GAPS Guidelines

GAP.9.3.2.0

A Publication of Global Asset Protection Services LLC

DUST COLLECTION SYSTEMS INTRODUCTION Any facility that handles materials in solid form will generate dust by abrading the solid. The dust generated this way is usually not produced in quantities large enough to be hazardous and can be addressed by regular housekeeping. If the dust in question is combustible, explosive or otherwise reactive, and is generated in large enough quantities to be dangerous, then it must be removed at its source. More recently, similar concerns have risen over dusts which are potentially toxic. Some dusts also contain adhesive materials leading to a residue buildup in, or even clogging of, dust collection systems. Some examples are pressboard, particle board, composites or dust coated with coolant or lubricating residues. At one time, simply exhausting such dusts to the outside atmosphere was an adequate way to address the problem. Environmental concerns now make it necessary to collect such materials before they pollute the surrounding area. Dust handling systems have therefore grown from simple dust exhaust systems to complex dust collection systems. Because of these constraints, some processes simply cannot operate if the associated dust collection systems are out of service. Although a dust explosion can be large enough to destroy an entire facility, even a relatively small amount of property damage to a collection system can cause a large business interruption loss. Consequently, major loss prevention and control attention to dust collection systems is justified. This section only covers fugitive dust collection systems. It does not address dust collection systems integral to production processes, such as pneumatic conveyors, spray dryers, etc. as these are specific to the individual process. This section does not cover electrostatic precipitators as these are addressed by GAP.9.3.2.1.

POSITION Management Programs Management program administrators should report to top management through the minimum number of steps. They should also institute adequate loss prevention inspection and audit programs to communicate program effectiveness to top management. This management feedback is a key feature of GAP.1.0.1 (OVERVIEW), Global Asset Protection Services’ (GAPS) total management program for loss prevention and control. In developing a program, pay particular attention to the following important areas: Material Hazard Evaluation Program Develop a test program to determine the pertinent physical and chemical properties of dusts that are likely to be generated. Choose test conditions that best represent all possible operating conditions. Include in the program the following tests:

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Minimum explosive concentration of (MEC) dust cloud – ASTM E1515 determines this.

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Global Asset Protection Services LLC and its affiliated organizations provide loss prevention surveys and other risk management, business continuity and facility asset management services. Unless otherwise stated in writing, our personnel, publications, services, and surveys do not address life safety or third party liability issues. The provision of any service is not meant to imply that every possible hazard has been identified at a facility or that no other hazards exist. Global Asset Protection Services LLC and its affiliated organizations do not assume, and shall have no liability for the control, correction, continuation or modification of any existing conditions or operations. We specifically disclaim any warranty or representation that compliance with any advice or recommendation in any document or other communication will make a facility or operation safe or healthful, or put it in compliance with any law, rule or regulation. If there are any questions concerning any recommendations, or if you have alternative solutions, please contact us.

GAP.9.3.2.0 • • •

Minimum ignition energy of dust cloud – ASTM E2019 determines this. Sieve analysis – ASTM D1921 determines the particulate size distribution of a product sample. Dielectric data – ASTM D150 determines the various dielectric properties needed to calculate the relaxation time.

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Maximum rate of pressure rise at optimum concentration - ASTM E-1226 determines this property, also known as Kst.

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Maximum explosion pressure at optimum concentration – ASTM E-1226 determines this property, also known as Pmax.

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Resistivity measurements – ASTM D257 determines various electrical properties of the material. Hot surface ignition temperature – ASTM E2021 determines this.

Minimum ignition temperature of dust cloud – ASTM E1491 determines this. As an alternative, obtain data from Material Safety Data Sheets provided by the supplier of the material. Process Hazards Evaluation Program While designing the various facility processes, consider the desirability of reducing the generation of hazardous dusts. Ideally, switch to noncombustible materials, or at least less combustible ones. If that is not practical, improved equipment may produce less dust, and consequently the amount that must be collected and handled will be less. Something as simple as increasing the particle size can markedly reduce the hazard. Determine the safe operating and potential upset conditions of all new or existing dust collection equipment used by the plant. Give special attention to static electricity and to dust buildup on hot surfaces as potential ignition sources. Interlock processes to shut down automatically and safely in the event that associated dust collection systems are inoperative or shutdown, or when their fire protection systems are activated. Operator Training Program Educate all operators in the hazards of the dusts involved and in the functions of the dust control equipment. Authorize operators to stop the process when any of this control equipment is out of order. Train operators in proper start-up procedures as well as manual emergency shut down procedures. Also train operators to respond to upset conditions and to be knowledgeable of required equipment maintenance and available fire and explosion protection systems Promote strict adherence to the written procedures. Housekeeping Design the housekeeping program to keep dust at predetermined safe levels throughout the facility. Adjust the number of cleaning personnel and equipment and the frequency of cleaning to match the dust accumulation rates under standard operating conditions. Establish an emergency procedure to immediately detect, report and clean up any unexpected dust concentrations. Use fixed vacuum systems rather than portable ones for cleaning. Avoid cleaning methods which tend to raise dust clouds such as vigorous sweeping or blowing down with compressed air or steam. If such methods must be employed, do so only with the following precautions:

• Vacuum the area thoroughly beforehand. • Shut down all electrical systems and any other ignition source. • Use only low pressure (15 psi [103 kPa]) compressed air or steam. • Ensure that no hot surfaces capable of igniting a dust cloud or layer exist. Duplication of Equipment In cases where a production curtailment would be critical, provide a backup dust collection system. As an alternative, install several separate smaller dust collection systems instead of one large unit.

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GAP.9.3.2.0 Separate duplicate units by barriers or distance. Keep multiple dust collection systems completely separate by avoiding the use of manifolding. Keep on site a supply of critical spare parts, such as filter bags or bag cages.

Location In descending order of desirability, locate dust collectors:

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Outside. In cutoff attached rooms with three outside walls. In cutoff rooms with one outside wall. In a cutoff interior room. In the same area as other occupancies but with as much distance as possible between the dust collector and any large value item.

In accordance with NFPA 654, dust collectors should be located outside unless protected with deflagration vents or an explosion suppression system. Deflagration vents must terminate outside the building or be equipped with a deflagration arrestor.

Operations Avoid the use of dust collection systems to pick up trash or scrap. If such use is unavoidable, arrange the system to separate the scrap and the dusts. For instance, installing coarse screens at the pick up points prevents scrap from entering the system, while coarse filters followed by fine filters separates scrap and dust after they enter the system. Install magnetic separators to remove ferrous materials at or near pickup and dump points. Avoid the recycling of collector exhaust back into the building. If such an arrangement is unavoidable, install High Efficiency Particulate Air (HEPA) filters so that no dust is carried back into the building. In addition, provide an isolation device to block the propagation of a fire or explosion into the building. Such devices include a local explosion suppression system or a very fast automatic valve.

Construction Materials Design and construct dust collection systems in accordance with NFPA 654. Use noncombustible construction in the building. See NFPA 220. Use noncombustible materials to construct dust collection systems. Construct dust collection systems of conductive materials and avoid the use of materials which tend to produce static electric charges such as plastic ducts. If it is not practical to use noncombustible bags in bag type filters, then use materials of the lowest combustibility possible. Use only nonsparking (nonferrous) metals for fan components. Structure Design structures so that they do not collect dust and are easy to clean. Avoid the use of roof supports members with extensive horizontal surfaces or any surface with an angle less than the angle of repose of the dust. Design the internal supports of large ducts so as to not form pockets which would collect dust. Design Design the dust collector fans to maintain negative pressure in the system to avoid dust leaks. Install the fans downstream of the dust collectors so that the dust laden air does not pass through the fans. Design and install electrical systems in accordance with NFPA 70.

Protection If a dust collection system is of noncombustible construction and the dust being collected is also noncombustible, no fire or explosion protection is needed. However, if the dust or the collection system construction is combustible, provide the sprinkler and hose station protection listed below. If

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GAP.9.3.2.0 the dust is determined to be explosive, also install explosion protection as needed. See also GAP.2.3.2 for protection of ducts of combustible construction. Automatic Fire Suppression Install automatic sprinklers inside the dust collection system. Use open head deluge systems activated by fixed temperature detectors for large high value bag houses. Use closed head sprinkler systems for all other dust collection system components. Design the sprinklers in accordance with 2 NFPA 13 and NFPA 15 to provide 0.25 gpm/ft2 (10.2 L/min/m ) over the entire internal area of bag type dust collectors. Install the heads in the clean air plenum and the fixed temperature detector in the clean air duct. Provide a density of 0.20 gpm/ft2 (8.15 L/min/m2) over the interior of the ducts, hoppers and cyclones. Design the dust collection system to withstand the weight of the maximum possible water containment. In lieu of that, install automatic drains. Be sure to extend sprinkler coverage beneath dust collection hoods which block ceiling sprinklers. Although a water based system is preferred, other extinguishing agents, such as CO2, dry chemical are acceptable, provided that they are designed and installed in accordance with the applicable NFPA standard. See NFPA 12 and NFPA 17 for details.

Courtesy of Fike Figure 1. Explosion Isolation.

Explosion Isolation Install explosion isolation valves in large dust collection systems as needed. Arrange these valves so that a single explosion cannot propagate from unit to unit destroying the entire dust collection system. Figure 1 shows such isolation. Since isolation valves do not protect the individual units where the explosion originates, install explosion relief or suppression on the individual units. Figure 2 shows an example of this. Explosion Relief Install adequate explosion relief throughout the dust collection system in accordance with NFPA 68. In descending order of desirability, do the following: •

For outdoor locations, install the relief panels directly on the dust collectors.

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GAP.9.3.2.0 •

For indoor locations, install explosion relief vents on the dust collector, vented through an explosion relief duct to an outside location. Limit the length of the duct to 10 ft (3 m) and maintain a cross section at least as big that of the relief panel throughout its length. Figure 3 shows an example of this.

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For indoor locations, install explosion relief vents on the dust collector, venting inside the building. Equip the vent(s) with a combined flame arresting and particulate retention device listed by a nationally recognized testing laboratory.

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GAP.9.3.2.0

Courtesy of Fike Figure 2. Explosion Isolation.

Courtesy of Fike Figure 3. Explosion Venting.

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GAP.9.3.2.0

Courtesy of Fike Figure 4. Explosion Suppression.

Explosion Suppression If adequate explosion venting cannot be provided, install explosion suppression throughout the dust collection system in accordance with NFPA 69. See Figure 4. Spark Extinguishing If the processes involved tend to produce sparks, install spark extinguishing systems in the ducts upstream of the collector in accordance with NFPA 69. See GAP.13.9.1. Hose Stations Provide manual extinguishment capabilities for the dust collection system. This should include:

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Sufficient doors or panels to provide access to all parts of the interior of the dust collection system. These doors or panels should be accessible by personnel by means of floor location, catwalks, platforms or fixed ladders.

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Install sufficient 1 ½ in. (40 mm) hose stations so that all doors or panels are accessible to at least one hose nozzle.

Interlocks Arrange interlocks on each dust collection system so that the upstream process can not be started unless the dust collection system is operating and so that activation of a fire or explosion protection system will return the system to a safe condition. This should include closing dampers, stopping rotary valves, and shutting down fans. Provide large fans with vibration monitor interlocked to shut down the systems if excessive vibration is encountered.

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GAP.9.3.2.0 DISCUSSION Dusts can be classified in many ways: combustible or noncombustible; conductive or nonconductive; fast burning or slow burning. This section starts with assumption that the dust in question is combustible since no further analysis is needed if it is not. For purposes of determining risk of static electric generating potential and electrical classification, it is necessary to know the conductivity of the dust. Metal dusts are highly conductive and therefore ground out static charges as fast as they are generated so their static electricity generating potential is low. They do not pyrolize so they do not ignite easily on hot surfaces unless the temperatures are very high. There are exceptions; zirconium, thorium and uranium have extremely low ignition temperatures. Metal dust fires in general are difficult to extinguish once started. Carbon based dusts, i.e., coal, agricultural and plastic dusts, are poor conductors. They hold static electric charges well so they are prone to static electric spark ignition. They are susceptible to pyrolysis so they will ignite if allowed to sit on a hot surface for extended periods of time. Fires in carbon based dusts are easier to extinguish than metal dusts, however. Because coal fired power plants deal with these issues, you can find additional information in GAP.17.12 and GAP.17.12.1. Explosion severity is measured by how fast the pressure increases in a standard enclosure when the dust in question is ignited. See Figure 5, Figure 6 and Figure 7. The dust hazard is classed as 1, 2 or 3 with 3 being the most severe. There is no direct correlation between the class and other parameters so a direct measurement is the only reliable indicator. Changing something as simple as particle size or moisture content can radically alter the explosion severity. Even if most of the particles in a given dust are large and relatively safe, movement of these particles will grind off much finer and more dangerous dust. Another factor that heavily influences the severity of a loss is static dust. This is the dust layer on exposed horizontal surfaces. When a dust explosion occurs, the first one is usually small. The shock from that initial blast shakes the static dust into the air creating a large dust cloud which then ignites to cause a much more severe secondary explosion. This explains the emphasis placed on housekeeping.

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GAP.9.3.2.0

Courtesy of Fike Figure 5. Typical Explosion Pressure Curve. Courtesy of Fike

Figure 6. Suppressed Explosion Pressure Curve.

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GAP.9.3.2.0

Courtesy of Fike Figure 7. Vented Explosion Pressure Curve.

Dust Collection Systems Dust collection systems include pickup points where the air and entrained dust is pulled into the system, ducts that convey the dust and air, fans that supply the air movement, and the dust collectors that separate the dust from the air. The dust collectors are mostly of the dry type. The dry type is further divided into cyclones and bag type dust collectors. See Figure 8.

Pickup Points The intake ends of a dust collection system are designed to fit the process and the equipment to which they are attached. A pickup can be a simple open-ended pipe located next to a grinding wheel or a large, complex hood covering the dryer and winders of a paper-making machine. A flexible hose and nozzle can also be connected to the dust collection system and used for manual cleanup just as a home vacuum cleaner is used. The principal hazard of a pickup point is a hood which blocks the spray patterns of ceiling sprinklers. If the hood is of combustible construction such as plastic, it adds to the combustible loading in the building.

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GAP.9.3.2.0

Courtesy of Fike Figure 8. Typical Dust Handling Process.

Ducts Ducts are used to convey air entrained dust from the pickup points to the dust collectors themselves. Ducts can be square, rectangular or circular in cross section and can vary in size from a few inches to several feet. Hazards associated with ducts include obstruction to ceiling sprinklers, combustible construction, and the combustibility of the contents of the ducts.

Fans Fans supply the air movement required to convey dust to the collectors. The hazard associated with fans is limited to providing a source of ignition. Ignition can be from hot bearings, electrical short circuit or sparking from a broken fan (if the wrong metal was used in its construction).

Dust Collectors Cyclone Collectors Cyclone collectors are conical units which operate by forcing air entrained dust to flow in a spiral path of decreasing radius. This increases the stream velocity and, therefore, the centrifugal forces on the dust particles, causing them to be thrown outward against the side of the cyclone. The dust particles then fall to the bottom of the cyclone and into a bin or hopper for disposal or recycling. The clean air exits at the center top of the cyclone. Cyclones vary in diameter from several feet to under one foot. Generally, the smaller the diameter, the higher the efficiency. Cyclone collectors work best on dust particles in the 10 to 300 micron range. They are, therefore, an economical way to handle coarse dusts. Because they contain no filter medium and have very little dust hold up, they are preferable to bag collectors for loss prevention purposes. To handle fine dusts, several small high efficiency units in series are needed. This increases the energy requirements of the driving fans.

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GAP.9.3.2.0 Bag Collectors Bag collectors (or bag houses, if large) are rectangular or cylindrical units which operate by forcing air entrained dust through a fabric filter. The fabric filter is called a bag because it is usually shaped into a long narrow cylinder closed at one end. The dust trapped by the bags is removed by mechanical shaking or by high pressure air pulses. The dust falls to a hopper for disposal or recycling. A variation on the bag filter is the tube type which uses plastic fibers arranged in tubes resembling auto engine air filters. They are cleaned by shakers and are most commonly used by foundries. Bag collectors work best for dust particles in the 0.01 to 20 micron range. They are, therefore, the most economical way to handle fine dusts. Barring combustible construction, the only hazards are those posed by combustible dusts, i.e., fire or explosion. Keep in mind, however, that combustible construction includes the bags. While noncombustible (or at least low combustibility) bags are available which are compatible with most dusts and operating temperatures, they tend to cost more. Even if the bags do not contribute significantly to a fire, they are likely to be badly damaged by a fire irrespective of the speed of the fire suppression system. Replacing bags in large bag collectors may take several days, not including order and delivery time, so it is wise to keep spare bags on hand.

Ignition Sources The most common ignition sources associated with fires and explosions in dust collection systems are static electricity and hot surfaces. Static Electricity Because most combustible dusts are carbonaceous, they are poor conductors and are therefore capable of holding a static charge. Since these particles are being carried by a moving air stream, they tend to generate and hold large static charges. If these charges ground out under the right conditions, a static electric spark strong enough to cause ignition can be generated. For this reason the need for grounding and bonding of all system components is accentuated. This is also one of the reasons that plastic ductwork is so undesirable. Hot Surfaces A layer of dust on a hot surface tends to trap the heat thereby increasing the temperature locally. As time passes the heat will pyrolize the dust until its ignition temperature is reduced to the temperature of the surface and ignition occurs. Others Less common but still important ignition sources are mechanical sparks, open flames, power tools, hot bearings and improper electrical systems.

Loss History Over a 20 year period, GAPS’ clients suffered over 700 losses in dust collectors. Of these, 97% were explosions or fires. Although fire losses were more than six times as numerous as the explosion losses, the average explosion loss cost more than three times as much as the average fire loss. Where adequate sprinkler systems were in service, the cost of fire losses was reduced by more than half. The presence of explosion relief or suppression caused a similar reduction in the cost of explosion losses.

Conclusions Due to the tightening of air pollution laws and employee safety regulations, manufacturers are being forced to become ever more efficient at removing dust particles from the air being discharged by their facilities. This in turn requires them to install large complicated dust collection systems throughout their plants. Because of the manner in which the ducts interconnect the various areas of the plant, any incident can effect the entire plant. The legal requirement to rebuild the dust collection system before restoring production can also cause an incident to lead to a prolonged business interruption.

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GAP.9.3.2.0 Additional material can be found in NFPA 61, NFPA 484 and NFPA 664.

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