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

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 that 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 must be 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 adequately vent the explosion. 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.

2007 NFPA-68 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 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 3 to 4 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 a necessary evil, 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 3 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 the above forms can be found in Appendix A to NFPA-68.

SUPPLIER AND CUSTOMER RELATIONSHIP

It is essential for the 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 the 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 adequately protect the collector, the plant and the employees.

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

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Michael Maxwell is vice president of engineering and technology and manager of applications for Griffin Filters LLC. He received a BS in Chemical Engineering from Iowa State University and an MBA from Lewis University. He has over 36 years experience in dust collection design, application and installation, and has been a member of the National Fire Protection Association since 1994.