Combustible Dust & Static Electricity Q&A
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.
It is recommended that the duct work design, construction, and installation be reviewed by someone with an appropriate mechanical engineering background. The resistance to internal pressure might not, however, be the only important consideration for ductwork. Often a relatively thin wall may well be able to withstand an internal “hoop-stress” pressure of 0.2 barg [about 2.9 psig], particularly for round ductwork. However, thin-wall ductwork might not be adequately self-supporting between hangers or saddles. Also, the possibility of accumulations of material [dust] in the duct – and accumulations of snow or ice on top of the duct – and the weight of such accumulations [and windstorms] also would affect the required strength of the ductwork.
A dust cloud explosion occurs by the introduction of a sufficiently energetic ignition source into a dust cloud of sufficient concentration that is suspended in an oxidant (usually oxygen in air). The minimum ignition energy ignition (MIE) is defined as the minimum electrical (electrostatic) spark energy that is capable of igniting a dust cloud atmosphere at its most easily ignitable concentration in air. Therefore, the maximum pressure caused by a dust cloud explosion can be estimated using the gas law and is based on the number of moles before and after the explosion (mostly N2 in air) along with the estimated maximum temperature. In contrast, thermal decompositions do not necessarily require oxidant to occur. A pressure rise caused by thermal decomposition depends on the number of moles of gas generated and the amount of heat released during decomposition.
Assuming that the by-product is in the form of a particulate solid material: Zinc, which comprises some 89 % of the said by-product’s composition, as well as iron, are combustible materials and if they are in the form of finely divided particulate solids, suspended in air in sufficient concentration, and subjected to a strong enough ignition source, will deflagrate or explode. Therefore, unless a representative sample of the by-product is subjected to an Explosion Classification (Go/No Go Screening) test according to ASTM E1226, Standard Test Method for Explosibility of Dust Clouds, to determine if the sample is explosible, you may consider the by-product to be an explosible material that could, under certain conditions, form an explosible dust cloud atmosphere.
Assuming that the by-product is in the form of a paste material: Consider conducting a Burn Rate (Fire Train, Flammability of Solids, Ignitability of Solids) test according to the UN Manual of Tests and Criteria. Powders of metals or metal alloys are considered to be readily combustible when they can be ignited and the flame of the reaction spreads over the whole sample in 10 min or less.
Based on the information provided, the following could be an explanation to what might have occurred:
Campers are warned not to use rocks or stones from a creek or river bed as the foundation for a fire [to keep a campfire away from combustible grass, moss, leaves, etc.], because the heat from a fire could cause the entrapped water in the pores or crevices in the rocks or stones to explosively vaporize, with consequent rupture or hazardous fragmentation. The same type of event could occur with cement or concrete, particularly if the cement or concrete were to contain “too much” water [so that the mixture could flow readily around reinforcing bars or other objects; thus, failing a “slump” test] or if the cement or concrete had not had enough time to completely “cure” [with incorporation of the water into the cement molecules, as hydrate].
Please see the response below in regards to the multiple questions asked here.
• Standard Polypropylene (PP) is a highly insulating material and during filling of large vessels with PP material (such as powders, granules and pellets) electrostatic discharges (known as cone/bulking brush discharges) can be expected. These cone discharges generally occur when filling grounded conductive vessels (e.g., grounded metal vessel). Although the energy of cone discharges is somewhat limited (depending on the vessel size, material size, material resistivity), cone discharges are capable of igniting flammable vapors, hybrid (vapor and dust) mixtures and combustible dust clouds with Minimum Ignition Energy (MIE) less than 25 mJ. If vessel is ungrounded, there is a possibility of generating a spark discharge from the vessel. To avoid such spark discharges, it is recommended to ground the vessels.
• Corrugated boxes could behave as semi-conductive material under ambient humidity conditions and could become grounded when placed on conductive grounded surfaces. However, under low relative humidity conditions, the corrugated box could become insulating and therefore could not be grounded.
• If the PP material is composed of large parts (9 mm – 50 mm) only as described in the question and there is no fine dust present in the PP material, the concern of dust cloud fire and explosion would not be present due to absence of fuel (dust). Electrostatic charges from the material surface inside the container could however still be present and could be a shock hazard for employees (for example if a grounded operator reaches into the container). In addition, the electrostatic field created by charged PP material could subsequently charge isolated conductive objects in the near vicinity unless the vessel is conductive or static dissipative and is grounded.
• The following tests could be considered for this situation:
o Consider determining the amount of dust (fine particulates) being present in the PP material. If dust could be present, consider determining the Minimum Ignition Energy (MIE) value for a representative sample of the finest PP material. If MIE is > 25 mJ, the risk of dust cloud ignition by cone discharges is minimal. If MIE<25mJ, additional safety precautions may be necessary. Expert advice should be sought.
o It is suggested to handle PP material in electrically grounded conductive or fiberboard containers. The resistance to ground of the conductive vessels should be periodically tested.
Data for the explosivity properties of salicylic acid are limited. Two references give Minimum Explosible Concentrations (MEC) of 15 and 25 g/m3. A reference gives a Minimum Ignition Energy (MIE) that is “less than 10 mJ”, and a Minimum Ignition Temperature (MIT) of 590oC. “Old” data for Maximum Explosion Pressure and Rate of Pressure rise are 84 psig, and 6,800 psi/second, respectively, as compared to 90 psig and 2,300 psi/second for Pittsburgh Coal Dust, and it is likely that these values are significantly “too low”. Since particle size and shape, moisture content, presence of contaminants, and test method amongst other factors can influence the explosion characteristics of the dust cloud, it is strongly suggested that a representative sample from your facility is taken and subjected to appropriate tests rather than basing very important safety decisions on published data.
A hot ember could have a very high temperature near 1,550F. In addition, a burning layer of milled straw may provide primary flame to ignite a dust cloud. A dust cloud explosion hazard would be deemed to exist if enough fines could be present in the process stream to create dust concentrations above the Minimum Explosible Concentration (MEC) during normal or abnormal condition. It is suggested to determine the MEC value (per the ASTM E1515 test method) for this material and then evaluate the process to determine if such concentrations could exist in the process stream. Generally speaking, the dust concentrations should be controlled below 25% of MEC value (to allow for system fluctuations) to abate the explosion hazard (by not having flammable atmosphere). However, if this cannot be ensured with a reliable certainty, explosion prevention (e.g., control of embers) and/or protection measures may need to be applied.
Controlling the hot embers would also be suggested to reduce the occurrences of fires. Consider the following options:
Dense particles – such as grit and metal – could be separated from lighter materials with a pneumatic/gravity separator as illustrated in Figure A.22.214.171.124(a) of the 2013 issue of NFPA 654 (with protection against fire in the particle collector). Also, a spark-detection and water-spray spark-extinguishing system could be installed, as described in Annex C of the 2006 issue of NFPA 654, and in paragraph 9.1.4 of the 2008 issue of NFPA 69.