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Dust Explosion Protection by Flameless Venting

Despite a much greater awareness of dust explosion hazards, numerous accidents continue to happen throughout the world, resulting in casualties, property losses and business interruption. Prevention constitutes the first line of defense, and remains an essential part of risk mitigation for dust explosions; in addition, explosion protection techniques (such as containment, venting, isolation and suppression) also need to be implemented to deal with consequences and effects of explosion when they do occur. While conventional venting remains the most common and popular method of protection used, its implementation is not always straightforward, especially for indoor equipment. Flameless venting has been developed for dust explosions, thanks to extensive large scale test programs. In comparison with conventional venting devices, the most notable benefits of using flameless venting devices are flame extinguishment and dust retention, resulting in blast, thermal radiation and noise minimization outside the protected equipment. Another distinct advantage is that it can be easily retrofitted to existing installations, without requiring significant changes to the process.

Introduction

In the last decades, dust explosions have resulted in a much greater public awareness and, in turn, in a demand for improved safety performances by the regulator in many industrial countries. In the United States of America, the Occupational Safety and Health Administration (OSHA) issued a new Combustible Dust National Emphasis Program (NEP) in response of the Imperial Sugar dust explosion that occurred in 2008 (14 casualties, 36 injuries).

For many years, the most common and popular method of dust explosion protection has been venting. In its simplest form, a vent is an aperture in the top or side of a vessel that provides a means of pressure relief during an explosion. While conventional venting is quite efficient and recommended for most of the cases, its implementation can pose concerns when dealing with indoor equipment. Indeed, the vent discharge must be directed out of the building by suitable ducting, to protect people in the vicinity of the vented vessel, and also to prevent a secondary dust explosion fueled by dust deposits. However, the reduced explosion pressure inside the vented vessel will be much higher because of this ducting creates flow restriction during the venting process. Some applications may need long ducts, possibly elbows, which require a much larger vent. Sometimes, venting is no longer a viable solution, and other protection methods should be evaluated. Even when the vented vessel is installed outdoors, the hazards from overpressure, thermal radiation, release of material outside the vessel, as well as noise (Wirkner-Bott et al., 1992a; Wirkner-Bott et al., 1992b; Wirkner-Bott et al., 1993; Schumann et al., 1995; Crowhurst et al.,1995; Crowhurst and Colwell, 1995; Forcier and Zalosh, 2000, Holbrow et al., 2000; Taveau, 2010) have to be taken into account. The implications of dealing with these hazards may require large restricted areas (Bernard et al., 1998), which may not be desired for some applications. Another technology of dust explosion protection has been developed (Going and Chatrathi, 2003) to overcome the above concerns. This technology, called flameless venting, has been adopted in guidelines and described in technical papers by several authors (Barton, 2002; Zalosh, 2005; Grossel, 2012).

Flameless venting

A flameless venting device is a passive element that typically includes an explosion vent panel and a flame-quenching unit that is inside a flanged metal frame; the frame attaches the element to the process equipment, such as a dust collector (Snoeys, 2008; Snoeys and Going, 2010; Snoeys, 2012). The flame quenching unit’s frame encloses layers of particle-retaining, high-temperature stainless steel mesh or carbon steel mesh. The flame-quenching unit may be cylindrical, rectangular, or square to fit the vent opening’s shape. Other types of flame quenching units are given in Annex A of EN16009 (2011).

During the early stages of an explosion inside the process equipment, the explosion vent panel opens. As the explosion expands, flame, burnt and unburnt dust discharge through the open vent into the flame- quenching element. Most of the dust is retained inside the element, and the energy (heat) dissipates as it travels through and is absorbed by the steel mesh inside the element. This reduces the burning fuel’s temperature below the fuel’s ignition temperature, extinguishing the flame and preventing flame propagation beyond the device. Only a small amount of dust passes through the flame-quenching element, and post-combustion gases from the explosion are safely vented through the device into the external atmosphere around it.

The flameless venting device not only extinguishes the flame and retains dust, but completely eliminates the need for explosion vent ducting and minimizes the vent relief area required for indoor explosion venting. Flameless venting devices are designed to suit particular storage and process equipment such as silos, bins, hoppers, dryers, mixers, dust collectors, cyclones, but also transport equipment such as belt conveyors, screw conveyors, and bucket elevators.

Conclusion

For many years, the most common and popular method of dust explosion protection has been venting. While conventional venting is quite efficient and recommended for most of the cases, its implementation is not always straightforward, especially for indoor equipment. Flameless venting has been developed for dust explosions, thanks to extensive large scale test programs.

In comparison with conventional venting devices, the most notable benefits of using flameless venting devices are flame extinguishment and dust retention, resulting in blast, thermal radiation and noise minimization outside the protected equipment. Another distinct advantage is that it can be easily retrofitted to existing installations, without requiring significant changes to the process.

When considering flameless venting, one has to consider the following:

  • The overall efficiency of the flameless venting device shall be determined by testing in order to calculate the required vent relief area.
  • Special attention must be paid to dusts that have the potential to block the flame quenching element (coarse or fibrous dusts).
  • Even though flameless venting devices greatly limit overpressures outside the protected equipment, it should be verified whether the building can withstand these pressure effects.

It is therefore recommended to work closely with an explosion protection manufacturer who can provide the appropriate recommendations and suitable equipment for the considered application.

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