Combustible Dust & Static Electricity Q&A
Various publically available references may contain data on the combustibility and explosibility of some common powders that are used in various industries. For example, you may find that organic materials of the types that you have listed are likely to be combustible and in the form of a finely divided dust cloud of sufficient concentration could give rise to dust cloud explosion hazards. On the other hand, inorganic materials such as salt are non-combustible.
However, published data on the explosibility of dusts must not be used for the determination of explosion prevention and/or protection measures. Factors such as test method, composition, particle size, moisture content may affect the results. Evaluation of the hazard of a combustible dust should be carried out by means of actual test data from a representative sample from your own process. If in doubt, you may consider performing a Go/No Go Explosibility Screening test in accordance with the ASTM E1226 (Standard Test Method for Explosibility of Dust Clouds) to determine if a particular material or a mixture of organic and inorganic materials is explosible or not. Depending on the result of the screening test, additional tests may or may not be necessary.
Dry ice blasting of objects that are either conductive (e.g. metals) but isolated from electrical ground or insulating in electrostatic terms (e.g. plastics) could generally be expected to result in the development of high surface potentials/voltages on such objects. Under certain conditions, the resulting electrostatic discharge(s) could have enough energy to ignite a combustible dust cloud atmosphere. However, if the dust cloud is formed in an oxidant (oxygen) depleted atmosphere because of the presence of CO2 such that the oxidant concentration is below about 10 percent, a dust cloud explosion will be unlikely. It is suggested that a hazard assessment is performed on this operation to ensure that adequate measures are in place for ensuring the safety of the operator(s) and the facility.
Depending on the explosion properties of the dust that will be collected in the dust collector there may well be a problem with having no explosion protection and (perhaps) isolation provisions for this dust collector. Although one may find some data on the explosion property of polypropylene dust in the open literature, it is strongly suggested that an explosion severity test (to determine the maximum explosion pressure, maximum rate of pressure rise, deflagration index (Kst Value) according to ASTM E1226, Standard Test Method for Explosibility of Dust Clouds) be conducted on the actual/representative finest dust sample that could be present in the dust collector. Based on the results of the laboratory test, it would then be possible to determine the need and design appropriate explosion protection, perhaps in the form of relief venting and ducting to a safe location on the outside of the building, flameless venting, or suppression. Additionally, one should consider isolating the explosion in the dust collector and preventing it from propagating to the upstream and downstream direction.
Unfortunately the question does not contain enough detail to enable one to provide an adequate reply. Generally speaking, static dissipative plastic bags are often used to either eliminate the possibility of occurance of “brush” or “propagating brush” type discharges. “Brush” discharges are capable of igniting flammable gas and vapor atmospheres but they have not been shown to ignite any dust cloud atmosphere. “Propagating brush” discharges on the other hand, could have energies as high as 1,000 to 2,000 mJ and are therefore capable of igniting not only flammable gas and vapor atmospheres but also combustible dust clouds. Depending on the minimum ignition energy, volume resistivity, and chargeability of the powder other measures such as grounding of operators and avoidance of the use of insulating hoses and plastic liners may be necessary. Conductive (metal) pipes and hoses for the conveying of the powder should also be considered. Additionally, all metal items of plant must be bonded and grounded. In some cases, the powder material itself can generate enough static charge to give rise to an electrostatic discharge which could be an ignition source. It is suggested that a process hazard assessment is conducted to:
- Identify areas of the facility where flammable atmospheres could exist under normal and/or abnormal conditions;
- Identify potential ignition sources that could exist under normal and/or abnormal conditions;
- Make recommendations for:
- Preventing the formation of flammable atmospheres in the plant and reducing the extent and duration of any flammable atmosphere that may be formed;
- Eliminating/controlling ignition sources (including the electrostatic discharges); and
- Protecting against the consequences of an explosion.
From your description, it is not clear whether the problem that you are describing is in the holding vessel at your facility or in the packaging that is received by your customer. I should mention that “altering the charge of the tank”, as is suggested in your email – assuming that the tank is constructed from a conductive (metal) material – would not be a practical option for a number of reasons including safety. If the holding vessel and/or the packaging is insulating (plastic), the dust buildup problem is possibly due to the vessel or the package being electrostatically charged prior to or during filling. In order to diagnose this problem correctly, I suggest that you consider conducting “volume resistivity” and “chargeability” tests on a sample of the fine particles and also determine the surface resistivity of the receiving vessel (if not metal) and the packaging material. Based on the results of these laboratory tests, practical measures for controlling the adhesion of the dust particles to the inside surfaces of the receiving vessel and/or packages can be devised.
Traditional talcum powder (Magnesium Silicate Hydroxide, CAS # 14807-96-6) is an inorganic mineral and is unlikely to support combustion. Corn starch on the other hand, is combustible and under the right conditions can give rise to a dust cloud explosion hazard. Based on the conditions described in your question, I suggest that you consider determining the ignition sensitivity and explosion severity characteristics through appropriate laboratory tests on representative dust samples. You should then consider conducting a dust explosion hazard assessment of your operations/processes involving combustible dusts with the objective of managing dust cloud explosion hazards through appropriate and practical explosion prevention and protection measures, including:
- Preventing the formation of explosible dust clouds in the plant and reducing the extent and duration of any dust clouds that may be formed;
- Implementing an effective housekeeping program,
- Eliminating/controlling ignition sources; and
- Protecting against the consequences of dust cloud explosions (e.g. explosion relief venting, explosion suppression, explosion containment, and explosion isolation).
Based on the information provided in your question and in the absence of any flammable gas or vapor atmosphere, you could consider using filter cartridges that are made entirely from insulating materials such as polypropylene, provided that the use of such filter does not give rise to “Propagating Brush” discharges. In some instances, the use of filter cartridges that are made entirely from non-conductive components is in fact preferred to cartridges that have some metal components, if the grounding of the metal components cannot be ensured at all times. Please note that this suggestion does not in any way negate the need for other measures that may be required to ensure safety from explosion hazards such as controlling other potential ignition sources and explosion protection for the dust collector.
Ammonium perchlorate is a strong oxidizer. The Department of Transportation (DOT) requires it to be shipped as either a 5.1 (Oxidizer) or 1.1D (Detonating Explosive). The requirement is that ammonium perchlorate ships as a 1.1D unless a DOT approved determination has been made that it is a class 5.1. The factors that normally determine the test outcome are particle size and method of achieving the particle size (precipitation vs. grinding). In our experience, 200 micron material is normally classed a 5.1, while 20 micron material is normally classed a 1.1D. Any ammonium perchlorate material with a significant distribution of fine particles below 20 micron will probably detonate if subjected to a fire while confined or a donor charge.
As far as the publically available data on the explosibility properties of ammonium perchlorate is concerned there is very little found in the usual references. A “layer ignition temperature” of 260°C is quoted in V. Babrauskas, “Ignition Handbook”, with no other dust-explosibility properties. This reference also gives a “dust cloud ignition temperature” of 400°C for ammonium nitrate, with no other dust-explosibility properties. In the same reference both ammonium perchlorate and ammonium nitrate are listed as “Dust found to be non-explosible”. It is therefore likely that ammonium perchlorate is similar to ammonium nitrate in that explosion only occurs as a “mass detonation”, in relatively large quantities. Note that the PEPCON [Henderson, Nevada] detonations on May 4, 1988, involved about 4,500 tons of ammonium perchlorate, with much of this material 200 microns or smaller.
It is unlikely that explosion or combustion could be initiated by credible electrostatic sources, but testing is strongly advisable to confirm this, and to determine the impact sensitivity. The finer material will generally have increased sensitivity. We would recommend a Process Hazards Analysis to fully evaluate your planned process and to address the hazards.
NFPA 664 does not get specific about the type of dust collector (bag-houses are typical in the wood industry) that should be utilized. NFPA 664 standard does, however, allow for performance-based designs and risk assessments in lieu of prescriptive methods. Also, the prescriptive controls are specific to those dust collectors with a deflagration hazard. It might be possible in some specific situations to make an argument for no deflagration hazard, for example when the dust cloud concentration in the dust collector is shown to remain below 25% of the Minimum Explosible Concentration (MEC) due to cleaning methods, amount of dust entering the collector per unit time, CFM of the blower, etc. and collection lines being kept free of deposits at all times.
I would like to start by reminding everyone that Minimum Ignition Energy (MIE) is a measure of the sensitivity of the dust cloud to ignition by electrostatic discharges only. An MIE >1,000mJ is normally an indication of a dust cloud that is not overly sensitive to ignition by electrostatic discharges. As such, bonding and grounding of metal sections of the equipment and system, as well as the avoidance of using plastic pipes/hoses, plastic lined/coated metal pipes and vessels, and insulating (plastic) containers for receiving highly charged powders, should reduce the risk of an ignition by electrostatic discharges. However, other ignition sources such as mechanical friction (heating/sparks), self-heating, and sparks from electrical equipment might still be present under certain conditions and might have enough energy to ignite the dust cloud or the dust layer. It is therefore recommended that an analysis of the process hazards be made by a competent authority and the ignition sensitivity of the dust cloud and perhaps the dust layer to all the identified ignition sources be determined by appropriate laboratory tests. Elimination of ignition sources may only be used as a primary basis of safety when all the ignition sources that can be present during normal and or abnormal operating conditions are identified and effective measures are then implemented and maintained for their elimination. It is also important to perform a risk assessment to ensure that the remaining risk is acceptable to the Authority Having Jurisdiction.