the principles of static...
TRANSCRIPT
Electrostatic Solutions Ltd www.electrostatics.net
THE PRINCIPLES OF STATIC ELECTRICITY
Dr Jeremy Smallwood
What we are trying to avoid…
• Ignition or explosion of flammable atmospheres – Gases – Liquid vapours – Dust clouds
• Electrostatic shocks to personnel
• Ignition of explosives or electroexplosive devices
Incident - vehicle refuelling in USA
16/12/2013
Video courtesy of Steve Fowler
www.esdjournal.com
Electrostatic Solutions Ltd www.electrostatics.net
THE PROBLEM
Shocks to personnel
• From charged objects – Isolated conductors charge up and discharge to
people
• Due to charged personnel – People charge up and get a shock when they
touch a conductor – They feel a shock if their body voltage was above
about 4000V
Ignitions of flammable atmospheres and explosives • For ignition, the ESD energy must exceed the material
Minimum Ignition Energy (MIE) • Different materials have different MIE • MIEs of gases and hydrocarbon vapours in air are
often around 0.1- 0.3 mJ • Some MIEs can be 0.02 mJ or less
– hydrogen-air – ethylene-air – oxygenated mixtures
• Dust clouds can have MIE from < 1mJ upwards – MIE varies with particle sizes and other factors
• Explosives can have MIE from <1mJ to >1J
Energy required to ignite a flammable atmosphere
Minimum Ignition Energy (MIE)
rich mixture
lean mixture
optimum mixture
Ignition energy
Fuel concentration in air
Upper flammability limit (UFL)
Lower flammability limit (LFL)
Region of ignition risk
Electrostatic Solutions Ltd www.electrostatics.net
ELECTROSTATIC DISCHARGE (ESD) SOURCES
Where does electrostatic charge come from? • Every material is made up of charge
– Negative electrons – Positive atomic nuclei
• These charges are normally present in balance, and their effects cancel
• When the local balance is disturbed, and we see more of one type, we may notice static electricity effects
• Charges are separated wherever two materials are in contact – Flowing liquids or particles in liquids – Solids making contact or rubbing – Happens many ways in process equipment, pipes and
containers
16/12/2013
Charging by contact and rubbing
ESD Detector Isolated metal plate
Electrostatic voltmeter
Electrostatic voltmeter sensor
Why don’t we always get static charge build-up? • if charge is dissipated
faster than it is generated, then no problem
• if charge is generated faster than it is dissipated then it builds up
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No problem Problem !
Insulators prevent charge moving away • Insulators stop charge
moving away and encourages static charge build-up
• Charge build-up gives high voltages
• Highly charged insulators in a hazard Zone may be the source of ESD • Plastic bags, boxes
• Glass (sometimes) • Dry paper & cardboard • Rubber
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Conductors allow charge to move away • A conductor is anything
which is not an insulator! • Conductors allow charge
to move around quickly enough to avoid charge build up
• Isolated (ungrounded) conductors are a strong source of ESD – E.g. people – E.g. metal objects
• Metal • Water • Damp card and paper • People
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Grounding
• We can “ground” or “earth” a conductor by connecting it to electrical earth by way of a wire or another conductor
• In order to work, a ground path must have conductors at every point along the grounding path – Like a chain, if a link is missing doesn’t work
• Grounding an insulator does not work as the charge cannot leave the insulator
• Conductive materials in a hazard Zone must be grounded!
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Charge generation and voltage
• Static charge Q generation acts as an electrical current source I
• The voltage build-up V is proportional to the current I and the material electrical resistance R
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R
I
V
Voltage and charge storage • If a static charge has built
up, voltage V is proportional to the charge present Q
• capacitance C is the relationship between voltage and charge – Capacitance depends on
local materials and geometry
• if Q is constant and C falls, V rises
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Q V
C
Charge decay time
• If a static charge has built up, voltage V is proportional to the charge present Q
• The charge leaks away at a rate I=V/R
• The charge and voltage theoretically decays to 40% of its initial value in the charge decay time τ = RC
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R V
C I
Charge decay time in materials
• In materials, the charge decay time is related to – Volume resistivity ρ – Conductivity σ = 1/ρ – Permittivity ε
• The charge decay time is τ = ρε = ε/σ – In liquids this is known as the “relaxation time”
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Induced voltages
• When a charged material approaches an isolated metal object E.g. a tool or metal part – The voltage on the isolated metal object rises – An ESD can happen when another conductor
touches it – The metal object then becomes charged…and
can again rise to high voltage and be the source of another ESD
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Induction of high voltages
ESD Detector Isolated metal plate
Electrostatic voltmeter
Electrostatic discharge (ESD) sources
• Brush and propagating brush discharges from charged insulators – Plastics – Rubber – Hydrocarbon liquids – Dusts & granules
The effect of air humidity
• Water vapour in the air tends to form a thin layer on material surfaces
• The water layer conducts electricity, increasing charge dissipation and reducing charge build up on insulators
• Static charge build-up tends to be reduced by air humidity, and is minimised above 40% rh
Beware of terminology
• “conductive”, “dissipative”, and “antistatic” have different meanings – In different industries – For different products
• Specify items and materials in terms of measureable parameters – Electrical resistance or resistivity – Resistance to ground
Electrostatic discharge (ESD) sources
• Sparks from charged conductors – Metals and other
electrically conducting solids
– Conducting liquids – Charged people
Different types of ESD have different ability to ignite materials
ESD type Occurs Will ignite Spark discharge Between two conductors Gas mixtures and dusts Propagating brush discharge
From insulator backed by a conductor
Gas mixtures and dusts
Brush discharge From insulators Gas mixtures Cone discharge Highly charged insulating
powder & granule cones in silos
Gas mixtures and low MIE dusts
Corona discharge Sharp edges on charged conductors
Not usually incendive
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Electrostatic Solutions Ltd www.electrostatics.net
HOW DO WE CONTROL STATIC ELECTRICITY?
In flammable atmosphere zones… • Avoid insulators that could
charge up – Source of brush discharges – Can induce voltages in
isolated metal parts – Area of insulators allowable is
severely limited depending on sensitivity of material
• Max 100cm2 for Zone 1 Group IIa & IIb hydrocarbons
• Avoid metal backed insulators that may become highly charged – Propagating brush discharges
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No insulators bigger than a CD case!
Ground all conductors • Avoid isolated metal parts
that could be charged by induction or triboelectrification – Source of sparks
• Charges on conductors can be conducted safely to earth by grounding – Never make a ground
connection within a flammable atmosphere – a spark could occur on contact!
• Insulators cannot be grounded
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Ground people through footwear and flooring • Ground personnel in
flammable atmosphere Zones via conductive or “antistatic” footwear and flooring
• A conductive or static dissipative floor is required – don’t use insulating coatings
such as epoxy – Keep it clean and avoid
contamination • PPE issued should be
compatible with use in a flammable atmosphere Zone – never remove clothing within
in a flammable atmosphere
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Beware of unusual electrostatic risks • E.g. steam
What went wrong - explained
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IEC 60079-32-1/TS/Ed.1 (2013): Explosive atmospheres Part 32-1: Electrostatic Hazards, Guidance
Thank you!
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