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, product loss, 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 explosion occurrence cannot be prevented by the elimination of combustible dust clouds and ignition sources potential, constructive explosion protection methods must be applied, to mitigate the consequences of the explosion to a non-hazardous level.
Asia consists of emerging and developing markets which have undergone numerous changes over the years. The growth of the economy has caused an increase in number of facilities as well as number of incidents that have occurred. Amongst the most severe known, recent incidents are the aluminum dust explosion in the automotive parts factory of the Zhongrong Metal Production Company in Kunshan, Jiangsu, China, and the explosion in 2016 in a dust collection device at a cement production line belonging to Qinghai Salt Lake Haina Chemical Company, Bejing, China.
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.
Many types of explosion venting devices are available. In general, venting devices should be made of lightweight material and have non-fragmenting or tethered construction to prevent missiles. Vents should also be fast acting. 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 is 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. A particle retention screen may be added to reduce the ejected amount of fines.
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. The maximum flow rate that can be achieved depends on the area of the valve, the cylinder pressure and the type of flow behavior.
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 or physical barriers 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.
Physical barriers – physical barriers can be passive systems or active systems. With passive systems, the valve mechanisms are inserted into the pipeline and the energy of the approaching explosion closes or opens the valve rapidly so the explosion propagation is halted or diverted away. With active barrier systems, stored energy is released upon explosion detection and drives the valve to closure.
Chemical barriers – chemical barriers are active systems that inject an extinguishing agent into the connection system where the agent remains intimately mixed with the combustible and will not immediately settle out. They can also be discharged slowly to give prolonged protection in combination with longer duration explosion events in large vented vessels.
When combined with a venting system, active physical and chemical barriers are released when the vents open. When combined with a suppression system, they are simultaneously activated from the same control system that triggers the suppression systems.
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 sources 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.
Fike® has over 70 years of experience with dust explosion protection and is equipped with a comprehensive in-house explosion testing laboratory and a large-scale remote testing facility which is used for research, analysis and for the testing of safety protection equipment. While all of Fike’s products have been tested, and approved under full-scale explosion conditions, this unique facility is often used to test specific industry scenarios.
For more information, go to www.fike.com
Top image: Fike fast acting isolation valves.