Industry Articles


Five Ways New Explosion Venting Requirements For Dust Collectors Affect You

What you need to know about the new NFPA 68 standard
on explosion venting for combustible dusts

By Lee Morgan and Tony Supine, Farr APC

Article first featured in Powder & Bulk Engineering magazine, July 2008

This article sums up what you need to know about the new NFPA 68 standard on explosion venting for combustible dusts. The information here can help you better understand what's changed in this revised standard and how it will affect your dust collection choices today and in the future.

The topic of combustible dust hazards in bulk solids processing plants has never been more -- pardon the expression -- explosive.

In February this year, a dust cloud explosion triggered a fatal blast and fire in the Imperial Sugar Company's Wentworth, Georgia, plant, generating a storm of media attention, public outcry, and governmental scrutiny. The blast, while more severe than most, was reportedly not a freak incident but the latest in a 25-year string of similar incidents at manufacturing facilities around the US.

3.2.2 Authority Having Jurisdiction (AHJ):
An organization, office, or individual responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure.
3.2.5 Shall:
Indicates a mandatory requirement.
3.2.6 Should:
Indicates a recommendation or that which is advised but not required.
3.2.7 Standard:
A document, the main text of which contains only mandatory provisions using the word “shall” to indicate requirements and which is in a form generally suitable for mandatory reference by another standard or code or for adoption into law. Non-mandatory provisions shall be located in an appendix or annex, footnote, or fine-print note and are not to be considered a part of the requirements of a standard.
3.3.2 Combustible Dust:
A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentration, regardless of particle size or shape.
3.3.4 Deflagration:
Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium.
3.3.7 Dust:
Any finely divided solid, 420 μm or 0.017 in. or less in diameter (that is, material capable of passing through a U.S. No. 40 Standard Sieve).
3.3.8 Enclosure:
A confined or partially confined volume.
3.3.10 Explosion:
The bursting or rupturing of an enclosure or a container due to the development of internal pressure from a deflagration.
3.3.20* KSt:
The deflagration index of a dust cloud.
3.3.27.1 Maximum Pressure (Pmax):
The maximum pressure developed in a contained deflagration of an optimum mixture.
3.3.27.2 Reduced Pressure (Pred):
The maximum pressure developed in a vented enclosure during a vented deflagration.
3.3.34 Vent:
An opening in an enclosure to relieve the developing pressure from a deflagration.
3.3.35 Vent Closure:
A pressure-relieving cover that is placed over a vent.

Figure 1 - NFPA 68: A Glossary of Terms

Since February, OSHA has contacted 30,000 companies that deal with combustible dusts, warning them of the danger of a deadly explosion if airborne dust particles contact an ignition source. Two members of Congress have also recently proposed legislation to help prevent dust explosions in manufacturing plants.

Explosions are a risk in many areas of a bulk solids processing plant, including the plant's dust collection system. The current industry focus on dust explosion hazards makes it a good time to examine the newly revised National Fire Protection Association (NFPA) standard focusing on this topic, NFPA 68: Standard on Explosion Protection by Deflagration Venting, and discover how it will affect your dust collection system. This standard, completely revised in 2007, is in effect now. You can purchase a copy of the full standard from the NFPA Web site.

This article's intent is to help you better understand what's changed in this revised standard and how it will affect your current and future dust collection choices. The following information reflects our best efforts at this time to comprehend and interpret NFPA 68 as it relates to dust collection systems that use closed-vessel (cartridge and baghouse) dust collectors. This interpretation - and eventually, the standard itself - will likely continue to evolve as industry experience with explosion protection grows over time.

The revised NFPA 68 affects how you handle your plant's combustible dust in five key ways:

  1. NFPA 68 has changed from a "guideline" to a "standard."
  2. You need to determine whether your dust is explosive.
  3. You need to commission a hazard analysis of your dust collection system.
  4. You need to maintain extensive documentation.
  5. You need to schedule an annual inspection of the dust collection system's explosion venting equipment.

Let's examine each of these topics in detail. As you read the following information, it may be helpful to refer to the related sidebar, "NFPA 68: A glossary of terms," which defines the NFPA 68 terms used in this article.