Deux Ex Atmospheres

How to start an article so broad and so important! Well as they used to say the best way to start is at the beginning.

So, what is an explosive atmosphere?

As we all know we are blessed with an atmosphere that sustains combustion. Without out it we would not have progressed so far as a species. In fact, we might not have progressed at all.

The most frightening thing however, is that when this life sustaining atmosphere changes, in particular with the introduction of other more combustible substances, and the introduction of an ignition source, then the combustion that it sustains will not be the gentle flickering of a flame, but a dramatic, immediate explosion.

Many industries utilise the materials, or produce such substances, that can, due to unregulated release, significantly change the surrounding atmosphere to cause such explosive atmospheres. The most significant of these have dramatically changed industries, countries and the public’s attitude to these industries.

I think it wise that I spend just a paragraph to identify the substances mentioned above. Of course, when we think of explosive atmospheres we immediately focus on gases. However, mists, dusts or vapours can also adjust the atmosphere to become explosive. These can of course occur in many working environments. Painting workshops, wood working workshops and flour mills are some of the more common.

“these events have caused loss of life, injury and expensive litigation and reputational damages”

It Cannot Happen to Me

Some of these events have caused loss of life, injury and expensive litigation and reputational damage. Unfortunately, it is often the nature of humans to still think “it cannot happen to me” or “not on my facility”. Therefore, these incidents are often repeated to small and large scales. During this article I will be looking at some of these incidents in brief detail, the human and technological factors involved and some of the strides forward we have made.

Now of course, due to this large consequence profile the states of the world have produced significant legislation to cover this subject and the subsequent approaches required. One of the most well known are the ATEX Directives of the EU.

ATEX Workplace Directive

These are Directive 99/92/EC or the “ATEX Workplace Directive” which covers the minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres and Directive 2014/34/EU or the “ATEX Equipment Directive” concerning equipment and protective systems intended for use in potentially explosive atmospheres.

So, how do we detect when the atmosphere begins to become explosive and what should our actions be when we detect it?

The above directives require that the employer classify where explosive atmospheres may occur into zones. These zones are based on the likelihood of such an explosive atmosphere occurring, and very importantly, its persistence if it does.

I will not go into the technical intricacies of producing this assessment and zone classification here, but the assessment of this risk should consider the following:

1. The hazardous properties of the dangerous substances involved
2. The amount of dangerous substances involved
3. The work processes, and their interactions, including any cleaning, repair or maintenance activities that will be carried out
4. The temperatures and pressures at which the dangerous substances will be handled
5. The containment system and controls provided to prevent liquids, gases, vapours or dusts escaping into the general atmosphere of the workplace
6. Any explosive atmosphere formed within an enclosed plant or storage vessel; and
7. Any measures provided to ensure that any explosive atmosphere does not persist for an extended time, e.g. ventilation

As we mentioned above there are various substances that can form an explosive atmosphere and of course there are various standards that can assist in the area of area of classification. BSE EN 60079/10 outlines the basic principles of area classification for gases and vapours whilst BS EN 61241/3, which was published in 2002 covers the area classifications for dusts.

Again, I shall not go into these documents here but to remind my audience that these are of course EU centric in legislation and guidance. As such they are to be used with caution and we should all ensure that our local legislation in our respective jurisdictions is followed as priority. Saying this we must also be knowledgeable of new innovations and approaches and where we can, implement them for the safety of all.

“the equipment required to be used in each area should meet the requirements of that area”

We have now zoned our facility and as such we must now look at the equipment we have in these areas. The obvious thing we need to do is to limit or even eliminate the sources of ignition. The equipment we choose should be based on the likelihood of occurrence and is constructed according to the zonal categorisation (1, 2, or 3) depending on the level of zone it is intended to be used.

Zone Classifications for Gases, Vapours and Mists

• Zone 0 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in form of gas, vapour or mist is present continuously or for long periods or frequently
• Zone 1 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally
• Zone 2 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only

Zone Classifications for Dusts

• Zone 20 – 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 – 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 – 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

Equipment Categories and Zones

• Zone 0 or Zone 20 – Category 1 equipment
• Zone 1 or Zone 21 – Category 2 equipment
• Zone 2 or Zone 22 – Category 3 equipment

I will again, try not to get too technical in this article, but just say that the regulatory structure for the design, construction and marking of such equipment is a subject all on its own!

Saying this, however, I would like to examine the use of equipment of different categories in different areas. Ideally, the equipment required to be used in each area should meet the requirements of that area. When a piece of equipment of a higher or lower category than that usually required would only be present in special circumstances, for example:

1. During maintenance operations where alternative effective precautions are provided to control the risk
2. Where workers can be excluded from the hazardous area, and will not be at risk from any ignition of an explosive atmospheres
3. Where equipment of the required category is simply not available, but a lower category can be used in combination with other protective measures to achieve the purposes of the regulations

Now we have equipment classification covered let’s take a quick look at where we cannot remove the explosion risks, how we should manage the sources of electrical sources of ignition.

We must examine the design of equipment and structures. For example, by:

• Avoiding hazards due to static electricity by bonding all conductors together and to earth
• Installing lightning protection systems appropriate to the construction and contents of structures
• Detuning structures capable of acting as RF antennae in explosive atmospheres within vulnerable zones of transmitters
• Application of a protective coating to cathodic protection isolating joints to prevent accidental contact

We should also be looking at the maintenance and inspection of our equipment and structures:

• Initial inspection before plant or equipment is brought into service
• Ongoing periodic inspection of explosion protected (Ex) equipment and/or systems
• Monitoring of static earthing
• Maintenance and inspection of lightening protection systems
• Periodic inspection of above ground isolating joints, for example in cathodic protection systems.

For those of you reading this that work in the industries regulated for hazardous atmospheres I am sure that the above are a rather simplistic view of the complexities involved, both technically, procedurally and systematically.

“this just gives us a glimpse of these challenges and the need for diligent investment, in equipment and people”

For many, this just gives us a glimpse of these challenges and the need for diligent and focused investment, both in equipment and people, needed to prevent catastrophic consequences.

Having a quick search on the prices of equipment Ex rated will show anyone what I mean by investment. However, we must be always careful on the reduction of standards of equipment, any equipment, there to warn us of atmospheric explosion and also, that purports to not pose an ignition risk. We cut corners at our own risk in this regard. The investment is often small in the face of the losses incurred if these systems fail.

Case Studies

We must now look briefly at when these systems and human factors have led to these undesired and most tragic events that we are trying to avoid. In these cases, I shall attempt to be concise but shall also refer to the more expansive resources for your future reference. If you are not familiar with these then I would recommend their study for we must all try to ensure that tragic history should not be repeated.

This very well-known explosion occurred at the largest oil-products storage deport in the United Kingdom with a capacity of about 273 million litres of fuel. The first and largest explosion occurred at 06:01 UTC near tank 912, which led to further explosions which eventually overwhelmed 20 large storage tanks.

The cause of the explosion was a fuel-air explosion in a vapour cloud of evaporated leaking fuel. The event measured 2.4 on the Richter scale and was reported as the largest of its kind in peacetime Europe.

Starting at 19:00 on the evening of 10 December 2005, Tank 912, towards the northwest of the main depot, was filled with unleaded petrol. At midnight the terminal closed, and a check was made of the contents of tanks, which found everything normal. Normally the gauges monitor the level of the fuel in the tank as it fills from the particular pipeline. From about 03:00 the level gauge for Tank 912 began to indicate an unchanging level reading, despite it being filled at 550 cubic metres (19,423 cu ft) per hour.

Calculations show that the tank would have begun to overflow at about 05:20. There is evidence suggesting that a high-level switch, which should have detected that the tank was full and shut off the supply, failed to operate. The switch failure should have triggered an alarm, but it too appears to have failed. Forty-one minutes later, an estimated 300 tonnes (330 short tons) of petrol would have spilled down the side of the tank through the roof vents onto the ground inside a bund wall – a semi-enclosed compound surrounding several tanks.

An overflow such as this results in the rapid formation of a rich fuel and air vapour. CCTV footage showed such a vapour flowing out the bund wall from around 05:38. By 05:50 the vapour started flowing off the site, near the junction of Cherry Tree and Buncefield Lane. Around 05:50 the rate at which fuel was being pumped into the tank increased dramatically.

Initially the fuel was pumped in at 550 cubic metres (19,423 cu ft) per hour, but it increased to about 890 cubic metres (31,430 cu ft) per hour. By 06:01, when the first explosion occurred, the cloud which was initially 1 metre (3 ft) deep, thickened to 2 metres (7 ft) and had spread beyond the boundaries of the site.

The report prepared⁶ following the incident found that the level gauge had stuck at random times after a tank service in 2005, but it did not concern maintenance contractors or site management. Further to this, a missing padlock on the independent shut-off valve, the failure of secondary containment and even tertiary containment led to the expansion of the incident. Wider management failings also found including:

• Management safety checks at the site were found to be deficient and not properly followed
• Site staff did not have control over the flow rates and timing of two of the three inlet sources, meaning that they did not have enough information to properly manage the storage of incoming fuel

It was also noted that there was a clear shift from process safety to process operations, including the reduction of wait times.

This was one of the largest events involving an explosive atmosphere in history and it highlights many of the human and technical factors that we have discussed, but predominately, the danger of reducing the focus of safety for the sake of production.

It is worthy of much more study and I encourage you to refer to the official report found at https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/238688/7491.pdf

The dust explosion is one that I have often found to be less focused on than the gas or vapour explosion. I am sure that my colleagues in the bulk food industry would challenge me on this, perhaps quite rightly.

One event that changed this for me was the explosion that occurred in Port Wentworth, Georgia in 2008. It was also one event that made me professionally focus on the need for stricter housekeeping.

The explosion occurred at 19:00 local time in what was believed to be a room where sugar was bagged by workers. The factory itself was antiquated, with much of the machinery dated, but with access to rail and shipping links it remained open. Leading up to the explosion the owners had some financial difficulties. Further, during the year before the Chemical Safety and Hazard Investigation Board (CSB) conducted a study into the risks presented by dust explosions after three fatal accidents had occurred.

The CSB had informed the OSHA of a number of recommendations which were partially implemented by 2008, however, they remained concerned about further fatal accidents.

The explosion occurred in the centre of the refinery, where bagging and storage facilities were fed completed product by a network of elevators and conveyor belts. Many of the buildings at this location were six to eight stories high with narrow gaps in between.

14 workers were killed in the explosion and over 30 injured.

The accumulation of sugar dust throughout the bagging area and in the atmosphere, coupled with poorly designed and constructed facilities were deemed to be integral in the explosion.

The operator also never practiced evacuation procedures and the extreme lack of emergency lighting meant that many personnel were confined to dark hallways and tunnels at the time of the explosion.

Again I would highly recommend reading the official CSB report to be found here https://www.csb.gov/userfiles/file/imperial%20sugar%20report%20final%20updated.pdf.

Conclusion

Explosive atmospheres can occur with a wider range of substances than first we think. Further to this the legal requirements and standards are complex and require focused corporate effort to adhere to. The investments in both correct and certified equipment, as well as a strong effort for human factors and procedure is required.

Without it, the lessons of the past disasters are doomed to be repeated. I sincerely hope that however brief this article is, it stimulates some conversation and thought on how we are all managing this most serious of risks in our respective industries. Let’s all continue the focus and also continue to adopt practical, innovative and forward-thinking approaches in the development of our practices, procedures and equipment to keep us all safe.

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References

1. www.hse.gov.uk/fireandexplosion/atex.htm#what accessed 2nd December 2022
2. Explosive Atmospheres – Classification of Hazardous areas (zoning) and selection of equipment (hse.gov.uk) accessed 2nd December 2022
3. Explosive Atmospheres – Classification of Hazardous areas (zoning) and selection of equipment (hse.gov.uk) accessed 2nd December 2022
4. Explosive Atmospheres – Classification of Hazardous areas (zoning) and selection of equipment (hse.gov.uk) accessed 2nd December 2022
5. Explosive atmospheres – Electrical, Control and Instrumentation (E, C&I) – HSE accessed 2nd December 2022
6. COMAH – Buncefield: Why did it happen? (hse.gov.uk) accessed 2nd December 2022