Fundamentals of Static Electricity

• Basic Concepts

• Calculation Methods

• Guidelines

• Case Histories

Fundamentals of Static Electricity

• How Do Charges Accumulate?

• How Do Accumulated Charges

Discharge?Discharge?

• How Do You Estimate Discharge Energy

in Relationship to the Minimum Ignition

Energy?

Flammable Dusts

Generally accepted rules of thumb for theminimum energies required to ignite gasand dusts are:

• Flammable gases need 0.1 mJ or more.• Flammable gases need 0.1 mJ or more.

• Flammable dust suspensions need 10 mJ ormore. Some chemical dusts with very smallparticles are known to have MIEs lower than 5mJ.

Flammable Dusts

• Acetamide

• Adipic Acid

• Aluminum

• Cellulose

• Coffee

• Corn• Aluminum

• Barley

• Carbon

• Corn

• Epoxy resin

• Iron

Flammable Dusts (Cont.)

• Milk

• Nylon

• Paper

• Steel

• Sucrose

• Wheat• Paper

• Polystyrene

• Starch

• Wheat

• Wood

• Zinc

Some MIE Values

Minimum Ignition Energies

Vapors mJ

Acetone 1.0

Acrolein 0.1

Benzene 0.2Benzene 0.2

Carbon Disulfide <0.1

Heptane 0.2

Toluene 0.2

MIEs

Additionally, a chemical dust may contain residual solventswhich usually have MIEs below 1 mJ. In the chemicalindustry, therefore, more attention needs to be devotedto the prevention of potential dust explosions. The addedsensitivity of chemicals may require the elimination ofsensitivity of chemicals may require the elimination ofeven low energy static electricity discharges (dependingupon the MIE of the chemical).

Under some circumstances inerting may also be required;inerting is the concept of decreasing the concentration ofoxygen to below the level required to supportcombustion (called the limiting oxygen concentration,LOC).

MIEs

Dusts have minimum ignition energies(MIEs) as previously shown. It is alsoknown that the MIE decreases as theparticle size decreases and it alsoparticle size decreases and it alsodecreases as the temperature isincreased. Some finely ground dusts areknown to ignite with MIEs below 5 mJ.

Flammable Dusts

Likewise, among the parameters that canaffect MIE is moisture content of the dust.More moisture in the dust elevates MIE,which is helpful.which is helpful.

This though, leads to another misconceptionor myth, pointed out in the “Deadly DustIII” video: that a humid environment canprevent dust explosions. In reality, theyhave occurred even during thunderstorms.

Flammable Dusts

Note then, that the ignition energy is aproperty of the dust, and not a function ofthe size of the plant or operation.

As pointed out in the “Deadly Dust III” video,As pointed out in the “Deadly Dust III” video,it is a misconception or myth that smallerplants can be immune to dust explosions.

MIEs

Additionally, a chemical dust may contain residual solventswhich usually have MIEs below 1 mJ. Hybrid systemsare those containing flammable vapors in addition todusts.

Under some circumstances inerting may also be required;Under some circumstances inerting may also be required;inerting is the concept of decreasing the concentration ofoxygen to below the level required to supportcombustion (called the limiting oxygen concentration,LOC).

MIE

Further information on the flammability ofdusts and gases can be found in NFPA61A, B, C, and D; 77, and 650.

Charge Accumulation

It is important to understand the concept of chargeaccumulation, because one method to preventstatic electricity ignitions is to prevent theaccumulation of charges. There are fourmethods for accumulating charges:methods for accumulating charges:

• Contact and Frictional,

• Double Layer,

• Induction, and

• Transport.

Contact and Frictional Charging

Contact and Frictional Charging occursduring:

• Dust transport; e.g., pneumatic transport.

• Pouring powders; e.g., down chutes.• Pouring powders; e.g., down chutes.

• Gears and belts; e.g., transporting chargesfrom one surface to another.

Double Layer Charging

Double layer charging is essentially frictionat interfaces on a microscopic scale.

• liquid—liquid

• solid-liquid• solid-liquid

• solid-solid

• gas-liquid

• gas-solid

Induction Charging

Induction charging is best described with an example: Ifyou walk up to a large metal vessel that is positivelycharged due to a previous charging operation, yourbody's electrons will migrate towards the positivelycharged vessel. If you touch the vessel, some of yourcharged vessel. If you touch the vessel, some of yourbody's electrons will be transported (spark) to the vessel.Your hand was positively charged by induction. Theresults could be a static electricity ignition. This exampleassumes that your shoes are nonconductive and/or thefloor is nonconductive.

Induction Charging

Charging by Transport

Charging by transport is the result ofcharged dust, liquid, or solid particlessettling onto a surface and transportingtheir charges to this new surface. In eachtheir charges to this new surface. In eachof these charge accumulation processes,the rate of charge accumulation is afunction of the rate of transportation.Lightening is an example of this type ofcharging phenomenon.

Electrostatic Ignitions

Electrostatic ignitions are the result oftransferring the accumulated charges toanother surface. If the energy of thisdischarge exceeds the MIE of a flammabledischarge exceeds the MIE of a flammablegas or dust, then the result is a fire orexplosion.

Electrostatic Ignitions

There are four types of static electricitydischarges which are relevant to dustcloud ignitions.

While their propensity to ignite dust cloudsWhile their propensity to ignite dust cloudsvaries, they would all have dischargeenergies >1mJ, so they would all beexpected to ignite flammable vapors andgases.

Propagating Brush Discharge

Propagating brush discharges are veryimportant; they are believed to be themajor contributor of static electricityignitions. They occur between a conductorignitions. They occur between a conductorand a nonconductive lining on anotherconductive surface.

Brush Discharge

• Between Non-Conductor and a Conductor

• Energetic (E < 5 mJ)• Energetic (E < 5 mJ)

• Breakdown Voltage > 4 kV

Brush Discharge (Con.)

• Lining Thickness Greater Than 2 mm

• Can Ignite Flammable Vapors and Rarely

Dusts

Conical Pile Discharge(Maurer Discharge)

• Between Sliding Solids and Charged Air

• Volume Greater than 1 m3

• Energetic ( E < 1 Joule)

• Powder with R > 1010 Ohm-m

Energy and Capacitance

The calculation methods for handling liquidsare well known. Methods for handlingsolids are not published as broadly. Thisslide shows the most important equationsslide shows the most important equationswhich are used when handling solids.

Energy and Capacitance

The objectives are usually:

(a)determine the accumulated charge, Q,expressed in coulombs,

(b)determine the capacitance, C, of the(b)determine the capacitance, C, of theobject or container contents, expressed infarads or coulombs per volt, and

(c)then compute the accumulated energy, E,expressed in joules (J) or milijoules (mJ).

Energy and Capacitance

This accumulated energy is then comparedto the minimum ignition energy (MIE) ofthe dust; this MIE is an experimentallydetermined property of the dust beingdetermined property of the dust beinghandled. This slide shows the calculationsfor the capacitance of a sphere,capacitance of a plate, and variousequations for the energy.

Energy and Capacitance

• Energy:

• Capacitance:

2 2

2 2 2

Q C V QVE

C

QV

C

• Capacitance of Sphere:

• Capacitance of Plates:

04r

rC

0r

AL

C

Energy and Capacitance

For a sphere, the capacitance is:

C = 4pereor

where,

C = the capacitance, farads or coulomb/voltC = the capacitance, farads or coulomb/volt

er = the relative dialectric constant which isthe property of a liquid or gas, unitless

Energy and Capacitance

eo = the permittivity constant; i.e.: =2.2x10-12

coul/volt • ft

or

= 8.8 x 10 -12 coul/volt m= 8.8 x 10 -12 coul/volt m

or

= 8.85 x 10-14 s/ohm cm

r = the radius, m

s =the time, seconds.

Energy and Capacitance

For a flat plate, the capacitance includes anA/L term where:

A = the plate surface area, m2A = the plate surface area, m2

L = the thickness of the plate, m.

Energy and Capacitance

The energy is computed using one of thesethree equations, where:

E = energy, joules

Q = charge, coulombs V is voltage, voltsQ = charge, coulombs V is voltage, volts

C = capacitance, coulombs/volt.

Energy and Capacitance

The accumulated charge is determined by

(a) multiplying the charge capacity for

a specific operation, coulomb per kg,

(b) times the feed rate, kg per second, and then(b) times the feed rate, kg per second, and thenmultiplying this charge rate, coulomb persecond, times the duration of the operation, s.

The next slide illustrates the charging capacity forvarious commonly used process operations.

Energy and Capacitance

The total accumulated charge is computedin coulombs by multiplying the charge

capacity, times the charging rate, times thetime of the operation:time of the operation:

Q = ( Charge capacity) ( Charge rate) ( time)

Q = (coulombs/kg)( kg/s)(s)

Q = coulombs

Accumulated Charge

Q= Charge Capacity x Charge

( )( )( )Coulombs kgQ s( )( )( )kg s

Q s

Q Coulombs

Accumulated Charge

Example: Determine the potential hazard ofpneumatically transporting a dry powder(dry powder with a particle size greater(dry powder with a particle size greaterthan 1 mm) at a rate of 30,000 kg/hr into ametal vessel which has a volume of 70 m3

Accumulated Charge

Given: The powder has a bulk density of 600kg/m3; the vessel has a spherical geometry; and70 m3 of the powder is charged into this vessel.The powder is flammable with an MIE of 20 mJ.

As illustrated, the radius of this vessel is 2.5 m,and the capacitance is 2.83 x 1040 coulomb pervolt. Notice that the dielectric constant ( er.) is forair, er =1, because a static electric dischargejumps across an air gap.

Accumulated Charge

The calculations on this slide use a chargingcapacity of 10-5 coul/kg that correspondsto the pneumatic transportation process.The final accumulated energy is 3.1 x 108The final accumulated energy is 3.1 x 108

joule which needs to be compared to theMIE of the powder (20 mJ). If there issufficient air (above the LOC) in thevessel, this operation would be veryhazardous. Notice that this energy is verylarge.

Prevent Ignitions

• This type of problem is minimized oreliminated by:

• Inerting

• Static Neutralizers

1. Passive Corona Device

2. Active Corona Device

3. Ionizing Blowers

Prevent Ignitions

Inerting is the process of decreasing theoxygen concentration to below the LOC byadding an inert gas; for example, nitrogenor carbon dioxide.or carbon dioxide.

Proper inerting to an oxygen level below theLOC eliminates the possibility of the dustexploding.

Prevent Ignitions

The Limiting Oxygen Concentration (LOC) isthe minimum oxygen required forcombustion of a dust cloud at anycombustion of a dust cloud at anyconcentration.

Prevent Ignitions

It is related to the particular dust anddetermined experimentally.

Values usually run in the range 8-15%.Values usually run in the range 8-15%.

Prevent Ignitions

Related to the LOC concept, is the concept ofoperating under vacuum, where appropriate.Vacuum operation is an alternate way ofVacuum operation is an alternate way ofreducing oxygen availability for combustion.

The vast majority of dusts will not burn ifatmospheric pressure is below about 40 mm Hg.

Prevent Ignitions

Passive corona devices are grounded rods orwires which are structurally mounted in the silo.Barbed wire has also been used successfully.Narrow diameter rods or wires (1mm in dia. orless) induce corona discharges which are aless) induce corona discharges which are adiffuse type discharge with energies lower thanthe MIE of dusts. Rods with larger diameters willinduce brush discharges which may haveenergies exceeding some dust MIEs. In allcases metal grounded and bonded constructionis necessary.

Prevent Ignitions

In one case history, a piece of tramp metal wasaccidentally dropped into the feed of a powder bin. Themetal tumbled down the solids pile and eventuallyapproached the grounded metal wall. A spark ignited aflammable dust suspension which produced a significantflammable dust suspension which produced a significantexplosion. This type of explosion has generatedimportant design standards for powder handlingsystems; i.e.: (a) use screens in powder chargingsystems to trap tramp metal and/or (b) use magnetictramp metal traps (good, of course, only for magneticmetals).

Prevent Ignitions

The following calculations illustrate thepotential for this tramp metal toaccumulate enough energy to ignite apowder, say a powder with a MIE of 10powder, say a powder with a MIE of 10mJ. It is assumed that this tramp metal is awrench with an equivalent diameter of 8cm (3.1 inch).

Prevent Ignitions

Similar to a previous calculation, thecapacitance is determined to be 4.4 x 10-12

coulomb per volt. The relative dielectricconstant, e , is again equal to one (for air)constant, er , is again equal to one (for air)

because the spark jumps through air.

Prevent Ignitions

The maximum field intensity is an empiricalvalue; it is 3 MV per m in air. As illustrated,this is converted to coulombs per m2 bymultiplying it by a unit conversion factormultiplying it by a unit conversion factor(8.8 x 10-12 coul/(V•m)).

Prevent Ignitions

The additional calculation computes theaccumulated energy in joules, whichcorresponds to this maximumaccumulated charge. As shown, thisaccumulated charge. As shown, thiscomputed energy, 33 mJ, exceeds theMIE of this dust.

Guidelines

How to Prevent Discharges:

1. Sparks: Ground and Bond

2. Propagating Brush: Keep U < 4 kV2. Propagating Brush: Keep Ud < 4 kV

3. Brush: Ud < 4 kV and Coating

Thickness < 2 mm

Guidelines

The preventive measures listed previously address

sparks from causes other than mechanical failure.

The “Deadly Dust III” video doesn’t cover the effects of

specific mechanical failure.

Explosion prevention for these types of events can be

effectively achieved by a strong Mechanical Integrity

element of a facility’s process safety management

program.

GuidelinesSpark discharges between two conductors can be

prevented by eliminating the air gap between them,

that is, bonding the two conductors.

Guidelines (Cont.)

When to Use Inerting:

• Hybrid Systems• Hybrid Systems

• Systems with Conical Discharges

Recommendations

• Understand Fundamentals and Apply

Guidelines

• Know Physical Characteristics of Dusts• Know Physical Characteristics of Dusts

Recommendations (Cont.)

• Use Team Approach to Solve Problems

• When in Doubt – Use Specialists• When in Doubt – Use Specialists