Part I of this series of articles How to Safely Store Combustible Dusts & Powders discussed how a hazardous area classification should be undertaken and how combustible dust explosions happen. This resulted in the assignment of a hazardous zone within or around items of processing equipment. Part I also considered the ignitability characteristics of powders and discussed various tests that can be undertaken depending on your dust situation i.e., dust accumulation, dust cloud ignition. The second part of this article series will discuss how combustible dust explosion and explosion hazard data should be used to ensure that fire and explosions from combustible dusts can be prevented.
What is combustion?
Combustion is simply an exothermic chemical reaction between a fuel and air or oxygen. For example, combustion of natural gas (methane) in air results in the combustion products of carbon dioxide and water vapour, and the release of a significant amount of energy. Hence any dust or powder which is not fully oxidised i.e., grain dust, will be capable of reacting with air and undergoing the combustion process.
As with most chemical reactions, generally some form of energy must be inputted to the fuel/air mixture to start the reaction – this is generally achieved by heating the reactants, but all the other potential energy sources (electrostatic sparks, mechanical sparks etc) discussed in Part I can achieve the same result.
The Relationship between Combustible Dust, Particle Size & Energy Sources
It is also common knowledge that chemical reactions generally occur more easily when the solid powder is of small particle size – hence why dust explosions occur. This is due to the surface area to volume relationship – the smaller the dust particles size, the larger the surface area. Also, any heat generated from the particle cannot be dissipated throughout the bulk of the powder, so the localised energy vapourises the solid particle.
But the potential energy source must have sufficient energy to start the combustion reaction. This is why understanding the specific ignitability characteristics of the powder is important – the minimum ignition energy and safe allowable temperature are key parameters.
If the energy generated in the processing operation does not exceed the energy required to start the combustion reaction, then ignition will not occur, and a fire or explosion cannot result.
If a generated dust cloud does not come into contact with a source at a temperature above the safe allowable temperature; or an electrostatic spark energy is less than the minimum ignition energy of the powder, then combustion will not occur.
Potential Flammable Dust Clouds and the Hazardous Area Classification
The Hazardous Area Classification has indicated where a potential combustible dust cloud may occur, and how likely they are of being present. The first aim is to ensure that flammable dust clouds do not exist. This can be achieved by ensuring the combustible powders are of large particle size; or are used in a wetted or paste form; or by the use of inerting to remove the oxygen, etc.
Reducing the probability of electrical or mechanical sources of ignition being present, or reducing the potential energy generated from such sources (so the energy is below the powder specific minimum ignition energy etc) can be achieved in various ways.
For example, ensuring all metal components and personnel are earthed will prevent spark discharges; reducing the presence or limiting the size of insulating components, or the addition of anti-static agents to high resistivity powders, can prevent electrostatic charge accumulation; reducing the tip speed of rotating equipment can prevent sufficient mechanical energy being developed.
Electrical/Mechanical Equipment and the ATEX Directives
Any electrical or mechanical equipment installed or used (if portable) within the hazardous zones is required to be of ‘special’ construction. The ‘special’ construction of the item of equipment is laid down in the essential safety requirements of the ATEX Directives and are certified as appropriate for the zone.
Essentially the construction ensures that sparking components either do not generate sufficient energy, or prevent the sparking components from coming into contact with a flammable dust cloud. There are different protection mechanisms used in the construction (for example, intrinsic safety, pressurisation, encapsulation etc).
This essentially means that the probability of the equipment acting as a potential ignition source for a flammable dust cloud present is low. The aim is to eliminate the potential of an item of equipment acting as an ignition source.
Flammable Dust Cloud & Ignition sources
The result of a fire or explosion is a combination of the likelihood of a flammable dust cloud being present, and the likelihood of an effective ignition source being present simultaneously. If a flammable dust cloud is present continuously (Zone 20), then the risk reduction is achieved by the probability of an ignition source being present.
In this case, so-called Category 1 equipment is constructed which essentially means that the equipment will not act as an effective ignition source in normal operation; in an expected malfunction state; or in a rare malfunction.
If a flammable dust cloud is present infrequently (Zone 22), then the risk reduction is primarily achieved by the probability of a flammable dust cloud being present. In this case, so-called Category 3 equipment is constructed which essentially means that the equipment will not act as an effective ignition source in normal operation only.
The equipment can also be constructed in such a way that the equipment cannot generate heat. Provided the maximum temperature that the equipment item is below the safe allowable temperature then safety is ensured.
Although, prevention or reduction of the generation of flammable dust clouds and the reduction of potential effective ignition sources should ensure safety, an actual ignition may still occur – the risk has been reduced but not eliminated. Therefore specific measures can be taken on your processing plant to deal with potential ignited events.
Dealing with an Ignited Event
Although we have done our best to prevent these events, they can still occur. In safety language what we have done is reduced the risk of fire and explosion to a very low level. We therefore now have to deal with the ignited event. This is often known as the Basis of Safety – the whole gambit, including organisational aspects to ensure that if a fire or explosion actually occurs, we do not hurt anyone.
The Basis of Safety
The Basis of Safety includes the Prevention of fires and explosions, and the Explosion Protection aspects, which is the subject of this article.
Explosion protection is generally achieved by one of three means:
- Containment – the effects of an explosion are contained within the equipment item. There are no external impacts of the explosion.
- Explosion Venting – the effects of the explosion (flame and pressure) are vented to a safe location.
- Explosion Suppression – the initial effects of the explosion are detected rapidly and extinguished.
Containment
The containment can be constructed in two ways – the first is explosion resistant plant (the explosion effects are fully contained, and the containment system suffers no damage. Therefore, it could be cleaned out and re-used.
Explosion Resistance
The second type is explosion resistant. Again, if a dust explosion occurs, the explosion is contained but the equipment may suffer some damage. The containment system may be bulged or distorted. Obviously, in such cases, the strength of the containment system will be different than the original design, and is likely to require replacement.
Normally containment as a Basis of Safety is little used in the powder handling sector – such systems are expensive to construct, but they are very effective (if designed correctly). Equipment that is routinely robustly manufactured, such as hammer mills etc often provide containment from explosion as well as containment of missiles etc.
Explosion Venting
Explosion venting is perhaps the most common protection seen in the powder handling industries. Many dust collectors have explosion venting as the explosion protection. It relies on an engineered passive system – a deliberately weak section of plant, with low inertia, which opens at very low pressures (known as PStat) very quickly and vents the flame and pressure effects to a safe area – often outdoors at high level to ensure people are not impacted during the venting.
If correctly designed, the original equipment is essentially undamaged because it only ‘sees’ a much reduced pressure (known as PReduced). The PReduced and PStat are important aspects, but the area of the vent needs to be correct as well. These factors can be designed into the system to ensure effectiveness.
Obviously during the venting process there may be some development of back pressure on the system, especially if the relief is ducted along tortuous ductwork runs. This has the effect of increasing the vent area required. Ideally relief ductwork should be straight and short (less than 3 metres).
Indoor Venting
Sometimes because of the restraints described above, the explosion vent is located indoors. When an explosion is vented a large flame, combustion products and pressure emanate from the opening which can cause serious harm to people. The combustion products (and indeed any entrained combustible dust) may be toxic causing potential additional hazards.
Traditionally, the area where the vent is became a cordon santitaire, with interlocked fencing preventing personnel access during operation. Now, special vents have been developed which prevent the passage of flame from the vent – so-called flameless venting. Although these prevent the flame from causing injury, the combustion gases and pressure still remain a potential hazard.
Explosion Suppression
The third method, explosion suppression is becoming increasingly common. It relies on detector systems to detect the initial stages of an explosion (either pressure or temperature etc) which on activation cause the rapid injection of an extinguishant into the system. Appropriate design by specialist providers is important especially with regards to the siting of the detectors in relation to the extinguishing point.
Whichever explosion protection method adopted also has to have means of preventing the propagation of the flame and pressure effects into upstream or downstream connected equipment. Flame and pressure is like water flow – it takes the route of least resistance. If such means of preventing propagation are not installed then a series of explosions within connected equipment items may occur.
Sometimes these isolation means can be fairly simple, for example, a choke in a screw conveyor or a rotary valve. Such systems allow a plug of powder to prevent the passage of flame etc. Prevention of flame flow in ductwork can simply be achieved by a flap valve – the valve is normally held open by the extracted airflow, but reverse flow will close the valve.
In other cases, it can be more complex, similar to the explosion suppression systems discussed above. Detection of the flame or pressure activates the operation of a quick acting valve to isolate sections of the plant.
Sigma-HSE
Sigma-HSE is an internationally recognised process safety company with ISO/IEC 17025 accredited combustible material testing laboratories. We provide fire and explosion data: 20L, MEC, MIE, LIT, LOC, electrostatics and thermal screening. Our safety engineers are experienced in DSEAR, ATEX and DHA across many process industries and are on hand to help you prevent combustible dust explosion, fire and other related hazards.
