1.0 When & How?

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 positive charge.

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 upon it.
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 spark.

Charge generation will take place during various processes like,

  • Rubbing wool on plastic
  • Pouring bulk material out of bag / container
  • Unrolling of a plastic or PVC film
  • 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 (insulating) materials.

2.0 There are various types of surfaces in contact, resulting in various combinations as mentioned below:

a) Solid - 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

3.0 Conditions 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 source.

(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 static electricity.

4.0 Detection / Measurement
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.

5.0 Evaluating the hazard
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 and grounding.
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

6.0 Control 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 near materials.
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 & clamp/clip.
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.

8.0 Filling the storage tanks & vessels:

a) Earthing of tanks, vessels & pipelines should be properly done. On the flanges of pipelines, proper bonding should be there.
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 be removed.
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.

10.0 Switch loading:
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 Loading'.

11.0 Combustible 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.

12.0 Shoes, 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.

13.0 Induction Charging
Charge can be transferred from one object to another without direct contact between them. This is known as Induction Charging.
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.

14.0 Remember :

  • 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 time.
  • 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.
Shri N.N. Parekh a graduate of 1965 in Electrical Engg. started his career with Surat Electricite co. and worked for eight years. Since June 1973 he has worked with IPCL, Vadodara in various capacities in LDPE project, Polymer Plants Aromatics Plants and utilities. For nine years he was heading Electrical wing of Engg. Services. During his tenure with IPCL he visited many countries. He was on deputation to Warri Refinery, Nigeria for 15 months. At present he is working as a Free Lancer. He is associated with Gujarat Safety Council for more than 15 years and working as a Faculty member for various topics on electrical safety in many training progremmes at industrial organisations.