In all industrial areas where flammable liquids or fine-grained, combustible bulk materials are filled, mixed or loaded, explosive atmospheres may form. In fact, such discharges are ubiquitous. However, what usually happens harmlessly in everyday life can have devastating consequences in an industrial environment – for example, when conveyor belts cause permanent friction. An earthing technique to minimise risk is therefore elementary.
The ignition hazard resulting from uncontrolled discharges in real applications depends on their form and the energy emitted. A distinction is usually made between:
- Tuft discharges occur when charged objects made of insulating material approach conductive objects. Due to the comparatively low energy density, this form of discharge can be reduced to a non-ignitable level by deliberately reducing the insulating surfaces.
- Corona discharges, which can form at field strengths of 3 MV/m and above on sharp edges or corners of conductive materials, also pose a relatively low ignition hazard.
- Spark discharge is the abrupt discharge between two charged objects. This is considered the most common source of electrostatic ignition and is avoided by earthing all conductive objects.
- The most energetic type of discharge is the sliding brush discharge, which typically occurs on thin insulating material surfaces such as foils or coatings. If the high amounts of energy generated by friction are released during a sudden discharge, they are capable of igniting all explosive atmospheres based on flammable liquids, gases or dusts.
- The fifth, industrially relevant type of discharge, the bulk material cone discharge, occurs when silos are filled with highly charged, insulating bulk material. The strength of the discharges, which occur between the upper part of the bulk cone and conductive silo walls, depends to a large extent on the grain size, conductivity and filling speed of the bulk material. (Stephan Schultz, R. Stahl, in technical journal “PROCESS”, 7/2021)
The standard “EN IEC 600792” and the “Technical Rule for Hazardous Substances 727” contain a number of different recommendations and regulations for the avoidance and safe discharge of dangerous electrostatic potentials. The regulations stipulate that all conductive equipment and objects must be earthed or have an equipotential bonding connection in order to limit charging to a safe level. Electrostatic earthing is considered to be ensured when the resistance of objects and materials to earth is less than one mega-ohm.
Observe Wear And Maintenance!
In principle, the prescribed electrostatic earthing can be realised with the help of simple cables and suitable pliers. However, especially in areas where mobile containers such as tank trucks, tank wagons or FIBCs (“Flexible Intermediate Bulk Containers”) are filled or emptied with flammable substances, passive earthing is risky. Due to frequent connection and disconnection of the clamps, abrasive ground contact or accidental rolling over of the cables with vehicles, the components are subject to high mechanical stresses. Even minor damage to the cables or contact elements can render a safety device ineffective.
In addition, corrosion, dirt or coatings lead to impairments of the conductivity between clamps and the object to be earthed. With passive, i.e. not actively monitored earthings, there is also the danger that interruptions of the conductive connection remain undetected.
Even though the monitoring of permanently installed earthing connections has not been a priority safety measure so far, this topic is becoming increasingly important due to the trend towards plant modularisation. It is therefore becoming a necessity for plants that use a large number of earthed applications such as filling and mixing stations or operate machines connected via pipe systems.
Photo: Jens Rother