Explosion Venting/Suppression Q&A
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.
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.