NFPA 654, the Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, is an all-encompassing standard on how to design a safe dust collection system.
NFPA 61, the Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, covers facilities engaged in dry agricultural bulk materials or manufacturing and handling starch.
NFPA 68, the Standard on Explosion Protection by Deflagration Venting, focuses on explosion venting on devices and systems that vent combustion gases and pressures resulting from a deflagration within an enclosure.
NFPA 69, the Standard on Explosion Prevention Systems, covers explosion protection of dust collectors when venting is not possible.
Mitigating Combustible Dust
In food manufacturing, it is critical to know the explosive potential of the dusts, gases, and dust/gas mixtures emitted during processing. NFPA states that a hazard analysis is needed to assess the risks and determine the required level of fire and explosion protection from combustible dust. The analysis can be conducted internally or by an independent consultant, but either way the authority having jurisdiction will ultimately review and approve the findings.
The first step in a hazard analysis is determining whether a facility’s dust is explosive. NFPA classifies dusts according to their explosibility, that is, their Kst values. Kst is the normalized maximum rate of explosion pressure rise, measured in bar m/s. A bar is a metric unit of pressure, which is slightly less than the average atmospheric pressure on Earth at sea level. (See Combustible Dust Properties list.)
NFPA Class ST1 dusts are rated below 200 Kst, Class ST2 dusts range from 200 to 300 Kst, and Class ST3 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. However, any dust above 0 Kst is considered to be explosive, and the majority of dusts fall into this category. If OSHA determines that even a very low Kst dust is present in a facility with no explosion protection in place, a citation will result, per OSHA’s NEP policy.
In addition to Kst, it is important to know other combustible dust properties such as Pmax (the maximum explosion pressure of a dust cloud, measured in bar) and Pred (the maximum pressure developed in a vented enclosure during a vented deflagration). These can be determined using ASTM E 1226-10, Standard Test Method for Explosibility of Dust Clouds.
A dust collection equipment supplier will need the Kst and Pmax values to correctly size explosion venting or suppression systems. Failure to provide this information will increase costs, since the supplier will have to use worst-case estimates of the Kst and Pmax values or may even refuse to provide the equipment.
Dust Collectors and Explosion Protection
Combustible dust explosions are a risk in many areas of a food processing plant, but one of the most common locations is the dust collection system itself. There are many types of devices and systems used to comply with NFPA standards for the explosion protection of dust collection systems, but they fall into two general categories: passive and active.
Passive devices include the following.
Explosion venting. Designed to be the “weak” link of the dust collector vessel, an explosion vent opens when predetermined pressures are reached inside the collector, allowing the excess pressure and flame front to exit to a safe area. It is designed to minimize damage to the collector and prevent it from blowing up in the event of a deflagration, thereby reducing the safety hazard.
Flameless venting. Designed to install over a standard explosion vent, a flameless vent extinguishes the flame front exiting the vented area, not allowing it to exit the device. This allows conventional venting to be accomplished indoors where it could otherwise endanger personnel and/or ignite secondary explosions. A safe area around the flameless vent still needs to be established due to the release of pressure and dust/gases.