29 November

Both USERS and MANUFACTURERS of FIBCs need to take action to prevent dust explosions caused by static electricity

A dust cloud of fine explosible particles in air with a concentration that is above its minimum explosible concentration (MEC) can be expected at some point when FIBCs are filled or emptied. The dust cloud could occur inside the bag – or even external to the bag in some cases. What’s more, movement of powder to the bag, inside the bag or on emptying, can generate static electricity. The combination of static electricity and a flammable dust cloud can result in an explosion or deflagration (flashfire) putting people and the plant at risk.  It should be no surprise, therefore, that FIBCs need to be type tested to avoid the electrostatic ignition risk – and that users are required by NFPA 652 to include FIBCs in their Dust Hazard Analyses (DHAs).

The amount of static generated and whether or not it accumulates on the bag or the powder, resulting in incendive discharges, depends on the properties of both the FIBC and the powder itself. It is therefore critical to ensure the correct type of FIBC is selected for the specific powders and circumstances in which it is to be used.

NFPA 652 requires that your Dust Hazard Analysis (DHA) has to include the electrostatic ignition hazards associated with the combustible particulate solids inside or surrounding the FIBC.

The 2019 edition of NFPA 652 Standard on the Fundamentals of Combustible Dust provides minimum general requirements to manage combustible dust fire, flash fire, and explosion hazards.  It also directs the user to other NFPA standards for industry- and commodity-specific requirements.  NFPA 652 requires that the owner/operator take the following steps:

  • Determine combustibility and explosibility hazards of materials
  • Conduct a Dust Hazard Analysis (DHA) which is a systematic evaluation of potential dust fire, deflagration, and explosion hazards and recommendation of measures for their management
  • Manage identified fire, flash fire, and explosion hazards
  • Establish written safety management systems

NFPA 652 requires that the Dust Hazard Analysis (DHA) also include the electrostatic ignition hazards associated with the combustible particulate solids inside or surrounding the FIBC.

The table 1 summarizes the requirements of NFPA 652 for use of FIBCs.

Table 1. NFPA 652 Requirements for Use of FIBCs

FIBC Type NFPA 652 Requirements
Type A
  • Must only be used with noncombustible powders or combustible powders with MIE >1,000mJ
  • Must not be used in locations where flammable vapors or gases are present
  • Must not be used with conductive dusts
Type B
  • Can be used for combustible dusts with MIE >3mJ
  • Must not be used in locations where flammable vapors or gases are present
  • Must not be used for conductive dusts
Type C
  • Can be used for combustible powders and in locations where flammable vapors or gases having MIE >0.14mJ are present
  • Conductive components (threads) of the FIBC must be electrically interconnected and terminate in a grounding tab.  The resistance from any of the conductive components to the grounding tab must be ≤107 ohms
  • Must be grounded during filling and emptying operations with a resistance-to-ground <25 ohms
  • Can be used for conductive dusts, provided that the dust is in contact with electrical ground
Type D
  • Can be  used with combustible powders and in locations where flammable vapor or gases with MIE >0.14mJ are present
  • Must not be used for conductive powders

Type B, Type C, and Type D FIBCs shall be tested and verified as safe for their intended use by a recognized testing organization in accordance with the requirements and test procedures specified in IEC 61340-4-4, before being used in hazardous environments”.

Intended use must include both the product being handled and the environment in which the FIBC will be used.

Table 2 summarizes the tests that are required by IEC 6130-4-4 for different FIBC Types.

Table 2. IEC 6130-4-4 Testing Requirements

FIBC Type Test
For all types of FIBCs
  • Minimum Ignition Energy (MIE) of the powder.  If MIE less than 20mJ, then
    • Electrostatic Volume Resistivity of the powder
    • Electrostatic Chargeability of the powder
Type B FIBCs
  • Electrical Breakdown Voltage of the fabric and liner
Type C FIBCs
  • Electrical Resistance between the grounding strap and any conductive location on the FIBC surface
Type D FIBCs
  • Incendivity of Electrostatic Discharges from the surface of the FIBC during full-scale filling and emptying operations
Liners for use in FIBCs
  • Surface Resistivity
  • Electrical Breakdown Voltage

According to NFPA 652, FIBCs that have not been tested and verified for type in accordance with IEC 61340-4-4, shall not be used for combustible dusts or in flammable vapor atmospheres.