Special Bulletin: NFPA 654, 484, 68, 69

OSHA Combustible Dust Emphasis Program and NFPA Changes

OSHA makes program changes due to a series of dust related explosions in the sugar, petrochemical and metalworking industries.

The "OSHA National Combustible Dust Emphasis" program was implemented to reduce the number of fatalities related to workplace explosions. Non-compliance with the program can and has already resulted in fines and has required process changes in the industries affected by the change. The main attention is focused on the following industries:

• Metal Dust such as aluminum and Magnesium.
• Wood Dust
• Coal and other carbon dusts
• Plastic dusts and additives
• Bio-solids
• Other organic dust such as sugar, flour, paper, soap and dried blood
• Certain textile materials

Local inspectors have the leeway to also look at any other type of dust deemed to be a potential explosion hazard. OSHA inspectors are specifically looking for potential ignition sources such as the accumulation of dust in the plant which could fuel a secondary and usually more powerful blast.

This could occur if an explosion occurred in a dust collector or some other piece of plant equipment, causing the dust accumulation on rafters, floors and other flat surfaces to shake loose and become airborne in a concentration that, if ignited, would cause the secondary explosion.

To that end, housekeeping becomes an area of vital importance. OSHA inspectors are looking for dust accumulations that are at least 1/32" thick and over a surface area exceeding 5% of floor area of any given room or plant. If the inspector deems it necessary, samples of dust must be sent to a testing laboratory to gauge their level of explosiveness. Testing is site and dust specific and generally accepted values for dust are no longer considered to be valid.

If the dust is considered to be a hazard, any dust collector located in the building or outside the building must now comply with all applicable NFPA standards. The program also emphasizes the need for prevention of dust clouds forming around equipment such as grinders, mixers or other dust producing operations. Thus, the need for properly equipped dust collection is further mandated within the program.**

NFPA 654-Standard for prevention of fire and dust explosions from the manufacturing, processing and handling of combustible particulate solids. **

Items of Interest:

• Particulate processing systems shall prevent fire or explosion from propagating from one process to another.
• Recycling of air-material separator exhauster to buildings shall be permitted if the system is designed to prevent both return of dust with an efficiency of 99.9 percent at 10 microns and transmission of energy from a fire or explosion to the building.
  • Interpretation - There must be dampers or a suppression system in any ductwork leading to and from a unit. Dampers or suppression system must be placed in a manner that there is no possible way an explosion could cause a secondary explosion.
  • Interpretation - Clean air can be brought back into the building so long as the efficiency is greater than 99.9%,and there is a damper or suppression system on the exhaust ductwork that meets NFPA standards.
• The design of explosion protection for equipment shall incorporate one or more of the following:
  • Deflagration Venting
  • Deflagration pressure containment systems in accordance with NFPA69
  • Deflagration suppression systems in accordance with NFPA69
  • Dilution with a noncombustible dust to render the mixture noncombustible
  • Deflagration venting through a listed dust retention and flame arresting device.
  • Isolation devices shall include the following:
  • Chokes (i.e. Screw conveyor)
  • Rotary valves
  • Automatic Fast acting valve systems in accordance with NFPA69.
  • Flame front diverters in accordance with NFPA 69.
  • Chemical isolation systems in accordance with NFPA69.
• Isolation devices shall not be required if a documented risk evaluation that is acceptable to the authority having jurisdiction determines that deflagration propagation will not occur.
• Dust Collectors shall be located outdoors. A risk evaluation shall be permitted to be conducted to determine the level of explosion protection to be provided. **
• Manifolding of dust collection ducts to air-material separators shall not be permitted unless interconnected on the same process stream or each duct is isolated prior to manifolding.
• Air material separators shall be constructed of non-combustible material.
• Access doors shall be provided and bonded and grounded.
• Continuous suction to minimize the escape of dust shall be provided for processes where combustible dust is liberated in normal operation.
• The dust shall be conveyed to dust collectors. **

NFPA 484, 2009 Edition-Standard on Combustible Metals **

SPECIAL NOTE: This standard only applies to Aluminum, Magnesium, Nobium, Tantalum, Titanium, Zirconium and other combustible metals. It does not apply to non-metallic dusts. Dust is considered to be explosive if it is a finely divided metal 425 microns (40 mesh) or smaller. (**)

Aluminum and Tantalum

• Ductwork shall be electrically bonded and grounded and moisture tight if exposed to weather or moisture. Plastic or non-conductive ducts shall not be used. Ducting Shall be as straight and as short as possible.
• Minimum duct transport velocities shall be 4500 feet per minute.
• Deflagration Venting shall be provided on ductwork. These vents shall relieve to a safe location outdoors. The only alternative is to use explosion isolation systems.
• Dry-type dust collectors shall be located outdoors and be provided with barriers or other means for protection of personnel.
• The area around the dust collector shall be posted with a sign that reads as follows: CAUTION: This dust collector can contain explosible dust. Keep outside the marked area while equipment is operating
• Dust collectors shall be constructed of metal. The system must also be bonded and grounded.
• Cleaned air may not be returned to the building.
• The dust collector shall have deflagration venting.
• Cartridge filters shall be manufactured to dissipate static electric charges.(Grounding Straps)
• Grinding systems shall not be served by the same dust collection system as buffing and polishing operations.
• Dust Collection Systems shall be for aluminum and aluminum alloys only. No other materials can be captured in the dust collector without a complete teardown and cleaning of the dust collection system
• Dust shall be removed from barrels or trays daily.

Magnesium, Nioburm, Titanium and Zirconium

• Dust shall be removed from barrels or trays daily.

Other Combustible Metal Dusts

• Dry-Type dust collectors shall be located outside of buildings.
• Grinding operations shall not be served by the same dust collection system as buffing and polishing operations.
• Individual machines with portable dust collectors can be used indoors when the object being processed or finished is incapable of being moved to a fixed hood or enclosure.
• For Plasma spray operations, the dust collectors must be located "at a distance from the point of collection".
• A documented risk evaluation acceptable to the authority having jurisdiction shall be conducted to determine the level of explosion protection to be provided for a dust collection system.
• Recycling of exhaust air into buildings shall be prohibited.
• Dry type collectors shall be fabricated of conductive material and grounded or bonded.
• Dry-Type dust collectors shall be equipped with deflagration vents.

Chapter 13 - SPECIFIC TO ALL METAL DUSTS

• Fire prevention, fire protection and emergency response is retroactive:
  • An inspection, testing, and maintenance program shall be implemented.
  • Records of the inspections shall be kept.
  • Fugitive combustible metal dust shall not be allowed to accumulate.

NFPA 68, 2007 Edition-Standard on Explosion Protection by Deflagration Venting **

Items of Interest and Interpretation**:

• Vent sizes shall be based on Kst and P-max of the dust mixture. Where the composition of the dust mixture is not certain, the vent size shall be based on the highest KST and P-max of all the components. Where actual material is not available for test, vent sizing shall be permitted to be based on Kst values for similar composition materials of particle size no greater than the specified particle size range per the chosen standard. When the actual material is available, the Kst shall be verified by test.
  • Interpretation: If your customer has a proprietary dust or a mixture they do not want to pay for testing on, use the Kst from the tables of known values showing the highest level in the mixture. The customer should provide you with documentation stating the maximum Kst they believe is present.
• If it is necessary to locate enclosures with deflagration vents inside of buildings, vent ducts shall be used to direct vented material from the enclosure to the outdoors. A vent duct shall have a cross section at least as great as that of the vent. Vent ducts with total lengths of less than one hydraulic diameter shall not require a correction to increase the vent area. Vent ducts shall be non-combustible and strong enough to withstand the p-max. Where external venting is not feasible, a device that operates on the principles of flame arresting and particulate retention shall be permitted to be used. This device shall be listed and within the tested Kst, P-max and enclosure type. The areas adjacent to the discharge point shall be clear of combustible dusts.
  • Interpretation: It is still okay to keep the dust collector indoors so long as it is vented in the proper manner or flameless venting is used. **

NFPA 69, 2008 Edition-Standard on Explosion Suppression Systems

Items of Interest and Interpretation**:

• The new NFPA 69 2008 Standard is not retroactive unless it is required by another AHJ (Authority having jurisdiction).
• Suppression systems are allowed.
• Active Isolation (automatic pneumatic dampers) or passive Explosion venting in the duct is required.

Interpretation: Use of explosion vents designed and engineered to meet all applicable standards. You have three choices to use on the intake and exhaust ductwork:

1. Automatic pneumatic dampers to prevent propagation of dust explosions back through the ductwork.
2. Passive explosion venting on the ductwork itself.
3. Suppression systems can be used (cannons) on the intake and exhaust.

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