The generation of electrostatic charges in the handling and transfer of flammable liquids causes frequently serious accidents.
Though commonly known, these dangers are not usually taken into consideration where safety measures are concerned. It is very important to avoid the generation of static charges in the presence of flammable atmospheres and to ensure its rapid elimination when created.
FORMATION OF STATIC ELECTRICITY
The formation of static electricity is mainly produced by:
Electrostatic dischargers can occur between metallic parts of different charges or between the surface of the liquid itself and a metallic part, such as a dipstick or stirrer in the presence of a flammable atmosphere inside a tank or in the area immediately surrounding the container.
PREVENTION MEASURES TO BE CONSIDERED
Control of flammable atmospheres.
All flammable liquids within an open container and above its flamepoint gives off a quantity of vapours capable of creating a flammable mixture with the air. For that reason, it is necessary to take into consideration that the risk won ́t be enoughly controlled if we only consider the elimination and control of the source of the ignition, as other risks exist apart from the risk of static electricity.
Control of the flow rate of liquid and the filling system of containers.
It is recommended to avoid high speed flow through pipes, and ensure a smooth surface in contact with the liquid specially controlling the presence of water or impurities which are notorious in the generation of static electricity.
Use of antistatic additives.
Used to increase the conductivity of the products, making it easier to discharge the static electricity generated.
Electrical installation and protected equipment.
The electrical installations, equipment and other electrical materials used in locations where the transfer and storage of flammable liquids takes place must be designed and installed in accordance with the complementary instructions Ml BT 026 of the electrotechnical regulations for low voltage (Directive 79/196/CEE) referring particularly to the guidelines for premises with a risk of fire and explosion. Also it is obligatory to comply with the regulation UNE 20-322-86 “Classification of locations with risk of explosion due to the presence of gases, vapours and flammable clouds.
Control of mechanical impact and other sources of ignition.
The proximity of dangerous atmospheres close to sources of ignition, other than sparks from electrostatic discharges, must be controlled as much as possible. Special attention must be paid to the effects of mechanical impacts which must be avoided at all cost.
The mechanical parts of pumping equipment, like the suction tubes of portable pumps and hose connections, should be constructed from antistatic materials, usually an alloy of aluminium and zinc (Al-Zn).
PREVENTION MEASURES TO BE CONSIDERED (CONT.)
Equipotential bonding cables and grounding:
it is imperative that static electrical charges are eliminated easily without causing danger. This is achieved fundamentally by the method of interconnecting all of the conductive surfaces, which may form an electric charge and connecting them at the same time to earth.
Control of resting time:
it is fundamental to maintain a waiting time, which is also called the resting time from the end of the transfer until the beginning of operations which may generate sources of ignition, such as opening of lids, collecting samples, etc., that might generate energy by impact or mechanical dents or even electrostatic sparks. These resting times provide a wide margin of safety in the working process and depend on the type of product being transferred.
For conductive flammable liquids the resting time should be a minimum of 30 seconds and for non conductive liquids with a resistivity greater than 1,011 Ωcm, the resting time should be a minimum of 1 minute.
Personnel work clothing.
The personnel that work in installations where transfer of flammable liquids takes place must not use clothes made from synthetic fibres and the clothing should be preferably of cotton, including the underwear. It is also advisable to limit the use of woollen clothing where possible.
Footwear and in this case the gloves, should also be conductive.
This will offer enough protection, provided that the floor is conductive, which also has to be taken into consideration.
The normal shoe with a leather sole offers, especially if the humidity is enough , a greater level of conductivity.
Nevertheless, for people with very dry skin or when using other insulating materials along with leather soles, metal rivets going through the sole are highly recommendable.
Control of ambient humidity and safe working procedures:
the maintenance of a relative humidity of above 60% is highly recommended in environments that may be flammable along with the capacity of those exposed to the risk to carry out safe working procedures.
Control of flammable atmospheres:
Detectors to measure the concentration of flammable vapours.
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