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Explosion Venting/Suppression Q&A

  •   What equipment requirements change when building equipment for ATEX zone 22 vs. a Class 2 Div 2 Group G dust explosive?

    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.

  •   What is the min Pmax or Kst numbers for explosion venting?

    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.

  •   Are all dusts potentially explosive? How can I tell if any of the dust we create during the manufacturing of our powdered laundry detergents is unsafe? Is there a listing of consultants or companies that can be hired to evaluate our particular situation?

    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.

     

  •   How can the Pred of an existing enclosure be increased to account for the back pressure created from vent ducts?

    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.

  •   Can a rupture disc be used for underwater applications for pipe systems?

    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.

  •   We have a 100 tn capacity silo for sugar packaging. Two vent panels have been installed on the top shell and have an adequate vent area but they are close to each other (not diagonally). Would it not damage the silo during an explosion as the pressure force will be acting only on one side of the silo?

    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.

  •   Can you confirm that it is not acceptable to use PVC piping (particularly with 90° bends in it) to vent a rupture disk for a process with an ST-2 powder having Pmax of 8.7 bar and deflagration index of 210 bar-m/sec?

    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.

  •   In a fuel -air environment, at atmospheric pressure, what is the minimum oxygen percentage required to create an explosive hazard?

    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.

  •   We are looking at a process to melt fine copper dust in an induction melting furnace. The dust is in metallic form and about 10 micron in size. It appears from the MSDS that copper is stable until about 700 °C but it melts at 1084 °C. Is there a potential explosion hazard with this very fine metallic particle between 700 and 1100° C? We are thinking about using a nitrogen blanket to minimize oxidation losses during melting. This will be a continuous process thus nitrogen blanketing will not be simple. Process rate is about 2 tns/hr.

    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.

  •   I need to meet NFPA requirements for a Class 1 dust. I will be adding a flameless explosion vent and need to know how do I determine if I need explosion isolation on the incoming pneumatic convey line?

    Per NFPA Standard 654 Section 7.1.4.1, 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.