formation of explosive atmospheres in the workplace OR avoid the ignition of explosive atmospheres;

* Conduct a risk assessment, including the likelihood of explosive atmospheres and a source of ignition;

* Classify the workplace into Zones, depending on the frequency and time that an explosive atmosphere is present in the form of gas, vapour, powder or dust. For example :

Zone 0 (highest risk) – an area in which an explosive gas/vapour atmosphere is present continuously, or for long periods;

Zone 1 (high risk gas) -an area in which an explosive gas/vapour atmosphere is likely to occur in normal operation;

Zone 2 (medium risk) – an area in which an explosive gas/vapour atmosphere is not likely to occur in normal operation, or, if it does occur, will only exist for a short time.

Zones 20, 21 and 22 have similar definitions but relate to cloudand dust atmospheres.

* Create and maintain an explosion protection document;

* Mark areas at point of entry;

* Provide staff with suitable and sufficient information, instruction and training on the appropriate precautions and actions to be taken to ensure safety;

* Select ATEX 94/9/EC-compliant equipment according to the intended Zones. Equipment such as lighting, process equipment, mechanical machinery, vehicles, access platforms, sweepers, etc. can all be selected as ATEX 94/9/EC-compliant for use in areas classified as hazardous under DSEAR/ATEX. Equipment for operation in hazardous areas is built to a level of protection known as a category, which matches the risk – e.g. Category 2C/Zone 1, Category 3C/Zone 2, Category 2D/Zone 21, Category 3D/Zone 22.

CASE STUDY 1: A SCOHISH DISTILLERY

At this particular site, an access platform is required for routine working in overhead areas. The hazard is ethanol (flashpoint: 13C) and although the work is carried out at height, the equipment operates in a hazardous area and has to be protected at the top of the platform and at ground level so that ft cannot be a source of ignition. The customer chose to manage the risk by converting the access platform so that ft provides audible and visual alarms upon detection of the vapour, followed by a fail- safe, controlled shutdown before a dangerous level b reached. The company also wanted to be sure that the system was able to detect a gas/vapour constantly and so chose a system that performs an automatic gas test on start-up and which automatically calibrates itself.

CASE STUDY 2: CARBOM-FIBRE INSULATION MANUFAQURER

This company recently undertook a risk assessment and area classification. Areas that were addressed inducted the solvents used for painting, housekeeping within the paint area, training of staff, and managing the storage of up to 40 barrels. The release of flammable volatile organic compounds (VOCs) from heating polymers was another concern where better ventilation was required to reduce the flammable atmosphere at source. Improving the handling of resin powder used for mixing with carbon fibre in a moulding process was also addressed and these process areas were classified as hazardous.

CASE STUDY 3: A CHEMICAL DISTRIBUTOR

This company in the north of England acts as the UK sales arm to some of the world’s largest chemical manufacturers, distributing products for adhesives and sealants, coatings and inks, food ingredients, health care, and plastics and rubbers. The company blends, stores, and distributes a range of flammable products, so flexibility and safety in its operation are paramount, as ft operates a top-tier COMAH site designed and staffed to ensure the integrity of its stockholding. The operation has been fully managed to comply with DSEAR/ATEX through risk assessment, efficient handling of flammable material, formally classified hazardous areas, signage, training, and the use of explosionproof equipment, such as forklrft trucks. A routine maintenance programme is also in place to ensure the integrity of equipment used in their processes.

References

1 For more information on both DSEAR and ATEX visit www.hse.gov.uk/fireandexplosion

Ian Ratcliff is managing director for the safety and environmental company Pyroban Ltd, based in Sussex. Ian is a Fellow of the Chartered Institute of Engineering and is responsible for three of Pyroban’s UK operating divisions that help manage the risk of explosion in industries ranging from oil and gas, chemical, pharmaceutical, food and flavours, logistics, and general manufacturing.

Copyright CMP Information Ltd. Dec 2006

(c) 2006 Safety & Health Practitioner, The. Provided by ProQuest Information and Learning. All rights Reserved.