Dust explosion strategies
Dust explosions are one of the most challenging, complex, dangerous, least understood hazards facing industry today. On one level it is not hard to understand what a dust explosion is: A loud noise, accompanied by a pressure wave and a lot of heat. But what is required for dust to constitute an explosion hazard?
Dust is a solid material-organic or unoxidized metal-not larger than 500 microns in cross section. Smaller particles make more explosive dust, and less noble metals are more explosive than more noble ones. In the simplest possible terms: oxygen + fuel = oxides + heat.
Dust must be agitated so that each particle is surrounded by an oxidant (usually air) to constitute an explosion hazard. Putting this dust into an enclosed space and concentrating it sufficiently supports the explosion. Lastly, a source of ignition is needed. When these factors meet, an explosion results. About 80% of all industrial dusts are explosive if oxygen and ignition are available.
There are a number of technologies and strategies available to deal with possible dust explosions. There is no one approach that is best for every circumstance. A blend of strategies is often optimal.
Strategies can be subdivided into those which mitigate the damage, but assume an explosion will occur; and those that seek to prevent the explosion from developing by controlling one or more of the contributing factors.
A dust explosion is a pentagon consisting of fuel (combustible dust), suspension (agitated dust), confinement (with an explosible concentration of dust), oxidizer (usually air), and ignition source (of sufficient strength and duration).
There are three avoidance approaches to dust explosions (assuming the pentagon can be avoided):
-Avoid ignition sources
-Avoid explosible dust concentrations
-Inerting to reduce oxygen concentration.
-There are also several ways to mitigate an explosion (assuming the pentagon could occur):
-Locate the vessel outside so that an explosion causes no consequential damage
-Locate the vessel inside, but adjacent to an outside wall so that a vent can be directed outside through a straight, short duct
-Locate the vessel indoors, but duct through the roof
Sources of ignition must be identified . Open flames are introduced from within a system or from an external source. Hot surfaces come from many sources, such as dryers, steam pipes, and heaters. Also consider the not-so-obvious possibilities such as blowers, fans, conveyors, milling machines, and other rotating machinery that could develop a hot bearing.
Another source of ignition is the introduction of heat between moving and nonmoving parts in process equipment. Mechanical impact is also a potential source of sparks. Static electrical charges must be prevented.
It is not possible to engineer a system that avoids the possibility of an ignition source for every eventuality. Because of the numerous possibilities, it is less than prudent to rely on avoidance of ignition sources as the only means of dust explosion prevention.
Avoid explosible dust concentrations. There are lower and upper limits to dust concentrations that support an explosible dust cloud. These limits are difficult to define because of the distribution of particle sizes in a cloud and the presence of fines.
In practical terms, a lower explosible dust concentration is in the range of 50-100 g/cu m and a maximum range is 2-3 Kg/cu m.
Inerting uses a gas such as nitrogen or carbon dioxide that displaces the air and with it the oxidant that supports combustion. This subject is complex with many variables.
Equipment must be sealed from the surrounding environment to contain the inerting agent and protect plant personnel. Loss of sealing has several negative consequences. The least serious is the usage rate for the inerting gas goes up substantially. Of greater concern is the possibility of a leak allowing a concentration of oxygen sufficient to support an explosion. Lastly, leaks or inattention could cause asphyxiation.
Venting is a simple principle. When the dust explodes inside a confined area, a deliberately weakened wall releases early in the pressure rise caused by the rapidly increasing temperature. Once this weak area is opened, burned and unburned dust and flame are allowed to escape the confinement so that the vessel itself does not experience the full rise in pressure. If the weakened area releases early enough and is large enough, the pressure remains sufficiently low inside the vessel to protect it from damage. The explosion is allowed to fully develop, but it does so outside the vessel.
From such a simple concept, venting has turned into a complex solution. How big does the vent really need to be? At what pressure should it burst? What part does vessel strength play in the venting solution? Where can the discharge of the vent be directed? How far will the blast travel?
National Fire Protection Association Guideline (NFPA 68) includes procedures that provide help in venting solutions.
Containment is very useful, especially where dust explosions are considered inherent or very likely to occur, where other methods are considered intrusive to the process, or where it is too easily rendered ineffective by externalities.
Containment is approached from two perspectives. First, equipment is designed to contain the maximum pressure exerted by the dust so that the elastic limits of the vessel’s materials of construction are not exceeded. Second, assume that the explosion will deform the vessel by exceeding those limits, but fall short of causing a breach. This technique assumes that the vessel would need repair or replacement after an explosion, but it provides a useful approach, especially where the vessel is relatively large and the cost of the first alternative is considered prohibitive.
In equipment where the possibility of explosion is inherent to the process and expected to be fairly frequent, the first design makes sense. In situations where other controls make the possibility of explosion unlikely, the second method is more practical.
Active suppression provides a permanently available, pressurized extingui-shing agent and a means to ensure fast discharge of the agent. The systems are active and rely on carefully positioned sensors, which are pressure, UV, or infrared optical. They must be selected with great care to be sensitive enough to protect, while at the same time not being so sensitive as to unnecessarily false trigger.
There are a number of strategies available that focus on (1) localizing the explosion, (2) isolating various areas of the system to prevent propagation, or (3) total saturation of the entire system.
Quenching pipe combines characteristics of inerting, containment, and venting. The operating principle is that in the event of an explosion in a protected vessel, a venting panel releases, burned and unburned dust and flame enter the chamber of the quenching pipe, and dust is retained on the surface of a mesh material. When the flame encounters the mesh, the temperature is quenched from at least 1500 C to approximately 90 C in a few milliseconds.
The device is actually performing three functions. First, as dust enters the quench tube the concentration increases rapidly in an area of limited oxygen; thus, the explosibility of the mixture is significantly reduced. Second, as the mesh filters the dust, fuel is removed from the deflagration. Third, the temperature is rapidly quenched because the mesh acts as a heat sink.-Edited by Ron Holzhauer, Managing Editor, 630-320-7139, email@example.com
About 80% of all industrial dusts are explosive if oxygen and ignition are available.
Mitigation and avoidance strategies are available to prevent or control the problem.
Magnitude of an explosion depends on dust surface area, shape, size, disbursal, turbulence, concentration, and several other factors.
There are a number of variables that affect the magnitude of a dust explosion. These factors include specific surface area of dust particles, shape of particles (cubes, spheres, fibers, etc.), distribution of particle sizes and shape in the cloud, disbursal of dusts (evenly spaced or clumped), amount of turbulence of the cloud, concentration of the particles, percent of moisture in dust, strength of ignition, length and duration of ignition spark, concentration of oxidant, heat of combustion of the dust, and total volume and shape of the vessel. Also consider the process and, in particular, what is being done with the product.
These factors make it impossible to determine more than empirical approximations to predict the outcome of any given explosion. In fact, it is safe to say that no two dust explosions are exactly alike, which makes the subject of mitigation complex and challenging.
Dust explosions have a tendency to propagate from the area of origination to adjacent locations. Explosions travel up and down stairwells, as well as from building-to-building through corridors and tunnels. Dust explosions move along the tops of I-beams via a layer of dust. To prevent this phenomenon, eliminate the availability of sufficient amounts of dust to support propagation.
Use isolation valves to stop propagation from vessel-to-vessel connected by piping. Dust explosions develop so rapidly that isolation valves must be able to close quickly. It is also important that enough space be allowed between the vessel where an explosion could occur and the placement of the valve.
The author is willing to answer technical questions concerning this article. Mr. Stevenson is available at 561-735-9556. The company web site is www.cvtechnology.com.