Download Static Electricity and Charge Accumulation

Static Electricity and Charge
Accumulation
Static electricity & charge accumulation
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case studies
Definitions - Types of materials
Conductive
– 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, dusts
Energy 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, vapors
Energy 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, dusts
Energy transfer--up to 100,000 mJ
Major 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, dusts
Energy transfer--up to 1000 mJ
Ignitability of discharges
Type of Discharge
Energy transfer Ignitability
Spark
< 10,000 mJ
gases, vapor, dusts
Brush
<
gases, vapor
Propagating Brush
4 mJ
< 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
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case 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. Nonconducting fluids
typically build up static
charge more rapidly.
Static electricity & charge accumulation
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case 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 current
For fluids the streaming current, Is, is calculated
using Eq. 7-12 for laminar flows.


12
 4.24 10 amp 
Is  
 f  Re u   r  

ft volt


s


where
 
f is Fanning friction factor Eq. 4-24 to 4-29
du 
Re 
Reynolds number

 r is dielectric constant Table 2.1
 is zeta potential values of 0.01 to 0.1 (worst case)
Streaming current
For turbulent flow, use Eq. 7-14.


14
 5.89  10 amp  d  u   r  
Is  


ft volt



s


where
 is the double layer thickness
 
  Dm
Dm is the molecular diffusivity
 r 0
 is the relaxation time 
c
 c is the specific conductivity mho/cm (Table 7-1)
Electrostatic Voltage Drops
For flow through a non conducting pipe (glass,
Teflon lined) a voltage drop can develop from
flowing liquid.
V  Is R
Where R is calculated from the conductivity of the fluid
L
R
CA
Where:
L is the length of non conducting pipe
 C is the specific conductivity of the fluid (Table7-1)
A is the cross sectional flow area
Charge Accumulation from Is
Charges can accumulate as a result of
streaming current:
dQ
 Is
dt
Assuming constant streaming current
Q  I st
Static electricity & charge accumulation
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case 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   kg 
Q
   s

kg

 s 
Charge capacities – solids handling
Table 7-5
Process
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
Capacitance
Q
C=
V
where
For a Sphere
For Plates
C  4 r 0 r
C   r 0 A
L
C is the capacitance, farads or coulomb / volt
 r is the relative dielectric constant which is a property of the liquid or gas (Table 7-1)
 0 is the permittivity constant=2.2  10-12
r is the radius of the sphere in m
A is the plate surface area in m 2
L is the thickness of the plate in m
Q is charge in coulomb
V is voltage in volt
coul
coul
s
 8.85  1012
 8.85  1014
volt  ft
volt  m
  cm
Capacitance of Various Objects
Table 7-6
Object
Small scoop
Bucket
Barrel
Person
Automobile
Tank Truck
Capacitance (farad)
5 x 10-12
10 x 10-12
100 x 10-12
200 x 10-12
500 x 10-12
1000 x 10-12
Static Energy Stored
2
Q
E
2C
CV 2
E
2
QV
E
2
coulomb 

units 
 (coulomb  volt )  Joule
coulomb
volt
2


units   coulomb
volt 
volt  
2
 (coulomb  volt )  Joule
units   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 coulombs
Compute 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:
 3V 
r

4



1
3
 3  70m 


4



3
1
3
 2.5m
Calculate capacitance:
C  4 r 0r
 r = 1 for air (Table 7-1) Spark jumps across air gap

C  4 (1) 8.85  1012 coul
volt  m

10 coul
2.5
m

2.83

10


volt
Example - solids handling - solution (cont.)
Determine mass fed:
kg
Feed  70m  600 3  42,000kg
m
3
Calculate charge accumulated (Table 7-5)

Q  105 coul
kg
 42,000kg   0.42coul
Example - solids handling - solution (cont.)
Calculate energy:
0.42coul 

Q
E

2C 2 2.83  1010 coul
2
2

volt

 3.1  108 J
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).
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
Hose diameter
Liquid conductivity
Liquid diffusivity
Dielectric constant
Density
Viscosity
MIE
20 ft
2 in.
10-8 mho/cm
2.2x10-5 cm2sec-1
25.7
0.88 g/cm3
0.60 centipoise
0.10 mJ
Example – fluid handling - solution
Procedure
Calculate voltage drop using V=IsR (Eq. 7-17)
Calculate R using Eq. 7-18
Calculate 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
2.54cm   610cm

 ft 
in.
  1in.  3.54cm   20.3cm
in.
L  (20 ft ) 12in.
A r
2
2
2
L
R

 C A 108

610cm
cm

2
20.3
cm


2
 3.00  109 
Example – fluid handling – solution (cont.)
Determine type of flow (laminar or turbulent)
 150 gallon

3
2



  1min 
ft
144
in
ft
min


u

15.3
s
  1in. 2
  7.48 gal   ft 2   60s 



2in  15.3 ft
0.88 g 3 



du 
s
7750cp
cm
Re 

 348,000


ft
g

 0.60cp 
 in s
cm3 



Hence Turbulent

 

Example – fluid handling – solution (cont.)
Calculate the streaming current:

 r 0

C
 25.7  8.85  1014 s cm   
  Dm =
108 mho

2
cm
2.2  10
-5
cm
22.7  10 s  = 7.07  10
s 
5


14
 5.89  10 amp  d  u   r  
Is  


ft volt



s


 
 22.7  105 s (Eq. 7-16)

-5
cm = 2.78  10 -5 in. (Eq. 7-15)
(Eq.7-14)


  2in  153 ft  25.7  0.1volt 
14
s
 5.89  10 amp 
7
Is  


1.66

10
amp
-5

ft volt
2.78  10 in 



s


 
Example – fluid handling – solution (cont.)
Calculate the voltage drop, accumulated charge and energy:
V  I s R  (1.66  107 amp )  3.00  109    498volts (Eq. 7-17)
Determine fill time
t
 300 gal   60 s min 
150 gal
 120s
min
Determine accumulated charge
Q  I s t  1.66  105 amp  120s   1.99  105 coulomb
Determine Energy
5
QV 1.99  10 coulomb   498volt 
E

 4.9mJ
2
2
This is greater than the MIE, so there is a fire or explosion hazard
Static electricity & charge accumulation
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case 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
m
dQ n
Q
   I s i ,in    I s  j ,out 
dt i 1

j 1
where
n
I 
i 1
s i ,in
is the current coming into the vessel
m
I 
j 1
Q
s
j , out
is the current flowing out of the vessel
is the charge loss due to relaxation

 is the relaxation time
Charge Balance
The charge flowing out of the vessel depends
on the charge already in the tank
Fj
 I s  j ,out  Q
VC
where
Fj is the rate of discharge through outlet j
VC is the container or tank volume
Q is the total charge in the tank
Charge Balance
Hence the charge balance becomes
m F
dQ n
Q
j
   I s i ,in   Q 
dt i 1

j 1 VC
If flows, Fj , streaming currents,  I s i ,in , and relaxation times,  ,
are constant, then this is a linear differential equation that has
the solution:
Q  A  Be  Ct
where
n
A
I 
i 1
s i ,in
 1 m Fj 
  
  j 1 VC 
n
I 
 1 m Fj 
B  Q0 
C   
m
 j 1 VC 
Fj 
1

  
  j 1 VC 
i 1
s i ,in
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
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case 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
Definitions
Types of discharges
Mechanisms of charge accumulation
– fluid systems - Streaming current
– Solids handling
Balance of charges
Bonding and grounding
Case studies
Case Studies from
a 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.
Situation
Solids Filling Operation
– 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.
Operator
Situation
– 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.
Situation
– 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.
Valve
Lined metal drum filling
Situation
– 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.
Situation
PE-drum filling
– 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.
Situation
Liquid Agitation
– 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.
Situation
Super sack filling operation
– 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 treatedsuper sack or charged operator by brush discharge.
Precaution
– Only packaging with sufficient antistatic treatment
should be used.
Filter
basket
A fine pigment was conveyed pneumatically
Situation
–
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.
Situation
Maintenance of a level indicator
– 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.