Nonconductive Plastic Containers and Stretch Film

        The use of nonconductive, plastic containers in potentially flammable locations may be an ignition hazard in plants. Static charge accumulations on such containers, caused by the transfer of poorly-­conductive liquids or solids, or by contact charging, cannot be dissi­pated by the bonding/grounding system.
        Contact (“triboelectric”) charging of a nonconductive container in a low humidity environment creates a spark ignition hazard by inducing charges in the liquid in the container. These induced charges may cause sparking, e.g., when the liquid is poured into a grounded safety can. Surprisingly, this hazard of charge induction is greatest when the liquid is “conductive”.
        Experience suggests the following precautions:

  • Fibre-board drums: No hazard of static accumulation, except for metal rims, which should be grounded during product transfer.

  • Kraft paper bags and plastic lined paper bags: No hazard with paper bags. Plastic-lined paper bags are usually not hazardous, but the static electrification for each bag/contents combination should be measured.

  • AlI-plastic bags and bags with removable plastic liner: Should be avoided unless measurements of electric field intensity at bag surface during product transfer is less than 5 kV/cm (12.5 kV/inch).

  • Plastic bottles and nonconductive drum liners: Are subject to the hazard of charge induction as a result of contact electrification. Precautions must be taken to minimize contact charging or to neutralize contact charges before use. Removal of plastic bottles from plastic bags may cause high contact charging. Electric field intensities greater than 5 kV/cm (12.6 kV/inch) at the surface of the bottle or liner should be neutralized before a “conductive” flammable liquid is put into the bottle. It is also important to avoid charging a plastic bottle than contains even a small quantity of a conductive, flammable liquid.

  • Stretch Wrap: Must be removed from pallets in a nonflammable location. This material is usually highly charged and represents a serious hazard in flammable locations.

  • Semi-bulk “supersacks”: Flexible Intermediate Bulk Containers (FIBC). FIBC’s are now categorized into types A, B, C & D. The type C bag contains thin conductive strips spaced closely together in the polypropylene weave. All strips are interconnected at the seams and via the lifting handles and labeled ground point. These conductive parts are designed to carry away any static electricity from the powders within. Type C bags have been proved to be safe for use in flammable atmospheres, providing they have been suitably grounded using a discharge lead and clamp.

  • Conductive plastic liners and containers: Although most plastic materials are non conductive, some conductive plastic liners and containers are now commercially available. Conductive plastic materials must be grounded during product transfer in flammable locations.

Bonding and Grounding Principles

Bonding and grounding is a very effective technique for minimizing the likelihood of an ignition from static electricity.
        A Bonding system connects various pieces of conductive equip­ment together to keep them at the same potential. Static sparking cannot take place between objects that are the same potential.

 

          Grounding is a special form of bonding in which conductive equip­ment is connected to an earthing electrode or to the building grounding system in order to prevent sparking between conductive equipment and grounded structures.1
        In potentially-flammable locations, all conductive objects that are electrically isolated from ground by nonconductors such as nonconductive piping or hoses, flexible hoses, flexible connections, equipment supports, or gaskets, should be bonded. An isolated, conductive object, can become charged sufficiently to cause static spark. Objects which can become isolated include screens, rims of nonconductive drums, probes, thermometers, spray nozzles, and high pressure cleaning equipment.
           Bonding and grounding cables must be durable and of low resis­tance. Connections of bonding conductors to process equipment must be direct and positive for portable equipment, un-insulated  copper or stainless steel, aviation-type flexible cable and single-point clamps, should be used. These clamps will make contact with metal surfaces through most paint, rust and surface contaminants. The single-point clamps are superior to the battery-type and “alligator” type clamps for making direct contact.
            This bulletin contains drawings of typical arrangements of bonding and grounding devices which should be used wherever solvents are handled. The large size of bonding and grounding cables is selected for minimizing mechanical damage rather than for current-carrying capacity. These cables carry microampere-level electrical currents. An arbitrary value of maximum resistance (e.g., 25 ohms) from each bonded object to the grounding bus is specified so that periodic checks of the bonding/grounding system with a simple ohmmeter can confirm that the system is intact and in direct contact with the bonded objects.
            Caution must be exercised in the installation of static grounding systems not to use as a ground, any part of the electrical current­ carrying system. Fires caused by electrical arcing from current feedback through the grounding system have occurred in plants where static-control grounds were tied into the electrical systems neutrals.

Inerting Methods and Procedures

 The introduction of an inert gas such as nitrogen into a ball or pebble mill or mixer will prevent a flash fire if an electrostatic spark occurs within a vessel. Care must be exercised that sufficient inert gas is introduced to adequately displace the oxygen throughout the entire vessel. The most common inert gases are nitrogen and carbon dioxide (C02).
        Two important considerations, when inerting, are gas pressure and gas velocity. High gas pressure could damage a closed vessel. To avoid overpressurization, a relief valve is recommended on the gas line to the mill.
        Inerting with carbon dioxide is potentially hazardous, and such systems must be carefully designed and installed. A C02 fire extinguisher should never be used to inert a vessel.
        Continuous automatic inerting systems are designed specifically for the coatings industry. These systems monitor the oxygen content in any mixing vessel and adjust the flow of inert gas to maintain a nonflam­mable environment within the vessel. This inerting method can be used in high speed dispersers as well as in ball and pebble mills.
        The section on “Inert Gas Systems” of NFPA 77, “Explosion Prevention Systems,” published by the National Fire Protection Association, discusses various factors involved in using inert gas. (Refer to Appendix A of NFPA 77.)  

1In order to successfully achieve this objective of the same ground potential for all metal objects when there are additional and/or redundant grounding systems, and particular, when there are supplementary grounding electrodes, all such grounding systems and electrodes should be connected together as required by the National Electrical Code, and by the NFPA Lighting Protection Code.

 

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