Date post: | 15-Jan-2016 |
Category: |
Documents |
Upload: | halle-parden |
View: | 250 times |
Download: | 1 times |
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
Static electricity & charge accumulation
DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling
Balance of chargesBonding and groundingCase studies
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.
Static electricity & charge accumulation
DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling
Balance of chargesBonding and groundingCase studies
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
Static electricity & charge accumulation
DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling
Balance of chargesBonding and groundingCase studies
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
Example – fluid handling – solution (cont.)
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
Example – fluid handling – solution (cont.)
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
Static electricity & charge accumulation
DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling
Balance of chargesBonding and groundingCase studies
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.
Static electricity & charge accumulation
DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling
Balance of chargesBonding and groundingCase studies
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 and Grounding
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.
Bounding and Grounding
Figure 7-7 and 7-8 should say “non” conductive hose.
Bounding and Grounding
Bounding and Grounding
Bounding and Grounding
Bonding and Grounding
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.
Static electricity & charge accumulation
DefinitionsTypes of dischargesMechanisms of charge accumulation– fluid systems - Streaming current– Solids handling
Balance of chargesBonding and groundingCase studies
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.