Explosion Venting/Suppression Q&A
Essentially, the European ATEX Zone 22 is defined as an area in which, under normal circumstances, dangerous and combustible conditions caused by a cloud of dust is not present or only present for a short period of time.
The American classification breaks it down further: Class II defines a hazardous area due to combustible or conductive dusts being present.
Division 2 narrows Class II to substances that are present only in abnormal conditions, such as container failure or system break down.
Group G defines the substances as flour dust, grain dust, flour, starch, wood, plastic and chemicals.
Equipment used in areas ATEX zone 22 should also be permitted in Class II Div2, but it is recommended that you consult with an expert in process safety/risk analysis or your Authority Having Jurisdiction to confirm what is required specifically for your application.
Anything over 0 Kst technically requires protection against the potential for a combustible dust explosion, but Kst, Pmax, MIE all have to be looked at. Once your dust is tested, NFPA 654 requires that a risk analysis be performed to evaluate your risk. And the “Authority Having Jurisdiction” for your facility—could be your insurance company, fire marshal, building inspector, OSHA, your company itself—makes the determination, based on the risk analysis, of what level of protection is required per NFPA standards and other facts and circumstances.
1. No. Some are some aren’t. In order to determine whether a dust is potentially explosive, it must be tested for a variety of parameters, including Kst, Pmax, MIE. Any dust with a Kst value of over 0 is considered potentially explosive. NFPA Standard 654 requires that dust be tested and that a risk analysis be conducted.
2. The dust must be tested for the factors noted above.
3. There is no particular list per se. There are companies that will do risk analysis, including insurance companies and there are manufacturers’ representatives that will review your situation and make recommendations. OSHA will also come in to do an evaluation of your facility if you’d like. And there are project engineers that will work with you to redesign/retrofit an existing process to comply with NFPA standards.
A vent duct effectively “leads” an explosion to the outside of a building and creates back pressure, the force of which depends on the length of the duct and properties of the combustible mixture. An enclosure needs to have a sufficient Pred to withstand this backpressure.
Pred, the reduced enclosure strength, is typically determined by taking 70% of the PES or design strength of an enclosure. Over time, due to corrosion, fluctuating pressure and stress in general, as the enclosure ages, the PES and Pred reduce from the original as-built design.
Pred can be increased by stiffening the enclosure in critical areas, like reworking all welded areas, re-enforcing the frame, and increasing the thickness of the sheet metal of the enclosure casing.
To be done correctly, a certified structural engineer would calculate and determine where/how the design strength has to be increased and by how much.
This may be a costly procedure. In many cases back pressure is avoided by simply installing an indoor flameless vent, that will, in the event of an explosion, safely vent an explosion inside without the need for ducting to the outside.
The design of a process rupture disc is such that it will release the overpressure from inside an enclosure based on the real pressure difference. If it is correctly designed, a rupture disc will release this overpressure whether it be under water or at high altitudes. The disc specifications will take into account all the atmospheric conditions, including location of application, temperature, etc.
Your concerns are well founded. The ideal installation in this situation is to have the panels located such that recoil forces will not have a damaging impact on the silo or other enclosure – in other words, across from each other. If, for any reason, the vent panels have to be installed on one side, then the enclosure must be sufficiently strong to withstand the potential recoil force.
PVC piping could be used so long as the strength (Pred) is high enough and it is sufficiently temperature resistant. The larger concern would be the 90 degree bends. Although NFPA standards do not definitively say you cannot have such bends, we don’t recommend it.
Generally–and I say this with some hesitation–if your fuel is gas of some kind, the minimum could be approximately 5%. If your fuel is dust of some kind, the minimum could be approximately 10-12%. For smoldering dust, it could be as low as 2%. Note that I say could be. These generalizations really don’t mean anything for a specific application. The answer always depends on the specific fuel and a LOC (Lowest Oxygen Concentration) must be determined for that specific fuel.
I would prefer to know more about your process to better understand why you are considering nitrogen blanketing for this application. Having said that, I recommend you have your dust tested even if you don’t find a Kst value (I didn’t locate one either!) Testing your dust will give you additional information from which you can make your decision.
Per NFPA Standard 654 Section 220.127.116.11, isolation is always required. “Where an explosion hazard exists, isolation devices shall be provided to prevent deflagration propagation between pieces of equipment connected by ductwork.”
NFPA standard 69 annex section A. 11.2. seems to provide an exception for 100 mm (4 in.) pipes, on the theory that, “Piping less than 100 mm (4in.) diameter is far less likely to provide a conduit for flame spread than larger diameters.” This same section starts by indicating that isolation is necessary unless:
1. A qualified risk analysis is performed
2. It is determined that the risk involved in not isolating is acceptable to the AHJ.