Problem:

• Cellulose, like a number of other dusts in industry, has flammable and explosive properties
• Cellulose dust exploded in a mill at one of a manufacturer’s plants
• The explosion penetrated from the mill into the production area, causing secondary explosions and a fire that completely destroyed most of the floor; witnessing the damage suffered, the manufacturer decided to avoid similar accidents in the future

Remedy:

• The company has decided to prevent similar situations in the future through a comprehensive approach to explosion and fire safety
• We installed safeguards in the production site and prepared a complete protection concept for a second, newly planned plant
• We talked over the client’s new investment ideas and proposed appropriate preventive solutions
  We prevent future explosions

Addressing explosion and fire risks by complex, integrated works

I should stress it strongly at the start of this article that although protection against explosions of dusts, gases and liquid vapours is a legal requirement, our experience shows that up to 80% of industrial plants fail to meet the minimum requirements of the so-called ATEX USER Directive and harmonised standards.

So, when do the procedures and safeguards required by law are most often implemented? Unfortunately, it is usually only in an emergency situation when:

• an explosion happens and this raises the awareness of the danger,
• an insurer audits an industrial plant and proves adequate safeguards are missing,
• the lack of safeguards is found by the Chief Labour Inspectorate, Fire Brigade or other state authorities.

In such events, comprehensive industrial fire and explosion risk response works become a priority. And while we at the WOLFF GROUP are keen to spread awareness of the risks arising from the processing of flammable and explosive dusts, gases and liquid vapours in industry, we are aware that this is a long-term process. Therefore, in the following article, we provide information about the complex work we carried out at our client’s site after a cellulose dust exploded to cause secondary explosions and fire at their site.

How it all started or the explosion and fire that changed everything

A dozen or so months before the project described here, one of the project owner’s plants exploded and caught fire that consumed a large part of the production area. The initial explosion took place at a cellulose mill. However, its effects were not confined to the inside of the unit, but the pressure and flames escaped into the production floor and caused accumulated dust to rise into the air and ignite. This resulted in a secondary explosion, which then developed into a fire.

This is how Wiesław Drosio of the Wołomin fire brigade described the incident at the time,

Probably the mill where cellulose processed was hit by the explosion. This ignited the cellulose on the mills and on the 15 m by 15 m area of the floor, as well as the cellulose in the bags

Another witness reported that he had heard a blast when he was in the shop, and when he came out, he saw dark smoke and flames. He concludes his account by saying,

There used to be a large metal shed but now it’s just gone!

Three people were injured and a large part of the production facility was completely destroyed. Following this incident, the WOLFF GROUP was chosen as the partner responsible for providing protection systems for the company’s two production sites against similar incidents in the future. It is worth noting at this point that the other plant is not yet completed, and our work to date has enabled all the protection systems to be included in the project.

Assessment of the current state

As part of our cooperation, we helped the project owner to identify the critical points in the production facilities that posed the highest risk of explosion and/or fire. In the next step, we started implementation work, which included designing a fire protection system, implementing explosion protection and prevention solutions in the form of dust monitors and a spark detection and extinguishing system. Meanwhile, we are also in constant touch over the design of the other plant and preparing to fit safeguards at it. You can read about the protection systems we implemented at the bottom of the page, but let us briefly talk about the situation that had led to the aforementioned project.

Explosive dusts or where explosion and fire hazards arise

To explain why dusts of apparently safe materials can be explosive, we will use a few examples.

We start a bonfire with the finest sticks which are much easier to light than a thick bough. They also burn faster and more dynamically. However, if we tied a handful of such sticks together in one dense bunch, we would have a similar difficulty in lighting it as with a single bigger stick of similar thickness.

The same applies when you want to light a thick braided rope. This task is by no means simple, at least until the rope is split into hundreds of independent strands.

Dusts follow this rule to an extreme degree. Many solid materials are difficult to ignite and the combustion take long or is virtually impossible to initiate. Everything changes when we fragment these materials. As a dust-air mixture wood, plastics, sugar and even some metals like aluminium and magnesium are explosive.

Below, you can read some detailed case studies that describe the causes and effects of various dust explosions, in different production facilities:

• Flour explosion [flour mill]
• Sugar dust explosion [sugar mill]
• Aluminium dust explosion [aluminium wheel manufacturer]

However, to find a number of articles that may broaden your knowledge of explosive dusts, click the links below:

• Combustible and explosive dust – STATISTICS
• Designation of dust explosion hazard zones – introduction

How can a dust explosion occur?

Let us reflect on this question using the example of dust collection systems, which are high-risk systems. Dust removal efficiency decreases over time, which is most often due to perforation of the filter cartridges. In this case, a mixture of air and explosive cellulose dust will be returned to the production sites. Dust will foul equipment and floors, increasing the risk of a similar explosion and fire that has already occurred at the other site of this project owner.

A risk also exists that before it falls, the dust-air mixture introduced into the area will come into direct contact with an ignition source, which will also result in an explosion and fire. In this case, the lingering dust that has time to build up before the explosion will provide fuel for subsequent secondary explosions and fire.

Just to remind you: the original explosion at the project owner’s production facility occurred inside the plant – the cellulose mill to be precise. Then, the explosion escaped to the outside, where it raised dust from the equipment and floor. It ignited the dust and so-called secondary explosions ensued turning into a fire.

“In my facility, there are no ignition sources” – Are you sure about this?

Now, a thought may be crossing your mind, “are there no ignition sources at my site?” However, the ATEX directive lists as many as 13 of these. Let’s look at just one of them – hot surfaces.
– Wait, there are no hot surfaces in my facility.
– Are you sure?

Conveyor belts, bucket elevators, other equipment with bearings, electrical equipment, motors, mills and other rotating machinery can heat up to temperatures exceeding the ignition temperature of the cloud or layer of dust in question. This was the case at Imperial Sugar Refinery where an overheated bearing led to the destruction of the entire factory. The death toll was 14; 36 more were injured.

What safeguards have we implemented to address the explosion and fire risks?

Explosion Hazard Assessment (EHA) and comprehensive advice to the developer and his subcontractor

Our works covered two plants, one existing and the other in the design phase. In the latter case, the project owner has so far benefited from our assistance with explosion safety advice. In addition to the commissioned Explosion Hazard Assessment, which we will prepare and submit to the fire brigade for approval, we are in constant contact with the client’s subcontractors who are designing the individual plant components. In this way, we have already suggested, among other things, which ventilation and heating system components to use so they are suitable for work in a hazardous area.

As for the existing plant, we also act in an advisory and partnership capacity. On the one hand, we advised the project owner on safeguards and, on the other hand, it consulted us on its new ideas for technological changes to the production areas so as not to introduce a new hazard into the workplace.

Explosion suppression – basic explosion protection

Cellulose dust is explosive, an undeniable truth that the project owner had unfortunately previously learnt ‘first-hand’ when an explosion and fire consumed part of its production floor. As a result, it decided never to allow a similar situation to occur again.

Vulnerable sites where explosions can occur are not only cellulose mills (an original explosion), but also dust collection plants. To adequately protect this equipment from the effects of an explosion, we proposed that an HRD cylinder explosion suppression system be used.

At first glance, explosion suppression often appears to be an expensive safeguard, however, when the long-term maintenance costs of the system are considered along with any downtime once the safeguard has been activated, it becomes apparent that HRD systems are often the most favourable option when analysing different security systems. It is worth mentioning at this point how to objectively analyse the offers from different protection system providers. The all-purpose measure of Total Cost of Ownership (developed by consultancy and advisory firm Gartner) is useful in this regard.

Explosion proofing with non-return flap valves

Whenever safeguards are applied to minimise the effects of an explosion, the isolation of the equipment and instruments to be protected must be borne in mind. Explosion isolation (decoupling, in other words) consists of disconnecting a device from the rest of the system in order to stop and prevent the further transmission of explosion.

In protected dust collection systems, we used passive isolation solutions in the form of non-return flap valves. Their operation uses the force of the explosion moving through the duct to close the flap automatically and contain the pressure and fire inside the duct.

The preventive solution to use was a spark detection and extinguishing system.

At the beginning of the article, we mentioned the three most common situations when companies decide to invest in explosion safety. Let me further remind you that these purchases are a legal requirement for every employer, but unfortunately, in our experience, 80% of industrial plants do not comply with the ATEX Directive.

Back with our project owner, let us recall that it took all the measures only after a cellulose dust exploded at one of its plants. Following these events, the insurer auditing the industrial plant pointed out the need to protect it against similar explosions and fires in the future by, among other things, installing a spark detection and extinguishing system on the cyclones, dust collection filters and buffer tanks as a preventive system to protect the aforementioned equipment from an ignition source (e.g. a hot particle) that could reach the dust collection system transported with the dirty air inside.

It is evident that for one owner, two of the three reasons coincided, which are the most common reasons for starting to implement explosion-proof systems.

The Atexon spark detection and extinguishing system used here is based on an extremely sensitive detector in the duct that is capable of sensing:

• single sparks
• streams of sparks (otherwise known as a spray or shower of sparks)
• black hot particles above 300 ^oC, that do not burn but are hot enough to cause an explosion.

As soon as a moving particle capable of causing an explosion is detected further down the duct, an extinguishing nozzle is activated to extinguish the spark or hot particle in the blink of an eye. In this way, we provided the project owner with a preventive solution to stop sparks from reaching the dust collectors, which would certainly have led to an explosion, and also meet the requirements the owner was imposed with by the insurer.

Dust level monitors

Regarding the two plants we provided safeguards for (one existing and the other in the design phase), the project owner undertook a project that was interesting from an energy-saving point of view. It decided that warm air from production would be returned to the production areas. This approach was intended to reduce energy consumption, which, with the continuing increases in the price of energy, is entirely understandable.

More importantly, it is estimated that the hot air return system project will pay for itself as early as the following winter.

The return of warm air presents a double risk – the risk of explosion and the risk to workers’ health at workplaces associated with the potentially increased levels of hazardous dust, which if inhaled, can damage the respiratory tract. We addressed those risks by using preventive dust level monitors.