Combustible Dust: What You Need to Know

The fact that more than 280 dust-related explosions or fires have occurred in the past 25 years, resulting in 119 fatalities and more than 700 injuries, has not gone unnoticed by regulatory agencies or organizations that provide guidelines aimed at eliminating or reducing the risk of explosions or fires. Part 1 of this series discussed the causes of dust-related explosions and fires and housekeeping methods to reduce the risk. This article will discuss related regulations and guidelines.

Both OSHA and the Chemical Safety and Hazard Investigation Board (CSB) rely on the National Fire Protection Association (NFPA) and its collection of codes, standards and guidelines to reduce the risk of dust explosions. NFPA publishes a great number of guidelines, standards and related documents designed to bring the latest scientific knowledge and understanding to those involved in dealing with fire hazards. Several of these documents relate directly to the chemical process industry and companies involved in handling combustible materials, including dusts and powders.

These documents periodically are updated, and between 2006 and 2009, in conjunction with the new OSHA combustible dust national emphasis program (NEP), NFPA revised several of its guidelines on fire and deflagration risk management and mitigation. Some of the guides and standards apply to specific industries, but two publications are general in nature and often discussed in conjunction with the OSHA NEP. These are NFPA-69 (dealing with explosion/deflagration prevention) and NFPA-68 (relating to venting of deflagrations to force the “hazard” to a safe area). Both of these publications are oriented towards protection of life, not protection of equipment. Therefore, subsequent damage to process equipment may result. With regard to dust collection equipment, NFPA-68 is more applicable.

Dust Collection

The NFPA standard on explosion protection by deflagration venting (NFPA-68) provides design criteria for applying pressure relief vents to enclosures to relieve the pressure buildup that accompanies the heat and flame generation during the rapid oxidation of fine dust materials. This document, first published in 1945 as a temporary standard, was the result of research into grain and industrial explosions.

In 1954, it was revised, designated a guideline and combined into one document of the accumulated knowledge and information available on dust explosion venting available at the time. This guideline used a series of “vent ratios” for determination of vent areas, based on the projected rate of pressure rise seen in testing of many dusts and powders and the volume of the enclosure or building.

Since 1954, the document has been revised several times, moving away from an emphasis on vent ratios for sizing relief vents to a system of formulas empirically derived from explosive testing conducted in Europe and the United States. These formulas incorporate factors related to the enclosure, or process equipment, strength and size (its ability to resist the pressure rise of a deflagration without venting and its volume), the characteristics of the vent to be used (its mass and means of attachment to the vessel) and the dust itself (its explosiveness, maximum pressure rise, etc.).

The 2007 version of this document brings about several critical changes from the previous version, many of which relate to OSHA's emphasis on the reduction of dust hazards and the NEP.

The vent design formulas completely have been revised, including the formulas and methods for modifying the vent area for enclosure geometry, vent panel mass, initial operating pressure, use of vent ducting, etc. The revised formulas more closely match testing results.

However, the formulas only are reliable when the data inputs are reliable. While NFPA-68 includes tables containing “explosion” data on many typical dusts (Kst, or rate of pressure rise, and Pmax, the maximum pressure developed in an unvented enclosure), it is suggested that this data not be used because it is too general in nature, and the user is encouraged to have a sample of the specific dust tested for its explosive characteristics. In cases where a sample of the actual dust cannot be obtained or tested, data from a similar material, or tabular data, may be used as long as the differences between the materials are considered and vent designs adjusted.