Brush Discharges Research Articles

Direct ignition tests of brush discharges for explosive methane-air and propane-air gas mixtures

The applied experimental set-up enabling measurements of current waveforms and charge transferred during direct ignition tests with brush discharges, was presented in the paper. Tests were conducted with ignition probe designed according to the IEC 60079-32-2 Standard for methane-air gas and propane-air gas mixtures. In the experiment 100 trials were conducted using sample consisted of fluorinated polyethylene. The sample was rubbed with sheep felt cloth. Charge transfer measured rates ranged from 50 nC to 260 nC and the current peak magnitude ranged from 0.5 A to 3.5 A. Magnitude of the current peak as a function of the transferred charge, recorded in the experiment, was presented in the paper. Two regions can be distinguished on the Q = f(dQ/dt) characteristic. There is a region where discharges do not cause ignition and the other one, where the results indicate ignition of gas mixture.

Ignition of a cloud of dry powder using super brush discharges

Ignition of a cloud of dry powder is a major concern in the field of industrial process safety. The different types of discharges are already defined (spark discharges, brush discharges, propagating discharges, cone discharges, corona discharges) such as their ignition properties in a gas or a dust atmosphere. For example, it is known that a classic brush discharge cannot ignite a cloud of dry flammable dust [6,13]. Glor and Schwenzfeuer performed direct ignition tests using brush discharges and defined that even if the energy released by this kind of discharge equaled the one of a spark, the power released by the brush discharge is too low to trigger an ignition.However, some doubts remained for super brush discharges. A brush discharge as a super brush discharge occurs between a charged insulating object and a conductive electrode. The main difference lies in the surface charge density reached on the insulator that is much higher for a super brush discharge than for a brush discharge. A high charge density can be reached for example using pipes of polyethylene individually charged by tribo-charging piled one above another. Such a configuration was evocated by Lüttgens [12] and tested by Larsen [11] who performed direct ignition tests in oxygen enriched atmospheres.This study is relevant with the actual safety problems since pharmaceutical and chemical powders are well known to generate electrostatic charges during their transport or handling and since the same configuration of independent polyethylene fibers can be found in flexible bulk containers that are one of the most common solutions to package this kind of powder.This paper presents the experimental set-up and the results of direct ignition tests performed with a polyethylene wax whose MIE is lower than 1 mJ at ambient conditions. The electric field reached at 1 m and the charge transfer were also registered and are described. Finally, numerical simulations are carried out to define the original surface charge density in order to help to understand the phenomenology of this discharge and its frequency of occurrence in industry.

Learning lessons from five electrostatic incidents

It is well known that electrostatic discharges can ignite mixtures of flammable gases with air and suspensions of combustible dust in air. For this reason, the prevention of electrostatic discharges is an essential part of measures to prevent explosions and fires. Incendive electrostatic discharges occurred in five cases in various chemical plants.The incidents involved were:1.Spark type electrostatic discharges from a leaking steam pipe.2.Spark type electrostatic discharges inside a charging chute for a solid.3.A spark type electrostatic discharge whilst transferring a solid powder from a bin, which led to ignition of the powder.4.A propagating brush discharge during pneumatic transfer of solid, which caused a severe electric shock to a process operator.5.A brush discharge inside an electrostatic precipitator which caused ignition of an aerosol of hydrocarbon.To avoid explosions and fires it is important to prevent incendive electrostatic discharges from occurring in industrial facilities. To help managers of chemical plants to do this, the incidents above are presented in the form of learning lessons including:•the mechanism of the generation and separation of electrical charges,•the mechanism of the electrostatic discharge,•the root causes of the incident,•the safety measures which are necessary to avoid a repetition.

Brush and propagating brush discharges on insulating sheets in contact with a grounded conductor

This is a report of an experimental investigation of the brush and propagating brush discharges on charged insulating surfaces in contact with a grounded backing plate to gain a guide for the risk assessment of ignition by the discharges on insulating coats and liners often used in chemical processes. Based on the result, we provide unified expressions of the charge transferred by brush discharge and of a criterion for the onset of propagating brush discharge with the estimation of their discharge energies for the quantitative analysis of ignition hazards associated with the coats and liners.

Ignition of a Cloud of Dry Powder Using Super Brush Discharges

Ignition of a cloud of dry powder is a major concern in the field of industrial process safety. The different types of discharges are already defined (spark discharges, brush discharges, propagating discharges, cone discharges, corona discharges) such as their ignition properties in a gas or a dust atmosphere. For example, it is known that a classic brush discharge cannot ignite a cloud of dry flammable dust (Glor & Schwenzfeuer, 2005; Schwenzfeuer & Glor, 2001). Glor and Schwenzfeuer performed direct ignition tests using brush discharges and defined that even if the energy released by this kind of discharge equaled the one of a spark, the power released by the brush discharge is too low to trigger an ignition.However, some doubts remained for super brush discharges. A brush discharge as a super brush discharge occurs between a charged insulating object and a conductive electrode. The main difference lies in the surface charge density reached on the insulator that is much higher for a super brush discharge than for a brush discharge. A high charge density can be reached for example using pipes of polyethylene individually charged by tribo-charging piled one above another. Such a configuration was evocated by Luttgens (Luttgens& Wilson, 1997) and tested by Larsen (Larsen, Hagen, & van Wingerden, 2001) who performed direct ignition tests in oxygen enriched atmospheres.This study is relevant with the actual safety problems since pharmaceutical and chemical powders are well known to generate electrostatic charges during their transport or handling and since the same configuration of independent polyethylene fibers can be found in flexible bulk containers that are one of the most common solutions to package this kind of powder.This paper presents the experimental set-up and the results of direct ignition tests performed with a polyethylene wax whose MIE is lower than 1mJ at ambient conditions. The electric field reached at 1 meter and the charge transfer were also registered and are described. Finally, numerical simulations are carried out to define the original surface charge density in order to help to understand the phenomenology of this discharge and its frequency of occurrence in industry.

A novel bipolar electrostatic ionizer for charged polypropylene granules used in a pneumatic powder transport facility

As a method of preventing or mitigating explosion due to electrostatic discharges during the loading of polymer granules in a metal silo, we have developed a novel bipolar electrostatic ionizer. In this study, we investigated experimentally a practical version of the bipolar electrostatic ionizer with a modeling test device to measure the charge-neutralizing current, and a full-sized pneumatic powder transport facility. The electrostatic discharges generated inside a silo while loading polypropylene (PP) granules were also observed visually with/without the novel bipolar electrostatic ionizer. As a sample, 300 kg of polypropylene granules 2–3 mm in size was used in this study. The specific charge of the polypropylene granules was clearly decreased by approximately 85 percent with the novel bipolar electrostatic ionizer. The brush discharges, as well as the incendiary bulk surface discharges, completely died out inside the silo when the novel bipolar electrostatic ionizer used.

Electrostatic Discharges from Non-Conductive Materials

In recent years insulating solid materials has been more commonly used in the industry. This is due to a variety of commercial reasons. As insulating materials tend to become electrostatically charged, handling of flammable materials in combination with plastics therefor is a potential ignition hazard.Industries handling flammable materials and / or combustible dusts should be risk assessed; a classification plan for explosive atmospheres as well as an explosion protection document shall be drawn up. Among other things sources of ignition in explosive atmospheres must be identified and assessed.Static electricity is an ignition source that is causing much concern as it is not easy to predict. Electrostatic brush discharges arise from charged insulating materials. Charge that build up can arise from contact charge when handling insulating solid material as well as when transporting bulk material (gases, liquids, solids) in insulating pipes, containers and other process equipment.

Charge transferred in brush discharges

Electrostatic discharges from surfaces of plastic materials can be a source of ignition, when appear in explosive atmospheres. Incendivity of electrostatic discharges can be estimated using the transferred charge test. In the case of brush discharges not all the energy stored at the tested sample is released and the effective surface charge density (or surface potential) crater is observed after the discharge. Simplified model, enabling calculation of a charge transferred during electrostatic brush discharge, was presented. Comparison of the results obtained from the simplified model and from direct measurements of transferred charge are presented in the paper.

The Contribution of Surface Potential to Diverse Problems in Electrostatics

Electrostatics spans many different subject areas. Some comprise “good electrostatics,” where charge is used for desirable purposes. Such areas include industrial manufacturing, electrophotography, surface modification, precipitators, aerosol control, and MEMS. Other areas comprise “bad electrostatics,” where charge is undesirable. Such areas include hazardous discharges, ESD, health effects, nuisance triboelectrification, particle contamination, and lightning. Conference proceedings such as this one inevitably include papers grouped around these topics. One common thread throughout is the surface potential developed when charge resides on an insulator surface. Often, the charged insulator will be in intimate contact with a ground plane. At other times, the charged insulator will be isolated. In either case, the resulting surface potential is important to such processes as propagating brush discharges, charge along a moving web, electrostatic biasing effects in MEMS, non-contacting voltmeters, field-effect transistor sensors, and the maximum possible charge on a woven fabric.

Experimental study on the relationship between the charge amount of polypropylene granules and the frequency of electrostatic discharges while silo loading

This paper is a report on the relationship between the charge amount (charge to mass ratio, q/m) of polypropylene (PP, 2–3 mm) granules and the frequency of electrostatic discharges that occur while loading a metal silo. The feedback control system was used in order to control the q/m of PP granules. The electrostatic discharges inside the silo were also observed using a conventional image-intensifier system. The charging control range for PP granules was from 0 to −12 μC/kg in the q/m. The results obtained from the experiments show that (1) two kinds of electrostatic discharges were clearly observed inside a metal silo while loading PP granules, i.e., brush discharges and incendiary bulk surface discharges; (2) the number of brush discharges and incendiary bulk surface discharges increased with the increase in the q/m of PP granules, almost reaching saturation; and (3) brush discharges and incendiary bulk surface discharges began to occur at the −1.16 μC/kg and −2.33 μC/kg points in the q/m, respectively, in this study.

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.

Identification and ranging of lightning flashes using co-located antennas of different geometry

A new technique for detecting and ranging lightning flashes up to approximately 100 km away is described. The technique uses a novel method of discriminating between lightning and non-lightning sources of quasi-electrostatic field change by comparison of the signal strength measured simultaneously on two co-located antennas, which is a function of their different geometry and exposure to the atmospheric electric field. In addition to detection of all types of lightning within range, the system also has the capability of detecting and classifying other sources of rapid electric field change associated with thunderstorm or shower cloud activity, such as from nearby corona brush discharge, strong space charge variability and charged precipitation. Real-time lightning flash detection and ranging combined with monitoring of initial storm electrification processes provides a comprehensive method of local severe weather warning. Since the operating frequency of 1–50 Hz lies below man-made sources of radio signals, the technique is considered especially useful for sites with strong radio interference.

ESD detection by transient earth voltage

We measured transient earth voltages (TEVs) accompanied with electrostatic discharges (ESDs) to investigate the possibility of ESD detection using the TEV. Clear TEV signals could be observed in spark, brush, and propagating brush discharges even when an ESD event detector failed. Therefore, ESD detection using the TEV may be significantly more effective as an auxiliary monitor, while the TEV signals accompanied with cone discharges could not obviously be detected. In addition, the features of the TEV depend on the type of ESDs; therefore, the TEV could also be used to identify the potential of the occurrence of problems and hazards by ESDs.

Charging of Materials Non-Metallic Used in Underground Mines Endangered by Firedamp and/or Coal Dust

The expansion parts and components of non-metallic in devices have become a fait accompli. In the case parts, components of non-metallic are used in atmospheres explosive and may be a potential source of ignition [1]. Ignition hazards due to static electricity from the charged non-conductive non-metallic materials are well known [2]. The prevalence protection metal parts and components of anti-corrosion exist in any device equipped with a metal casing. Inherent risks associated services are caused by static electricity. Electrostatic phenomena arise in many situations, in a variety of environments and industries. The occurrence of charge static in many cases unexpected is unwanted and requires the use of expensive preventive measures. Brush discharge and propagating brush discharge (e.g. composite coating about non-conductive electrostatic properties of an isolated base conductive) create explosion hazard in underground endangered by firedamp and/or coal dust [3].

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.

Partial inerting – A possible means of eliminating the brush-discharge-ignition hazard with explosive gases and vapours?

It was found that the brush discharge ignition hazard with IIA gases/vapours (propane) can be eliminated by reducing the oxygen/nitrogen volume ratio in the atmosphere to the order of 15/85. The condition is that the diameter of the earthed electrode ≤ 40 mm. Published information on whether an oxygen/nitrogen ratio of 15/85 is acceptable as regards the oxygen deficiency hazards is contradictory. With IIB gases/vapours (ethylene) the oxygen/nitrogen ratio must be reduced to at least 10/90 to eliminate the brush-discharge-ignition hazard. Lowering the oxygen/nitrogen ratio to this level will undoubtedly present a substantial oxygen deficiency hazard to humans. ► Brush discharge ignition of IIA gases can be eliminated if the O 2 /N 2 volume ratio is reduced to ≤15/85. ► The condition is that radii of curvature of earthed electrodes ≤20 mm. ► Whether or not this is acceptable in terms of oxygen deficiency hazards to humans is unclear. ► With IIB gases an O 2 /N 2 ratio << 15/85 is required for eliminating the brush discharge ignition hazard. ► When assessing the brush discharge ignition hazard all MIEs must be determined by identical methods.

Experimental investigation on the measurement deviations in recording brush discharges from insulating surfaces by Coulombmeters

Abstract The total charge on a charged sheet before and after a provoked brush discharge, their difference “ C ”, the induced charge “ A ” on different probes immediately before the discharge and their transferred charge “ B ” measured by commercially available Coulombmeters have been measured for probes from 2 mm to 25 mm in diameter including human thumb and shielded probe. The brush discharges were provoked on PTFE discs 10 cm–20 cm. The results obtained can be used for correct estimation of the incendivity of brush discharges between 10 nC and 90 nC.

An innovative and advanced oxidation process for effluent treatment through wet tube-type electrostatic precipitation

The degradation of hazardous pollutants (organic pollutants/non-biodegradable) in waste water was investigated with wet tube-type electrostatic precipitator (WESP). A WESP equipped with brush discharge electrodes was investigated under various operation conditions to characterize the current/voltage-curves and it was compared with the state of the art empirical correlation mentioned in literature. Experimentally obtained current/voltage data was modeled to develop an adjusted empirical correlation for brush type discharge electrodes. Investigations show that corona current is a function of operation voltage and geometry of discharge electrode. Formation of oxidizing reagents like ozone was investigated and the process was optimized.Degradation of several pollutants like acetone, EDTA and phenol was recorded at specific corona current. A series of investigations was conducted to determine the optimum process conditions without sparking. Degradation rate of organic pollutants is higher in the dominating region of ozone formation.WESP equipped with brush discharge electrode produce an enhanced/uniform corona which is capable of eliminating pollutants from off-gas and wastewater simultaneously.

Static Hazards of the VAST

The vertical atmospheric storage tank (VAST) is a large, mechanically weak tank often used to store static accumulating flammable liquids. Explosion of a single VAST can destroy an entire tank farm via rocketing and domino effects. This article provides a new methodology for assessing electrostatic hazards. Appendix A describes flammability aspects, including a new analysis of flash point measurement errors. Appendix B discusses two recent VAST explosions. Recommendations are made for improving the safety of VAST operations. Electrostatic analysis starts with our previously published model expressing the surface potential at the center of the liquid surface “Vmax” in the form of an infinite series of Bessel and hyperbolic functions. A truncated one-term approximation with an improved scale factor “κ” is provided to facilitate spreadsheet calculation. Using at least the first three terms of the series expansion, thousands of individual calculations were made for a wide range of tank dimensions, filling rates and liquid relaxation times, to determine the maximum potential “V∗max” attained during filling, which occurs at some critical depth “Dcrit”. It’s shown there exists a critical width “wcrit”, typically 0.75-1.5 m, at which V∗max reaches a maximum for a given flow rate and relaxation time. At widths above the critical width, V∗max becomes independent of flow rate and, provided the aspect ratio (H/w) of the VAST exceeds 2/3, V∗max becomes independent of height (H) as well. The worst case for VAST filling is a width near “wcrit” plus an effective liquid relaxation time ∼30 s (measured conductivity ∼1 pS/m). We derived very simple expressions for the critical width “wcrit”, the critical liquid depth “Dcrit”, the V∗max yielded by a given charging current “IC”, and the charging current needed to yield a particular V∗max. Of particular interest were the conditions for V∗max = 25 kV, the recognized threshold for incendiary brush discharges. Assuming a worst case 30 s relaxation time and a maximum allowable V∗max of 25 kV, we examined three different expressions for the charging current “IC” and determined the minimum “vd” product (flow velocity x inlet pipe diameter) that could theoretically produce this potential. Equations and graphs are presented showing that the maximum allowable vd product increases with increased VAST width (beyond the critical width) and varies with the assumed charging equation. It’s shown that the classic “Schon” equation isn’t conservative and that charging currents are likely to be much higher than predicted, especially when the diameter of the inlet pipe is less than about 0.1 m (4-inch) and the liquid has a high charging tendency. The “vd” expressions and graphs should assist in the development of Codes of Practice. The uniform charge density assumption used in our model is most valid as conditions approach worst-case, since for widths near the critical width the jet mixing time constant at typical flow rates is small compared with the assumed “worst case” 30 s relaxation time constant. However, in practical cases, jetting from bottom side entry inlets may greatly increase charge densities at the liquid surface. This plus other non-idealities such as water and sediment in tank bottoms are discussed with reference to practical VAST operations.

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.