1.0 INTRODUCTION
Whenever two dissimilar materials come in contact, a transfer of electrons from one material to the other takes place. As a result, one material acquires an excess of –ve charges and the other an equal excess of +ve charges. If the two materials are now separated, each will carry an excess of charge, and electrostatic charging has taken place. If both materials are conductors (e.g. 2 metals), the high conductivity allows equalization of charges to take place along the contact surface during the separation so that no measurable excess charge is present on complete separation. However, when one of the material involved is non-conductor (e.g. plastics), static charge occurs.
NOTE
In the case of dust particles or liquid droplets moving in gases, charging is always to be expected, irrespective of the conductivities of the substances involved.
When charging process is complete, the excess charge will start to equalise themselves. The discharge take place exponentially and is dependent on the resistance of the charged material to earth.
An electrostatic discharge is capable of causing ignition when the energy when the energy released is greater than the minimum ignition energy of a flammable mixture present at the time. For a mixture to be flammable, the ratio of the concentration of the flammable material to be concentration of oxygen must lie between the lower and upper explosive limits. The vapour mixture present above a liquid is only flammable if the liquid temperature exceeds the flash point. However, one should remember that the temperature of liquid droplets (in aerosols) can rise very rapidly above the flash point due to their small heat capacity.
2.0 GENERAL PROTECTIVE GUIDELINES :
2.1 Earth all conductors :- A large number of hazards caused by electro discharge can be eliminated through earthing of all conductors.
2.1.1 Earth all pieces of equipment
2.1.2 Interconnect building structures with conductors.
2.1.3 Earth insulated metal parts, e.g. flanges in plastics pipelines.
2.1.4 Place metals drums on conductive floors.
2.2 Earth the personnel –The human body is a conductor with regard to static electrically and can carry charges around .
2.2.1 Install conductive floor coverings.
2.2.2 Wear conductive floorwear in areas where an explosion hazards exists and electrostatics consideration get priority.
( A man walking with insulating footwear, normally provided to technicians attending to faulty electrical equipment, generates static charge due to separating effect between soles and floor. The man is charged being conductor and the charges cannot flow away because of insulating soles)
2.3 Avoid and prevent charging by using conductive materials.
2.4 Keep the velocity low when conveying fluids, gases and powders.
2.4.1 Limit the flow velocity of liquids and of gases containing dust particles.
2.4.2 Avoid turbulence in non-conductive liquids.
2.5 Avoid formation of explosive mixtures
2.5.1 Remove gases, vapours and dusts by suction at the source.
2.5.2 Work with liquids at temperature atleast 100c below their flash point.
2.5.3 Charge flammable liquids to vessel by suction, and discharge by nitrogen pressure.
3.0 SPECIFIC PRECAUTIONARY MEASURES :
3.1 Working with flammable liquids :- Chargeable, flammable liquids in order of decreasing hazards.
Carbon disulphite----- Ether ---- Petrol ---- Benzene ----- Hydrocarbons ---- Ester ---- Chlorinated hydrocarbons ------ Low Ketones and alcohols.
3.1.1 Hazards
Charging of the liquid when it flows along the walls of transfer lines (pipes) or flexible tubes. Charging of pipe or tube or adjacent parts outside the pipe. Discharge of sparks between metal parts e.g. tube fittings and containers.
3.1.2 Measures
3.1.2.1 Avoid formation of explosive mixtures within the pipe (tube, hose) completely filled.
3.1.2.2 Keep velocity low when discharging liquids into containers.
Generally, velocity below 1.0m/sec. S considered safe using 25 mm to 50 mm diameter pipe.
3.1.2.3 Keep the liquids pure. Dust and droplets of water are charge carriers and can generate considerable charge.
3.1.2.4 Flexible tubes, with or without wire coils, made of conductive materials, tubes having conductive connection with the metal flanges or screwed joints, metal hose with lining are recommended. The electrical resistance between the two couplings, (for all types of flexible tubes or hoses) should not exceed 106 ohms.
3.1.2.5 Earth metals flanges fitted on glass or plastic pipes having nominal diameter of 50 mm. Or over.
Use of plastics flanges (need not be earthed) is recommended.
3.2 Filling and emptying drums by hand : Transfer of flammable liquids to other smaller containers.
3.2.1 Hazards : Charging of liquid and the container due to separation (flow). Discharge of sparks between metal parts- drum and the can or between can and funnel.
3.2.2 Measures :-
3.2.2.1 Earth metal drum or place them on earthed metal grating.
3.2.2.2 Take the filling pipes or funnel right down to the bottom to avoid splashing, spraying.
3.2.2.3 Earth operator. Provide conductive shoes.
3.3 Transfer of solvents from vehicle (tankers) to storage tanks :
3.3.1 Hazards : Charging of the liquid due to flow and charging of the vehicle. Discharging with sparks between conductive parts e.g. between hose fittings and vehicle, or between driver and tank or vehicle.
3.3.2 Measure :
3.3.2.1 Connect vehicle and tank with earthed conductor.
3.3.2.2 Use conductive tube/hose.
3.3.2.3 Wear conductive shoes.
NOTE : When filling plastic can (over 5 litres capacity) or non-conductivity containers charges accumulated in the liquid can drain off only very slowly.
3.4 Fillings of agitator vessels with flammable solvents :
3.4.1 Hazards : Charge develops in liquid due to splashing, whirling. Discharges with sparks above the liquid surface are possible.
3.4.2 Measures :.
3.4.2.1. Inertize vessel with N2. Use of CO2 is not recommended due to formation of ‘dry ice'.
3.4.2.2 Extend filling tube right down to the vessel or let it be as near to the side-wall.
3.4.2.3 Draw the solvent with the aid of vacuum.
3.5 Feeding of solid material to an agitator vessel or reaction tank pre-loaded with flammable solvent :
3.5.1 Hazards : Charge develops due to stirring process, feeding of the material. Discharges with sparks at the manhole or above the surface of the liquid.
3.5.2 Measures :-
3.5.2.1 If possible, load solid first.
3.5.2.2 If possible cool solvent atleast 100c below its flash point.
3.5.2.3 Add powder through rotary valve or such equipment which prevent direct fall of a large quantity of powder.
3.6 Operations at the open manhole of a filed vessel
3.6.1 Hazards: Charged reaction masses inside the vessel/agitator tank can discharge with sparks via a metal object such as a sampler ignition hazard is near the manhole where solvent vapours mix with air.
3.6.2 Measures :
3.6.2.1 Before opening the manhole, stop agitator and wait for five minutes. Intertise the vessel or switch on exhaust ventilators, if fitted on the vessel, for a few seconds before opening the manhole.
3.6.2.2 Use sampler beaker or measuring rods made of insulting material. Wooden measuring rods are acceptable. Keep sampling beakers clean.
3.7 Centrifuges :-
3.7.1 Hazards : High charges develops due to
I) High speed of the rotating parts,
II) Use non-conductive liners or coatings, e.g., synthetic fibre cloths, rubber-lined bins,
III) Intensive separating process between solid and liquid components and
IV) The presence of flammable vapour or aerosols with high turbulence.
V) Spark capable of ignition will cause fire or explosion.
3.7.2 Measures :
3,7.2.1 Centrifuge machines with insulating internal coating must not be used with flammable solvents.
3.7.2.2 All metal parts must be earthed.
3.7.2.3 Intertise the centrifuge, when called for.
3.7.2.4 withdraw filter cloth (synthetic fibre) slowly in order to prevent charging.
3.8 Handling of powders – Mixing, crushing, grinding, drying, transfer :
3.8.1 Hazards : When powders are whirled up, the individual particles become charged owing to the separating process irrespective of the conductivity of the substance. Parts of apparatus coming in contact with the charged powder themselves become charged. Build up of high charge can lead to ignition of powder and a first explosion can cause secondary explosion when more dust whirls up.
3.8.2 Measures :-
3.8.2.1 Earth all conductive parts of apparatus or machines even if they do not come in drier and bonding of trolleys with drier wall, earthing of drier and bonding of trolleys with drier wall, earthing and bonding of ventilation exhaust ducts, to be provided.
3.8.2.2 Provide dust extractor system. Operate as cleanly (dust-free) as possible. Keep the plant and the work-place clean. DO NOT allow accumulation of dust in the room, on the walls, ceilings, electrical junction boxes etc.
3.8.2.3 Dry dust is ‘dangerous’ Moistening the material, when possible, improves conductivity.
Note :- To fill powdered materials into plastic bag or drums with plastic lines, is, as such, not hazardous.
However, when such packing are emptied, sparks may be generated which may not have sufficient energy to ignite dusts, but they will ignite flammable solvent vapours or gases if enclosed in the room.
3.9 Spray drying, fluidized bed drying :
3.9.1 Hazards : High charging of substances (powder or granules) owing to the high speed separation of the particles in dry atmosphere – in the drier or in the dust separators (cyclone) of filter. Charging of parts of apparatus that come into contact with the powder/granules. Discharge with ignition especially if flammable solvent vapours are present.
3.9.2 Measures :
3.9.2.2 Bond and earth all metal parts of the casing of the drier.
3.9.2.2 Earth internal metal installations such as sieves, spray nozzles, retaining baskets, prop rings……..
3.9.2.3 If conductive filters are used, the filter cloth must contain fine metal fibers which are spun into the individual thread of the fabric. Filter cloth with separate leads or wires are dangerous; if such a lead or wire bracks, a spark gap is created .
4.0 GENERAL RULES
4.1 Metal pipe clamped to the building structure may be counted as earthed.
4.2 Apparatus such as agitator vessels, receivers, filters etc. made of metals which are firmly connected to metal pipes or stands count as earthed. Covers of such apparatus screwed firmly to the lower part, even if there is n insulating gasket in between, count as earthed.
4.3 Drive-belts (V-belts and flat belts) must be conductive.
4.4 Plastic flanges for glass pipe do not require earthing. Metal flanges with 50 mm and more diameter are to be earthed. Individual metal bolts need not be earthed.
4.5 Do not provide casing or coating made of conductive material on plastic pipelines.
4.6 The paint on metal pipelines can have an insulating effect, if the individual parts are completely painted painted before assembly.
4.7 A charged product can charge the metal bowl or the container which cannot discharge to earth because of its insulating wheels.
The man wearing insulating shoes (with plastic or crepe rubber soles) is charged when handling ‘charged’ metal container, and on approaching an earthed object (e.g. pipeline) he can cause a discharge with sparks. In an explosive atmosphere even changing of uniform (personal clothing) made of synthetic fibre can cause sparks due to separation effect.
Recommend
Use of conductive wheels
Use of safety shoes with leather shoes.
Use of cotton uniform with a directive not to change uniforms within the
manufacturing areas.
( In India, a lightweight, ankle high industrial boot with an anti-skid nitrile rubber sole that is antistatic, is now being manufactured by one of the leading shoe industrial units.
For those who require to work in highly sensitive area use of this type of safety shoe is recommended.) use of Rupture Panel of drier, and locking arrangements on the drier doors, vents for distillation columns, centrifuges, driers….. are recommended. Vents should be straight, without bends, and rupture disc should be of suitable design.
