Combustible Dust: What You Need to Know

Part 2

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.

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 dust collector (or 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.

Vent location on dust collector

The next item to be considered is where the vents on the dust collector housing will be placed. In past versions of NFPA-68, vents could be placed on the clean side and dirty side of the housing with the intent to provide the majority of the vents on the dirty side. The 2002 version, classified as a guide, began to address the issue of vent location, with two sets of formulas for vent sizing: one for vents located completely below the filter bags and one for vents with only a portion of the vent located below the filters.

In this new 2007 edition, now designated as a standard, three possibilities are considered. The first is for vents located completely below the filter bags in an extended dusty air plenum. For this situation, the volume of the housing to be used is the dirty volume below the filter bags, unless the bags are spaced farther apart, in which case the dirty volume of the entire housing is used (volume below the filter bags plus the volume between the filter bags).

The second situation is where the vents are located, with all or some portion of the vent above the bottom of the filter bags. In this case, the volume used is the entire dirty volume below the baghouse tubesheet (total volume below the tubesheet minus the interior volume of the filter bags). However, the filter bags next to the vent must be removed, shortened or restrained so the filters will not obstruct the vent opening at all in the event of a deflagration.

The final case is where at least some portion of the vent(s) is located below the bottom of the filter bags, but bags are located next to the vents. The total volume below the tubesheet (dirty volume plus the interior volume of the filter bags) must be used and adequate barriers installed at the vents to prevent the filter bags and cages from entering the vent opening.

Physical geometry of the dust collector

Deflagrations in open enclosures generally expand in a spherical manner. When confined in one or two directions, the energy created causes the flame front and pressure wave to accelerate in the open direction(s). If the enclosure (dust collector) is long and narrow, the flame front and pressure wave can accelerate to the point where the velocity exceeds the speed of sound. In this case, the deflagration transitions into a detonation, similar to what is seen with high explosives, and it becomes impossible to vent the explosion adequately. Therefore, the length-to-diameter ratio of the housing, the true diameter or hydraulic diameter in the case of non-cylindrical enclosures, is evaluated and vent sizing and location are determined to prevent any possible transition of the deflagration. This same procedure is used for providing proper venting of ductwork handling combustible materials.

In addition, when ductwork is used to vent an indoor baghouse to an outdoor safe location, a backpressure develops in the housing due to the expanding pressure/flame in the housing while the combustion products are trying to vent through the ducting. This backpressure, when calculated, can be very high. Therefore, in most cases, it may be necessary to make indoor baghouses cylindrical to withstand this increased pressure.

Standard Additions

Two significant additions to the new standard reflect concerns highlighted in the CSB study and the OSHA NEP and inspection procedures.

The first addresses the problem of flame fronts migrating from their initial source to ignite combustible materials in other parts of the plant. Besides OSHA's emphasis on housekeeping to eliminate dust accumulations within the plant, NFPA mandates the means to prevent a deflagration from moving down ductwork to other process or non-protected equipment and machinery. Either rapid-acting dampers must be installed to close ahead of the flame front to isolate the baghouse or other vessel, or chemical suppressant systems must be installed to extinguish the flames inside the ductwork to prevent igniting dust clouds in other upstream or downstream equipment.

For small baghouse systems, this is not too difficult. However, for larger systems, where the ductwork may easily exceed three to four feet in diameter, it is difficult to find suitable isolation dampers, and suppressant systems will need to be used.

Even though the tendency is for the initial combustion to “feed” on dust in the incoming ductwork and “climb” backwards through the plant, it is essential that isolation be provided in the clean-side ducting as well, in the event the filter bags may burn and dust can escape through the clean-side plenum to the fan and discharge stack, to other downstream processing equipment, or back into the plant if the discharge air is recycled into the plant to conserve heat and energy.

The second critical addition is an emphasis on inspection, maintenance and training. In the past, pressure relief doors and rupture panels were looked at as necessary evils, required to protect the baghouse and reduce the risk of explosions elsewhere in the plant, but the kind of device you install and forget unless it is activated or used. However, because neglected vents can fail to operate effectively, NFPA-68 now requires a regular inspection and maintenance plan to include written instructions, inspection forms with sign-off requirements and maintenance of these forms for review by plant and public safety personnel for a minimum of three years.

The third critical addition is a requirement for the equipment or relief vent supplier to provide a vent closure information form to document the design and sizing basis for the vent. This form should be kept safe along with the inspection and maintenance record forms. Examples of these forms can be found in Appendix A to NFPA-68.

Critical Relationships

It is essential for customers and suppliers to work closely together to ensure the deflagration vents are properly designed, sized and installed. This requires that certain critical design information be made available to the vent supplier, and the vent supplier must provide proper documentation, including sizing criteria, design basis and safe operating and maintenance instructions, to the customer. Proper design starts with a full understanding of the material, or dust, being handled in the process equipment.

The customer must provide accurate information concerning the process design and operation so that the dust collector can be properly integrated with the other equipment. In addition, the customer must supply the dust collector provider with complete information on the explosive characteristics of the dust. This will, most likely, require all dusts to be tested by an acceptable testing facility to establish their Kst and Pmax values. The dust collector supplier must use these established values to determine the appropriate vent area to protect the collector, plant and employees.

Likewise, the baghouse supplier must work with the customer to provide required design information, proper maintenance and inspection procedures and recommendations for upstream and downstream isolation equipment.

Michael Maxwell is vice president of engineering and technology and manager of applications for Griffin Filters. He earned a bachelor's degree in chemical engineering from Iowa State University and a master's degree in business administration from Lewis University. He has more than 36 years of experience in dust collection design, application and installation and has been a member of the NFPA since 1994.

This article originally appeared in MHM's sister publication, EHS Today).

This two-part series examines the causes of dust explosions, their devastating impact and the measures suggested by OSHA, the National Fire Protection Association and the Chemical Safety and Hazard Investigation Board to eliminate them. Part 1 appeared in the January issue of MHM.

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