Rolf K. Eckhoff

Prevention and mitigation of dust explosions in the process industries: A survey of recent research and development

Abstract The paper covers three aspects, viz. basic research, applied research and development on prevention and mitigation of dust explosions in industrial practice, and testing of ignitability and explosibility of dusts. With few exceptions, only works published from 1990 and onwards are referenced, i.e. publications that are not discussed in the previous survey (Eckhoff: ‘Dust Explosions in the Process Industries’, 1991). Increased knowledge about the numerous facets of the dust explosion hazard has created a justified request for a more differentiated approach to the design of preventive and mitigatory measures in industry. In this process, cross-fertilization between fundamental research and applied research and development is essential. Computer simulation models, carefully calibrated against experiments, will become useful tools in a not too distant future. The use of expert systems requires adequate quality assurance.

Auto-ignition of CH4air, C3H8air, CH4/C3H8/air and CH4/CO2/air using a 11 ignition bomb

Abstract The distinction between auto-ignition and hot-surface ignition of a given gas is emphasized. In ideal auto-ignition there is no diffusion of heat or matter. Published information on auto-ignition temperatures (AIT) of multi-component fuels in air is scarce. This also applies to North Sea natural gas, of which CH4, higher alkanes and CO2 are essential components. In the present experimental laboratory-scale study, AIT of four types of hydrocarbon mixtures ( CH 4 air , C 3 H 8 air , CH4/C3H8/air and CH4/air/CO2) have been measured using a 11 ignition bomb. The experimental method ensured that the gas mixtures studied were of known composition and homogeneous in concentration. The gas mixture was admitted to the pre-evacuated ignition bomb in the form of a turbulent jet when the bomb wall had reached the desired temperature. Ignition was recognized as a sudden pressure rise in the bomb a few seconds after the gas flow into the bomb had stopped. The minimum AITs for CH 4 air and C 3 H 8 air were found to be 640 °C and 500 °C, respectively. The AIT of CH4/C3H8/air decreased with increasing propane content and total fuel concentration. A fuel concentration region was discovered for which CH4/C3H8/air and C 3 H 8 air with the same ratio of propane to oxygen gave the same AIT. Reducing the oxygen content of a CH 4 air mixture by adding CO2 gave, under the present experimental conditions, a systematic increase of AIT with increasing CO2 content. The role of the CO2 was probably essentially that of an inert diluent. It has been known for a long time that the ‘minimum hot-surface ignition temperature’ is not a constant for a given gas mixture, but highly dependent, by several hundred degrees centigrade, on the dynamic state of the gas, the geometry and material of the ignition surface, and the mode of heat supply to the surface. The direct application of AIT values to assess industrial hot-surface ignition risks may therefore be unduly conservative. Consequently there is a need for general mathematical models that can predict minimum ignition temperatures for various practical situations in industry. Such models will have to contain sub-models of ignition chemistry, fluid mechanics and heat and mass transfer.

Minimum ignition energy (MIE) — a basic ignition sensitivity parameter in design of intrinsically safe electrical apparatus for explosive dust clouds

Abstract In general terms, the purpose of any safety standard is to define borderlines between safe and unsafe conditions, with reasonable safety margins. The electrical spark ignition sensitivity of dust clouds (MIE) varies over at least eight orders of magnitude. Therefore, in the case of intrinsically safe electrical apparatus to be used in the presence of explosive dust clouds, substantial differentiation of the minimum requirements to prevent ignition by electrical sparks is needed. The present paper proposes a method by which adequate differentiation of required maximum permissible currents and/or voltages in intrinsically safe electrical circuits to be used in explosive dust clouds can be achieved. In essence, the concept is to use conservative first-order ignition curves, calculated or estimated from the experimental MIE value of clouds in air of the actual dust. Charts to be used for design purposes are given in the paper. Internationally standardised test methods allow MIE for clouds of any dust to be determined, at least down to the range of a few mJ. There is, however, a need for a supplementary method covering the range of lower energies, down to 0.01 mJ.

Critical dimensions of holes and slots for transmission of gas explosions: Some preliminary results for propane/air and cylindrical holes

Abstract Critical hole diameters for explosion transmission from a primary virtually closed chamber into an ambient gas cloud were determined. Most of the present experiments were conducted with a 1-l primary chamber. The motivation for the study is two-fold. First, results from this kind of experiment are of direct practical use in further improvement of design and maintenance procedures of “flame-proof” electrical equipment. However, such experiments can also contribute to improvement of the general understanding of the mechanisms of flame propagation in turbulent, premixed gases. The preliminary experimental results presented confirm that the minimum tube diameter for flame transmission depends strongly on the location of the ignition point. The generally accepted limiting values are conservative, in the sense that they can only be approached if the ignition source is located in a narrow zone in the vicinity of the entrance to the transmission hole. This must be taken into account if flame-proof equipment is to be designed on the basis of risk analysis. An observation related to mechanisms of turbulent flame propagation in premixed gases in general was also made. In the case of ignition far away from the transmission hole, i.e. for high gas velocities through the hole at the moment of flame front arrival at the hole, the re-ignition probability for a given hole diameter was in fact higher for off-stoichiometric propane concentrations than for concentrations close to stoichiometry. The average chemical reaction rate in the primary chamber had its peak in the region of stoichiometry, and hence the pressure in the primary chamber at the moment of flame front arrival at the transmission hole entrance, was also at its maximum in that concentration range. Therefore, the average turbulence intensity in the potential re-ignition zone, and hence the rate of entrainment of cold unburnt gas by the hot jet was also at its maximum. Hence, a main reason for the observed effect may be that at stoichiometry more efficient cooling by cold-gas entrainment, compared with at leaner and richer mixtures, more than compensated for the faster chemical reaction.

Maize starch explosions in a 236 m3 experimental silo with vents in the silo wall

Abstract Large-scale wall venting experiments were conducted in the same steel silo of 22 m height, 3.7 m diameter and 236 m3 volume used in previously published roof venting experiments. Maize starch test dust (125 kg), was blown into the silo at the top through a conventional pneumatic pipeline. The dust injection process lasted for 28 s, whereafter the air flow in the pipeline was terminated and the ignition source activated. The dust concentration in the silo was measured during dust injection. Pressure as a function of time and flame speed at different locations in the silo were recorded during each explosion experiment. In the case of an uncovered vent of 4.4 m2 in the cylindrical silo wall close to the silo top, the maximum explosion pressures in the silo were generally considerably lower than with a 3.6 m2 roof vent, and even somewhat lower than with a roof vent of 5.7 m2. Lower turbulence, and hence combustion rate in the dust cloud in the silo, due to more restricted flow out of the vent, may be the reason for this. On the condition that adequate precautions are taken to prevent destructive effects of reaction forces, venting through the silo wall may therefore be preferential to roof venting even from the of view of minimizing the explosion pressure.

Possible sources of ignition of potential explosive gas atmospheres on offshore process installations

Abstract This paper highlights the basic features of various ignition sources and some principles for prevention of ignition of potential explosible hydrocarbon gas atmospheres in industrial practice. Ignition by hot solid surfaces, heat from mechanical impact and friction, electric discharges and jets of hot gaseous combustion products is discussed in detail. The possibilities of effective ignition sources in industrial practice being generated by stray electric currents, cathodic corrosion protection currents, static electricity, lightning, high-frequency electromagnetic waves, optical and radioactive radiation, ultrasonic radiation and heating by compression are discussed.

Ignitability and explosibility of polyester/epoxy resins for electrostatic powder coating

Abstract The use of electrostatic powder coating is expanding. In view of the dust explosion hazard related to this process, a comprehensive investigation of ignitability and explosibility properties of 11 polyester/epoxy resin powders used in electrostatic powder coating has been carried out. The powders differed with respect to the ratio of polyester to epoxy, pigment type, pigment content, density and particle size distribution. The powders were tested in the closed Hartmann bomb for establishing the maximum explosion pressure and the maximum rate of pressure rise, in the open Hartmann tube fitted with the CMI electric spark generator for measurement of the minimum ignition energy, and in the ‘Nordtest Fire 011’ apparatus for determination of the minimum explosible dust concentration. Attempts were also made at conducting some explosibility tests in the Swiss closed 20-litre spherical vessel, but severe blocking problems in the dust dispersion system were encountered. Particle size distributions of the powders were determined using a laser diffraction method, and the specific surface areas were measured by nitrogen adsorption. All the powders gave approximately the same maximum explosion pressures in the Hartmann bomb, whereas the maximum rate of pressure rise decreased with increasing pigment content and particle size. Clouds in air of all the resins had quite low minimum ignition energies, from below 3 mJ to approximately 20 mJ. Except for one or two powders the minimum ignition energy increased fairly systematically with particle size. There was no systematic influence of the pigment content, although the powders with the highest pigment contents also had the highest minimum ignition energies. The minimum explosible dust concentration increased systematically with increasing pigment content, in such a way that the concentration of combustible material at the minimum explosible dust concentration was nearly the same for all the dusts, and close to the minimum explosible concentration of gaseous hydrocarbons like methane and propane.

Sizing of dust explosion vents in the process industries: Advances made during the 1980s

Abstract To prevent damage to humans and property caused by dust explosions in the process industries, several kinds of precautions may be taken. Because of its favourable features, both technically and economically, venting has become a widely used method for controlling dust explosions. It is therefore unfortunate that the literature on dust explosion vent sizing is still in part contradictory. It is generally appreciated that the vent area needed to keep the pressure in a dust explosion in a given enclosure below a given limit. is determined by the rate of heat release in the explosion. However, this rate depends not only on specific dust properties, such as chemistry and fineness, but also on several properties of the dust cloud that are determined entirely by the industrial process in which the cloud is generated and ignited. Taking these properties into account has become a central issue during the 1980s. This paper reviews a number of realistic large-scale experimental investigations conducted during recent years. The experimental results are compared with the corresponding vent areas recommended by several different vent sizing methods in current use. A differentiated method for vent sizing, based on data from experiments conducted under realistic industrial conditions, is proposed. The continued need for conducting further realistic experiments in various types of full-scale vented indusrial enclosures is emphasized.

Design of electrical equipment for areas containing combustible dust: Why dust standards cannot be extensively harmonised with gas standards

Abstract Over recent years, the idea has emerged within the IEC (International Electrotechnical Commission), as well as within the standardisation system of the European Union, that it may be beneficial to harmonise design concepts for electrical equipment for areas containing combustible dusts, with those for areas containing combustible gases and vapours. The harmonisation idea has been encouraged by the European Union “ATEX 100a” Directive, which suffers from insufficient differentiation between combustible dusts, combustible mists, and combustible gases/vapours. This deficiency probably originates from focusing on the extensive similarity of combustible dust clouds, mist clouds and gas/vapour clouds when it comes to ignition and burning properties. However, these similarities are of little significance unless there is an explosible cloud in the first place. And this is where dusts, mists and gases/vapours differ substantially, as discussed in detail in the present paper. It is suggested, therefore, that the idea of extensive harmonisation of design concepts for dusts with those established for gases/vapours be put aside (e.g. IEC Committee draft standards for “Ex i” and “Ex p” for dusts, as well as a proposal for a new “Ex m” dust standard). Instead, the safe design of electrical equipment for areas containing combustible dusts should mainly be based on two simple concepts, use of enclosures that keep the dust out to the required extent, and measures that keep the temperature of any surface in contact with dust clouds or layers sufficiently low to effectively prevent ignition. This is in full accordance with the current philosophy in European standardisation as expressed clearly in EN 50281-1-1 and -2: “The ignition protection is based on the limitation of the maximum surface temperature of the enclosure, and on the restriction of dust ingress into the enclosure by the use of “dust tight” or “dust protected” enclosures”. The same philosophy has been prevailing in USA for quite some time. It is indeed to be hoped that Europe will also maintain this sensible approach, and revise the “ATEX 100a” directive accordingly.

Influence of liquid and vapourized solvents on explosibility of pharmaceutical excipient dusts

Hybrid mixtures of a combustible dust and flammable gas are found in many industrial processes. Such fuel systems are often encountered in the pharmaceutical industry when excipient (nonpharmaceutically active ingredient) powders undergo transfer in either a dry or solvent prewetted state into an environment possibly containing a flammable gas.