Appeared on Powderbulk.com
January 2016 By John Micheli, Middough Inc.
Airborne combustible dust in the right concentration and exposed to an ignition source will ignite and create a deflagration (or rapidly traveling flame front). If that deflagration occurs within a confined space, such as a manufacturing process vessel, the pressure buildup can cause the vessel to rupture or explode. In some cases, such an explosion can spread well beyond its point of origin. A primary explosion can disturb settled dust from nearby surfaces and disperse the dust into the air. This larger combustible dust cloud can then ignite and create a secondary explosion with potentially devastating consequences for plant workers and property
In response to a series of fatal dust explosions, in 2007 OSHA issued a National Emphasis Program (NEP)1 (and reissued it in 2008) to focus on combustible dust safety at manufacturing plants. The NEP is broad and covers most industries, listing multiple NFPA standards, ASTM tests, and Factory Mutual industry guidelines as applicable references in its appendices. These references are regularly updated, so to ensure that your new or existing plant is safe and compliant with regulations, you should use the most current versions rather than the versions that were available when the NEP was issued.
Your insurance carrier has an experienced loss-control staff that can provide technical assistance or advice regarding dust hazards. Contact your insurance carrier early during a capital project to discuss potential hazards and mitigation methods. This will allow you to include the carrier’s safety recommendations in the project’s capital cost estimate.
While you may need outside engineering or consulting help to meet all current regulations, there are many things you can do yourself to minimize the possibility of a dust explosion at your plant and comply with the instructions, industry standards, tests, and guidelines identified in the NEP.
Any organic, food, pharmaceutical, agricultural, or metal dust is likely to be combustible. Don’t assume, however, that dust from an inorganic material won’t burn or support burning; many inorganic materials are combustible or can act as an oxidizer to support combustion.
If your plant uses or processes combustible dust, it’s your responsibility, per NFPA 68,2 to know and keep on file the dust’s material physical properties, including explosion characteristics. Keeping a file of material safety data sheets (MSDSs) may not be sufficient, since most MSDSs for bulk solids don’t include information about combustibility or dust characterization. If you purchase the material, the material supplier should provide this information. If the supplier doesn’t provide the information, you must have the material tested.
For venting, extinguishing, or containing dust explosions for most materials, you need to know the following properties:
- Deflagration index (KSt), which is the maximum rate of pressure rise (measured in bar meters per second) per ASTM E12263 testing methods.
- Maximum explosive pressure (Pmax), which is the maximum pressure (measured in bars) created by a dust explosion within an enclosed vessel per ASTM E1226.
If you’re handling a material with a high K value (<200 bar meters per second) and a high Pmax (>7 bar) or if your process has special operating conditions, material storage concerns, or other factors that could ignite the material, you may need to know some or all of the following additional material properties:
- Minimum explosible concentration (MEC), which is the lowest concentration of suspended dust required for combustion per ASTM E1515.
- Limiting oxygen concentration (LOC), which indicates the lowest percent oxygen concentration that can support combustion of the dust.
- Minimum ignition temperature (MIT), which is the lowest temperature that will ignite a suspended dust cloud per ASTM E1491 or dust layers on a hot surface per ASTM E2021. These are two different values, and the material’s combustibility and your plant’s circumstances will determine whether you need to know both characteristics.
- Minimum ignition energy (MIE), which is the lowest- energy electric spark required to ignite a suspended dust cloud per ASTM E2019.
- Resistivity and conductivity, which are measures of how strongly the material resists or conducts the flow of electric current per ASTM D257. Resistivity and conductivity indicate how readily the material can build up and retain electrostatic charge during transport and storage.
Consult a corporate safety manager and your insurance carrier to determine which properties you need to know for your material and application. You can find the explosion characteristics of many commonly used bulk solid materials online or in standards such as NFPA 61 and 68. Only use this published information to get an approximation of a material’s explosion characteristics; however, never use the information for equipment or venting design. The published values may not accurately represent your specific material’s properties.
Be sure to also determine the explosion characteristics of materials that result from intermediate process steps in your operation. This includes materials that are blended with other ingredients, crushed, agglomerated, or treated. If you purchase any preblended or custom-made batches of minor or micro ingredients, you should test those materials as well.
If your plant changes ingredient suppliers, be sure to request the ingredient’s physical properties from the new supplier. The new ingredient may be chemically the same as the previous ingredient but may have different physical and explosion characteristics. For example, spray-dried milk powder from one supplier may have a significantly different bulk density, flowability, Pmax, or KSt than milk powder from another supplier depending on the method of spray atomization, the liquid feed conditions at the dryer inlet, the drying temperature, and the flow patterns inside the dryer. Also, if you expand your product line by changing an existing product’s formula, be sure to know the explosion characteristics of any new ingredients.
Practice Good Housekeeping
Poor housekeeping is probably the largest contributor to dust explosion or fire risk, and dirty, dusty conditions in your plant will likely be cited during an inspection as an OSHA violation of the General Duty Clause. Housekeeping is also the area over which you have the greatest amount of control when attempting to minimize your dust hazard exposure. There is, however, a human tendency to neglect housecleaning or cleanup work unless clear rules or expectations are in place. (Just ask the parent of any teenager.)
Workers will typically attend to their manufacturing duties but may not completely perform cleanup activities unless specifically directed or unless a problem occurs that’s too big to ignore, such as a major spill. Workers may overlook minor spills or dust displacement and leave the cleanup for the following shift.
Never use compressed air to clean up a bulk solids spill or remove accumulated dust. Compressed air just redistributes the material to other areas, and the resulting dust cloud could be sufficiently dense to create a combustible dust hazard or a respiratory hazard to workers in the area. Only use an intrinsically safe vacuum cleaning system rated for Class II, Group G, Division 1 environments to remove combustible dust. You can also use water to hose down an area and remove any loose or accumulated dust if doing so is safe for the equipment and facility.
The following areas are common sources of combustible dust accumulation in bulk solids manufacturing plants:
Spills. When a bulk solid material spill occurs, protocol should be to clean it up as soon as possible. Spills or accumulated material will normally be around bag dumps and bulk bag unloading stations or near transfer points between equipment, including:
- Inlets and outlets of bins, silos, tanks, and rail and truck hoppers
- Spouts between equipment
- Conveyor inlets and discharges
Transferring material from one location to another causes a corresponding displacement of air. Usually, the material moves downward by gravity, while the displaced air moves upward. The upward-moving air can entrain dust, which, unless the air is contained or removed by a dust collection system, will settle in the surrounding area. Since the air movement is vertical, the dust can be carried high and settle on high surfaces in the area. Also, air currents and drafts may cause the dust to disperse over a larger area, so the dust may settle on elevated surfaces in an extended zone around the spill or transfer point.
A dust layer that’s opaque (that makes seeing the underlying surface difficult), is significant and should be removed as soon as possible. Disturbing a dust layer just a few millimeters thick can easily create a dust cloud that could ignite and cause a deflagration. Conduct regularly scheduled inspections and clean off high surfaces in rooms or areas that contain bulk solids handling equipment.
Be sure to observe the area from a high vantage point to determine where dust has accumulated. This may sound obvious, but I’ve often seen areas that looked clean when viewed from the ground but that had dust buildup on topof pipes and horizontal surfaces. If there are no high vantage points, such as walkways or platforms, use a scissors lift, portable steps, or a ladder. Typical high surfaces include:
- Tops of equipment
- Building structural members
- Tops of storage cabinets
- Tops of pipes
- Tops of HVAC or process ductwork
- Electrical panels and cabinets
- Lighting fixtures
- Sprinkler heads
Over time, dust can accumulate in hidden or seemingly enclosed locations. Periodically check hidden areas such as interstitial spaces above ceilings or below floors. Electrical panels may not be properly sealed in areas near bulk solids handling equipment, and dust may enter and accumulate inside the panels over time. Have qualified personnel regularly open and inspect electrical or control panels near such equipment in accordance with approved electrical safety guidelines. Accumulated combustible dust in electrical cabinets or panels should be removed since ignition sources such as electrical sparks or hot surfaces may be present. Replace door gaskets and seal conduit connections as necessary. In severely dusty environments, you may need to pressurize the panel or replace it with an enclosure that’s properly rated for the conditions.
Since light airborne dust will often follow the air currents in the area, always inspect HVAC return- air intakes for accumulated dust. A noticeable dust buildup on a return’s louvers or grillwork may be an indication that dust is entering the HVAC return-air system. Excessive dust in the HVAC system could create nuisance fire alarms, since smoke detectors may not be able to distinguish between airborne dust and smoke particles. Also, if airborne dust is causing nuisance fire alarms in the HVAC system, dust is likely accumulating inside the ductwork as well. This can become a fire hazard over time, particularly in ductwork that contains heating elements, which could ignite the accumulated dust. Inspect return- air ducts periodically and clean them out as necessary. To prevent dust from entering return-air ducts, install inlet filters on the return-air intakes. Applications handling moisture-sensitive materials often use a desiccant wheel system to dehumidify the process airstream. Such systems have a steam- or electrically heated hot-air regeneration section. If the regeneration air intake is in an area where combustible dust is present, the system could accumulate dust on the heating surfaces and that dust could possibly ignite. Regularly check to ensure that inlet air filters are present, properly positioned, and clean.
If your facility has a packaging operation, you’ll have dust from handling, cutting, and forming the cardboard cartons or cases. Over time, cardboard dust can accumulate in the immediate area if housekeeping isn’t thorough. Cardboard dust is combustible, light, and can easily become airborne around powered conveyors and other equipment with moving parts. Packaging equipment will often use compressed air for pistons, actuators, vacuum generators for suction cups, and blow-offs, and the air released from these could distribute cardboard dust over a wider area. Packaging equipment handling cardboard usually has electrical wiring or components that may not be rated for dusty environments and should be cleaned frequently to prevent dust accumulation.