Electrostatic Ignition Hazards Research Articles

Minimum ignition energy of hydrogen-air mixtures at ambient and cryogenic temperatures

The ignition and combustion of hydrogen in air is considered more hazardous compared to other fuels due to the lower minimum ignition energy (MIE) and the wider flammability range. Spark discharge is the most common type of electrostatic ignition hazard. There is a need in validated safety engineering tools to accurately calculate MIE in a wide range of temperatures from atmospheric to cryogenic which are characteristic for hydrogen systems and infrastructure. Current MIE assessment methodologies rely on the availability of experimental data on quenching distance and/or laminar burning velocity and thus are mostly empirical correlations. This prevents their application beyond the limited number of experimental data, i.e. to arbitrary composition of the hydrogen-air mixture at arbitrary temperatures including cryogenic. This work aims at the development of a model able to accurately predict MIE for hydrogen-air mixtures with arbitrary initial composition and temperature. Cantera and Chemkin software are used to calculate the properties and unstretched laminar burning velocity of hydrogen-air mixtures. The flame thickness is found to well represent the critical flame kernel in the suggested model. The model is validated against experimental data on MIE for mixtures at ambient and cryogenic (down to 123 K) temperatures. Results show that the effect of flame stretch and preferential diffusion shall be considered to accurately predict MIE for lean hydrogen-air mixtures, which was not possible for previous models.

Iron oxide/aluminum/graphene energetic nanocomposites synthesized by atomic layer deposition: Enhanced energy release and reduced electrostatic ignition hazard

Nanocomposites consisting of iron oxide (Fe2O3) and nano-sized aluminum (Al), possessing outstanding exothermic redox reaction characteristics, are highly promising nanothermite materials. However, the reactant diffusion inhibited in the solid state system makes the fast and complete energy release very challenging. In this work, Al nanoparticles anchored on graphene oxide (GO/Al) was initially prepared by a solution assembly approach. Fe2O3 was deposited on GO/Al substrates by atomic layer deposition (ALD). Simultaneously thermal reduction of GO occurs, resulting in rGO/Al@Fe2O3 energetic composites. Differential scanning calorimetry (DSC) analysis reveals that rGO/Al@Fe2O3 composite containing 4.8wt% of rGO exhibits a 50% increase of the energy release compared to the Al@Fe2O3 nanothermite synthesized by ALD, and an increase of about 130% compared to a random mixture of rGO/Al/Fe2O3 nanoparticles. The enhanced energy release of rGO/Al@Fe2O3 is attributed to the improved spatial distribution as well as the increased interfacial intimacy between the oxidizer and the fuel. Moreover, the rGO/Al@Fe2O3 composite with an rGO content of 9.6wt% exhibits significantly reduced electrostatic discharge sensitivity. These findings may inspire potential pathways for engineering energetic nanocomposites with enhanced energy release and improved safety characteristics.

Electrostatic Ignition Hazards Associated with Dissipative Dust Filters

In present guidelines and codes of practice it is strongly recommended to use dissipative filter media in dust separators, if the minimum ignition energy of the powder is below 3 mJ and/or a hybrid mixture may be present. Despite being in compliance with this rule and despite the filter media passed the relevant tests, burnt product was found in a dust separator of pharmaceutical production. Extensive investigations have shown that the demands made in the present standards on dissipative filter clothes are not sufficient to prevent ignition of deposited powder and ignition of an explosive atmosphere. Proposals for additional tests and requirements are presented in the paper.

Hazard assessment of high speed slurry blending using computer modelling of electric fields and potentials

Generalised published guidelines indicated a new high shear slurry dispersion system should include inerting of the vessel headspace to avoid electrostatic ignition hazards. This paper describes how computer modelling of electric fields and potentials, combined with experimental measurements on laboratory scale equipment, was used to show that inerting is not warranted in this case. Ultimately the conclusion was that, subject to certain restrictions, an incendive electrostatic discharge in the new blender could be considered so unlikely that inerting or other protective measures would not be required.

Electrostatic ignition hazards in insulating or dissipative tubes and hoses for pneumatic transfer of powders—Measurements and model calculations

When transferring powder through pipes or hoses made from insulating material, propagating brush discharges cannot be excluded. To calculate the limit value of the resistivity of the insulating material, below which no propagating brush discharges will occur, the charging current due to the powder transfer must be known. This charging current has been determined experimentally. Based on analytical calculations and computer models limit values for the resistivity of the hose material are derived from these experiments.

Electrostatic ignition of sensitive flammable mixtures: Is charge generation due to bubble bursting in aqueous solutions a credible hazard?

Experiments have been conducted to gain insight into the credibility of sparging aqueous solutions as an electrostatic ignition hazard for sensitive hydrogen/air or fuel/oxygen mixtures (Minimum Ignition Energies of ∼0.017mJ and ∼0.002mJ, respectively, compared to ∼0.25mJ for hydrocarbon/air mixtures). Tests performed in a 0.5m3 ullage produced electric field strengths between 125 and 560Vm−1 for air flows of 5–60lmin−1, respectively, comprised of 2–4mm diameter bubbles. Field strength can be related to the space charge and fitting to an exponential accumulation curve enabled the charge generation rate from the air flows to be estimated. This was observed to be directly proportional to the air flow and its magnitude was consistent with literature data for bubble bursts. The charge accumulation observed at laboratory scale would not be a cause for concern. On the basis of a simple model, the charge accumulation in a 27m3 ullage was predicted for a range of air flows. It is apparent from such calculations that ignition of hydrocarbon/air mixtures would not be expected. However, it would seem possible that field strengths might be sufficient to cause a risk of incendive spark or corona discharges in moderately sized vessels with sensitive flammable mixtures.

Modelling of Electrostatic Ignition Hazards in Industry: too Complicated, not Meaningful or only of Academic Interest?

Modelling of Electrostatic Ignition Hazards in Industry: too Complicated, not Meaningful or only of Academic Interest?

Modelling of electrostatic ignition hazards in industry examples of improvements of hazard assessment and incident investigation

Abstract Computer modelling and simulation of electrostatic phenomena is nowadays not very often used in industry for an assessment of ignition hazards due to static electricity. With 4 examples (silo filling and assessment of occurrence of cone discharges, calculation of capacitances of screws and flanges for assessment of earthing and bonding requirements, influence of sample geometry on the surface resistance and surface resistivity and assessment of requirements for plastic hoses with a metal spiral for pneumatic transfer of powders) the usefulness and benefit of model calculations of electrostatic phenomena for the assessment of electrostatic ignition hazards is demonstrated.

Electrostatic ignition hazards due to pneumatic transport of flammable powders through insulating or dissipative tubes and hoses – New experiments and calculations

Abstract During transportation of powder through pipes or hoses made from insulating materials, electrostatic discharges can occur. To define the resistivity limit of the insulating material for the avoidance of propagating brush discharges, the charging current due to the powder transfer was determined experimentally using different types of hoses and bulk material. The subsequent modeling shows that propagating brush discharges will not occur if such hoses are constructed from dissipative materials or when the resistance measured from the inner surface to the conductive structure of the hose multiplied with the contact area of the electrode is below 4 MΩ m².

Experimental study of minimum ignition energy of lean H2-N2O mixtures

Ignition energies for short duration (<50ns) spark discharges were measured for undiluted and nitrogen-diluted H2-N2O mixtures of equivalence ratios ϕ=0.15 and 0.2, dilution of 0% and 20% N2, and initial pressures of 15–25kPa. The ignition events were analyzed using statistical tools and the probability of ignition versus spark energy density (spark energy divided by the spark length) was obtained. The simple cylindrical ignition kernel model was compared against the results from the present study. Initial pressure has a significant effect on the width of the probability distribution, ranging from a broad (P=15kPa) to a narrow (P=25kPa) probability distribution indicating that the statistical variation of median spark energy density increases as initial pressure of the mixture decreases. A change in the equivalence ratio from 0.15 to 0.2 had a small effect on the median spark energy density. The addition of 20% N2 dilution caused a significant increase in the median spark energy density when compared to no dilution. The extrapolation of the present results to atmospheric pressure, stoichiometric H2-N2O indicates that the electrostatic discharge ignition hazards are comparable to or greater than H2-O2 mixtures.

An assessment of electrostatic ignition hazards associated with fuel flow through plastic pipes at roadside filling stations

Abstract The use of electrically insulating synthetic materials, such as high-density polyethylene (HDPE), for fuel pipelines and other fuel handling components is now widespread. It is well reported however that the flow of fuel under certain conditions in plastic pipes can produce measurable levels of electrostatic charge. This paper describes in detail a full-scale experimental test rig simulating a filling station layout – in particular the conditions existing during road tanker delivery of high-charging fuels. It presents data from a series of electrostatic measurements, discusses results and draws conclusions with regard to existing safety margins and precautions for safe operation.

Electrostatic ignition hazards with plastic pipes at petrol stations

Occasional electrostatic ignitions have been associated with the use of buried, non-conductive (insulating), plastic fuel pipes at petrol stations. The incidents are rare and normally involve only minor flash fires but there is scope for escalation and it is prudent to prevent them.This paper analyses the electrostatic processes associated with the use of non-conductive plastic pipes at petrol stations to establish what ignition mechanisms are plausible. To do this it considers details of the reported incidents alongside the results of published voltage measurements, observations of the typical spread of streaming currents recorded in gasoline handling and theoretical estimates of the voltages on non-conductive pipes. The analysis suggests that the greatest risk is posed by incendive sparks from unbonded conductors but that incendive brush discharges from insulating pipe surfaces are also possible. In all cases the charging mechanism was by fuel flow. Careful earthing of all conductive items would eliminate incendive sparks from unbonded conductors but the hazard from incendive brush discharges would remain. By using plastic pipes with earthed conductive or dissipative inner linings the hazards from both incendive sparks and brush discharges arising from liquid flow charging can be prevented. Hazards arising from charging mechanisms other than liquid flow can be controlled by applying suitable operational procedures whenever flammable atmospheres might be present.

Grounding resistance for control of static electricity ignition hazards

The flow of hydrocarbon product in pipes generates static electricity at density levels that may be sufficient to cause sparks and ignition when product is delivered to a tank. This paper describes the formulation of a method that can be used to control such electrostatic ignition hazards during tank filling operations. The grounding current for a grounded metallic tank being filled with charged liquid was found. The grounding current found was used to find the grounding resistance limit that ensures that dischargeable energy is always below the ignition level. The analysis presented in this paper was focused on cylindrical tanks; however, the method can be extended to any shape charged liquid container whose electric potential is known.

Chargeability of ethanol–petrol biofuels

Chargeability of ethanol–petrol biofuels during refuelling has been studied in real dispenser environment in order to assess safety risks due to fuel charging at fuel filling stations. Two biofuel blends were studied: E10 containing 10 vol-% of ethanol and 90 vol-% of petrol, and E85 containing about 85 vol-% of ethanol and 15 vol-% of petrol. Charging of standard 95 Octane petrol was studied as a reference. The results show that the charging of E85 is negligible and no charge will be accumulated as long as the fuel dispenser system is properly grounded. In the case of refuelling with E10, charge is accumulated but the level of total charge is still so low that no real electrostatic ignition hazards exist due to fuel charging at filling stations as long as the system is properly grounded. Electrostatic ignition hazards due to fuel charging are real only for standard petrol fuel.

An investigation of the occurrence of propagating brush discharges from Type B flexible intermediate bulk containers with liners

Flexible Intermediate Bulk Containers (FIBC) are generally divided into groups (A–D) based on their electrostatic ignition hazard. The key distinction between a Type A FIBC and a Type B FIBC is that the latter is incapable of producing highly incendive propagating brush discharges. This ability is normally evaluated by measuring the breakdown voltage, with the criterion for a Type B (according to the latest standard, IEC 61340-4-4:2005) being that the result must be less than 6 kV. However, the use of liners is specifically excluded, though in practice many powders transported in FIBC require a liner to be used, typically due to transport regulations. The presence of a liner clearly influences the breakdown voltage, and hence the propagating brush discharge risk from the package. The logical extension of the general Type B requirement would be that a liner with a breakdown strength of <6 kV could prevent the occurrence of propagating brush discharges; unfortunately, as the liner material and thickness are usually dictated by strength and compatibility requirements, a higher breakdown voltage is often unavoidable. Despite this, experiments on typical Type B FIBC/liner combinations together showed that even if the breakdown strength of the liner is >6 kV, propagating brush discharges may still not be possible from the bag/liner combination, though with thicker liners it was shown that propagating brush discharges could still occur. A relationship is proposed to identify the potential for a propagating brush discharge based on the FIBC fabric thickness, liner thickness and liner breakdown strength.

Measurement of charge transfer during filling and emptying FIBC

Filling and emptying normal FIBC (flexible intermediate bulk containers) in hazardous areas represents a high electrostatic ignition hazard. Manufacturers of FIBC have therefore designed different measures to reduce the ignition risks from static electricity. Depending on the measures applied FIBC are categorized into types A, B, C and D. End of 2005 a new international standard for testing type D FIBC has been published. Type D FIBC have now to be certified according to this standard. Since this standard requires comprehensive ignition testing on a test rig especially designed for this purpose, industry is looking for a simpler test method, which can be used for the development of new FIBC fabric and in production quality control. In the present paper it is attempted to apply the method of charge transfer measurement to assess the ignition probability of static discharged from FIBC during filling and emptying and to correlate the results with the ignition tests performed on the standard test rig.

Metastable Dissociation of Anions Formed by Electron Attachment

A comprehensive analysis of metastable dissociation of 2,4-dinitrotoluene (DNT) parent anions formed by attachment of electrons of controlled energy is presented. We characterize the energy dependence and kinetic energy release of the reaction which competes with autodetachment. A surprising finding is a highly exothermic metastable reaction triggered by the attachment of thermal electrons which we relate to the well-known electrostatic ignition hazards of DNT and other explosives. Quantum chemical calculations are performed for dinitrobenzene in order to elucidate the process of NO abstraction.

Electrostatic spark ignition of sensitive dust clouds of MIE

Accidental electrostatic sparks in industrial plant producing/handling powders/dusts occur whenever a non-earthed electrically conducting object has been charged tribo-electrically to a high voltage and suddenly discharges its energy to earth via an air gap of appropriate length. When assessing the electrostatic spark ignition hazard in an industrial plant, the parameters of prime concern are the capacitances C of electrically conducting plant items that may become charged tribo-electrically, the voltages U to which they may become charged, and the minimum electric spark ignition energies ( MIE) of the dust clouds of concern. Whenever 1 2 C U 2 > MIE , there is a possibility of accidental electrostatic spark ignition. Current standard apparatuses for determining MIE of dust clouds have a lower spark energy limit of 2–3 mJ. In an investigation by the present authors, discussed in detail elsewhere, a new spark generator capable of producing synchronized capacitive sparks of energies down to the order of 0.01 mJ was developed and used for testing a selection of ignition-sensitive powders for MIE. Several of the MIEs found were 1–2 orders of magnitude lower than the lower energy limit of current standard test apparatus. Other experiments by the present authors, also reported elsewhere, have shown that quite low MIEs can be found for some dusts even with a less optimal synchronization mechanism, which may occur accidentally in practice. The main object of the present paper is to discuss possible practical concerns arising from the finding that clouds in air of some dusts can have very low MIEs. In such cases, one may have to pay attention to even minor C values, i.e. minor plant items. Alternatively, with larger C values, even quite low voltages may give rise to hazardous spark discharges. However, some types of fine metal powders of low MIEs will quite readily form electrically conductive layers on the solid surfaces with which they make contact. Hence, electrostatic spark ignition inside process equipment containing such dusts may be less probable than in the case of process equipment containing non-conducting dusts of correspondingly low MIEs. There may be a need for a new standard test method for determination of MIEs of dust clouds in the <1 mJ range.

Dust and electrostatic hazards, could we improve the current standards?

Dust explosions are recognised as very major industrial hazards. A meticulous attention should be engaged towards their analysis, prevention and control. A number of causes can trigger a dust explosion. Static electricity is one of the ignition sources. The current safety standards for determining electrostatic ignition hazards of dusts, such as electrical resistivity and charge decay time, are currently used in the process industries. Nevertheless, the standardisation of the proposed protocols suffers mainly from the looseness of a clearly definite description concerning the experimental conditions. The content of the paper is focused on the influences of the time, the voltage, the temperature, the relative humidity and the compaction of the dust on the two precedent parameters. The obtained results clearly show the necessity to improve the current standards. A proposition to complete the conditions of the normalised protocols is suggested.

Electrostatic ignition hazards in the process industry

Fires and explosions continue to be triggered by the accumulation of electrostatic charge in different branches of the process industry. This is the case even though nowadays the electrostatic phenomena that give rise to ignition hazards in industrial practice are generally well known. This paper presents various reasons as to why this should be so and draws attention to the conflicts of interest, which arise, when other requirements need to be met. In addition, current trends and new results with respect to the assessment of electrostatic ignition hazards are reviewed. They include (i) incendivity and characterisation of discharges from insulating surfaces with and without contact to a conductive surface, (ii) incidents affecting the present assessment of occurrence and incendivity of propagating brush discharges in practice and (iii) characterisation, measurement method and ignition hazard assessment of modern packaging systems such as flexible and rigid intermediate bulk containers (FIBC and RIBC). Finally, the consequences of Directives issued at the European and national levels are appraised with respect to electrostatic ignition hazards.