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Index 1. What is Static Electricity ?
2. Origins of Static Electricity
3. What are the hazards of static electricity ?
4. Prevention Ways to Control Electrostatic Hazards
5. Practical Safety Instructions
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Unit 1
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Definition Static electricity is a surface phenomenon which causes by separation & aggregation of material’s charges which is in contact with each other and then separated.
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How does it generate? 1. The surface electrical charges of the objects try to balance each other by the free flow of electrons between themselves when they are in contact.
2. After separating, objects are left with either an excess or deficiency of electrons which causes both objects become electrically charged.
3. If the mentioned charge don’t have a path to ground, objects won’t be able to move and become “ static “ .
4. If static electricity is not rapidly eliminated , the charge will build up.
5. It will eventually develop enough energy to jump as a spark to some nearby grounded or less highly charged object in an attempt to balance the charge.
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Some sources of static electricity generation
Movement of human body / Conveyor belts /Reeling or unreeling of paper or plastic sheets
Solid / Solid
Discharge of wet steam / Liquid sprays , including water Gas / Liquid
Discharge of carbon dioxide / Filling of containers with powder
Gas / Solid
Stirring of immiscible liquids / Settling of drops of one liquid through another
Liquid /Liquid
Flow of liquids through pipelines / Splash filling of tanks Liquid / Solid
Examples Materials
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More examples 1. Flashover / lightning 2. Spraying / Coating 3. Blending / mixing 4. Flow of dust or small, solid objects in a pipe 5. Screens in a moving steam of liquid 6. Inside of pipes carrying moving liquid, dust, or small solid objects 7. Transfer of liquid fuels from one tank to another 8. High – velocity flow of any liquid in a pipe 9. Flow of liquids through a screen
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10. Conveyor belts made of rubber, leather, or plastic 11. Movement of material in a pile of dust or of fine, solid particles 12. Rubbing of explosives 13. Plastic covers and containers of flammable materials 14. Metal or plastic chutes and funnels used with flammable dusts or liquids 15. Steam – cleaning of tanks in which there are flammable vapors 16. Nylon or wool carpet in the vicinity of flammable materials 17. Personnel – borne charges 18. Conductors in the vicinity of magnetic fields
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Physics The amount of energy stored or discharge :
E : energy in Joules
C : capacitance in Farad
V : potential in Volts
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Releasable energy of walking over a carpet in dry air
Capacitance for a person is approximately : 100 Pf
Produced voltage between person & carpet : 50000 v
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Unit 2
Origins of Static Electricity
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2.1. Double Layer Charge Work Function : Essentially energy for separation an electron from the surface of materials to infinity.
Work Function of insulated objects is high. Work Function of objects with the more conducting electrons is low. Work Function will be decreased by increasing temperature.
While two dissimilar objects are in contact with each other at the same temperature, interchanging of electrons would produced on their interfacial areas.
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Electrons transfer from donor materials with the lower work function to acceptor materials with the higher work function.
Results : 1. Materials with the higher work function will receive the excess charge of electrons on their surface.
2. Capacitance of the system will be increase because of the minimum distance between layers and this causes charge polarization on material’s interfacial areas.
Helmholtz Theory
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2.2. Electrostatic Induction
Fig1‑ Uncharged objects position
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2.2. Electrostatic Induction
Fig 2 – Induced charge on conductor by Negative electrons
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2.2. Electrostatic Induction
Fig 3 – Grounding conductor and flowing of negative induced charges to earth
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2.2. Electrostatic Induction
Fig 5 – relocation of positive charges on the conductor surface
Fig 4 – Equivalent remained positive charge on conductor
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2.2. Electrostatic Induction
Fig 6 – Remained positive charges on conductor Fig 5 – relocation of positive charges on the conductor surface
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2.2. Electrostatic Induction
Fig 6 – Remained positive charges on conductor
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2.3. Static Charges on Clothes & Body Excess charges is usually aggregated on human body.
we are concerned here with the below conditions: 1. Sparkly draining from body ; it causes fire in flammable areas or may lead to electrostatic shock.
2. Gas discharging from clothes ; particularly when it occurs in the most sensitive flammable areas such as Acetylene Air mixture or Hydrogen – Air mixture
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2.4. Charging of Flowing Liquids
Double layer charge will be produced on liquid / liquid interfacial area , as solids.
Presence of most ions is needed for charging of liquids ; While a liquid is transferring through a pipeline into a tank , if positive ions of liquid remain on the surface of pipeline then negative charges will produce a diffuse layer in liquid.
Amount of charge swept by liquid is depended on volume resistivity and separation velocity of liquid from pipeline.
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Conductance property of liquids is usually expressed by electrical conductivity.
Conductivity is measured in reciprocal ohms and the unit used to be called mho and the SI name for the unit is Siemens.
between the V 1 when the application of S 1 Conductivity is to flow. A 1 points causes
Conductivity
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Relaxation : The dissipation of a charge
Relaxation time : The time taken for a hydrocarbon liquid to lose about 2/3 of its charge
It is not possible to give a time for the complete loss of charge ; voltage and therefore also the current flowing from the charge decreases continuously.
Some Definition
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Unit 3
What are the hazards of
Static Electricity ?
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1. Great potential for discharge 2. Surprise & involuntary response of a person who receives the static charge 3. Creation of an arc and ignition of certain vapor or dust mixture in air 4. Creation of sparks in an explosive or flammable atmosphere. 5. Fire and explosion
Hazards
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Four conditions must be met that static electricity be hazardous :
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 )
4 The spark must occur in an ignitable vapor or dust mixture.
What are needs to make static electricity hazardous?
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Unit 4
Prevention Ways to Control Electrostatic Hazards
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Hazardous static charges can be avoided in two ways :
1. Preventing the generation of static electricity 2. Draining it away so that a charge cannot be build up.
It would be wrong to think that the generation of static electricity can be completely eliminated , so we must :
Minimizing the build up of charge
Prevention ways
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1. Bonding & Grounding
2. Humidification
3. Static collectors
4. Additives
How can static electricity be controlled?
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Bonding : Connecting two or more conductive objects with a conductor, such as cooper wire, that equalizes the potential charges between them.
Bonding & Grounding
Bonding is also connecting the various parts of equipment and containers that are electrically separated .
Fig 7 ‑ When bonded Together, both Objects share Same charge
Bond Wire
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Grounding : Connecting one or more conductive objects directly to earth using ground rods, cold water cooper pipes , or building steel .
Grounding drains the static charges away as quickly as they are produced.
Note that bonding does not eliminate the static charge
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A relative humidity of 60% to 70% at 700 F may prevent paper or layers of cloth and fibers from sticking together.
A high relative humidity, however, is no guarantee against the accumulation of static electricity.
Humidification of air in closed environment may help, but is usually costly and less effective in reducing risk .
Humidification
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Static charge build up is greater when the air has low moisture content.
Don’t rely solely on humidification as a control measure in areas where there are flammable liquids , gases or dusts.
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A static collector works by its closeness to the source of electrostatic generator ;
If a discharge occurs , it is captured by the highly conductive collector which this results in preventing long hot spark .
To be effective , collectors must be properly grounded.
Static Collectors
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Using of antistatic additive , is one of the other controlling ways against generation of static electricity ;
The additive increases the conductivity or lowers the resistance of the liquid .
The ones used for aviation fuels will typically increase the
conductivity to greater than 50 pS/m
Additive
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Unit 5
Practical Safety Instructions
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1 electrical conduit systems 2 plastic pipes 3 gas or steam pipes 4 dry pipe sprinkler systems 5 lightning rods 6 metal storage racks 7 building support beams
A designated ground source is preferred
Static grounds must not be made to :
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Establishing a good conductive path by connectors :
1 Remove all dirt, paint , rust , or corrosion from areas where connections are to be made
2 Use connectors that are strong enough for the job ; use connectors with higher conductivity
3 Use flexible connectors where there is vibration or continuous movement
4 Connect metal to metal
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Controlling static electricity on people : 1 Conductive flooring ;
Conductive floors reduce risks in hospital operation rooms, where anesthetic gases may be explosive under the right condition.
2 Conductive cloth and footwear ; In locations where static charge from clothing could be dangerous, workers
wear conductive clothing, particularly shoes. All of these items must be free of dirt and other contaminant .
3 Cotton or linen clothing instead of wool , silk , or synthetic material ;
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4 Static discharge from clothing fabric ; U.K. Standards of safety clothes which is used in flammable areas : Where the minimum ignition energy of atmosphere is more than 0.2 mJ: use every types of clothes if a proper grounding of people is established by conductive floor and antistatic shoes. Resistance between person and earth does not be over than 100 mega ohms by flooring and shoe This resistance does not be lower than 50 kiloohms where there is an encounter danger with main voltage
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Where the minimum ignition energy of atmosphere is less than 0.2 mJ Resistance between person and earth does not be over than 1 Mega ohms by flooring and shoe. Surface resistivity of cloth must not be over than 50 Gohms
Mentioned surface resistivity of clothes with natural fiber such as cotton and linen will be existed if relative humidification of area be 65% or more.
Using of antistatic agents for clothing Dacron such as polyester and nylon is necessary.
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Hydrocarbon Liquids and Hazards Reduction ways Conductivities of Hydrocarbon liquids which generate hazardous charges :
The corresponding relaxation times are :
Lowering the velocity of the liquid by using a pipe line of larger diameter ; The surface to volume ratio will also be reduced but this does not have a significant effect.
When dispensing flammable liquids, both bonding and grounding are required ; ensure that the receiving container is bonded to dispensing container before pouring the liquid, and the dispensing container is grounded
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Alternative locations for bonding wire
fastened to pipe
Alternative location for bond
(only if inherently connected
with the loading piping)
Product flow
Tank truck with no charge on
outside surface
Ground
Figure 8 Bonding and Grounding while Top Loading a Tank
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Prevent of presence of any immiscible liquid (such as water ) in flowing of hydrocarbon liquids ; they increase the charge in the receiving vessel by as much as 50 times.
The maximum velocity in the pipe line should be 1 m/s , if the liquid is contaminated . If the hydrocarbon is completely dry , then higher velocities can be used:
2 m/s at the lowest conductivity
7 m/s at the higher conductivity
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Passing liquid through a pump increase charging considerably particularly if it goes through a microfilter ; A length of earthed conductive pipeline should be provided between pump and tank .
The residence time in the pipeline should be about three times the relaxation time of the liquid ; but not less than 30 s which would be sufficient for any liquid with a conductivity of 2 pS/m or more
The residence time can become inconveniently long for liquids with very low conductivities ; required time can be reduced by the use of antistatic additive.
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Safety notifications for tanks and fueling hazards If there is a flammable atmosphere in the tank ;
At least 30 min should be allowed after filling has stopped for the charge to relax , before any manual dipping or sampling is allowed
Splash filling will generate charges as well as releasing flammable vapor and spray ;
The filling velocity should be restricted to 1 m/s until the outlet of the fill line is covered to a depth of at least 0.5 m
“ This velocity applies also to the filling of road and rail tankers.”
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Visual notification
1. Warning Signs
2. Warning Tapes
3. Warning Labels
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Warning Signs
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Warning Tapes
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Warning Labels
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References 1. Electrostatic Hazards; Gunter Luttgens & Gunter Luttgens & Norman Wilson Norman Wilson 2. Health and safety for engineers ; 2006 2006 edition edition 4. Industrial Accident Prevention Association – static electricity ; Revised Revised 2006 2006 5. www.sceinc.com 6. www.labelsourceonline.co.uk 7. www.esdjournal.com
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