Industrial Bulk & Processing March/April 2004
Many applications for FIBC require the use of electrostatic protected bags to prevent the ignition of flammable powders that may be transported in them or of flammable vapours that may be present when they are being discharged. For many years, industry has used a classification scheme based on four types of FIBC distinguished according to their area of use and method of protection. Type A are plain FIBC without any means of electrostatic protection. They are intended for use when no flammable powder or vapour is present. Type B are similar to Type A, but they must be constructed from materials that have an electrical breakdown strength of less than 4 kilovolts. They are intended for use where flammable powders may be present, but where there are no flammable solvents or vapours. Dry powders with minimum ignition energy (MIE) of greater than 3 millijoule may only be ignited by very energetic discharges called propagating brush discharges (PBD). Based on work by Maurer et al , thin films with breakdown strengths below 4 kilovolts did not produce PBD. These results have been applied to FIBC materials, hence the requirement for Type B.
When more sensitive powders, or flammable gases and vapours are present, additional electrostatic protection is required. For such applications Type C or Type D FIBC are required. Type C are fully conductive bags, or bags made from fabric containing an interconnected grid of conductive yarns or tapes. To achieve full electrostatic protection Type C bags must always be connected to earth when used in the presence of a flammable atmosphere. Indeed, if a Type C bag were to become isolated from earth, it not only fails to provide protection, it adds to the danger because the isolated conductors can generate high energy spark discharges easily capable of igniting a flammable atmosphere. Type D FIBC were pioneered by LINQ Industrial Fabrics Inc in response to major industrial users that required a greater degree of safety than could be offered by Type C. By their very nature as transportable containers, FIBC cannot always be earthed. Human intervention is required to re-establish proper earth connection whenever a Type C bag is filled or emptied. Human error is inevitable and the consequences can be disastrous as evidenced by explosion incidences contained in Britton’s paper of 1993 . Type D bags eliminate the danger from human error by providing continuous electrostatic protection without the need of an earth connection. LINQ introduced Crohmiq blue™ as the first effective material to provide continuous electrostatic protection for FIBC without earthing, and the Type D FIBC was born. During the past decade Crohmiq blue™ has established a faultless safety record with over 8 million bags used around the world without incident. Over 2 million of these bags were bags reused after wet or dry process refurbishing. It is fair to say that Crohmiq blue™ has become synonymous with Type D.
Until recently, the type classification scheme, although widely known, had not been formalised in any kind of standard. Codes of practice such as BS 5958 and ZH1/200 did include guidance on the use of FIBC, but they did not differentiate between FIBC with the type classification system, nor did they address the safe use of FIBC not connected to earth. As a consequence of the safety record established for Type D FIBC by Crohmiq blue™, CENELEC has included Type D in the latest code of practice to be published in Europe: CLC TR 50404:2003 Electrostatics – Code of practice for the avoidance of hazards due to static electricity. For the first time, this code of practice establishes formal definitions for Types A, B, C and D and just as importantly gives users guidance on when and how to use each type. The guidance is summarised in Table 1.
|Bulk Product In FIBC
(MIE of Powder)
|Substances in Surrounding Atmosphere|
|No Flammable Atmosphere||Flammable Dust Atmosphere||Explosive Gas/Vapour
(Group IIA or IIB)
|Greater than 1000 mJ||Type A, B, C or D||Type B, C or D||Type C or D|
|3 mJ to 1000 mJ||Type B, C or D||Type B, C or D||Type C or D|
|Less than 3 mJ||Type C or D||Type C or D||Type C or D|
Table 1. Guidance on when different types of FIBC should be used (from CLC TR 50404:2003)
As the Table shows, for the most hazardous situations, when sensitive flammable powders may be present inside FIBC, or when sensitive dusts, gases or vapours are present outside FIBC, full electrostatic protection is required. In these situations safety can be achieved equally by using Type C or Type D. The code of practice emphasises the importance of providing a proper earth connection for Type C FIBC whenever they are filled or emptied, and that electrical continuity between all conductive elements within the bag shall be maintained by suitable design requirements. The resistance between any and all conductive elements shall be less than 10^8. There are no special design requirements for Type D. However, according to CLC TR 50404:2003 a Type D FIBC must be qualified as safe for use in a flammable atmosphere by demonstrating that it does not produce incendiary discharges. The International Electrotechnical Commission (IEC) is currently developing an International Standard for evaluating the electrostatic protection of FIBC intended for use in flammable atmospheres. For FIBC that are required to be earthed (i.e. Type C), evaluation is in the form of a simple resistance measurement to ensure electrical continuity throughout the bag and to its earth bonding point. There is no single measurable parameter for FIBC not required to be earthed
(i.e. Type D). Instead the draft IEC standard employs a test procedure based on the use of a gas probe. A gas probe is a device, which streams a flammable gas mixture around an earthed spherical electrode. When the probe is brought up to an electrostatically charged surface, the earthed sphere may initiate a spark or brush discharge through the flammable gas mixture (see Figure 1). To be qualified as safe, any spark or brush discharge produced from the FIBC surface must not ignite the flammable atmosphere when charged under conditions expected during normal operation.
Thorough testing is required
The FIBC under test must be charged in a way that accurately simulates how it is charged in practice, both in terms of the rate of charging and the distribution of the charge. In use, FIBC are charged as a result of the inflow and outflow of charged powder. This is simulated in the IEC method by the use of polypropylene pellets. The pellets will naturally become charged through their interaction with transport pipes and chutes, a process known as tribocharging. However, the nature of the testing demands that the rate of charging should be representative of the worst case likely to be found in practice. For this reason, the natural pellet charging is enhanced by the injection of charge from a high voltage corona. As one might expect, such testing is not just performed once. To be certain that an FIBC will not produce incendiary discharges, at least 200 approaches with the gas probe must be made encompassing all components of the FIBC. Furthermore, the test conditions must be as severe, or more severe than may be expected in practice. The severity of the test is determined by
Charging currents vary according to the process involved and at worst will be in the region of 1 to 3 mA. As a minimum safety requirement, Type D FIBC must be tested at least under these conditions. Any FIBC not able to complete this test procedure, under conditions of realistic severity, without producing a single ignition cannot be qualified as Type D. In addition to pioneering Type D technology, LINQ Industrial Fabrics has played an active role in developing appropriate test methods. The test facility at LINQ’s South Carolina headquarters is fully compliant with the draft International Standard and LINQ’s technical experts are participating members of the joint IEC/ISO Working Group.
International standardisation is of vital importance to any industry. To quote from IEC’s mission statement: “The IEC’s multilateral conformity assessment schemes, based on its international standards, are truly global in concept and practice, reducing trade barriers caused by different certification criteria in various countries and helping industry to open up new markets. Removing the significant delays and costs of multiple testing and approval allows industry to be faster and cheaper to market with its products.” With the publication of CLC TR 50404 in 2003 and the development of the IEC testing standard, industry will have a formalised assessment scheme that ensures the electrostatic safety of FIBC and will provide guidance on the selection and safe use of FIBC for different applications.