Both NFPA and FM use this Kst value in formulas to calculate the amount of explosion vent area required for a dust collector. Class 1 dusts are below 200 Kst, Class 2 dusts range from 200–300 Kst, and Class 3 dusts are rated above 300 Kst. As a rule of thumb, when dusts approach 600 Kst, they are so explosive that wet collection methods are recommended. In addition to Kst, other important measurements that factor into the standard include “Pmax” (the maximum pressure in a contained explosion) and “Pred” (the maximum pressure developed in a vented enclosure during a vented deflagration).
Table 1 compares the Kst values of a number of common chemical dusts. A comprehensive compendium is available online at http://www.hvbg.de/e/bia/gestis/expl/index.html. This Web site contains a European database that lists the combustion and explosion characteristics of more than 4,000 dusts. The data provided, however, aren’t a substitute for the required dust testing.
Figure 1 -- Standard explosion vent:
Figure 1 shows a standard explosion vent that has been sized for 200 Kst and manufactured to NFPA standards. For highly combustible dusts, vent sizing and ducting requirements become more complex and may require special calculations and equipment modifications to achieve compliance as we’ll discuss later.
3. You must commission a hazard analysis (also called a risk evaluation). Chapter 4 of NFPA 68 introduces a new hazard analysis requirement, stipulating that a plant will have to commission a risk evaluation for the dust collection system and keep the report on file, to show to the local fire marshal or other officials at a moment’s notice. It’s possible that some equipment suppliers or independent rep agencies may start to offer this service. Given how time-intensive the risk analysis is, however, we anticipate that a whole cottage industry of consultants will spring up to meet the newly created demand.
Figure 2 -- Staged explosion: After
4. You now must maintain extensive documentation. A hazard analysis isn’t the only documentation required under NPFA 68. In Chapter 11, the standard outlines many other types of documents that now must be maintained to satisfy the local fire marshal or other AHJ. Section 11.2 lists 19 different types of documentation that must be kept on file. You can readily obtain some of these — such as manufacturers’ equipment data sheets, instruction manuals and specifications. Others — including a combustible material (dust) properties test report, user documentation of conformity with applicable standards, and employee training requirements — will present more substantial challenges to the plant or safety engineer in charge.
5. You now must schedule an annual inspection. Chapter 11 also stipulates that an inspection shall be performed on explosion venting equipment at least annually and possibly more often, based on documented operating experience. The objective of this inspection is simply to determine that all components of the system are correctly operating. Section 11.4 outlines a 16-point vent inspection with this objective in mind. The plant owner or operator also must verify in writing that the production process material hasn’t changed since the last inspection. You must file inspection reports with the other documentation listed in Section 11.2.
For inspections and documentation, responsibility again rests with the end-user. Although the new requirements are challenging for everyone, they will especially tax smaller processing facilities that don’t have dedicated safety personnel.
Design considerations The primary purpose of incorporating explosion venting in a dust collector is to save lives, not property. A well-designed explosion vent functions as a “weak element” in the pressure envelope of the equipment, relieving internal combustion pressure to keep the collector from blowing into pieces. Figure 2 depicts an actual staged deflagration in a dust collector equipped with explosion venting.
Explosion venting usually will save the collector from becoming a total loss, although major internal damage may occur. Nonetheless, if personnel remain safe, the explosion protection system has done its job. Collectors for combustible dust applications are typically, though not always, located outside and are designed to vent away from buildings and populated locations (Figure 3).
NFPA 68 includes several chapters of detailed information on design requirements for the use of explosion venting equipment. The most important areas of change and concern are:
Performance-based design option. Chapter 5 states that if there’s another method for protecting the device from explosions that’s acceptable to the AHJ, you can use that method instead of NFPA standards. You must document the design methodology and data sources and maintain them for the life of the dust collector.