Using Pneumatic Conveying to Reduce Your Combustible Dust Explosion Risk
Combustible dust can be a severe hazard to your plant and workers, but a pneumatic conveying system can reduce your plant's risk for a combustible dust explosion. This article explains the five elements required for a dust explosion and shows how a pneumatic conveying system allows you to control those elements to reduce your plant's dust explosion risk.
Combustible dust is a critical concern for any industry that processes and conveys bulk solid materials. Changes in regulations have brought this issue into even greater focus. The NFPA has issued stricter safety standards to prevent and mitigate explosions and fires in plants where combustible dust is present. Several NFPA standards pertain to combustible dust in various individual industries, but the NFPA’s intent is to have consistent standards across industries and materials.
Under the right circumstances, nearly any type of dust can be combustible. Even some materials that aren't normally considered combustible can produce combustible dust. Generally, particles smaller than 40 mesh can create a hazard if they’re allowed to accumulate or become airborne. Materials that produce combustible dusts include sugar, coal, aluminum, wood, graphite, corn, titanium, paper, and many others. Even in this short list of examples, you can see that many industries deal with combustible dust. As shown in Figure 1, combustible dust explosions have impacted a broad cross-section of industries.
The Dust Explosion Pentagon
Five elements are needed for a dust explosion to occur, as shown in Figure 2: combustible dust, oxygen, an ignition source, dispersion of the dust in the air, and a confined space in which the dust cloud is contained. These five elements are called the dust explosion pentagon.
Removal of any leg of the
combustible dust pentagon
can prevent an explosion.
The first three elements are needed to ignite a fire (or deflagration), while the last two elements can turn that fire into an explosion. If any one element is removed or suppressed, an explosion can't occur.
Pneumatic conveying addresses all five dust explosion elements and allows you more control over the elements than typical mechanical conveying methods, such as belt, vibratory, chain-drag, or screw conveyors or bucket elevators. With pneumatic conveying, you can more effectively remove elements of the dust explosion pentagon from your process, helping you to prevent a combustible dust explosion and meet NFPA standards. Let’s explore how pneumatic conveying allows you to control each dust explosion element.
Avoid Fueling the Explosion
Combustible dust created from processing your material provides the fuel for a dust explosion. Eliminating dust completely can be very difficult in any conveying system, but a pneumatic conveying system can help you minimize this fuel source.
Dust is created when your material’s particles break or degrade during handling. Many mechanical conveying methods tend to facilitate that breakage due to vibration and particle-to-particle or particle-to-equipment abrasion. Depending on your material, you may be able to use dense-phase pneumatic conveying to minimize this breakage and produce less dust. Unlike dilute-phase pneumatic conveying, which uses low-pressure, high-volume air to convey material at high velocity, dense-phase pneumatic conveying uses high-pressure, low-volume air to convey the material at low velocity. The material moves through the conveying line in a series of slugs in which there’s very little interparticle abrasion or impact with the conveying-line wall. This slug flow can be much gentler and less abrasive on your material and create less dust than dilute-phase pneumatic conveying.
Dust accumulation in the workspace or in your conveying system can also provide fuel for a fire or explosion. If your conveying system allows material to drop out of the process stream and accumulate in the workspace or conveying system, this accumulation can create an explosion hazard. Unlike mechanical conveying, pneumatic conveying offers 100 percent material containment with no transfer points or other locations for material to accumulate. Also, a pneumatic conveying system can include air injection ports to add motive air into the conveying line at points where material might be prone to accumulate in the conveying line. This additional airflow keeps material moving along the conveying line and prevents material accumulation that could fuel an explosion.
Eliminate Ignition Sources
Ignition sources come in many forms, as shown in Figure 3. The ignition source for a fire or explosion in a mechanical conveying system can be anything from a spark produced by an out-of-alignment system component to an overheating bearing or belt. Visually monitoring your entire system to find these potential causes of ignition can be difficult.
Pneumatic conveying eliminates the major ignition sources, minimizing the possibility of an explosion. With pneumatic conveying, few moving parts come into contact with your material or other equipment. Since there’s no friction between moving components, a pneumatic conveying system reduces the chance of sparking and heat sources that could ignite the material. Often, pneumatic conveying systems also incorporate automated controls and monitoring devices, which optimize process safety. These precision controls can monitor the system and stop potential equipment problems before they can develop into an ignition source.
Take Oxygen Out of the Equation
It's difficult to imagine a way to keep oxygen out of any bulk solids manufacturing process. We're surrounded by it! But you don’t need to eliminate oxygen completely to prevent a dust explosion; you just need to reduce the oxygen concentration to below your material's limiting oxygen concentration. The limiting oxygen concentration is the volumetric percentage of oxygen in the conveying gas below which combustion cannot occur. This “safe” oxygen level varies greatly depending on the material and particle size but is typically less than 15 percent, as shown in Table I. Keeping the oxygen in the conveying gas below the material's limiting oxygen concentration eliminates the possibility of an explosion.
In a pneumatic conveying system, this is done by inerting. Inerting uses an inert gas, such as nitrogen, rather than air for the conveying gas. This reduces the oxygen percentage to a safe level and removes the dust explosion threat for the vast majority of bulk-solid materials. Inerting a mechanical conveying system would be very difficult, however, because a mechanical conveying system isn’t completely enclosed like a pneumatic conveying system.
Manage Your Dust Dispersion
Combustible dust needs to be in specific concentrations in the atmosphere in order to be an explosion hazard. The NFPA has done deep research on combustible materials and has developed specific formulas for calculating these concentrations. It’s important that you have your dust tested to determine safe concentration levels for your material and application.
With a mechanical conveying system, the surrounding air isn’t moving, so dust can hang in the air and reach dangerous concentrations. A pneumatic conveying system encloses the material and dust and keeps it moving, allowing you to control the air-to-material ratio and ensure that a dangerous dust concentration doesn’t develop. This is impossible with an open mechanical conveying system without the use of an extensive dust collection system and sheathing.
Contain the Dust Cloud Safely
The final element of the dust explosion pentagon is containment of the dust cloud. As stated earlier, in a pneumatic conveying system, the material is 100 percent contained, which may seem to present a hazard. In a properly designed system, however, that containment is controlled to keep your explosion risks to a minimum. Pneumatic conveying allows you to contain dust safely and collect it from the system so it doesn’t accumulate in the workspace and create a hazard. Remember, containment is only a concern if all other elements of the dust explosion pentagon are present. A properly designed pneumatic conveying system can prevent that from occurring. Work with an experienced pneumatic conveying system supplier to ensure that your system efficiently and effectively conveys your material while keeping your plant and workers safe.
See www.nfpa.org and www.osha.org for detailed information on combustible dust standards and explosion prevention.