An Overview On Static Electricity-syimahanifah.blogspot.com

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An overview on static electricity Ibrahim Mohamed Shaluf Department of Chemical Engineering, Faculty of Engineering – Sbrata, University of 7th April, Al-Zawia, Libya

212 Abstract Purpose – This paper aims to provide graduate students, researchers, governmental and independent agencies with an overview on static electricity. Design/methodology/approach – Static electricity has been studied by researchers, academicians, company specialists, governmental and independent agencies. Static electricity incidents have been collected from several sources such as the technical, general articles, internet web sites, and internal reports. The static electricity definition, incidents, hazards, and static electricity prevention have been reviewed. The static electricity incidents have been arranged and classified into fire, and explosions. Findings – Static electricity can be the cause of problems in many areas of industry. It presents a source of ignition for flammable gases, liquids and powders. It can cause fires and explosions in tankers, aircraft and petrochemical plant and in printing, pharmaceutical, food products and explosives industries. Originality/value – This paper presents an overview on static electricity, the incidents, and the methods to prevent static electricity generation and accumulation. Keywords Electrostatics, Fire, Hazards, Explosions Paper type General review

Disaster Prevention and Management Vol. 17 No. 2, 2008 pp. 212-220 q Emerald Group Publishing Limited 0965-3562 DOI 10.1108/09653560810872514

Introduction Static electricity is a potential source of ignition wherever there is a flammable mixture of gas or of powders or dusts (Lees, 1996). Static electricity is created when two objects or materials that have been in contact with each other are separated. When in contact, the surface electrical charges of the objects try to balance each other. This happens by the free flow electrons (negatively charged particles) from one object to the other. This causes both objects to become electrically charged. If these charges do not have a path to the ground, they are unable to move and become “static”. If static electricity is not rapidly eliminated, the charge will build up. It will eventually develop enough to jump as a spark to some nearby ground or less highly charged object in an attempt to balance the charge (Industrial Accident Prevention Association, 2006). An atmosphere becomes flammable when the ratio of oxygen to combustible material in the air is neither too rich nor too lean for combustion to occur. Combustible gases or vapors will accumulate when there is inadequate ventilation in areas such as confined spaces. Flammable atmospheres may also be formed by chemical reactions. These occur when surfaces are initially exposed to the atmosphere or when chemicals combine to form flammable gases (The University of Tennessee, 2006). Static can be a cause of problem in many areas of industry. It presents a prospective source of ignition for flammable gases, liquids and powders. It can cause fires and explosions in tankers, aircraft and petrochemical plant and in printing, pharmaceutical, food products and explosives industries. The chemical, petrochemical industries and the petrol filling stations have experienced fire and explosion incidents due to static electricity. The incidents have been rearranged and

reviewed. It has been noted that eight explosion incidents occurred in storage tanks and crude oil storage tankers in the period of 1953 to 1969 resulted in 29 fatalities. Overall, 303 fire and explosion incidents occurred during the filling of containers (storage tanks, road, rail tanks and drums) in the period of 1979 to 1988 resulted in five fatalities and 100 injuries. The majority of incidents have occurred in earthed containers. Earthling alone does not eliminate the hazard of static electricity. A total of 243 fire incidents occurred in petrol station worldwide in the period of 1993 to 2004. The incidents are sufficient to indicate the importance of static electricity as an ignition source. This paper presents an overview on the static electricity, the incidents and the methods to prevent the static electricity generation and accumulation. Static ignition incidents Electrostatic spark in the presence of flammable atmosphere (gas, vapor, and aerosol or dust cloud) can present a fire or explosion hazard. An analysis of the incidents statistics shows that 11 of the incidents (4 percent) were known to be caused by electrostatic sparks. (In Germany 9 percent of all reported incidents were caused by static sparks.) It is normally very difficult, after an incident, to establish exactly what caused the mishap. Ignition sources such as welding or mechanical failure are more easily traceable than an electrostatic spark. The large percentage (27 percent) of incidents with unknown sources of ignition probably has a significant number that can be attributed to statistic (Ball, 1996). Lees (1996) summarized some incidents which occurred due to static ignition. Explosions have occurred due to generation of static charge by the discharge of carbon dioxide fire protection systems. Such a discharge caused an explosion in a large storage tank at Biburg in Germany in 1953, which killed 29 people. In 1954, a large storage tank at Shell refinery at Pernis in the Netherlands exploded 40 minutes after the start of pumping of tops naphtha into straight-run naphtha. The fire was quickly put out. The next day, a further attempt was made to blend the materials and again an explosion occurred 40 minutes after the start of pumping. The cause of these incidents was determined as static charging of the liquid flowing into the tank and incendive discharge in the tank. These incidents led to a major program of work by Shell on static electricity. An explosion occurred in 1956 on the Esso Paterson during loading at Baytown, Texas, the ignition being attributed to static electricity. Another incident involving a carbon dioxide discharge occurred in 1966 on the tanker Alva Cape. In 1969, sever explosions occurred on three of Shell’s very large crude carries (VLCCs): the Marpesa, which sank, the Mactra and the King Haakon VII. In all three cases tanks were being cleaned by washing with high-pressure water jets, and static electricity generated by the program process was identified as the ignition source. Following this set of incidents, Shell initiated an extensive program of work on static electricity in tanker cleaning. Explosion due to static ignition occur from time to time in the filling of liquid containers, whether storage tanks, road and rail tanks or drums, with hydrocarbon and other flammable liquids. Abbott (1988) pointed out that the published statistics revealed that there were 303 incidents of fire or explosion reported to the Health and Safety Executive during the period 1979 to 1988. These incidents resulted in five deaths and 100 injuries. The majority of incidents have occurred in earthed containers. Earthling alone does not eliminate the hazard of static electricity. Renkes (2006) reported the fire at refueling sites that appear to be static related incidents in the USA. Renkes pointed out that in the period of 1992 to 2006 there

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have been 166 fire incidents that caused the damage to vehicles and stations. These incidents resulted in one fatality and 68 injuries. In the period of 1993 to 2004, there were 243 reported incidents of fires breaking out at petrol stations around the world (Australian Government, 2006). These incidents are sufficient to indicate the importance of static electricity as an ignition source. Table I summarizes the incidents which have resulted from static electricity.

214 Static electricity in industrial processes The industrial situations in which undesired static charges are generated are largely those in which two surfaces move relative to each other, with initial contact followed by subsequent separation. When the surfaces are separated, one body tends to be left with a positive charge and the other with a negative charge. If the bodies are good conductors of electricity, the charge moves quite freely and both bodies are effectively restored to their original uncharged state through the last points of contact at separation. But if one or both of the bodies are poor conductors, the charge does not flow freely and booth bodies retain charge after separation. There are many industrial processes that involve surface contact, movement and separation of poorly conducting materials. These processes may be classified in terms of: the phase involved, e.g. gas-solid; the general type of system, e.g. dusts and powders; or the particular type of processes or equipment, e.g. pneumatic conveying. Some systems in the processes industries where static effects are important are listed in Table II. The hazard of static electricity occurs in the process industry in: fluid handling operation such as pipeline flow, setting of drops, agitation, filling of storage tanks, filling of tankers; powder and dust and pneumatic conveying; in sprays and mists such as in steam cleaning and steam leaks; moving equipment such as conveyer

Incident type

Incident number 1 2 1 1 3 303

Table I. Summary of the static ignition incidents

Explosion Explosion Explosion Explosion Explosion Fire and explosion Fire

243

Table II. The systems in process industries where static effects are important

Liquid – solid Liquid – liquid Gas – liquid Gas – solid Solid – solid

Facility Large storage tank Large storage tank Loading tanker Tanker Crude oil tankers Filling of containers (storage tanks, road, rail tanks and drums) Petrol stations

Date

Location

Casualties

1953 1954 1956 1966 1969 1979-1988

Germany The Netherlands USA USA The Netherlands UK

29 fatalities – – –

1993-2004

Around the world

–

5 fatalities and 100 injuries

Flow of liquid through pipes, filters splash filling of tanks Mixing of immiscible liquids; settling of drops of one liquid through another Cleaning with wet steam; spraying with water; leakage of wet steam Pneumatic conveying; fluidized beds Belt derives; conveyers belt; reeling of papers or plastics; human body

belt and bucket elevators, and the human body. Static electricity is essentially a phenomenon of low current but high voltage. The static charge may discharge causing an incendive spark or it may give a less hazardous corona discharge spark or may leak away to earth. Sparks from good conductors are more incendive than are those from poor conductors. The hazard of static electricity may be estimated by comparing the energy of an electrostatic charge with minimum ignition energy of flammable gas mixtures and powder or dust suspensions (Lees, 1996). Static generation Static charge build up is a result of physically separating a poor conductor from a god conductor or another poor conductor. When different materials touch each other, the electrons move across the interface from one surface to the other. Upon separation, more of the electrons remain on one surface than on the other; one material becomes positively charged and the other negatively charged. If both the materials are good conductors, the charge buildup as a result of separation is small because the electrons are able to scurry between the surfaces. If, however, one or both of the materials are insulators or poor conductors, electors are not as mobile and are trapped on one of the surfaces, and the magnitude of the charge is much greater. Common industrial examples are pumping a nonconductive liquid through a pipe, mixing immiscible liquids, pneumatically conveying solids, and leaking steam that contacts an ungrounded conductor. The static charges in these examples accumulate to develop large voltages. Subsequent grounding produces large and energetic sparks. For industrial operations where flammable vapors may be present, any charge accumulation exceeding 0.1mJ is considered dangerous. Static charges of this magnitude are easy to generate; the static buildup created by walking across a carpet averages about 20mJ and exceeds several thousand volts (Crowel and Louvar, 2002). An electrostatic discharge occurs when two materials at different potentials or polarities come close enough together to generate a charge transfer. In an explosive environment this sudden transfer of charges may be energetic enough to be an ignition source. To prevent these ignitions, one must understand: . how charges accumulate on objects; . how charges discharge by means of charge transfer; and . how to estimate the resulting energy discharged in relation to the minimum ignition energy (MIE) of the explosive environment. Charge accumulation There are number of mechanisms by which an electrostatic charge can build up. The main mechanisms are summarized as follows: (1) Contact and separation: If two different solid materials are rubbed together, so that there is first contact and then separation of their surfaces, charges build up on the surfaces of the two materials, one material having a charge of one polarity and the other a charge of opposite polarity. (2) Induction. Induction charging occurs only where the body is a conductor. If a body which is an isolated conductor is placed in an electrical field, charges of different polarity are induced on opposite sides. If then an earthed electrode

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touches, or even approaches close to, this body, the charges close to the electrode flows away, leaving the body with a charge of opposite sign. (3) Double-layer charging. Charges separation occurs on a microscopic scale in a liquid at any interface (solid-liquid, gas-liquid, or liquid-liquid). As the liquid flows, it carries a charge and it leaves a charge of opposite sign on the other surface, for example, a pipe wall. The factors which influence the accumulation There are some factors which influence the magnitude of charge accumulation in separation charging and these include: . the temperature of the surface; . the permittivity; . the number and density of contact points; . the electrical conductivity; . the higher the conductivity of the materials, the greater the extent to which any charge generated is neutralized; . the speed of separation; and . changes in the condition of materials. Minimum ignition energy (MIE) The minimum ignition energy (MIE) is the minimum energy input required to initiate combustion. All flammable materials (including dust) have MIEs. The MIE depends on the specific chemical or mixture, the concentration, pressure, and temperature. Experimental data indicated that the MIE decreases with an increase in pressure. The MIE of dusts is, in general at energy levels somewhat higher than combustible gases, and an increase in the nitrogen concentration increases the MIE. Many hydrocarbons have MIEs of about 0.25mJ. This is low compared with source of ignition. For example, a static discharge of 22mJ is initiated by walking across a rug, and an ordinary spark plug has discharge energy of 25mJ. Electrostatic discharges, as a result of fluid flow. Also, have energy levels exceeding the MIEs of flammable materials and can provide an ignition source, contributing to plant explosion (Crowel and Louvar, 2002). The MIE of the flammable materials are listed by the Department of Labor (1990). Flammable atmosphere An atmosphere becomes flammable when the ratio of oxygen to combustible material in the air is neither too rich nor too lean for combustion to occur. Combustible gases or vapors will accumulate when there is inadequate ventilation in areas such as confined spaces. Flammable atmospheres may also be formed by chemical reactions. These occur when surfaces are initially exposed to the atmosphere or when chemicals combine to form flammable gases. An atmosphere becomes flammable when ratio of oxygen to combustible material in the air is neither too rich nor too lean for combustion to occur. Combustible gases or vapors will accumulate when there is inadequate ventilation in areas such as confined spaces. Flammable atmospheres may be formed by chemical reactions. These occur

when surfaces are initially exposed to the atmosphere or when chemicals combine to form flammable gases. Combustible dust concentrations are usually found during loading, unloading, or conveying coal, grain, fertilizers or other combustible materials. The explosion from these concentrations occurs when high amounts of static electricity accumulates at low humidity readings and causes a spark which ignites the combustible mixtures present in the air. Also, desorption of chemicals from the inner linings of surfaces of a tank or vessel may produce a flammable mixture. An example of desorption can occur when propane is emptied from a tank. After the removal, the walls may desorb some remaining gas and create a flammable mixture in the tank (The University of Tennessee, 2006). The hazards of static electricity The main hazard of static electricity is the creation of sparks in an explosive or flammable atmosphere. These sparks can set off an explosion or fire. The danger is greatest when flammable liquids are being poured or transferred. For static electricity to be a hazard, four conditions must be met: (1) there must be a means for a static charge to develop; (2) enough energy must build up to cause ignition; (3) there must be a discharge of this energy (a spark); and (4) the spark must occur in an ignitable vapor or dust mixture. Hazards identification The flow diagram in Figure 1 helps to assess the electrostatic fire/explosion hazard for any industrial plant or process. The hazard arises when there is a simultaneous presence of flammable fuel/air atmosphere and an energetic ignition source. The diagram is followed through completely to provide protection. Static electricity prevention Static charge buildup, resulting sparks, and the ignition of flammable materials are an inevitable event if control methods are not appropriately used. The design objective is to prevent the buildup of charges on a product (liquid or powder) as well as on surrounding objects (equipment or personnel). The static electricity control and prevention have been discussed by researchers and agencies. Wells (1997) pointed out that the industrial plant should be earthed and electrical continuity maintained at all times. The resistance to earth must be less than 10V. Other precautions include reducing resistivity by ionic agents, injecting charged ions, adjusting the humidity to promote leakage of charge, bonding all containers and eliminating flammable atmospheres. High flow-velocities, e.g. . 1 m/sec, should be avoided particularly for two-phase flow, and valves of maximum bore may be installed. Static can cause by letting material splash into a vessel, so a deflector merged. Areas of high charge generation should be located away from explosive mixtures to allow for charge decay. Department of Labor (1990) discussed the guidelines of controlling of static electricity in industry in detail. The American Petroleum Institute (API, 2006) and the Petroleum Equipment Institute recommend the motorists with precautions and

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Figure 1. Flow diagram for the assessment of electrostatic hazard

guidelines to avoid potential problems with static electricity at the gas pump by staying outside the vehicle during refueling, and to follow all safe refueling practices during the routine gasoline tank fill-up. Crowel and Louvar (2002) presented the methods which can be adopted to prevent the static electricity. The methods are summarized as follows: (1) Prevent charges from accumulating to dangerous levels by reducing the rate of charge generation and increasing the rate of charge relaxation. This method is generally used when handling liquids.

(2) Prevent charges from accumulating to dangerous levels by designing the system to include charge reduction by means of low-energy discharges. This method is generally used when handling powders. (3) When dangerous discharges cannot be eliminated, then prevent the possibility of an ignition by maintaining oxidant levels below the combustible levels (inerting) or by maintaining fuel levels below the LFL or above the UFL. Measures to mitigate the consequences of an explosion are also options for consideration (for example, deflagration venting and explosion suppression). Conclusion An overview on static electricity has been presented and the following have been noted: . The incidents that occurred due to static electricity have been rearranged and reviewed. . It has been noted that eight explosion incidents occurred in storage tanks and crude oil storage tankers in the period of 1953 to 1969 resulted in 29 fatalities. . A total of 303 fire and explosion incidents occurred during the filling of containers (storage tanks, road, rail tanks and drums) in the period of 1979 to 1988 resulted in five fatalities and 100 injuries. The majority of incidents have occurred in earthed containers. Earthing alone does not eliminate the hazard of static electricity. . Overall, 243 fire incidents occurred in petrol stations worldwide in the period 1993 to 2004. . The incidents are sufficient to indicate the importance of static electricity as an ignition source. . If materials being handled or processed are likely to generate charge, there are a number of precautions which need to be taken to avoid risk of ignition of flammable atmospheres. These are well described in British Standards (1991). . If there are any doubts about the charge generating capabilities of particular materials or processes or about the ability of standard approaches to adequately and economically handle problems then it is necessary to make appropriate electrostatic measurements and/or seek professional advice. References Abbott, J.A. (1988), “Survey of dust fires and explosions in the United Kingdom 1979–1988”, European News Letter, Vol. 8. API (2006), For Immediate Release, available at: www.pei.org/news/static.htm (accessed July 2006). Australian Government (2006), Static, available at: www.astb.gov.au/publications/2005/pdf/ static_fire.pol (accessed 9 June, 2006). Ball, R. (1996), “Electrostatic ignition hazards short course on fire and explosion”, paper presented at University of Leeds, Leeds 23-24 September. British Standards (1991), Control of Undesirable Static Electricity BS5958 Part I and II, British Standards.

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Crowel, D.A. and Louvar, J.F. (2002), Chemical Process Safety – Fundamentals with Applications, 2nd ed., Prentice Hall, Englewood Cliffs, NJ. Department of Labor (1990), Guidelines for the Control of Static Electricity in Industry, Occupational Safety and Health Service Department of Labor, Wellington, available at: www.osh.dol.govt.nz/order/catalogue/799.shtml (accessed July 2006). Industrial Accident Prevention Association (2006), A Health and Safety Guideline for your Workplace – Static Electricity, available at: www.whscc.nf.ca/resource/iapa/static electricity.pdf (accessed July 2006). Lees, F. (1996), Loss Prevention in The process Industries, Butterworth-Heinemann, Oxford. Renkes, R.N. (2006), Stop Static, available at: www.pei.org/static/fire_reports.htm#year (accessed June 2006). The University of Tennessee (2006), Environmental Health & Safety – Confined Space, Available at: http://web.utk.edu/ , ehss/training.htm (accessed July 2006). Wells, G.L. (1997), Safety in Process Plant Design, George Godwin, London. Further reading Chubb, J.N. (2006), The Control of Static Electricity, available at: www.jci.co.uk/ControlStatic.html (accessed June 2006). Corresponding author Ibrahim Mohamed Shaluf can be contacted at: [email protected]

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