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Explosion Venting/Suppression Q&A

  •   We work with several difference powders in the pharmaceutical manufacturing industry (weighing, blending, tableting, etc.). Many of the explosivity numbers (Kst, dP/dT, etc.) help determine how severe an explosion would be, should it occur. However, when evaluating chemical powders to see if we can handle them in our facility, if we test and know the MEC of each chemical (to simplify the testing and maximize cost effectiveness), and can keep the dust levels well below the MEC, do we have to know any of the other explosivity testing values (Kst, Pmax, dP/dT, MIE, etc.) when trying to ensure an explosion won’t occur?

    In theory, this seems to be the true. And, yes, if you have total control of all aspects of the process and can control the dust concentration through every aspect of the process, then you should not need to do more–in theory. NFPA Standard 69, 2014 edition, even says so. See Chapter 8, Deflagration Prevention by Combustible Concentration Reduction.

    Here’s the problem:  the dust concentration changes throughout the process. Can you really assure yourselves you can control the concentration such that you are always maintaining a level that is well below the MEC? Think about what happens at your dust collectors. Do you really have control of the amount of dust that accumulates on the filters/bags? Will there be a “mishap” there? I am always concerned about the human factor: will a broken filter bag be detected and replaced immediately? Will filter bags be replaced immediately at the precise time they need to be? All this and much more during the process can affect the MEC and the ability to control your dust concentration.

    If your authority having jurisdiction is willing to accept this approach based on documentation that shows you can and will control the dust concentration to maintain a level well below the MEC, then you are good to go.

  •   If a plant is processing multiple (pharmaceutical) products that feed to a single dust collector, what would the recommended sampling plan be in order to send samples for Kst/MIE/MEC testing. The concern is that it cannot be guaranteed what combination of materials will be present and that they may not represent the worst case sample for testing. Also, will Kst/MIE/MEC results be sufficient for evaluating the design requirements for suppression and dust collector venting?

    Generally, we recommend that whatever sampling system you use, you do so with a goal of getting the worst case scenario: highest Kst, highest pmax, lowest MIE, Lowes MEC. Generally, we can say that mixing dry powder will likely not result in a synergistic reaction, so usually the individual parameters of each material you use can be your guide to determine the worst case scenarios.

    Having said that, I recommend you contact Fauske & Associates, Burr Ridge, IL. (630) 323-8750, or your preferred source for testing dusts. They will provide you with a step by step approach to collecting the dust given your specific applications, material combinations and circumstances. You need to let them know if there are solvents involved and any other complicating factors. Remember, by the time the dust is received by Fauske there may not be any trace of the solvents, so complete information regarding the possibility of a hybrid mixture in the process is important.

  •   During the processing of solid hexamine we go thru a phase in which the material has a Kst of >500 with a Pmax of 134 psig. Is there a process of designing isolation and/or suppression at these levels?

    While indoor mechanical venting and mechanical isolation systems would not be an option, there are suppression systems that might be able to help you with your application. We would need a full dust testing report with an exact Kst, Pmax, Pred, MIE, hybrid mixture yes/no. (The test results would also help us review whether explosion panels/venting to the outside might be an approach to explore, if your process is outside or located near an outside wall.) In all likelihood, your specific dust would also need to be suppression-tested. If you want more information please give us a call or give us a phone number where we can reach you.

  •   When you must use a Ta burst disc, what metallurgy do you recommend for the rupture disc holder? Many rupture disc suppliers mix and match metals to save money on the holders, what is Rembe’s opinion in this regards? What are the galvanic affects in utilizing different metallurgies in strong halide environments?

    A rupture disc and holder are one safety/relief system. As such, all “wedded” parts should be made of the same or similar metal as determined by corrosion resistance. In situations in which Tantalum is selected to be the ultimate choice for a rupture disc, the holder should be, at minimum, steel with a Tantalum coating or manufactured in an alloy such as Hastelloy C or B. In my opinion, based on past experience, due to diffusion, the holder or any other part should not be coated with Teflon, Halar, PFA or similar coatings.

    A more precise recommendation requires detailed information about your process and chemistry.

  •   How prevalent is the application of explosion venting in electrostatic precipitators and is there a failure rate established for explosive venting devices?

    Electrostatic precipitators are not typically used with combustible materials.

    In some rare cases in which traces of combustible gases or material with a low MIE could enter into the air stream  of an electrostatic precipitator, explosion vents have been added. The vent area is calculated in accordance with NFPA 68 as with all other enclosures/applications. Once a pressure rise occurs due to a combustion, just as with any other enclosure containing combustible material, the panel will open and release the pressure at a defined Pred.

    The panels are all batch-tested during manufacturing, so a failure-to-open or an open-too-late will not happen, so long as calculations are correct, installation is correct, and the area surrounding the enclosure is not in any way impeding the release of the  explosion. We would need more information about your specific application—indoor/outdoors, etc.—to give you any additional guidance and you are more than welcome to contact us if we can assist.

  •   We had a community college install a dust collection system inside a room next to their welding area. There are 32 welding stations and three grinding stations (with aluminum, carbon, and stainless steel dust) go into the collector. Can this collector be inside without venting and what is the chance of fire or explosion?

    I see two questions here: first, can a dust collector be located inside under the circumstances you describe and the answer, in theory, is yes. Please check NFPA Standard 484, that strongly prefers dust collectors be located outside for metal dust applications. However, there is always the performance-based design option if either your customer’s dust collector is already located inside the plant or if the application, circumstances, cost-effectiveness etc., require the dust collector be located inside.

    The second question is does the dust collector require venting, whether it is inside or outside and, again the question is yes.

    Per NFPA standards 654, 68, 69, 484, dust collectors that are part of a process with combustible dust must be protected, either with chemical or mechanical suppression and isolation. There are many facets to the requirements, but suffice it to say that you are definitely dealing with combustible dusts, so the process you describe must have venting/suppression and isolation.

    If you put the dust collector inside, and it is located very near an exterior wall, the process might be able to be ducted to the outside, using a regular explosion vent. If not, the options for indoor suppression of metal dusts are limited but available.  REMBE is the only company, to date, as far as I am aware, that has ATEX approved Q-Rohr 3-6T/6T-AL indoor/outdoor flameless venting systems that can be used with metal dusts.

    If you place the dust collector outside, explosion panel(s) is (are) required.

    Regardless of the decision that is made regarding the location of the dust collector, a careful calculation under NFPA 68 needs to be made to assure of proper sizing of the mechanical suppression systems you decide to use.

    If you want more information, please give us a call–we are more than happy to assist in determining the best solution for your particular process.

  •   Would the discharge screw of a storage silo be considered as a pipe with regards to the application of NFPA-68-2013 section 8.12? Should we consider the fill volume ratio in the screw to determine effective pipe size for a deflagration to propagate trhough the screw? Should we always plan for a rotary valve to isolate the screw discharge from downstream transfer systems?

    Enclosures containing combustible material should always have some type of explosion protection—venting and isolation or other means in accordance with, at minimum, NFPA 654, NFPA 68, and NFPA 69.

    To avoid flame propagation in case of an event, either to avoid secondary explosions in connected enclosures, or to avoid fire balls from entering areas where people can be harmed, inlets and outlets have to be closed by isolation equipment or, in the case of longer connecting pipes, by explosion venting as described in NFPA 654 , Annex E1.

    The discharge of a storage silo many times is realized by screw conveyors, which could work as isolation devices, depending on the type of construction and if they are completely filled with product – drag conveyors may never fulfill this requirement. To avoid critical situations, in many cases, rotary air locks are installed at short distances, so yes you should, as a general proposition, plan for a rotary valve that complies with NFPA standards for explosion protection to isolate the screw discharge from the downstream transfer systems. In fact, it is recommended that you isolate both the inlet and the discharge.

    NFPA 68, Chapter 8.12 describes the handling of pipes shorter than 6 m and diameters less than 0.3 m. In the case of interconnected enclosures and larger diameters, the connecting pipe must either be completely isolation or vented as described in NFPA 654, Annex E1. Very small volumes can be added to the larger enclosure to be vented.

    I would be more than happy to review your specific application so as to make a more comprehensive and definitive recommendation.

  •   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.

    Update 5/14/14: REMBE has provided an update to this question:

    Note:  since this question first arose, there is a newly ATEX certified flameless venting system option available for protecting against combustible metal dusts—the REMBE Q-Rohr-3-6T. This might also be an option for this copper dust application, should the results of Kst testing indicate the dust is combustible.


    Original answer: 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.

  •   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.