Testing an FIBC with the gas ignition probe
Wolfson Electrostatics News
Big Bags without 'big bangs'
The use of flexible intermediate bulk containers (also known as FIBCs, big bags, jumbo bags, etc) has increased enormously throughout manufacturing industries where large quantities of powdered, granular or pelleted materials are handled. The bags are usually cubic in shape and may hold up to 1.5 m3 of bulked product.
Many processes, for which big bags are used, may contain sensitive flammable media such as solvent vapours or dust clouds with low ignition energy. The fact that these bags which are traditionally manufactured from woven polypropylene can generate high levels of static electricity when filling and discharging is now well recognised and as a consequence efforts have been made by FIBC manufacturers to design bags, which are inherently antistatic.
ATEX directive 137, on minimum requirements for improving protection of workers potentially at risk from explosive atmospheres, was required to be implemented by member states of the EU by the end of June 2003. Since part of this directive is the avoidance of the ignition of explosive atmospheres, compliance with ATEX requires the use of an effective 'antistatic' FIBC in operations where flammable atmospheres cannot be ruled out.
A number of FIBC manufacturers around the world are now turning their attentions to developing the so-called Type D antistatic bag. In this case the fabric comprises woven polypropylene for mechanical strength but is chemically treated in order to modify its electrostatic properties. In addition, antistatic or partially conductive threads may be incorporated into the weave. The advantage of a Type D antistatic FIBC is that by careful design, incendive electrostatic discharges from the fabric surface can be ruled out. An electrostatic safety audit on this type of FIBC has been developed by Wolfson Electrostatics and is often requested by potential customers wishing to use big bags safely in zone 1 and other hazardous environments. The main approach of the Wolfson Electrostatics audit test is to attempt a controlled ignition of a sensitive propane/oxygen/nitrogen atmosphere during filling and discharging the bag with highly charged polymers. A special gas ignition probe has been developed for use in the test and FIBC designs from Europe, East Asia and Australia have been appraised.
Footnote: Type D antistatic FIBCs are designed to be static safe from the point of view of electrostatic discharges from the fabric surface even when used unearthed. It is appreciated, however, that if a sensitive flammable atmosphere exists the FIBC itself is only one potential source of electrostatic ignition and full antistatic precautions should be taken on all materials within the atmosphere. View the 'Guidance for Plant Engineers - FIBCs' .pdf file (opens a new window - 114 kb).
For further information on FIBC safety contact Wolfson Electrostatics.