static electricity

Static ElectricityStatic Electricity

Static ElectricityStatic Electricity

Hazards & Hazards & ControlsControls

Static Electricity - Generation

• When ? When two dissimilar materials come in contact with

each other or they remain in contact while they are in motion there is a transfer of electrons between them which results in Static electricity generation.

If any one of these materials is a non conductor then

accumulation of electron occurs on them.


• How ? All materials are made of atoms. Atoms contains

positively charged Protons neutrally charged Neutrons in Nucleus.

Electrons are moving in different orbits around nucleus. Various orbits can accommodate different no.s of Electrons e.g. 2,8,18,18,32…

Atoms have tendency to become stable by accepting or donating electrons from the outer orbits.


• How ? Sometimes electrons from outer orbits are separated

from stable atoms due to rubbing action between them.

Materials with deficiency of electrons becomes positively charged & excess of electrons becomes negatively charged resulting into potential difference between them.

Static ElectricityPrecautions for plant safety

Remember, Static Electricity is one of the lessacknowledged reasons for fire and explosion inChemical Processing.

Its generation is not realized and accumulation is

not identified till it discharges in the form ofElectrical spark.


Liquid flowing in pipelines.

If the liquid or the pipeline is nonconducting, it leads to the charge build-

up inliquid or pipeline or isolated conductingparts.


Discharge of liquid or powder from pipe.

Pneumatic conveying of powders. If thepipeline is non-conducting, then both

pipe and also the powders get charged.


Pouring of liquid or powder into/ from a container.

While pouring liquids or powder from a container,if the container or the material is non-conductingwill lead to accumulation of charges. Addingmaterial into reactor through man-hole isdangerous.


Spraying of liquid through a nozzle.

Spray drying, spray painting etc. are some to the

operations of this type. This is the reason for powder coating the job is kept charged to

attract paint particles.


Stirring of material in a reactor.

If the material is non-conducting or the reactor is

glass-lined, the charges will build up in liquids. The

problem is more pronounced if the reactor is glass

lined.


Use of normal plastic containers and liners.

Normal plastics have high resistance, this leads tocharge generation and accumulation. Plastic containers can not be easily discharged by Earthing and Bonding. Antistatic/ Static dissipative plastics are available.


Belt moving on a pulley

If the belt used is normal and not antistatic,then it will lead to the build of charges on the

belt.

Characteristics of Materials

• Materials have tendency to release electrons

LeatherGlassWoolHuman hairAluminum

• Materials have tendency to accept electrons

• Hard Rubber• Vinyl (PVC)• Silver• Polyester• Acrylic

Examples of Static Electricity Generation

• Static Electricity is usually generated when two dissimilar materials are rubbed against each other. e.g.

Wool on plastic

Plastic comb on dry hair

Shoes sole on carpet

Static Electricity Generation in Industry

• Solid – Solid

Between Belt & Pulley

While Grit / Sand Blasting

Charging dust or powder in Hopper


• Solid – Liquid

Liquids flowing through pipeline

While Filling liquids in Storage tanks


• Solid – Gas

Steam coming out from pin hole

Gas coming out from nozzle.


• Liquid – Liquid

Water settling in Hydrocarbon filled tan

• Liquid – Gas

Air bubble rising through tank filled with liquid

Static Electricity Generation in Industry during various operations

• Loading - Unloading

While loading or unloading materials from tankers, drums, carboys there are chances of Static charge generation.

Generation of charge depends on type of container & materials which are to be unloaded.

Static Electricity Generation in Industry during various operations

• Agitation, Mixing or Blending

While Agitation operation there are chances of rubbing actions between material & Agitator or Reactor.

During mixing rubbing action can also occur between two materials which are to be mixed.

Accumulation of Static charge

• Generation of Static charge is not a hazard on its own.

• Accumulation of Static charge occurs when out of two material, one is non conductor.

• Accumulation of charge also occurs if generated charge is not discharged or rate of generation is higher than rate of dissipation or discharge.

• This process is also known as Electrostatic Induction

Accumulation of Static charge

Examples

Plastic drums Non Conductive Filter bags

Plastic Suction devices

Electrostatic Discharge

• Accumulated Electrostatic charge can discharge in following ways:-

SparksBrush DischargeCorona DischargeLightning like Discharge.


• Sparks

Discharge occurs between two conductors.

Discharge occurs in a sudden burst.

Radius of curvature is >=5cm

Equivalent Energy=0.2 mJ


• Brush Discharge

Discharge occurs between conductors & charged conductive surface.

Rapid Discharge gives overall impression of brush.

Luminous phenomenon emitted from conductor ends in a gap.

Radius of curvature is 0.5cm

Equivalent Energy=3 mJ


• Corona Discharge

Discharge occurs between conductive point & charged conductors.

Luminous phenomenon emitted from conductive point gives shape of ‘Corona’.

Radius of curvature is <1mm.

Equivalent Energy<0.1 mJ


• Lightning Discharge

Discharge occurs similar to phenomenon of lightning.

In industry, chances of such discharge are rare

Discharge can occur in case highly charged dust clouds or betn. two dust clouds

Equivalent Energy=Very High

Hazards of Static Charge

• In industry, main hazard of Static charge is Fire or Ignition. Static charge can be hazardous if following condition are fulfilled :-

• Generation of Static charge• Accumulation of Static charge• Electrostatic discharge.• Presence of flammable source• Required minimum Ignition energy

Conditions for Fire Explosion

• For propagation of Fire or Explosion three elements are required.AirCombustible material Ignition Source with required Ignition

Static Electricity provides source of ignition with required ignition energy

Minimum Ignition Energy

Minimum Ignition Energy (MIE) is lowest Energy required to ignite combustible mixture in air.

Static charge provides this energy when discharged after

accumulation.

MIE is different for different material

MIE of various materials in Air { in mJ}

Hydrogen-0.01, Methanol-0.14, Acetone-1.15,

Normal dust>10

Human Sensitivity Against Electrostatic Discharge

• If human body comes in contact of Electrostatic discharge the effect on the body depends on amount of energy discharged.

• Severity of shock is not as great as power electricity• For Example

Discharge Energy of

0.002 J – Perceptible

0.25 J - Severe Shock

10 J - Fatal

Control Measures to Prevent Hazards of Static Charge

• Earthing & Bonding

Generated Static charge doesn’t pose any hazards if it is grounded through earthing.

Earthing is to be provided on pipelines, storage tanks, drums or carboys, vessels to ground generated static charge.

Size & type of Earthing should decided considering total static charge to be grounded


• Earthing

Earthing wires, strips are to connected with Earth pits. Design of Earth pits is to be done as per IS-3043:1966 Continuity of Earthing & Resistance of Earth pits are to be measured regularly . In case of Large storage tanks two separate earthing are provided on opposite sides.


• Bonding

Bonding is provided to carry over generated static charge if non conductive materials are placed between two conductors.

Bonding is done on pipe line flanges where there are chances of static charge due to flow of material inside

Continuity of bonding strips & clits are to be checked regularly.


• Use of conductive material

Static charge generation can be prevented / reduced by use of conductive material.

Use of conductive liquids

Avoid use of plastic bags or containers

Use conductive pipes

Use conductive filter bags in dust filter if dust is flammable.


• Non-Conductive liquids

Aliphatic & Aromatic H/C

Ethers & Esters

Carbon disulphide

Carbon tetrachloride

Petrol, Benzene, Xylene

Cyclohexane,

Chloroform

Conductive liquids H/C with‘Polar’ group Alcohols,Aldehydes, ketones

• Methanol, Ethanol• Acetone• Isopropanol• Glycol• Formic Acid


• Avoid Explosive Mixture Formation

Remove Explosive dust, vapors, gases by use of vacuum suction devices

Maintain inert atmosphere by nitrogen or other inert gas.

Use nitrogen pressure for charging flammable materials.


• Consider Relaxation time

It is time required for discharge of static charge. Relaxation time is different for various liquids

Liquid Relaxation Time

Toluene 21 s

Hexane 46 h 42 min

Heptane 0.18 s

Methanol 9*10-6 s

Ethanol 1.6*10-4 s

High velocity causes rubbing action & separation of electrons.

Normally 1-1.5 m/sec can be maintained.

Turbulence is to be avoided


• Keep the velocity low

• Avoid Splash filling

Splash filling of liquids is to be avoided.Dip pipe is to be used for filling


• Maintain Humid Atmosphere

Static charge generally discharges through air

Conductivity of humid air is slightly more than dry air which helps in early discharge

• Avoid charging on human body

Human body is considered as conductor

Conductive floors and footwear's are to be used.


• Use Antistatic Additives

Antistatic additives increases conductivity of materials or decreases the resistance which helps in discharge of static charge.

Additives should be chemically inert & should not react with parent material.


• Measurement of Static Electricity

Static Electricity can be measured using Electro Static voltmeter.

Presence is demonstrated by sparks, shocks or attraction/repulsion

Measurement is important for comparing it with Minimum Ignition Energy of combustible material present in surrounding.


• Follow standard procedures / practices

Standard procedures & practices are to be followed while handling static charge generating materials.

Plastic drums are not to be used

Earthing is to be provided to drums while manual loading/unloading.


• Conclusion Mainly following points/conditions are to be considered for avoiding hazards of Static Electricity

Avoid / Reduce Static charge generation by use of conductive material.

Provide proper earthing / bonding for discharge of Static charge generated

Avoid Explosive mixture formation in areas where chances of Static charge generation

Case study

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static electricity

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static electricity

static electricity

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outer orbits

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