When dissimilar materials come in contact and separate,
(i.e. they are in motion and maintain contact with each
other), there is transfer of electrons from one material
to the other. There is build up (accumulation) of electrons
on the other material. The excess of electrons is called
negative charge and deficiency of electrons is called
When very heavy cloud of charge tries to break the air
insulation (or mixture of air & gas/vapors) and
find its way to earth, it results into static electricity.
Flow of charged cloud to the earth causes spark. In
the hazardous area (containing hydrocarbon gas or vapours),
this spark can cause explosion or fire.
If one of the materials is non-conductor or electrically
insulated conductor, static electrical charge can accumulate
Opposite charges attract each other and same charges
repel each other. Attraction can be measured as a potential
difference or voltage. Larger the voltage, greater the
flow of electrons across the air gap, causing larger
Charge generation will take place during various processes
- Rubbing wool on plastic
- Pouring bulk material out of bag
- Unrolling of a plastic or PVC
- Flow of liquid through a pipeline
- Spraying of a paint or impact
of dust particles on an equipment wall.
Measurable charge will
be created only if one of the materials involved is
non-conductor. All solids having surface resistance
higher than 1011 ohms are regarded as non-conductive
2.0 There are various types of surfaces in contact,
resulting in various combinations as mentioned below:
- solid - grit blasting, pneumatic conveying
of powder & pellets belts & pulleys
b) Solid - liquid - Liquid in pipeline
c) Solid - gas - Hydrocarbon gas
or CO2 gas, coming out from nozzle
d) Liquid-Liquid - Water settling
in oil / Hydrocarbon tanks
e) Gas-liquid - Released gas or air
bubble rising in a large tank
for generation of static charge:
There are following Conditions for generation of electrostatic
hazard. If all conditions are fulfilled, there will
be fire or explosion.
- Rate of generation is higher than
the rate of dissipation.
- The gas mixture around the spark
is of flammable nature.
- Accumulated charge is sufficient
enough to cause a spark with intensity, sufficient
to ignite the gas mixture
- The proportion of flammable gas
and air (or oxygen) is within the explosive limit.
For the last condition,
it is required to know the higher & lower explosive
limits for that particular gas / vapor.
a) Lower Explosive
Limit (LEL): It is the minimum concentration of vapor
or gas in air (or Oxygen) below which propagation
of flame does not occur on contact with an ignition
(b) Upper Explosives Limit (UEL): It is the maximum
concentration of vapor or gas in air (or Oxygen) above
which propagation of flame does not occur on contact
with an ignition source. (Concentration is expressed
as % v/v in air at 1 bar pressure)
Products that are better
conductors are also better generators of static electricity.
They discharge also rapidly. Hence, their 'Relaxation
Time' is also very short. Relaxation Time is the time
(in seconds) to remove 63% of the charge. Refined flammable
liquids like petrol, jet fuel etc. become charged with
Shock, sparks or attraction/repulsion of light particles
surfaces usually demonstrate the presence of static
electricity. To measure the accumulated static charge,
electrostatic voltmeter is widely used in industry.
Severity of the shock is not as great as that from power
electricity, but involuntary movement may cause injury.
In some industries like printing, rubber, pharmaceutical,
paper, PE sheet packaging, etc where lightweight materials
are handled, materials stick together or stay apart.
Storage, handling, use & processing of flammable
and/or combustible materials (liquid, gas, vapor, and
dust cloud) should be assessed properly to decide further
steps needed to safeguard the installation against fire
or explosion caused by static charge.
Basic Steps required are as under:
a) Identifying locations
where charge separates & accumulates.
b) Assessing the ignition hazards at these locations.
c) Identify ungrounded conductive objects or poorly
grounded objects, including personnel.
d) Identify materials that could serve as electrical
insulators & could interfere with proper bonding
e) Identify those locations that might pose a static
electricity hazard, even if there is no evidence of
accumulation of charge, at the time of the evaluation
of hazard :
Various steps are there, to control the hazard caused
by static electricity. It must be understood that, generation
of static charge cannot be stopped. One has to take
necessary steps, to control the hazard.
In case of pellets, dusts or powders, static charge
generation is caused by operations like sieving, pouring,
mixing, grinding and pneumatic transfer. In liquid handling,
transfer through hoses and pipes, agitation of two phase
mixtures and settling of two phase mixtures, filtration
etc. cause static charge generation.
The steps required are as under:
a) Draining the accumulated
charge by bonding and earthing.
b) Avoid splash filling.
c) While designing pipeline, 'Relaxation Time' should
be kept in view.
d) Maintaining high relative humidity in the atmosphere
e) Controlling flow / velocity. For pipes upto 200-mm
dia, less than 7 Mt per second and for higher sizes,
velocity less than 1.5 Mt per second are safe.
f) Provide nitrogen (or other inert gas) blanketing
in the area. (Agitators, tanks etc.)
g) Provide ionized atmosphere in that area. (This
is very costly and also requires radio active source,
hence normally not adopted.)
7.0 Filling drums & containers
a) Drum or vessel
to be filled should be earthed with flexible wire
b) Avoid splash filling. Inlet fill pipe should reach
near the bottom.
c) As far as possible, avoid use of non-metallic pipes
and drums. If non metallic drums & pipes are used,
provide proper earthing. Bare metallic wire should
be wrapped around the non-metallic hose and both ends
of the wire should be connected to earthing. It is
safer to place the non-metallic drum on earthed metal
plate. Earth the drum also by earth clip.
d) If drum filling is to be done on the weighing scale,
platform should be earthed separately.
e) During filling, a small vent plug on the top should
be kept loose to eliminate pressure build up.
the storage tanks & vessels:
a) Earthing of tanks,
vessels & pipelines should be properly done. On
the flanges of pipelines, proper bonding should be
b) Gauging or sampling shall be carried out only after
surface turbulence is subsided.
c) Floating roof tank & inert gas blanketing should
be used after proper requirement and suitability.
d) Air entrapped (during pumping operation) should
e) Liquid surface should be free of loose material
or floating object.
f) Cleaning or gas freeing of tank or vessel by steam
or water should be done only after proper earthing.
9.0 Loading / Unloading of flammable liquids for Road
and Rail tankers :
a) Tanker should be earthed first, before opening
the dome. Metal surface at E- point should be thoroughly
cleaned of paint.
b) Before filling is started, to avoid splash filling,
ensure that the fill pipe has reached the bottom.
c) Once tanker is filled, wait for 5 to 10 min, before
removing the earthing clamp.
d) If tanker is to be unloaded, wait for five min.
after connecting the earthing. Dip rod shall be lowered
only after this period.
Numbers of explosions or fires have occurred in tank
truckers or tank cars while they were being filled.
The most common cause is 'switch loading'. A tanker
contains some flammable vapours, (such as gasoline vapours)
from previous load and is then filled with liquid of
higher boiling point, such as gas oil. The gas oil is
not flammable at ambient temperature. Therefore, no
special precautions are necessary to prevent formation
of static charge.
If the tank is filled quickly or even filled with splash,
a static charge is formed. Spark jumps from the liquid
to the wall of the tank, igniting gasoline vapours present
after previous unloading. Such loading of different
material (after unloading one type) is called 'Switch
powder, dust, polymer granules
Static electricity is generated in conveying and free
falling of combustible powder / dust, polymer granules
(pellets) etc. Many explosions and fires have been reported
in pneumatic conveyer system, silos, dust collection
systems etc. To avoid such fires, effective earthing
and bonding of metallic conveying pipelines and silos
should be ensured. If solid material is containing free
flammable material, then it is advisable to install
a degassing system with proper interlocking. Other alternative
is to have inert gas blanketing inside silos and conveying
pipes. Any deposit of material on the inner wall of
the pipes should be removed regularly.
clothing & flooring:
a) For the safety
against static charge hazard, total resistance between
the body and ground should be less than 108 ohms.
b) Where risk of exposure to mains electricity exists,
resistance to ground should be more than 50,000 ohms,
but less than 100 Mega ohms.
c) In the presence of the flammable atmosphere, clothing
should not be removed. Avoid use of polyester clothing
in flammable area.
e) Conductive floor (made up of metal standing plate)
gives good safety. Such floor is normally provided,
where operator has to charge the additives or chemical
(in powder or dust form) in a reactor agitator.
Charge can be transferred from one object to another
without direct contact between them. This is known as
When a conducting article is placed in an electric field,
the internal charges are free to move. Since unlike
charges attract, the article polarizes, so that one
side is effectively positive and the other negative.
When the conductor is touched with a grounded rod, only
half charge flows to the ground. One polarity remains
effectively bound by its attraction to the object. When
they are separated, the conductor is left with a net
charge. It can be seen that, this phenomena can lead
to the accumulation of a high charge on personnel working
in the vicinity of an electrostatic charge. Even though
there is no direct contact with the charged object,
person working near an insulating plastic belt or highly
charged insulating powder, gets charged.
- We can not prevent the generation
of static charge, we can only reduce its hazard
by discharging it.
- If charge is formed on non-conductor
or ungrounded conductor, they can remain for some
- Most of the pure hydrocarbons
are non-conducting liquids.
- Most liquids containing oxygen
atoms in their molecules are good conductors.
- Static charge formed on good conductors
flows to the earth very quickly.
- Proper earthing and bonding give
good protection against static charge hazard.
- IT IS VERY UNSAFE TO CARRY OUT
ANY MODIFICATION WITHOUT PROPER STUDY OF CONSEQUENCES
OF THE CHANGE.