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A Scientific Review of Dust Collection - Part 10

Fires, Explosions, Hazards

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next several months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Flammable and Explosive Powders

Powders have a range of hazardous characteristics from very dangerous to somewhat hazardous. A powder by itself may have an oxidant that requires no oxygen from the atmosphere to produce an explosion. Examples are propellants such as chemicals that inflate automobile air bags and gunpowder. Another category is dust that is very easy to ignite and requires no ignition source. There is a form of carbon that is so fine that when it is hurled into the air from a shovel, it will burn before the dust hits the ground. The degree of danger is often related to the fineness of the dust and to a small quantity of it that has an extremely large surface area in contact with the atmosphere. Another example is in diesel engines where there is no need to provide an ignition source in the engine since the liquid mist consists of very fine droplets that are in contact with the surface area of air.

Most explosive dusts and powders fall into the categories where they can produce an explosion. An explosion is defined as a process in which the fire will propagate itself and develop high pressure in a confined space. There are lower and upper explosion limits. If the concentration of dust is below the lower explosive limit, ignition will not create and explosion. The heat produced is not sufficient to affect the adjoining particles. Also, there is a dust concentration limit that would usually be explosive, but there is not sufficient oxygen to burn the adjoining particulate. Therefore, the flame front is blocked by their own dust particles. The key is in evaluating the explosion hazard since the combustion must be rapid. For instance, fine aluminum powders are very explosive with minimal explosive limit concentrations. However, very coarse aluminum powders will not even sustain combustion when some external heat is applied.

Mechanics of Explosions

When a mixture of gas and compound is mixed in explosive concentrations, combustion is rapid and radiates from the source of ignitions as sonic or supersonic velocity. The face of this combustion layer is called the "flame front". It travels until it reaches a solid wall or runs out of combustible material. In a confined space the pressure will rise immediately. In a shaker collector, one possible ignition source may be from a spark that was generated by static charges during the shaking of the bags in the cleaning process. There are two kinds of explosions, primary and secondary. In an air venting system that is connected to various machines, the dust concentration at one machine may be explosive and provide an ignition source which will trigger a primary explosion. The flame front may travel through the ducts to the collector and dislodge all of the dust on the filter bags which causes the dust concentration to be explosive. The flame front will then trigger an explosion in the collector which is a secondary explosion.

Explosion Vents

In the design of dust collectors handling explosive dusts, the usual approach is to provide vents that will direct the explosion out of the collector to prevent or minimize damage to the collector. There are several designs of explosion vents.

  • Membrane where a known strength of membrane will rupture at a present pressure.
  • Hinged or restrained low inertia panels that are held with springs that can be adjusted to different pressure points.

To provide guidance in controlling the rate of dust combustion and in reducing the structural effects on the dust collectors and related equipment, the National Fire Protection Association has written NFPA-68 called "Guide for Venting of Deflagrations". Historically, vent ratios were defined in a simple formula such as 40:1. However, in recent years, most of the types of dusts were tested for combustibility and for the rate of combustion. A value called "Kst" was generated and it became part of the deflagration index for the particular dust. This "Kst" value is expressed in the units of bar-m/second.

The definition of deflagration is given as the propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium. Almost every standard type of dust has been evaluated and has been given a "Kst" value in the deflagration index. The higher the Kst number, the faster its rate of combustion. By following the specific rules found in the NFPA-68 Guide a "Kst" value is determined and is used to calculate the total venting area that is needed for each individual application. Each specific application may be somewhat unique; therefore, the dust application should be reviewed by appropriate authorities such as insurance carriers or local government agencies.

Shaker Mechanical Cleaning Collectors

As mentioned earlier, a shaker collector could auto ignite during the shaking process. It is inevitable that the inside of the collector will pass between the upper and lower explosive limits during the cleaning process. To reduce this hazard, the bags are treated to ground the media which involves the coating of the bag fibers with a conductive coating and to ground the bags to the housing. The biggest risk is during the shaking process since the inventory of dust on the bags is much higher than for the pulse jet units which results in the collector becoming more vulnerable to secondary deflagrations.


Fabric Pulse Jet Collectors

The reverse jet dust collectors have an inherent advantage in collecting dusts that have an electrostatic charge. The best way to dissipate charges from a powder collecting on the surface of a filter media is to reverse the flow through the dust with a gas that is not ionized. The cleaning system process removes charges continually. They are inherently also much more resistant to secondary explosions. Figure 10-1 shows a cylindrical bag. When the bag is 41/2 inches in diameter and 96 inches long with a high ratio reverse air system, the total quality of air injected into the bag during a cleaning cycle is approximately one cubic foot. This dust laden air forms a hollow cylinder one inch thick. The cylinder of dust then falls down into the hopper.

Secondary Explosion Damage Potential

The engineering approach is to reduce energy produced in a deflagration. The energy is dependent on the availability of fuel and the availability of oxygen in the air.

  1. Reduce oxygen available by using more compact high ratio advanced technology fabric designs and by using collectors with multiple hoppers to reduce hopper volume on product side of the collectors.
  2. Reduce the fuel that is available to fuel the fire or "conflagration". A bag with smooth finish felted media, such as a bag with eggshell or singed finish, will reduce the dust inventory. Maintain a low pressure drop through the filter media as indicated by the readings on the magnehelic gage which is accomplished by frequently pulsing the air cleaning valves. A low pressure drop through the filter media correlates to a minimum amount of dust cake / inventory. Another option would be to use PTFE laminated bags and increase the cleaning frequency so that the dust inventory can be reduced to minimum levels.
  3. For cartridge and pleated bag media, keep the pressure drops low when passing air through the cartridge media by cleaning the media frequently.
  4. Remove the dust from the hopper as fast as it is collected so that there will be less fuel to feed a conflagration.

Other Operating Techniques

Sometimes dusts from other processes can be mixed in the air stream before the collector in order to gain a less combustible mixture. For instance, if one machine produces an inert dust and another machine produces a combustible dust, the mixture will be less combustible than single combustible dust.

Fire in Collectors

There are many dusts and powders that will burn but the rate of combustion is low. Since the dust is the fuel and oxygen is present, the design of most dust collectors unfortunately is actually an efficient furnace design. The velocity of air across the media is such that fires are fanned very effectively. By careful selection of filter media and by observing good operating techniques, the reduction of deflagration can be achieved which will help reduce or maybe prevent fire damage.

Causes of Fires

There are two main methods to ignite fires. One is by sparks entering the collector and the other is by spontaneous combustion.

Sparks

Sparks and flaming debris (i.e3. cigarette butts) are often drawn into the hoods and ducts of a venting application. It might seem that a long dust run would cool the sparks or debris enough to prevent a fire from occurring, but there are causes where sparks have travelled over a hundred feet, continued through a cyclone, and finally arrived inside the dust collector and started the dust and bags on fire. Also, the fire will start while the venting is in full operation. For example, when a campfire burns, sparks can be seen rising over the fire. These sparks consist of heavy particles, yet they will often be lifted in the rising, smoky air at very low velocities. Actually, the relatively heavy and dense spark surrounds itself with a layer of hot air (Figure 10-2) which allows the solid spark particle to be more buoyant than the surrounding air. In the case of sparks traveling in a duct, the spark is also surrounded by buoyant hot air and the spark travels easily a long distance through the duct. In addition, the duct system and cyclones are designed for smooth flow through the pipes; therefore, the spark dust does not drop due to gravity nor does it spin out by the centrifugal forces that are generated in a cyclone.

Spontaneous Combustion

When fume dusts are collected, they are very fine with huge surface areas. When the dust collector is shut down, the fumes may continue to oxidize and this oxidation produces heat. When the dust collection system is operating, the heat from oxidation is removed by the flow through the filter elements. When the flow is stopped, these types of dusts can cause hot spots to develop in the filter cake, and when the flow is again started, the "hot spot" may ignite a fire because the velocity through the bags creates an ideal process to fan the flame.

Extinguishing Sparks

The requirement is to cool the spark. The hot gas must be stripped from the spark and gas flow around it must develop a difference in velocities. Actually, sparks can be cooled in a fraction of a second and a good method to accomplish this is to induce a turbulent air flow into the system. Instead of the process air moving smoothly and in a straight line, eddies are formed in the gas stream. These eddies travel through the ducts and will strip any air from the spark particle which will cause the cooling of the spark. Some methods to induce turbulent flow are listed below:

  1. An abrupt transition by changing duct sizes will generate turbulence.
  2. Single or multiple orifice plates will cause turbulence. Often these are installed near the hood inlets. The pressure drop across a transition is related to average velocity. At 1000 feet per minute, the pressure drops due to transition will be approximately 6% as compared with a transition at 4000 feet per minute.
  3. A square elbow with no turning vanes will also induce turbulence.
  4. A designed spark trap.

Sprinkler Systems

The underwriters often dictate the installation of water sprinkler systems in collectors, especially when the air is recirculated into the workplace. When the fire is completely out, it is important to make sure that the water flow from the sprinkler system is stopped. If the water flow is not stopped, the hopper and bins of the collector could collect significant amounts of water and may cause structural damage to the collector itself and surrounding structures. In some cases, the water damage could exceed the fire damage.



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