Problem:

• Shredding and grinding pallets fastened with nails generates explosive wood dust, as well as hot particles and sparks
• The dust, together with hot particles, is conveyed to three dust collectors, the equipment which is statistically highly prone to explosions
• Before contacting our company, the project owner had already ordered dust collectors that were not suitable for Ex zones

Remedy:

• We changed the location of the dust collectors so that a possible explosion did not pose any risk to property or to the life and health of workers
• We used explosion isolation solutions to prevent the explosion in the filter from spreading to the other parts of the system
• We used a spark detection and extinguishing system to minimise the risk of hot particles from the pallet grinding process entering the filters

Project owner’s primary business objective: Minimising production waste and turning it into a source of new, additional profits

It has long been known that any process that aims at minimising production waste can reduce overall production costs. This waste may come directly from the production process, or the source of its origin may lie elsewhere. This was true in the case of a corrugated cardboard and cardboard packaging manufacturer.

As with many manufacturing plants, the company uses many thousands of wooden pallets annually throughout the process of receiving raw material, production, storage and transport. Some of them is naturally subject to wear, and as waste should be sent to a recycling company. However, the project owner manufactures paper products which also involves the daily processing of wood fibres into cardboard and then cardboard packaging. With this in mind, the plant managers decided to build on their expertise and diversify the business by opening a line to process old, broken pallets into blocks for new pallets.

During this diversification, explosion and fire safety concerns were raised.

The pallet block manufacturing process involved shredding and grinding wooden pallets, followed by shaping and pressing the prepared raw material into blocks. In order to extract the wood dust that will be generated during production, the project owner purchased three dust collectors to be installed inside the production area.

The whole project was going according to plan until they realised that this wood dust could pose a serious explosion and fire hazard during its extraction. Then, the project owner contacted us to help them verify the risk and provide a protection system for the new production line.

Important fact: dust collection does not eliminate the risk of explosion

The use of dust collectors in production processes where dust may be released is important not least because dust is harmful to workers’ respiratory tracts. At the same time, dust collection systems are required when the dust being extracted has explosive properties. However, there is still a large group of people in industrial plants who do not realise that dust collection does not eliminate the risk of explosion – the risk still is there, but it is transferred with the dust from the workplaces to the dust collection system. This is why, in line with the statistics we know, dust collectors are one of the groups of equipment where explosions most often occur in industrial environment.

In a free on-line workshop, Zbigniew Wolff talks more about the risk of dust explosions in dust collection systems, and how to protect these systems from the effects of an explosion.

What to do when a dust collector is not suitable for use in an EX zone and cannot be replaced?

The task we faced was not a rare case. In fact, it could be said that we often encounter a similar pattern of owner’s needs for explosion safety solutions. Namely, it is still very common for firms to approach companies such as the WOLFF GROUP only when at least some of their new equipment has already been ordered and nothing can really be done about it, even if the equipment is not suitable for work in hazardous areas. Our task then often requires an approach with a great deal of understanding of the customer’s situation in order to come up with a solution where the purchase of another piece of equipment (this time, ATEX-compliant) would be the last resort. This was also the case here.

Our client’s original concept was to install three dust collectors around the production area. Because of the considerable risk of an explosion in the dust collector, we had to propose a different, safer location, so that a possible explosion would not pose a risk to the life and health of workers and other parts of the plant. We selected and proposed a suitable site outside the production floor, in the open air, in an area where there was no other equipment and where there were no employees. This way the risk of an explosion in the dust collector no longer posed a real danger to others. What remained was to isolate the dust collectors from the rest of the plant to avoid the risk of explosion movement and propagation to other parts of the plant.

How to ensure explosion isolation on dust collectors?

The first step of the blast isolation work was to draw up a hazard and risk analysis. It proved that an explosion in the filters could retract up the ducts that supply contaminated air to the dust collector. On the other hand, after considering the construction of the filters and the effects of an explosion in them, we found that the highest risk on the clean side of the filters is the propagation of a fire through the piping that conveys clean air back to the production area.

We therefore proposed two separate isolation solutions to match the diagnosed threat. On the clean side, we used RE120-rated fire doors, which are fully sufficient to extinguish a possible fire within 2 hours of an explosion. On the dirty side, the explosion would constantly encounter fuel for further propagation as dusty air moves along these pipelines. For this reason, we used RNV explosion-proof non-return flap valves. A newer version of the SNR valves, they provide better insulation performance, and they can also be used downstream of pipe elbows, which is not possible with standard SNR valves.

What explosion safety steps are required by the ATEX USER Directive?

So far, we have described how we ensured effective protection for owner’s workers and plant from the effects of an explosion by taking the filters outside the production area and using a full isolation of the dust collectors on both the dirty and clean sides. However, our role was far from over, given that these actions require the last, third step in the sequence of actions that an industrial plant should take to ensure safety.

According to the ATEX USER Directive, an industrial company should follow the procedure:

• Reduce the possibility of creating an explosive atmosphere,
• Reduce the potential for ignition sources of explosive atmospheres,
• Protect instruments and equipment from the effects of an explosion in order to protect the life and health of employees and company property.

The first step is always only partially feasible for dust explosion hazards in industry; simply put, production processes will always generate dust, so excessive dust and dust accumulation must be properly prevented. This is possible by applying, among other things, systems for collecting dust from air at workplaces.

How to eliminate the risk of ignition sources in explosive atmospheres?

Very little is needed to ignite an explosive atmosphere and initiate an explosion – usually a small spark is capable of causing an explosion as long as it reaches an area with a dust level within the explosive limits for the dust in question. Standard actions that every company should take include:

• Use non-sparking tools
• Fit only explosion-proof electrical engineering devices, lighting, and heating equipment in the required places
• Use equipment designed for use in potentially explosive atmospheres, i.e. the equipment whose construction prevents the occurrence of sparks during production processes,
• Safeguard against static electricity by, among other things, using earthing systems for all processes involving bulk/liquid product transfer / overflow.

Are you in need of objective arguments to use in your talk to management about explosive safety? You will concact.

Can raw materials also be a source of ignition in an explosive atmosphere?

Sometimes the product itself, or its processing, can, at some point, turn into a source of ignition in an explosive atmosphere. This was the case for a tea manufacturer, discussed in a separate article, as well as the cardboard and cardboard packaging plant described here.

Let us recall that the diversification of production using waste, consisting of worn wooden pallets, involved the shredding and grinding of these pallets in the first stage of the process. Pallets themselves, of course, consist not only of wooden planks and blocks, but also nails and sometimes other metal parts. Consequently, when analysing this process, we were certain that this would pose an additional spark hazard. If the dust collection system then pulled in such a spark or hot particle along with the dusty air, the effect in the form of an explosion would occur almost immediately. Our assumptions were confirmed by a controlled trial run of the system carried out by the project owner. There was a micro fire almost immediately.

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

We proposed that the project owner use a system that immediately detects and selectively extinguishes sparks in the ducts. More importantly, the proposed solution does not bring the production line to a standstill, an important feature from the point of view of production continuity, since, according to our estimates, these types of sparks will occur very frequently in the dust collection ducts.

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°C 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’s new production line with a preventive solution to prevent sparks reaching the dust collectors, which would otherwise certainly have led to an explosion.