Features Health and Safety Sawmilling
Hazardous Material

It is well known that wood is a combustible material, and combustion incidents do happen in industries processing wood. But when two sawmills in British Columbia exploded in three months earlier this year, killing four people and injuring dozens more, the potential for dust explosions suddenly received a lot more attention.

Dust explosions occur whenever a cloud of fine and dry combustible material (dust) is ignited. How easily a dust explosion can be created depends on the kind of material, but also on the particle size and the moisture content of the dust. The finer and drier a dust is, the easier it is to create a dust cloud; the easier it is to ignite that cloud, the more violent the ensuing explosion is.

Fine dust can be generated in many places, but it is a feature of most wood processing plants that fine product is generated in many places. Obvious sources of dust would be sawing, chipping, planing and similar processes. Other plants use the fine material generated elsewhere as their feedstock, such as in biofuel production and energy plants.

Sometimes the “fine” material appears to be fairly coarse, more like chips than dust. However, even when chips are formed there always is a finer fraction of dust in there. Besides, when handling chips and similar materials, more dust will be created by attrition and breakage of the chips and one will have to assume that enough dust is available for the generation of flammable dust clouds.

Such clouds can arise in a plant as part of normal operation, such as in a dust extraction system, or as an abnormal event, such as when a dust deposit is raised into a cloud. Recognizing the potential for the formation of flammable dust clouds is the first step towards the elimination and control of dust explosion hazards.

Conditions Required for Dust Explosions
In many respects dust explosions are similar to gas and (solvent) vapour explosions: in both cases the fuel (gas, vapour, dust) is dispersed in air to form a flammable atmosphere.

Provided that the concentration of the fuel in the air is in a suitable range (the flammable or explosible range), an ignition will result in a flame that propagates throughout the whole mixture. This generates a lot of hot combustion gases, which will be experienced as a fireball or flash fire by an outside observer because the hot gases expand. If the combustion takes place in a closed volume (vessel, room) this expansion cannot take place and the pressure increases instead. When the pressure exceeds the strength of the enclosure, the wall will rupture and we call the event an “explosion.”

Although there are many similarities between gas/vapour and dust explosions, there are also clear differences, which are caused by the fact that even the finest dust is much larger than gas or vapor molecules, so that the mixing in the case of dust is less ideal. The conditions for a dust explosion can be summarized
as follows:

• The dust must be combustible (as far as dust clouds are concerned, the terms “combustible,” “flammable” and “explosible” all have the same meaning and could be used interchangeably);
• The dust must have a particle size distribution capable of propagating flame; this is often given as a particle size smaller than 420 micrometre, but this is not a clear cut-off and sometimes particles with a larger dimension can still participate in a dust explosion;
• The dust must be dispersed (forming a cloud in air);
• The dust concentration must be within the explosible range (above the Minimum Explosible Concentration);
• The atmosphere in which the dust cloud is present must be capable of supporting combustion (usually the oxygen in the air); and
• The available ignition source must have sufficient energy to initiate combustion.

Several of these conditions are the same as for a fire: a fuel, with air (oxygen) and an ignition source. The difference between a (dust) fire and a (dust) explosion is the mixing of the fuel and the air in suitable concentrations. This will lead to a much faster combustion, which has been termed flash fire above. Adding confinement will turn the flash fire into an explosion. Thus the well-known fire triangle is converted into an explosion pentagon.

Assessment of Dust Explosion Hazards in Your Facility
In order to eliminate and control dust explosion hazards in a plant, it is essential that the hazards be identified correctly. A systematic approach is necessary to ensure that all relevant factors and situations are correctly identified, and that the control measures are realistic for the process in question.

Such a systematic approach to identifying dust cloud explosion hazards and taking measures to ensure safety in any facility involves:

Determining the dust cloud’s ignition sensitivity and explosion severity characteristics through appropriate laboratory tests on representative dust samples;

• Identifying areas of the facility where combustible dust cloud atmospheres could exist under normal and/or abnormal conditions;
• Identifying potential ignition sources that could exist under normal and/or abnormal conditions;
• Preventing the formation of explosible dust clouds in the plant and reducing the extent and duration of any clouds that may be formed;
• Taking measures to eliminate/control ignition sources; and
• Taking measures to protect against the consequences of dust cloud explosions.

One of the hardest parts in the assessment process is to envisage all the conditions where dust clouds could arise, not only in normal operation, but also during startup, shutdown, cleaning, maintenance and in case of some process upset or equipment malfunction. Similarly, lack of experience often leads to the dismissal of potential ignition sources as non-viable, with the consequence that the control measures will not address that particular type of ignition source. “That will never happen here”, “we have done it like this for 20 years” and similar statements have proven to be wrong so often that they are regularly classified as “famous last words.”

There are many different types of potential ignition sources, and it can be challenging to identify all of those in a plant. However, the ignition sources that have been found to be the cause of the majority of explosions in dust handling/processing plants include welding and cutting, heating and sparks generated by mechanical equipment failure, sparks generated by mechanical impacts, hot surfaces, open flames and burning materials, self-heating, electrostatic discharges, and electrical sparks. Which ones of those are relevant in a particular facility depends on the kind of equipment and processes involved.

Measures to protect against the consequences of explosions, usually called “explosion protection measures” include explosion relief venting, explosion suppression, explosion containment, and explosion isolation. All these measures have in common that they protect a single enclosure (vessel), and do not prevent the explosion from occurring. When executed correctly, each of these measures will provide adequate protection against a defined hazard – another reason to make sure that the hazard identification is done correctly.

While explosion protection measures are an essential part of an explosion control strategy, the main focus must always be to prevent the explosion where possible. In that respect, preventing the formation of a flammable atmosphere is generally more reliable than ignition source control and must therefore be the first priority. Usually this is done by first avoiding the presence of the dust, and then avoiding the dispersion into a cloud. An alternative would be to remove the oxygen from the equation: “inerting” prevents the combustion by reducing the oxygen content in the atmosphere to a low value that no longer sustains the combustion. This is only suitable for closed systems (to reduce the inert gas consumption) that can effectively exclude the presence of people. After all, nitrogen is one of the deadliest gases used in industry.  

Dust Explosion Hazards in Wood Processing Industries
The systematic hazard assessment process mentioned above is not a “global” one. In every facility the conditions vary from one process or area to the next, and this must be incorporated in the hazard assessment. This requires that the hazard assessment consider the various processes in the facility in detail, while making sure that the possible interactions between processes and areas are taken into account.

One area that always demands special attention, but is often overlooked, is the general workplace. From a process and production point of view, there should be no dust in most of the workplace. Any dust that is released into the workplace is a potential concern. Even apart from fire and explosion hazards, that dust could cause problems with the equipment, could lead to occupational hazards such as health issues, and will cost money to clean up. If that dust is combustible, such as is the case with wood dust, the problems multiply. By the nature of the dust, the finest dust will tend to end up on elevated surfaces such as overhead beams and equipment. From that vantage point, this dust can be dispersed into a cloud that can fill a significant part of the building – a far greater volume than any of the process equipment, made up of the most ignitable and explosible fraction of the dust and in a room where personnel may be present
.
A common scenario is that a small incident, such as a small “primary” explosion somewhere or in some vessel, disturbs the dust on the overhead surfaces and creates a large dust cloud, which is then ignited by the flame from the primary explosion. The “secondary” explosion in the workplace will then disturb more dust, leading to a chain reaction of dust explosions that can run through a facility and destroy it completely. Secondary dust explosions cause by far the greatest damage and more casualties than primary explosions, and there is virtually nothing that can be done against them except to prevent them by eliminating the fuel in the first place.

Another area that is always worth considering in detail is any dust extraction system, as it is designed to collect the dust and therefore the fuel is present as part of the normal process. Dust extraction systems also tend to connect a lot of equipment, so an explosion propagating through the extraction ductwork can emerge in many locations and cause follow-up incidents there. Important design considerations for any dust extraction system are the design of the ducts and the conveying air velocity in the various ducts. It is important to prevent dust deposits in the ducts, since they can be raised into a cloud in the case of an incident which will help the explosion propagate further through the ducts.

The Basis of Safety for Dust Cloud Explosion Hazards
After the hazard assessment has been carried out and the preventive and protective measures have been implemented, it is important to record the “Basis of Safety” of the operations. The Basis of Safety is exactly that: what makes the process safe. It usually is a combination of technical and organizational measures. Prevention of explosions is generally more desirable than explosion protection, for obvious reasons.

Often more than one Basis of Safety is implemented for the same process. Just because, for example, a dust collector is fitted with explosion protection, it does not follow that preventive measures are not needed. Instead, the conditions should be such that preventive measures ensure that the explosion protection will not be called upon very often, and preferably never, in the lifetime of the plant. Similarly, if dust clouds cannot be prevented all the time, ignition source control may be considered as a Basis of Safety. Nevertheless, dust cloud prevention should be as good as possible so that one does not just rely on ignition source prevention.

The Basis of Safety for a plant or process is specific to that particular installation, and may differ from the Basis of Safety chosen for another installation that is broadly similar. The choices depend on the exact circumstances. A factor that often remains unspoken in the choice of Basis of Safety is what happens if an incident occurs. The systems may have prevented casualties and damage, but will there be downtime before the plant can start up again? And if so, is that interruption acceptable?

When discussing Bases of Safety, we tend to put a lot of emphasis on hardware: specification of electrical equipment, explosion protection systems, etc. However, the “software” is at least as important. Installation and maintenance are often considered in this respect, but “training” and “management of change” are often overlooked in this respect.

Employees can be a significant safety asset if adequately trained, not just in their jobs, but also in awareness of dust explosion hazards, recognition of the conditions leading to dust explosions and the explosion prevention and control measures implemented in their facility.

A good management of change procedure is vital for the safe operation of a plant. Many incidents happen when changes are made to an installation. This can be a temporary change or it could be a permanent one. For example, increasing the throughput of a plant by removing bottlenecks and increasing processing speeds will probably increase the amount of dust that is generated and that may no longer be effectively controlled by the existing dust extraction system.

Wood dust is a combustible material, and when fine and dry enough it creates a dust explosion hazard. Understanding that this is the case in many wood processing facilities enables the assessment of the explosion hazards, the identification of suitable preventive and protective measures, and the formulation of a suitable Basis of Safety.

Pieter Zeeuwen is a Senior Process Safety Specialist, Explosion & Process Safety, at Chilworth Technology, Inc. in Princeton, N.J. He can be reached directly at 609-799-4449 or pieter.zeeuwen@dekra.com.