A dust explosion is the rapid combustion of fine particles suspended in the air within an enclosed location. The danger of an explosion is ever-present in industrial processes that handle combustible dust. While not all incidents receive notoriety, most result in economic loss due to downtime, lost production capabilities, and process equipment destruction or damage.
Some explosions injure or kill workers and non-plant personnel. Statutory requirements by the NFPA in the United States and by ATEX in the European Union mandate the use of explosion protection. If the explosive environment cannot be avoided by the elimination of combustible dust clouds and potential ignition sources, constructive explosion protection methods must be applied, to mitigate the consequences of an explosion to a non-hazardous level.
Dust explosions can happen when a sufficient amount of fine combustible dust becomes airborne and finds a hot spot that ignites the dust/air cloud. Dusty product typically involved in dust explosions are dry food products, grains, coal, wood, pharmaceuticals and even metal dusts from polishing or grinding operations. The fine dust particles act as fuel, and intermixed with air, can sustain a very rapid combustion with flame speeds up to 5 meters per second or more. When this happens inside dust processing equipment such as dust collectors, conveyors, dryers, mills, sieves, mixers or storage equipment, the combustion will rapidly heat up the process air and generate pressures up to 10 bars or more. The casing of the process equipment is typically not strong enough to withstand the explosion pressure and ruptures. Flame, pressure and metal parts fly off several meters throughout the facility and damage and hurt nearby people and equipment. When dust layers are present on facility beams, support structures or floors, they are whirled up by the ejected pressure wave, become airborne and further sustain the dust explosion. In the most dramatic incidents, complete dust processing facilities have been blown up resulting in multiple fatalities and complete plant destruction.
The first step in preventing dust explosions is to try to prevent explosive dust clouds from being created. Although this is often not possible inside process equipment (unless the process air is inerted by nitrogen), prevention of unnecessary spillage of dust on floors and facility structures surrounding the processing equipment is very manageable. Contained processing, leak tight casing design and dust cleaning operations will reduce considerably the likelihood that a dust explosion will arise, as well as reducing the extension of the incident in case the dust explosion occurs.
The second step is to eliminate potential ignition sources. The most frequent ignition sources that are reported are hot spots and sparks arising from mechanical friction or collisions. Correct and robust design of mechanical movement, slow movement or temperature supervision on areas where friction is expected are the most applied prevention methods. As well as ignition sources from mechanical friction, there are also electrical sources of ignition which include sparks from electrical circuits and electrostatic discharges. To prevent this from happening, all electrical equipment used in areas where combustible dust clouds may arise should carry the correct protection approvals according NFPA or ATEX. Electrostatic discharge can be prevented by correctly and robustly earthing and bonding all metal parts of processing equipment as well as the careful and restricted use of non-conducting materials that come into contact with combustible dust. Finally, there are explosions which have been ignited by auto-combustion and self-heating of large piles of stored product, such as wood or grains. In these cases, supervision of self-heating by temperature, infrared or combustion gas sensors needs to be applied.
The final step is the application of constructive mitigation measures. These are designed to limit the consequences to non-hazardous levels, in case a dust explosion still happens, despite the preventive measures taken. Mitigation techniques include explosion venting, suppression and explosion isolation.
The oldest, most widely used protection method is explosion venting. The technique is simple – a sufficiently large opening relieves the build-up of destructive explosion pressure by allowing the combustion by-products to flow out at a rate high enough to prevent the pressure from rising above the pressure capability of the protected equipment.
Venting devices should be made of fast-acting, lightweight material and have non-fragmenting or tethered construction to prevent missiles. In some applications, the speed of response can be critical because the pressure continues to rise as the vent opens; the pressure can increase considerably above the nominal vent-opening pressure even if the full venting area is available immediately.
Venting has several strengths: it’s a passive method and the equipment is relatively inexpensive, it’s easy to install, has low maintenance requirements and can be rapidly replaced. However, the method also has several weaknesses: it is difficult to protect equipment inside buildings because combustion by-products must be released or vented to a safe location (preferably outside), it doesn’t prevent fire damage, it can’t be used if toxic or environmentally harmful materials will be vented to the atmosphere, and in some cases, very large venting areas may be needed for low-pressure structures.
With flameless venting, a box with flame filter material is installed on top of the vent, allowing the vent to open within the box, forcing the combustion by-products to pass through a flame filter where it is extinguished. Flame filters typically consist of porous metal packages, with pore size small enough and package thickness large enough to cool the combustion by-products down to such a level that further combustion is halted.
Since no flames are vented, flameless venting is an often-used technique for indoor installations, as there is no risk of secondary explosions. Also, nearby operator injury is prevented if a small safety distance is respected. As with venting, very low maintenance efforts are required.
The adverse effect however is that the outflow of combustion gases may be hindered by the flame filter. This leads to higher internal pressures that can exceed the equipment capability which needs to be compensated by using larger venting areas.
Explosion suppression uses extinguishing or suppressant agents to stop the combustion process and prevent deflagrations from generating damaging overpressures. If suppression is to succeed, a large excess of suppressant agent must be mixed through the protected volume very rapidly. Within a time interval of milliseconds, the deflagration must be detected, the suppressor valve opened and the suppressant discharged and mixed into the vessel volume. Larger vessels need a longer time interval; smaller vessels a shorter time interval.
A typical explosion suppression system consists of a detector, electrical control circuitry and a suppressant container. Explosion detection is achieved by optical detectors or pressure detectors. Pressure detectors will not respond until the pressure has risen to a detectable set level. Optical detectors signal within milliseconds once the flame is visible. The most time-consuming step is the flow of the suppressant agent from the cylinder through the opened valve and its dispersion into the protected volume.
Because space and time are important for suppression to be successful, the number, type, activation-levels and proper location of detectors and containers is essential to provide rapid dispersion in the protected volume.
Explosion suppression has several strengths: it allows complete containment of process media and assists in controlling any ensuing fire, it reduces the propagation of the flame front to other process equipment, it can be used indoors near personnel and it can be integrated with other protection methods and shutdown procedures.
When deflagrations occur, the effects are frequently transmitted to other vessels or locations that are connected by piping, ducting or conveying systems. These connections become deflagration transmission pathways.
The ability of flame to propagate away from the ignition point can cause it to travel in the opposite direction of the flow in flowing systems. Flame propagations can also lead to pressure piling, where the pressure builds up in adjoining vessels prior to the flame arriving. As a result, the ensuing deflagration in connected vessels starts at an increased pressure and flow turbulence with more serious consequences – both in terms of the rate of combustion and the final pressure. Even if the connected vessel is protected, the protection method may be ineffective because it was designed for less severe initial conditions.
That’s when you use explosion isolation. Explosion isolation systems use chemical barriers with extinguishment powder or physical barriers (active and passive rapid acting valves) interposed into lines leading to and from vessels. The objective of these barriers is to prevent the propagation of pressure and flame to additional equipment or operating locations. The design challenge is to determine the right position of the barrier, not too close to the vessel, so the barrier is in place before the explosion reaches its location, but also not too far from the vessel, so that the explosion violence and associated pressure piling has not reached a point where it could destroy pipeline or barrier functions.
Applying effective explosion protection systems
Applying effective explosion protection systems for dust handling processes requires a holistic approach. Each equipment or process part under ignition risk requires basic protection, consisting of venting, flameless venting, suppression or a combination thereof. Interconnections on the other hand require explosion isolation. However, basic protection and isolation are interdependent. In certain cases, the primary protection can be applied so that explosion propagation is made impossible and isolation is not required. In other cases, a process apparatus is an integral part of an interconnection. Also, certain choices of basic explosion protection can make explosion isolation impractical or impossible. The challenge lies in correctly understanding how the process works, where the dust cloud and potential ignition source are present and how an eventual explosion would develop. As a result, explosion protection methods should be chosen with care, matching the most appropriate techniques against the application. You should adopt a systems approach to explosion protection, rather than considering each method separately.
For more information, go to www.fike.com