Static Electricity and Charge Accumulation. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation.

Static Electricity and Charge Accumulation

Static electricity & charge accumulation

DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling

Balance of chargesBonding and groundingCase studies

Definitions - Types of materialsConductive– A material incapable of retaining a significant electrostatic

charge when in contact with earth and having a volume resistively equal or lower than 104Ω•m

Dissipative– A material incapable or retaining a significant amount of

electrostatic charge when in contact with earth and having a volume resistivity higher than 104Ω•m but equal to or lower than 109Ω•m measured at ambient temperature and 50% relative humidity.

Non-conductive– A material having a volume resistivity higher than 109Ω•m

Spark discharges

Discharging of static electricity between two conductors.

Spark Discharge

Generation of Spark Discharges.– Charge accumulation at a

conductive object.– Field strength exceeds the electric

strength of the ambient atmosphere.

Ignitability--gases, vapors, dustsEnergy transfer--up to 10,000 mJ

Brush discharge

Brush Discharge

Generation of Brush Discharges– Conductive electrode moves towards

a charged nonconductive object.

Nonconductive lining or surface must have a breakdown voltage greater than 4 kV and a thickness greater than 2 mm.Nonconductive coating can be a layer of the powdered solid.Ignitability--gases, vaporsEnergy transfer--up to 4 mJ

Propagating Brush Discharge

Propagating Brush Discharge

Generation of Propagating Brush Discharge– Bipolar charging of the high resistivity

material (non conducting) that is lining another conductor.

– Field strength exceeds the electric strength of the high resistivity material. Non conducting lining must have breakdown voltage greater than 4 kV

Ignitability--gases, vapors, dustsEnergy transfer--up to 100,000 mJMajor contributor to static electricity ignitions.

Cone Discharge

Cone Discharge

Generation of Cone Discharge.– Vessels larger than 1 m3.– Relatively fast filling rate, greater than

0.5 kg/s.– High resistivity (>1010Ωm) bulk product,

larger than 1 mm diameter.– Charge accumulation in the bulk

product.– Field strength exceeds the electric

strength of the ambient atmosphere.

Ignitability--gases, vapors, dustsEnergy transfer--up to 1000 mJ

Ignitability of discharges

Type of Discharge Energy transfer IgnitabilitySpark < 10,000 mJ gases, vapor, dusts

Brush < 4 mJ gases, vapor

Propagating Brush < 100,000 mJ gas, vapor, dusts

Cone < 1,000 mJ gas, vapor, dusts

Corona < 0.1 mJ some gases with low MIE




Charge Accumulation

Whenever two dissimilar materials come in contact, electrons move from one surface to the other. As these materials are separated and more electrons remain on one surface than the other,one material takes on a positive charge and the other a negative charge.Mechanisms for Charge Accumulation:– Contact and Frictional– Double layer– Induction– Transport

Contact and Frictional Charging

Dust transport– e.g. pneumatic transport of powders/solids

Pouring powders– e.g. pouring solids down chutes or troughs

Gears and belts– e.g. transporting charges from one surface to

another

Double layer charging

Caused by friction and movement at interfaces on a microscopic scale.– Liquid-liquid– Solid-liquid– Solid-solid– Gas-liquid– Gas-solid

Induction charging

When an isolated conductor is subject to a electric field a charge polarity develops on the object. If the object is grounded then the charges closest to the grounding source flows away leaving the body with a net charge of opposite sign.

Charging by Transport

Results from a charged dust, liquid or solid particles settling onto a surface and transporting their charges to this new surface.The rate of charge accumulation is a function of the rate of transportation.

Fluid handling operations

Many fluid handling operations can generate static electricity. This becomes a problem when non conducting pipes (glass or Teflon lined) are used without adequate bonding.

Fluid flow into vessels

When fluid flows into a vessel it carries a charge with it which can build up in the tank if the tank is not properly grounded.Routine inspection of grounding minimizes the change for fire or explosion due to a spark discharge from the charged tank.

Splash Filling

When non conducting fluids (or solids) free fall through air they pick up a significant static charge.When there is spraying or splashing static electricity can build up.This can be a source of sparks

Spraying of Liquids

When fluids are spayed in air a static charge can built up fairly rapidly in some fluids. Non-conducting fluids typically build up static charge more rapidly.




Streaming current

When a liquid or solid is flowing, there is a transfer of electrons from one surface to another as they flow past each other.

Streaming currentFor fluids the streaming current, Is, is calculated using Eq. 7-12 for laminar flows.

124.24 10

Re

where

is Fanning friction factor Eq. 4-24 to 4-29

Re Reynolds number

is dielectric constant Table 2.1

is zeta potential values of 0.01 to 0.1 (wors

s r

r

ampI f u

ft volts

f

du

t case)

Streaming currentFor turbulent flow, use Eq. 7-14.

14

r 0

5.89 10

where

is the double layer thickness

is the molecular diffusivity

is the relaxation time

is the specific conductivity mho/cm (Table 7-1)

rs

m

m

c

c

amp d uI

ft volts

D

D

Electrostatic Voltage Drops

For flow through a non conducting pipe (glass, Teflon lined) a voltage drop can develop from flowing liquid.

Where is calculated from the conductivity of the fluid

Where:

is the length of non conducting pipe

is the specific con

s

C

C

V I R

R

LR

A

L

ductivity of the fluid (Table7-1)

is the cross sectional flow areaA

Charge Accumulation from Is

Charges can accumulate as a result of streaming current:

Assuming constant streaming current

s

s

dQI

dt

Q I t




Accumulated charge from solid handling

Solid geometries are almost always ill defined, so need to be based on empirical calculations. Solid processing operations have different empirically determined charge capacities.

Q=Charge Capacity X Charge Rate X time

coulombs kgQ s

kg s

Charge capacities – solids handling

Table 7-5Process Charge Capacity (coulombs/kg)Sieving 10-9 to 10-11

Pouring 10-7 to 10-9

Grinding 10-6 to 10-7

Sliding down incline 10-5 to 10-7

Pneumatic transport 10-5 to 10-7

Capacitance

0 0

Capacitance For a Sphere For Plates

Q C= 4

Vwhere

C is the capacitance, farads or /

is

r r

r

AC r C L

coulomb volt

-12 12 140

the relative dielectric constant which is a property of the liquid or gas (Table 7-1)

is the permittivity constant=2.2 10 8.85 10 8.85 10

is the radius of the sp

coul coul s

volt ft volt m cm

r

2

here in

is the plate surface area in

is the thickness of the plate in

is charge in

is voltage in

m

A m

L m

Q coulomb

V volt

Capacitance of Various Objects

Table 7-6Object Capacitance (farad)Small scoop 5 x 10-12

Bucket 10 x 10-12

Barrel 100 x 10-12

Person 200 x 10-12

Automobile 500 x 10-12

Tank Truck 1000 x 10-12

Static Energy Stored

22

22

coulomb units ( )

2

units ( )2

units ( )2

QE coulomb volt Joule

coulombCvolt

CV coulombE volt coulomb volt Joulevolt

QVE coulomb volt coulomb volt Joule

Calculations

Determine the capacitance, C, of the object or container contents, expressed in farads or coulombs per volt.Determine the accumulated charge, Q, expressed in coulombsCompute accumulated energy, E, expressed in J or mJ.Compare to the MIE of the dust or vapor.

Example – Solids handling

Determine the potential hazard of pneumatically transporting a dry powder (dry powder with a particle size greater than 1 mm) at a rate of 30,000 kg/hr into a metal vessel which has a volume of 70 m3.Given: The powder has a bulk density of 600 kg/m3; the vessel has a spherical geometry; 70 m3 of powder is charged into the vessel. The powder is flammable with a MIE of 20 mJ.

Example – solids handling - solution

Determine radius of sphere:

Calculate capacitance:

11 3 333 3 702.5

4 4

V mr m

0

12 10

4

= 1 for air (Table 7-1) Spark jumps across air gap

4 (1) 8.85 10 2.5 2.83 10

r

r

C r

coul coulC mvolt m volt

Example - solids handling - solution (cont.)

Determine mass fed:

Calculate charge accumulated (Table 7-5)

33

70 600 42,000kg

Feed m kgm

510 42,000 0.42coulQ kg coulkg

Example - solids handling - solution (cont.)

Calculate energy:

This is much greater than the MIE of the powder. If there is sufficient air this would be very hazardous.This is the total charge that could go into vessel while filling. Multiple discharges would occur, certainly there would be conical pile discharges (unless grounded).

228

10

0.423.1 10

2 2 2.83 10

coulQE J

coulCvolt

Example – Fluid Handling

Determine the voltage developed between a charging nozzle and a grounded tank and the charge accumulated during the filling process at 150 gpm.

Example – Fluid Handling (cont.)

Additional information:– Non conducting hose length 20 ft– Hose diameter 2 in.– Liquid conductivity 10-8 mho/cm– Liquid diffusivity 2.2x10-5 cm2sec-1

– Dielectric constant 25.7– Density 0.88 g/cm3

– Viscosity 0.60 centipoise– MIE 0.10 mJ

Example – fluid handling - solution

Procedure

Calculate voltage drop using V=IsR (Eq. 7-17)

Calculate R using Eq. 7-18Calculate Is using Eq. 7-12 or 7-14

Calculate Q using Q=Ist

Calculate E=(QV/2)

Compare to MIE

Example – fluid handling – solution (cont.)

Calculate the Resistance

222 2

9

8 2

12 . 2.54(20 ) 610.

3.541 . 20.3.610

3.00 1010 20.3C

in cmL ft cmft in

cmA r in cminL cm

RA cmcm


Determine type of flow (laminar or turbulent)

3 2

2 2

3

3

150 144 1minmin 15.37.48 601 .

2 15.3 0.88 7750Re 348,000

0.60

Hence Turbulent

gallonft in ftu sgal ft sin

ft gindu s cpcmft gcp in s cm


Calculate the streaming current:

14

50

8

2-5 5 -5 -5

14

14

25.7 8.85 1022.7 10 (Eq. 7-16)

10

= 2.2 10 22.7 10 = 7.07 10 = 2.78 10 . (Eq. 7-15)

5.89 10 (Eq.7-14)

5.89 10

r

C

m

rs

s

scm s

mhocm

cmD s cm ins

amp d uI

ft volts

ampI

7

-5

2 153 25.7 0.11.66 10

2.78 10

ftin voltsamp

ft involts

Example – fluid handling – solution (cont.)Calculate the voltage drop, accumulated charge and energy:

7 9

5 5

5

(1.66 10 ) 3.00 10 498 (Eq. 7-17)

Determine fill time

300 60 min 120150 min

Determine accumulated charge

1.66 10 120 1.99 10

Determine Energy

1.99 10

2

s

s

V I R amp volts

sgalt s

gal

Q I t amp s coulomb

couloQVE

4984.9

2This is greater than the MIE, so there is a fire or explosion hazard

mb voltmJ




Balance of Charges

When you have a vessel that has multiple inputs and outputs, you can determine the charge accumulation by a charge balance.Consider streaming currents in, charges carried away by flows going out, and charge loss due to relaxation.

Charge Balance

, ,1 1

,1

,1

where

is the current coming into the vessel

is the current flowing out of the vessel

is the charge loss due to relaxation

is the relaxation ti

n m

s si in j outi j

n

s i ini

m

s j outj

dQ QI I

dt

I

I

Q

me

Charge Balance

,

The charge flowing out of the vessel depends

on the charge already in the tank

where

is the rate of discharge through outlet

is the container or tank volume

is the total char

js j out

C

j

C

FI Q

V

F j

V

Q

ge in the tank

Charge Balance

,1 1

,

Hence the charge balance becomes

If flows, , streaming currents, , and relaxation times, ,

are constant, then this is a linear di

n mj

s i ini j C

j s i in

FdQ QI Q

dt V

F I

, ,1 1

01

1 1

fferential equation that has

the solution:

where

1

1 1

Ct

n n

s s mi in i inji i

m mj Cj j

j jC C

Q A Be

I IF

A B Q CVF F

V V

Charge Balance

This relationship is used to determine the charge developing in the tank as a function of time relative to an initial charge of Q0.The capacitance of the vessel is calculated as before (typically assume equivalent spherical vessel).The static energy stored in the vessel is then calculated from E=Q2/2C.Examples 7-9 and 7-10 demonstrate using this relationship.




Bonding and Grounding

Charge buildup is always possible when you have moving fluids or solids. The potential for discharge is always present.We can eliminate sparks if we ensure that all parts of the system are connected with a conductor

Bounding and Grounding

Historically there was little problem when piping was all copper, stainless steel or iron. The problem comes when pipes or vessels are glass or Teflon lined or made from polymers or connected with non-conducting gaskets.There has always been a problem when you are pouring either liquid or a solid through an open space i.e., a filling operation.


Bonding– Is the connection of a

conducting wire between two or more objects.

– The voltage difference between the two objects is reduced to zero, however they may have a voltage difference relative to ground or another non connected object

Grounding– Is the connection of a

conducting wire between a charged object and the ground.

– Any charge accumulated in the system is drained off to ground.


Figure 7-7 and 7-8 should say “non” conductive hose.





Grounding Glass-lined Vessels

Glass and plastic lined vessels are grounded using tantalum inserts or a metal probe.This is less effective if fluid has low conductivity.

Dip Legs to Reduce Splash Filling

To eliminate the static charge that builds up from a fluid free falling through air, a dip leg is used. Note hole to prevent back siphoning.An angle iron can also be used so fluid runs down the angle iron instead of free falling.




Case Studies froma production plant

Following are a series of case studies of accidents that actually happen at BASF and Dow and shared with the SACHE Chemical Process Safety Workshop participants.

Solids Filling OperationSituation– A non-conductive bulk product is fed

out of 25 kg PE-bags in a vessel, in which a flammable liquid is being stirred. During shaking of the the just empty bag an ignition occurred.

Cause– All handling of non-conductive solids

or bulk products may generate static electricity. Due to contact charging of the sliding bulk product, both the bulk product and non conducting package materials became charged. Brush discharges form the surface of the bad ignited the vapor/air mixture.

Precaution– Either fill into a closed, inerted

vessel or avoid charge generation.

OperatorSituation– An operator filled a non-conductive bulk

product out of 25 kg PE-bags in a solvent free mixer. Exhaust system operated. All equipment grounded, the floor was dissipative, the operator wore dissipative footwear. During pouring the product in the reaction vessel explode.

Cause– The plastic wrap that held the sacks on

the pallet was on the floor and the operator was standing on it. This allowed a static charge to build up in him.

Precaution– Always guarantee ground connection.

ValveSituation– A ball-valve is installed in a waste gas

collecting system. During usual production an explosion occurred; the pipe system was destroyed.

Cause– A valve consists of conductive and

non-conductive parts. Conveying of dust suspensions or droplets may generate charge accumulation on the ball and/or shaft if not bonded to the grounded housing. Spark discharge from charged ball to housing caused explosion.

Precaution– Guarantee ground connection of

conductive equipment.

Lined metal drum fillingSituation– A pure liquid was filled in a steel drum with

an inner plastic liner. To avoid splash filling a short funnel was inserted in the spout. The nozzle, the drum and the weighing machine were all grounded. Despite having an exhaust system there was an explosion during drum filling.

Cause– Electrostatic charge generation at the

surface of the non-conductive coating cannot be transferred. The funnel had sufficient capacitance was insulated from the ground by the PE lined filler cap. Spark discharge from funnel caused explosion.

Precautions– Guarantee ground connection of all

conductive equipment.

PE-drum fillingSituation– A mixture of water and hydrocarbon was

separated; the water phase was released from time to time into a PE-drum located below the separator. During such a release a fire occurred on top of the PE-drum.

Cause– Splash filling the PE-drum generated charge

accumulation at the wall material. The unintended release of a small amount quantity of hydrocarbon generated a flammable atmosphere in the drum and an ignition by brush discharges occurred.

Precaution– Install a level indicator so that an unintended

release of hydrocarbons does not occur.

Liquid AgitationSituation– After intense mixing, a non-conductive flammable

dispersion was poured from the mixing vessel into a PE-drum just positioned below. The exhaust system was in operation, and to avoid charge accumulation a grounded rod was inserted. During drum filling a fire occurred.

Cause– Intense stirring of non-conductive liquids or

multiphase liquids leads to charge accumulation. Splash filling in the non-conductive drum led to high charge accumulation on the inner walls of the drum and brush discharges from wall to grounded rod.

Precaution– Need to have another exhaust system and filling

method since an explosive atmosphere and static electricity are formed at the same time in the same location.

Super sack filling operationSituation– A reactor vessel was purged with N2 and feeding

toluene was started. During the feeding operation a resin was prepared for pouring from an “antistatically treated” super sack via the filling port. The exhaust system was operating. Just at the beginning of pouring the bulk product into the vessel, an explosion occurred.

Cause– Charge build up was generated both by splash filling

the liquid and pouring the bulk product. Flammable atmosphere in the gas space of the vessel was avoided by N2 purging, but the fast release of the bulk product ejected toluene/dust/N2 mixture up into the air where ignition occurred from either a spark discharge from the charged-insufficiently treated-super sack or charged operator by brush discharge.

Precaution– Only packaging with sufficient antistatic treatment

should be used.

Filter basketSituation– A fine pigment was conveyed pneumatically

from a jet mill to a filter. The product settled in the filterhousing was set on fire and transported through the rotary valve in a silo. All conductive parts were properly grounded.

Cause– The pneumatic conveying and the collection of

charged fine particles usually generates high charge accumulation in filters. Extremely high charging at the rubber coating of a metal flange generated a propagating brush discharge. Settling particles were ignited and fell into the powder heap.

Precaution– In systems where high charging rates are

possible, the combination of conducting and non-conducting materials must be avoided. Replace rubber gasket with a conducting one.

Maintenance of a level indicatorSituation– A level indicator at a pressurized vessel was

blocked. Usual maintenance procedure is the fast release of product in a pail until the connection between indicator and vessel is cleared. During such a procedure a fire occurred and two persons were injured.

Cause– The release of a pressurized liquid generates highly

charged droplets thus generating both an explosive atmosphere in the surrounding and brush discharges between the opened valve and the surface of the non-conducting pail used.

Precautions– For effective cleaning a fast release is required. To

avoid ignition the procedure needs to be changed to discharge the pressure in a waste gas collecting system.

Case Studies

Those who ignore history are doomed to repeat it.Those who ignore case studies are likely to repeat the same operational behavior and are doomed to experience the near miss, the serious, or the fatal accident.

Static Electricity and Charge Accumulation. Static electricity & charge accumulation Definitions Types of discharges Mechanisms of charge accumulation.

Documents

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