Causes of Static Electricity in Packaged Goods Storage
Static electricity is generated by two main factors:
One is internal, namely the conductive properties of the material;
The other is external, namely, friction, rolling, and impact between materials.
Many commodity packaging materials have internal conditions for generating static electricity. Furthermore, storage operations such as handling, stacking, and covering are essential, which inevitably lead to friction, rolling, and impact between packages. The friction between plastic packaging during stacking can easily generate static electricity.
The Hazards of Static Electricity in Packaged Goods Storage
High static electricity potentials accumulate on the surface of packaging, making it highly susceptible to static sparks. These hazards manifest in two main ways:
One is the risk of explosion.
For example, if the packaged contents are flammable, the vapors emitted by these materials reach a certain ratio with air, or if the solid dust concentration reaches a certain level (i.e., the explosion limit), an explosion can occur upon contact with static electricity sparks.
The other is the risk of electric shock.
For example, static electricity can generate high-potential discharges during handling, causing operators to experience uncomfortable electric shocks. This is a common occurrence when handling plastic-packaged goods in warehouses. The intense friction during handling and stacking can generate high-potential static electricity discharges, and workers can even be knocked unconscious by static electricity.
Preventing the Hazards of Static Electricity on Packaged Goods in Warehousing. The following methods are generally used to prevent and control the hazards of static electricity in packaged goods storage:
1. Minimize the generation of static electricity on packaging. For example, when handling flammable liquids, limit excessive shaking in the packaging drums, control loading and unloading methods, prevent leakage and mixing of different oils, and prevent water and air from entering the drums.
2. Take measures to quickly dissipate any static electricity that does generate, preventing its accumulation. Examples include installing good grounding devices on handling tools, increasing the relative humidity in the workplace, laying conductive flooring, and spraying conductive paint on certain tools.
3. Apply a certain amount of countercharge to charged objects to prevent the buildup of static voltage (e.g., using an inductive static neutralizer).
4. In some cases, static electricity accumulation is unavoidable, and the rapid buildup of static voltage can even generate static sparks. In these cases, measures should be taken to prevent the discharge of static electricity without causing an explosion. For example, inert gas can be filled in the storage area for flammable liquids, alarms can be installed, and efficient exhaust systems can be used to keep the amount of flammable gas or dust in the air below the explosion limit.
5. In areas with fire and explosion hazards, such as chemical storage areas, workers should wear conductive shoes and anti-static work clothing to promptly dissipate static electricity. From the perspective of static ignition hazards, bulk bags are generally classified into four categories based on their construction. This classification system is widely used in Europe.
In June 2003, the European Committee for Electrotechnical Standardization (CENELEC) published document CLC/TR50404, "Electrostatics - Code of Practice for the Avoidance of Hazards Due to Static Electricity." This comprehensive standard for electrostatic handling across various industrial sectors includes a chapter detailing the safe use of bulk bags. The standard classifies bulk bags into four categories: Type A, Type B, Type C, and Type D.
Type A bulk bags have no special static safety features and are therefore not recommended for handling sensitive, flammable dusts and powders. Furthermore, they should not be used in the presence of dust clouds or flammable solvent vapors. These bulk bags are typically made of ordinary woven polypropylene cloth, which is an insulator. Sometimes, depending on the application requirements, Type A FIBCs are lined with inner bags or coated.
Type B FIBCs are similar to Type A FIBCs and are made of ordinary woven polypropylene fabric. However, the breakdown voltage of the fabric used in Type B FIBCs cannot exceed 4 kV. This means that Type B FIBCs are immune to propagating brush discharges. This is an important classification; it means that the discharges that can occur in the FIBC are low-energy brush discharges. If propagating brush discharges can be excluded and the brush discharge energy is 4 mJ, it is reasonable to believe that this type of FIBC is safe for use with flammable gases with an ignition energy of no more than 4 mJ. Similarly, this type of FIBC is safe for use with combustible dusts with an ignition energy of no more than 4 ng. However, Type B FIBCs are not suitable for use with flammable hydrocarbon vapors. It is important to note that some factory-made FIBCs meet the Type B classification criteria and can still cause accidents. For example, a Type B bag may meet the Type B standard when tested by a testing agency. However, in actual use, due to the coating on the inner liner and the bag body, the breakdown voltage exceeds 4kV, effectively turning the Type B bag into a Type A bag.
Type C bulk bags are designed for sensitive and flammable environments, including those with flammable hydrocarbon vapors. These bulk bags are made of conductive fabric or woven fabric with a conductive/antistatic coating. Conductive fabric is essentially a woven fabric interwoven with conductive fibers/tapes. In some designs, the conductive threads are parallel and spaced 20mm apart. In other designs, the conductive threads are woven into a network with perpendicular intersections. The conductive threads are typically conductive tapes or conductive metal wires.
Type D bulk bags have antistatic or static dissipative properties and do not require grounding. Most Type D bulk bags currently on the market are made with fine semi-conductive threads interwoven into the fabric. Unlike Type C bulk bags, these semi-conductive threads are parallel but not cross-linked. This type of bulk bag may also have a static dissipative coating. Since fires and explosions caused by bulk bags are generally attributed to static electricity, in order to solve this problem, some new "static safe" bulk bags have been developed and commercialized.