Five Ways New Explosion Venting Requirements
For Dust Collectors Affect You (continued)

How NFPA 68 will affect your explosion venting design

When a dust collector is designed with explosion venting, the primary purpose is to save lives, not property. A well-designed explosion vent functions as a weak element in the equipment's pressure envelope, relieving internal combustion pressure (back pressure) to keep the collector from blowing up into pieces. The vent's function is illustrated in Figure 2, a series of photos showing a staged deflagration in a cartridge dust collector equipped with an explosion vent. Typically, the collector is located outside and designed to vent away from buildings and populated locations, as shown in Figure 3.

While explosion venting will usually save the dust collector from being a total loss, the collector can still sustain major internal damage. Nonetheless, if personnel remain safe, the explosion venting equipment has done its job.

NFPA 68 includes several chapters with detailed information on design requirements for explosion venting equipment. In the following subsections, rather than attempt to summarize all of these requirements we'll instead focus on the most important areas that have changed or are of most concern to bulk solids processors.

Figure 2 - Staged deflagration in cartridge dust collector with explosion venting.

Figure 3 - Cartridge dust collector with explosion venting to vent explosion away from building.

Performance-based design option

Chapter 5 in NFPA 68 describes a performance-based design option, which specifies that if another method for protecting your dust collector from explosions is acceptable to the AHJ, you can use that method instead of one specified in NFPA 68. You must document and maintain this optional design method and its data sources over the collector's service life.

An example of a performance-based design option is conducting actual explosion tests of your dust collector to show that it will stand up to certain pressure conditions, instead of using the back-pressure calculations for collectors in NFPA 68. Some dust collector suppliers can provide this testing using a combination of field tests and full-scale dust collector laboratory tests. This approach can sometimes yield more accurate real-world performance data than the calculations provided in NFPA 68.

Sizing vents and vent discharge ducts.

For many years, explosion vents were sized using simple ratios - that is, for a given dust collector volume, 1 square foot of explosion vent area was needed. The agricultural and wood products industries were among those that commonly used these ratios. However, these old formulas no longer apply; instead, you must use the new design criteria in NFPA 68.

Chapters 7 through 9 provide the calculations you now must use for properly sizing explosion vents, vent discharge ducts (also called vent ducts), and other components. A reputable dust collector supplier will follow the vent sizing equations in Chapter 8 and be able to supply a calculations sheet that becomes part of the documentation you keep on file to prove your plant's compliance.

Regarding vent discharge ducts, Chapter 6 stipulates that "vent ducts and nozzles with total lengths of less than one hydraulic diameter shall not require a correction to increase the vent area" (6.8.4). (Hydraulic diameter expresses a noncircular opening's area.) This means, for example, that if you have an explosion vent with a hydraulic diameter of 40 inches, you can use a 40-inch-long vent discharge duct without risking damage to the collector from increased back pressure. However, for a duct more than 40 inches long (or whatever length is equivalent to the vent's hydraulic diameter), you must follow much more stringent calculations for vent size to compensate for the estimated increase in back pressure to the collector.

Therefore, when you require longer vent discharge ducts -- such as when the collector will be located inside -- and the standard calculations no longer apply, you'll have to work with your supplier to verify your dust's KSt value, vent discharge duct length, and strengthening requirements. When designing a collector for such an application, the performance-based design option may come into play.

Chapter 6 also notes that "to prevent snow and ice accumulation, where the potential exists, and to prevent entry of rainwater and debris, the vent or vent duct shall not be installed in the horizontal position, unless any of the alternative methods in 6.5.2.3.1 are followed" (6.5.2.3). This means that if your dust collector has horizontally mounted filter cartridges and horizontally mounted explosion vents, you may need to take extra steps to achieve compliance.

The accepted "alternative methods" for protecting horizontal vents are fixed rain hats, weather covers mounted at an angle to shed snow, and deicing devices such as heated vent closures (pressure-relieving covers). If you use one of these methods, your vent may require additional safety components and testing. For example, if you use a weather cover, the standard says that you must use restraints and design and test the cover to prevent it from becoming a free projectile in an explosion. The other option is to eliminate horizontal explosion vents altogether by using a dust collector with vertically installed cartridge filters and vertically mounted explosion vents.

Figure 4 - Flame-quenching device

Also new in NFPA 68 are sections allowing flameless venting inside buildings (6.9, 10.6). Flameless venting devices allow you to vent an explosion safely indoors without allowing any flame (or pressure front) to escape from the collector. Devices that meet this standard are commercially available in various configurations. A flameless device for quenching explosions, as shown in Figure 4, is a viable option to ducted explosion vents, but it's not recommended for toxic applications such as potent pharmaceutical dusts because of the risk that dust can be released into the room where venting occurs.