Propagating Brush Discharges Research Articles

Complete Prevention of Contact Electrification by Molecular Engineering

Summary Electrostatic charges are easily generated on surfaces during contact electrification. Although these invisible charges have emerged as a new dimension in mediating the functions of surfaces, such as energy conversion, liquid transport, reactivity, and adsorbability, the accumulation of charges on surfaces can also pose many undesirable consequences. Despite notable progress, existing approaches in engineering antistatic surfaces suffer from limitations such as the need to modify bulk materials or for delicate control of patterning on surfaces that rely on the neutralization of generated charges. Herein, we report a general toolbox for designing antistatic coatings by leveraging on chemically heterogeneous components with electron-donating and electron-accepting functions, i.e., N-(2-aminoethyl)-3-aminopropyltrimethoxysilane and 1H,1H,2H,2H-perfluorooctyl trimethoxysilane, to molecularly engineer the surface potential to achieve an electrostatic homogeneity and completely prevent charge generation. Our approach is general, which allows the facile fabrication of antistatic coatings on various materials, even flexible and curved, with good re-writability and transparency.

Estimation of the energy released during propagating brush discharge

Propagating brush discharges are highly energetic and pose risks to the control and indicating equipment, hazardous atmosphere and human health. The nature of the PBD poses some difficulties, as one electrode, is non-conductive. Direct measurements of the voltage requires specialized equipment. Presented method uses voltage measurements on a low inductance RC circuit. PBD were also recorded with fast camera at 200 K pps. Some dependencies were observed, e.g. the energy released is usually higher for negative polarity. For chosen PVC test samples (0.2 mm thick), typical energies actually released to the atmosphere were between dozens and hundreds of mJ.

Assessment of electrostatic discharges in insulating venting lines

In the field of protection against an explosion, every plant where flammable chemicals (liquids, gases, powders) are handled should build an explosion protection document. This document is a powerful tool to assess explosion management on the plant. Among other parts, it must include an ignition source analysis that assess the probability of explosion for every single place where a flammable atmosphere is expected.The ignition sources to be considered as listed in various standards such as the EN 1127-1 (Secteur interdisciplinaire de normalisation, 2012) and the IEC 60079-0 (IEC, 2012). The trickiest ignition sources are often static electricity and sometimes, the general measures that could be defined for a fully new plant are extremely difficult to implement on an existing plant.A common case deals with venting lines. Those lines are sometimes made of insulating plastics while a flammable atmosphere remains possible under normal operation. A strong charge build up can be generated on the inside surface of those lines and several hazards might happen. The charge build up in the inside part of the line will generate an electric field in its vicinity and will charge by influence the conductive parts that are not grounded. In that case, a spark discharges can occur in the working area. If a flammable atmosphere takes place at that point, an ignition could occur. The second main hazard would be a propagating brush discharge across the thickness of the venting line. In that case, an ignition of the vapours inside the pipes is possible.This presentation describes an experimental approach to measure the charge build up generated during the different process steps of one chemical firm. The results showed large differences depending on the process step and a hazard of propagating brush discharge was identified at some point.

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.

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.

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.

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.

A Simulation Model for Understanding Propagating Brush Discharges: Comparison of Theory and Experiments

Propagating brush discharges can release large amounts of energy. From the point of view of safety, this feature makes them among the most destructive electrical discharges. This paper provides a survey of recent work in the modeling of propagating brush discharges. Experimental data from various authors are compared with a discharge model in which areas of a charged surface are modeled as capacitive cells, and bridges between cells are modeled as spark gaps driven by the tangential electric field between cells. A theory that explains the physics of this particular type of discharge is presented. Charge transfer and post-discharge charge distributions are compared between the theory and the experimental data of several authors.

Tribocharging of Particle Contaminants Evaluated as an Ignition Source in Oxygen-Enriched Environments

Abstract Contamination particles make an oxygen system more vulnerable to fire hazard as they provide a large surface area with reduced minimum energy of ignition. Adiabatic compression, resonance, solid friction, flow friction, and promoted ignition have acted as the source of ignition in oxygen systems. In this paper, static electricity generated in solid particles is proposed to be another possible source of ignition. It is known that the movement of particles in a system results in the buildup of static charges in them. This static electricity, if supported by suitable geometric configuration and grounding of metal projections, may lead to propagating brush discharge (PBD) and provide the energy needed for initiation of fire. Particles with low auto-ignition temperature, particularly in dispersed state, may help in initiating the fire, while metal particles may aid the propagation because of their high combustion energy. In this paper, we have determined the velocity profile utilizing the FLUENT code and calculated the charge generated in particles, exploring the ignition probability of equipment in oxygen environment through electrostatic discharge. We have also attempted to perform preliminary calculation to determine charge density and the electric field strength required for brush discharge. Computational fluid dynamics analyses on globe valves reveal the presence of probable areas of particulate accumulation in the valve. The decrease in minimum ignition energy (MIE) in the environment of high pressure, high temperature, high purity of oxygen, low humidity, small particles, high dielectric constant, low thickness of insulating layer, and high resistance of the deposited dust layer facilitates occurrence of PBD, which may cause ignition. Adiabatic heating may aid combustion by lowering the MIE of particles. Experiments have been proposed for verification of the hypothesis and analytical calculations presented by the authors. The paper may give a new insight into the possible causes of oxygen fire.

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.

Propagating brush discharges in flexible hoses

Propagating brush discharges can occur inside conductive pipework with an insulating liner when pneumatically conveying powders. Flexible plastic hoses with embedded wire spirals are frequently used for pneumatic transfer, but it has not been clear if they have the same problem. This work shows that propagating brush discharges can occur in structures modelling such hoses. Further work is required to establish if mitigating factors are present in actual hoses. Nevertheless, the advice from this work must be that such hoses should not be used for pneumatic transfer of flammable powders unless the plastic is static dissipative or conductive.

Effective method for measuring the energy of propagating brush discharges

Due to their very high energy propagating brush discharges are the most dangerous electrostatic discharges. As it is known, they usually occur on thick insulating layers on which significant charge can be accumulated. Discharge energy is a function of several parameters, like the surface charge density (influenced by the thickness of the layer and its relative permittivity), the size of the charged surface, etc. This paper represents a laboratory model to examine the energy of propagating brush discharges. Based on these measurements list of results were evaluated to estimate the energy of the generated propagating brush discharges. This energy is especially important to determine how incentive these discharges are. By the help of the model the effect of the polarity of the original charging, the surface charge density of the sample and the geometry of the sample have been examined.

Analysis of a dust explosion caused by several design errors

AbstractA chemical reaction process began by feeding a combustible fine powder into a reactor through a charging hopper. The reactor and the charging hopper were fitted with a nitrogen inerting system to prevent dust explosions. Surprisingly, a dust explosion occurred when feeding the powder into the reactor from a flexible intermediate bulk container (FIBC).On‐site investigations found that the design of the inerting system allowed a sufficient amount of air to enter the charging hopper. There were no mechanical, electrical, and thermal sources of ignition inside the closed reactor–charging hopper system before the explosion. Electrostatic discharges appeared to be the ignition source; however, the ignition source could not be identified without further analysis by conducting tests. Therefore, various dust explosion and electrostatic tests were conducted to determine (1) the minimum amount of electrostatic discharge energy required to ignite the dust cloud and (2) the likelihood of various types of electrostatic discharges to ignite the dust cloud under the existing design and process conditions. The test results showed that the powder suspended in air at the optimal dust concentration requires &lt;3 mJ of spark energy to ignite. A glass‐lined pipe at the bottom of the charging hopper was found to be the most likely ignition source. Laboratory tests demonstrated that the ignition was the result of a propagating brush discharge from the glass‐lined pipe. © 2005 American Institute of Chemical Engineers Process Saf Prog, 2006

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.

Observation of Propagating Brush Discharge on Insulating Film With Grounded Antistatic Materials

A propagating brush discharge (PBD) usually occurs on a highly charged thin film with a grounded metal backing plate and often becomes energetic enough to ignite a flammable gas/vapor and even a dust cloud. In an actual situation, a PBD is seen on the surface of a large-sized flexible intermediate bulk container (FIBC) during its filling and/or emptying processes in an industrial site when the fabric of the FIBC is highly insulating. Anticipating that a PBD could occur even on an antistatic FIBC if it were used with an insulating inner bag, various types of antistatic fabrics for FIBCs in contact with an insulating polyethylene terephthalate (PET) sheet were put to the test. An antistatic fabric was grounded, and the PET sheet on the fabric was charged with a corona charger. A PBD was then generated by approaching a grounded spherical electrode. The discharge patterns were observed, and the currents and charge amounts were measured. A thin metallic fiber woven on an insulating fabric was found to drastically enhance the transferring of charge during a PBD, probably because of the corona discharges emanating from the metallic fiber. The resistivity of the fabric also affected the charge transference. The incendiary nature of each mode of PBD was discussed.

Ignition hazard due to static electricity in particulate processes

The paper gives a review of the state of the art in the field of powder ignition by discharges due to static electricity. After a short introduction, emphasis is given to most recent results from research on powder ignition by different types of discharges occurring during powder handling and processing. It is explained why minor changes of the equipment or of the particulate processes have a huge effect on the energy of spark discharges and thus on the ignition hazard. Discharges associated with charge accumulation on high resistivity powders and granules and on insulating walls of equipment are discussed, and the most recent results concerning their incendivity are reviewed. In this context, cone discharges, brush discharges, propagating brush discharges and corona discharges are dealt with. An assessment of the ignition hazard due to the build up of static electricity cannot occur without taking into account the relevant parameters characterizing the initiation and the course of dust explosions. The progress achieved in this field (measurement and interpretation of minimum ignition energy data, role of bulk resistivity, presence of flammable gases or vapours—so called hybrid mixtures, etc.) is also shortly reviewed.

Propagating brush discharge initiation of dust layers – A new test method

In many powder handling processes, powder can be deposited onto a metal or conductive surface layers. If the powders are very good insulators, the deposited layer can build up electrostatic charges to the level of local electrostatic breakdown creating a propagating brush discharge. An explosion hazard can exist if the powder fines cling to the insulating layer in process. The occurrence of a discharge on the insulating layer, either by internal electrostatic breakdown or by moving a grounded object toward the insulating layer, can loft the dust into the air. This discharge may also have sufficient energy to ignite the dust cloud causing catastrophic explosion results. A new test method has been developed to simulate this occurrence. Its goal is to determine the electrostatic discharge energy in the propagating discharge required to ignite the lofted dust. Propagating brush discharge having energies less then 30 millijoules have ignited dust deposits on insulating surfaces backed up by grounded conductive material. A method description and some test results are presented in this paper.