static electricity - hazard and control

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ChilworthSafety and Risk Management P. Ltd.

Tel: Mumbai Office 022-66942350 Delhi Office 011-26136979 Email: [email protected]

Electrostatic Ignition Hazards –Assessment & Control

presented byAlok Singh

Chilworth Safety & Risk Management P. Ltd.

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Electrostatic Ignition Hazards –Assessment & Control

Course Content:

- Understanding electrostatic hazards- Flammable atmospheres- Identifying static hazards- Control of static electricity- Discussion

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COURSE OBJECTIVES

• You will learn

– how electrostatic charge is generated

– how to recognise electrostatic hazards which cause fires and explosions

– how to control electrostatic charge to reduce the risk

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UNDERSTANDING ELECTROSTATIC HAZARDS &

BACKGROUND

Understanding Definitions

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ELECTROSTATIC EFFECTS

• Electrostatic effects are everywhere:

– hair– clothes– car– balloon– TV, computer monitor

• Electrostatic effects:

– are commonplace

– are predictable

– can be controlled

-+++

++

+

+

-- -

---

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STATIC ELECTRICITY

• Uses

– photocopying

– surface coating

– printing

– electrostatic precipitator

• Problems

– clinging

– dust attraction

– powder segregation

– nuisance / shock

– ignition (fire / explosion)

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INTRODUCTION – DEFINITIONS

Conductive: The ability to allow the flow of an electric charge; possessing a conductivity greater than 104 pS/m or a resistivity less than 108 ohm-m

Antistatic/ static dissipative: Capable of dissipating a static electric charge at an acceptable rate (charge generation rate less than charge relaxation rate)

Nonconductive/ Insulating: The ability to resist the flow of an electric charge (charge generation rate greater than charge relaxation rate)

Electrostatic Discharge: Release of static electricity in the form of a “spark”, “corona discharge”, “brush discharge”, or “propagating brush discharge” that might be capable of causing ignition under appropriate circumtances

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INTRODUCTION – DEFINITIONS

• Surface Resistivity: Describes the ability of a solid surface to transmit electric charges. Units of Surface Resistivity are Ohm/square.

• Volume Resistivity: Volume resistivity is defined as the resistance of unit length of a sample of unit area. It is thus a property of the material itself, regardless of the dimensions of the sample.

Volume Resistivity, pv= R A/d

pv is the volume resistivity, R is Resistence (ohms), A is the sample area and d its length. The unit of Volume Resistivity are Ohm meters

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Static electricity is a (stationary) surplus or deficit of electrical charge

Electrostatic charge is a surface effect

Two kinds of charge are known: Positive and Negative

Unit of electrical charge is COULOMB, (C), equal to 6.24 × 1018 electrons

STATIC ELECTRICITY

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ELECTRICAL FIELD STRENGTH

Two kinds of charge are known; Positive and Negative.

Under static conditions,

– separated like charges exert a mutual force of repulsion

– separated unlike charges exert a force of attraction

The field of force associated with charge is referred to as an ELECTRIC FIELD, in units of volts per meter ( most often indicated by kV/m).

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Flammable Atmosphere

Charge Accumulation

Charge Generation

ASSESSMENT OF STATIC HAZARDS

Enough Energy

Discharge Mechanism

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IDENTIFICATION OF ELECTROSTATIC HAZARDS

•FLAMMABLE ATMOSPHERES•CHARGE GENERATION•CHARGE ACCUMULATION•ELECTROSTATIC DISCHARGES

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FIRE TRIANGLE

OXIDANT FUEL

IGNITION SOURCE

OXIDANTA substance which supports combustion.

FUELSolid, liquid (vapor or mist) or gas capable of being oxidized.

IGNITION SOURCEAn energy source capable of initiating a combustion reaction.

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CONDITIONS FOR EXPLOSION

OxidantOxidant

ConfinementConfinementMixingMixing

FuelFuel Ignition sourceIgnition source

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Information for assessing electrostatic caused fire and explosion hazards of fuels include:

FLAMMABLE CHARACTERISTICS• Flammable Limits (LFL and UFL)• Flash Points• Limiting Oxygen Concentration

IGNITION SENSITIVITY• Minimum Ignition Energy, MIE

CONSEQUENCES OF IGNITION• Maximum Explosion Pressure, Pmax• Maximum Rate of Pressure rise, (dP/dt)max

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FLASH POINT DATA

M A T E R IA L F L A S H P O IN T C

C L O S E D C U P

O P E N C U P

A c e tic a c idA c e to n en -b u ta n o lM e th a n o lT o lu e n e

4 3 -94 41 6 7

4 0-1 7 2 9 1 0 4

o

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FLAMMABILITY OF DUSTS

• Can only be measured• Test usually based on

vertical tube • In UK:

– “Group A” = flammable– “Group B” = non-flammable– for T > 110 °C furnace test

needed before a Group B can be confirmed

• Group B powders may burn or be thermally unstable

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MIE of Some Gases & Vapors

Material MIE(mJ)

Propanol 0.650Ethyl Acetate 0.460

Methane 0.280Propane 0.250Ethane 0.240

Methanol 0.140Acetylene 0.017Hydrogen 0.011

Carbon Disulfide 0.009

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MIE of Some Dust Clouds

Material MIE(mJ)

PVC 1,500Zinc 200

Wheat Flour 50Polyethylene 30

Sugar 30Magnesium 20

Sulfur 15Aluminum 10

Epoxy Resin 9Zirconium 5

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EXPLOSION LIMITS

• Fuel - air mixtures are only flammable within the flammable range

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FLAMMABILITY LIMITS (vol.%)In air

LEL UEL

Hydrogen 4.0 75.0

Methane 5.0 15.0

Propane 2.1 9.5

Ethylene 2.7 36.0

Carbon monoxide 12.5 74.0

Toluene 1.2 7.1

Methanol 6.0 36.0

Ethanol 3.3 19.0

IPA 2.0 12.7

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EXPLOSIBLE CONCENTRATION FOR DUST

• Lower explosion limit (LEL) for dusts– usually 30 - 100 gm-3

– visually dense cloud – measured in 20 litre

sphere

• UEL not measurable or useful– several kg m-3

2 m

40 g/m³ Coal Dust

25W

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LIMITING OXYGEN CONCENTRATION FOR GASES

• Varies with the material

• Can be estimated from the LEL

• Typically 8 - 12 vol.%

• If unknown, assume 5 vol.%

• Examples are for N2 as inert gas

MATERIAL LOC (%)

Acetone 13.5

Benzene 11.0

Carbondisulphide 5.0

Hydrogen 5.0

Methanol 10.0

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LIMITING OXYGEN CONCENTRATION FOR DUSTS

• Generally higher than for gases

• Typically 10 - 15 vol.%

• If unknown, assume 5 vol.%

• Examples are for N2 and Flue gases as inert gas

MATERIAL LOC(%)

Rye Flour 13.0

Organic Pigment 12.0

Methyl Cellulose 10.0

Beta-naphthol 9.5

Sulphur 7.0

Paraformaldehyde 6.0

Aluminium 5.0

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Charge Generation


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CONTACT CHARGING

When two materials make and then break contact, the contacting surfaces acquire a net charge, with one becoming negative and the other positive

In practice, electrostatic charging is due to multiple frictional contact/rubbing between surfaces - - - - - - - - - - -

+ + + + + + + + + + +

+ + + + + + + + + + + +

+ + + + + + + + + + + +

- - - - - - - - - - - -

- - - - - - - - - - - -

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A factor affecting electrostatic charging is the “type” of contact two materials make, i.e. friction, force and velocity of

contact

In liquids electrostatic charging depends on the presence of ions in the liquid. Ions of one polarity could be absorbed at the interface and they then attract ions of opposite polarity which form a diffuse layer of charge in the liquid, close to the surface. When the liquid is moved to the interface, it carries away some of this diffuse layer and a net charge is developed on the liquid, provided it is sufficiently non-conductive to prevent recombination

CONTACT CHARGING

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CONTACT CHARGING – EXAMPLES

POWDER HANDLINGSieving Pouring Mixing Grinding Pneumatic Transfer

LIQUID HANDLINGLiquid transfer through pipesAgitation of two phase mixtures, Settling of two phase mixture,Filtration

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Charge Accumulation

Charge Generation


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Conductors Insulators

ELECTROSTATIC CHARGE ACCUMULATION - EXAMPLES

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ELECTROSTATIC CHARGE ACCUMULATION

Electrostatic charge can accumulate on:

Isolated Metal Plant

Personnel - Personnel can attain potentials of 10 to 50kV- Maximum discharge energy = 100 mJ

Non-conductive Material (Surface Resistivity > 1011 Ohm/Square)- Accumulation of static charge- Insulation of conductive items- Charge retention on liquids & powder in non-conducting containers

MIE - Gases

MIE – Dusts

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Electrostatic charge can accumulate on:

Bulk Liquids - Liquid conductivity > 100 pS/m, No or LITTLE static charge generation Liquid Mist Powder Dust Cloud

If water vapor is present in air, it absorbs onto surfaces and forms a slightly conducting surface layer.

The extent to which the water absorbs and the increase inconductivity depends on the nature of the surface and the

humidity of the atmosphere.

ELECTROSTATIC CHARGE ACCUMULATION

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Conductivity of Some typical LiquidsRef. Plant/Operations Progress (Vol. 11, No. 2)

Non-conductive 0.1 Xylene Non-conductive < 50 Gasoline (unleaded)Non-conductive < 1 Toluene Semi-conductive 500 Isopropyl Ether Semi-conductive 100 Gasoline (leaded)Conductive 4.4 × 107 Methyl Alcohol

Conductive 2 × 107 n – Butyl Alcohol Conductive 6 × 106 Acetone

Conductive 2.9 × 104 Ethyl Acetate

COMMENT CONDUCTIVITY (pS/m)

LIQUID

LIQUID CONDUCTIVITY

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Charge Accumulation

Charge Generation


Enough Energy

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ELECTROSTATIC DISCHARGES

• Types of discharges• spark• brush• propagating brush• corona• cone (bulking)

• Effective / available energy depends on the source of the discharge

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SPARK DISCHARGE

Origin• charged isolated conductor

Energy• E = 0.5 x C x V2

• in practice up to a few 100 mJ

Incendivity• gases, vapours, dusts and

mists• as long as E > MIE

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• Between earth and

• isolated metal drum (nylon wheels on trolley)

• isolated person (insulating footwear)

• isolated pipe section (insulating gaskets)

• isolated conducting liquid (in plastic drum)

SPARK DISCHARGE - EXAMPLES

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• Caused by

• charged insulating bulk container

• charged insulating liquid

• charged dust cloud

• charged thunder cloud

BRUSH DISCHARGE - EXAMPLES

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Origin• high charging situations on high resistivity materials• insulator with metal close by

Energy• not established• > 1 Joule (1000 mJ)

Incendivity• vapour• gases• dusts• mists?

PROPAGATING BRUSH DISCHARGE

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• High charging level on thin, highly insulating material:

• pneumatic conveying though coated metal duct

• internally coated cyclone

• insulating bulk container

• high speed conveyor belt

PROPAGATING BRUSH DISCHARGE - EXAMPLES

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CONE (BULKING) DISCHARGE

Origin• highly charged powder in bulk

Energy• up to ~25 mJ for fine powders• higher energies for granular

materials

Incendivity• vapours• gases• sensitive dusts

MIE - Gases MIE – Dusts

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Charge Accumulation

Charge Generation


Enough Energy

Discharge Mechanism

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DISCHARGE MECHANISM

• Need a discharge path

• Difficult to predict when and where

• Assume that a discharge is possible

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CONTROL OF STATIC ELECTRICITY

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CONTROL OF STATIC ELECTRICITY

• Avoid or minimise charge generation

• Prevent charge accumulation

• Remove charge before hazardous potentials are reached

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EARTHING PRINCIPLES

• Earthing avoids charge accumulation on electrical conductors and prevents electrostatic sparks

• Connecting the point of charge generation will also prevent sparks

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• Large items are usually connected to the structure– Often no extra

earthing needed

– Bonding straps are needed where items are insulated (e.g. rubber mountings)

• Structure connected to mains earth

• Bonding across joints no longer considered necessary in many cases

PLANT STRUCTURES, PIPELEINES &LARGE FIXED ITEMS

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EARTH MOVEABLE EQUIPMENT

• Drums, tankers, trucks, feeders, drum pumps etc.

• May be earthed through contact with a suitable floor (R< 1 MΩ)

• Floor resistance may be variable - use a earth lead & clip

• Make the contact BEFORE operations are started

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Grounding Clamp Used For Road Tankers

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Ground Proving Unit

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Bonding and Grounding of Metal Flange Coupling on Insulating Parts

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Bonding and Grounding Not Good Practices

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• Total resistance between the body and ground should be < 100 M ohm

• Where risk of exposure to mains electricity exists, resistance to ground should be:5 × 104 ohm < R < 1 × 108 ohm

PERSONNELEarth people through their footwear and low resistance floors

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FLOOR MEASUREMENT

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NON-CONDUCTIVE FLOOR?

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CLOTHING

Any clothing, including those made from insulating materials, may be worn PROVIDED:

• Minimum Ignition Energy of flammable atmosphere is > 0.2 mJ

• Operator is grounded (R < 100M ohm)

• Clothing is not REMOVED in the presence of the flammable atmosphere

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MAINTENANCE

• Footwear– test regularly - preferably every shift– ensure insulating insoles are not worn– test samples from suppliers– replace any footwear with high resistance

• Floors– keep floors clean and free of deposits– floor paints are often insulating– employ good housekeeping

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LIQUIDS

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HAZARDOUS OPERATIONS

The following operations require specific control measures:

– gauging and sampling– tank cleaning– road and rail tanker discharging– flexible hoses– centrifuging

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CONTROL OF STATIC HAZARDS –LIQUIDS

Static electricity hazards can arise in various liquid handling operations including filling, sampling, filtration and mixing. The following suggestions can reduce the electrostatic ignition hazards

• Use Electrically Grounded Conductive Plant All items of plant including pipes, vessels, containers etc. should be electrically conductive and/or static dissipative and grounded.

• Increase Liquid Conductivity An antistatic additive may be used in very small concentrations in order to raise the liquid conductivity

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TYPICAL PROCESS

StreamingCurrent (Is)

is = 10-10 to 10-4 A

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CONTROL OF STATIC HAZARDS –LIQUIDS

Continued:

• Control of Liquid Entry to the Vessel Liquid should enter a vessel through a dip leg with submerged ends or bottom inlet point

• Filters and valvesUse valves with maximum diameter possible. Locate filters and valves as far as possible from the entrance tothe receiving vessel

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Static charge can build up on powders with Volume Resistivity > 109 ohm.m or if powder is in insulating or ungrounded conductive vessels. Depending on Minimum Ignition Energy of the powder cloud consider:

Volume Resistivity Vs. Minimum Ignition Energy

Volume resistivity < 109 ohm.mNo static charge accumulation if powder is handled in grounded conductive plant

Volume resistivity > 109 ohm.mNo electrostatic ignition risk in grounded conductive plant if minimum ignition energy is greater than 25 mJ

POWDERS

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AVOID CHARGE GENERATION POWDERS

• Increase conductivity

• Reduce transfer velocity

• Often these are not possible as they are intrinsic to the process– materials conductivity is naturally high - plastics– pneumatic conveying needs high velocities to work

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EARTHING RODS FOR POWDERS

• Remove charge from powder surface

• Must be in place before starting filling

• Not suitable when solvents present

Non Conductive Plastic Liner

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VESSEL CHARGING

• Charging powder in vessel containing a flammable solvent, or charging solvent wet powder, is potentially hazardous

• Plastic drums or liners are often used

• Flammable atmosphere around charge opening

hazardous charging operation

Plastic drum or plastic liner or metal drum withplastic liner

for solids containing flammable solvent or pre-charged flammable solvent

Vessel grounded

Plastic funnel

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NON-CONDUCTIVE SOLID MATERIALS

• Volume resistivity > 109 Ω.m

• Surface resistivity > 1011 Ω

• Charge relaxation time > 20s (often much longer)

• Often hydrophobic

• Examples – polyethylene

– polypropylene

– PTFE

– PVC

– epoxy

– etc.

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CONTROL MEASURES

• Use bonding straps to earth isolated conducting objects

• Segregate non-conductive solid materials from flammable atmospheres

• Replace non-conductive solid materials with conductive or static dissipative material

• Static dissipative materials must be earthed

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Any Questions?

static electricity - hazard and control

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