What You Need to Know about Dust Explosions and Your Facility, Part 2

(Read part 1 here)

The fact that over 280 dust-related explosions or fires have occurred in the past 25 years, resulting in 119 fatalities and over 700 injuries, has not gone unnoticed regulatory agencies or organizations that provide guidelines aimed at eliminating or reducing the risk of explosions or fires. Part 1 of this series (EHS Today, December 2009) 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 Review 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 and not protection of equipment. Therefore, subsequent damage to process equipment may result. With regard to dust collection equipment, NFPA-68 is more applicable.

NFPA-68-2007 AND DUST COLLECTION

The NFPA Standard on Explosion Protection by Deflagration Venting (NFPA-68) provides the 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 was published in 1945 as a temporary standard, the result of research into several grain and industrial explosions.

In 1954, it was revised, designated a guideline and combined into one document 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 are adjusted.

VENTING

NFPA-68-2007 is the standard to use for designing explosion (deflagration) vents for any size, or type, of enclosure or building where combustible dusts are handled or processed. However, this standard also includes some special requirements relating to the design and installation of deflagration vents on dust collectors. These special requirements take into account the separation of conveying gases and dusts into a “clean side” and a “dirty side” of the unit as well as the obstruction to venting posed by the internal filter bags and their support cages. Without getting into the finer details of the standard, a summary of these considerations is given below.

Dust collector location in plant: The first item to consider is the location of the dust collector itself. The NFPA standard recommends placing the baghouse outdoors, where it can vent to a safe area without additional risk to the plant or personnel. If the baghouse must be located indoors, it should be placed as close to an external wall as possible. Then the deflagration vents can be placed on the side of the housing next to the exterior wall and a vent duct can be supplied to move the products of combustion to an outside location. If the baghouse cannot be located near an external wall, or other location where the pressure relief vent can be ducted to a safe area, a flame suppressant device may be considered. These devices allow the pressure wave created by the original deflagration to be vented into the room where the equipment is located, but suppress the flame front associated with that pressure wave, thereby preventing the original flame from igniting any accumulated dust that may be displaced in the building. If the dust collector is vented outdoors, either through a duct or from an outside location, the vents should be positioned so that they do not allow the expelled material from being drawn into the plant through windows, ventilation equipment, doorways, etc. In addition, the flame and pressure wave should be directed away from locations where personnel may be working. This can be done by the use of deflector plates, shields or other means as long as the vent itself is not obstructed.