Explosion Venting/Suppression Q&A
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.
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.
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.
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.
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.
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.
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.
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.
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.