TESTING & CERTIFYING THE SAFETY OF STATIC PROTECTIVE FIBC
TESTING & CERTIFYING THE SAFETY OF STATIC PROTECTIVE FIBC by Dr. Paul Holdstock, Texene LLC.
The following article was written prior to the publication of the second edition of IEC 61340-4-4 in January 2012. The description of the testing and certifying procedures for Type D FIBC contained in the article are still valid because the second edition of the standard did not change these requirements.
The main changes in IEC 61340-4-4 Ed. 2.0 with respect to the first edition are:
a) Adoption of a type classification system for FIBC based on four types: A, B, C and D.
b) Guidance for safe use of FIBC in relation to hazardous areas and hazardous zones defined in IEC 60079-10-1 and IEC 60079-10-2 is added (similar to the guidance given in CLC/TR 50404).
c) Resistance to groudable points limit for Type C FIBC is reduced from 108 ohm to 107 ohm.
d) Resistance to groundable points and electrical breakdown voltage measurements on FIBC shall be measured at low humidity only.
e) Requirements for labelling FIBC are changed to improve clarity and ease of recognition by end users.
· Only official classification, i.e. Type A, Type B, Type C or Type D, can be used on FIBC labels.
· Labelling as D+, Dplus, CD, etc. is not permitted.
f) Classification, performance requirements and guidance for safe use of inner liners in combination with FIBC are added.
g) An informative annex giving guidance on test methods for quality control and inspection testing is added.
Industry is now familiar with the different types of static
protective FIBC known as Type C and Type D. Perhaps less well know is
how the different types are tested and certified for safe use in
hazardous flammable environments. Type C FIBC, which rely on a
conductive path to ground to safely dissipate electrostatic charge can
be tested by measuring electrical resistance. However, until recently
manufacturers had their own ways of doing this, some of which were not
entirely suitable. For example, one commonly used method involved
placing the FIBC flat on a table and measuring the resistance of the
fabric surface using an electrode arrangement of a cylindrical
electrode surrounded by a ring electrode. Whilst this is a standard
arrangement for many textile applications, it is of limited use for
measurements on FIBC. The electrode arrangement only measures across a
small area of fabric, and so it can only be used to confirm that small
areas of fabric are sufficiently conductive. Such an arrangement cannot
determine the integrity of ground connections throughout the entire
FIBC. Even if measurements are made with widely spaced electrodes, i.e.
point-to-point resistance measurements, the fact that the FIBC is lying
flat on a table may cause connections that would not be there when the
FIBC is opened out and filled with product.
Testing of Type D FIBC was even more irregular. Unlike Type C FIBC, where the mechanism of charge dissipation is directly related to a single, measurable physical quantity, i.e. resistance, no such relationship exists for Type D FIBC. Charge dissipation in Type D FIBC is a result of a process that is dependent on geometrical and electrical parameters. Consequently, a variety of measurements were used by test laboratories in an attempt to determine safety performance. Measurements such as surface potential, charge decay time, charge transfer, etc. were all used. The problem was that none of these measurements could really answer the fundamental question of whether or not the ungrounded FIBC could be used in flammable environments without producing incendiary discharges. It was, therefore, almost impossible for end users to make a meaningful decision about the safety of Type D FIBC, or indeed how one FIBC compared to others.
Ignition testing has become the preferred test procedure for Type D FIBC. The principle behind ignition testing is to charge an FIBC, present a flammable atmosphere and try to provoke an electrostatic discharge. Several test laboratories offer ignition testing, but there are variations in their test parameters and conditions. An FIBC that passed in one laboratory may have failed in another.
IEC 61340-4-4 - The Standard for Testing Static Protective FIBC
In 1999, the International Electrotechnical Commission (IEC) Technical Committee on Electrostatics (TC 101) launched a project to standardise test procedures for static protective FIBC. The project culminated in the publication in 2005 of the International Standard IEC 61340-4-4:2005 Electrostatics - Part 4-4: Standard test methods for specific applications - Electrostatic classification of flexible intermediate bulk containers (FIBC). This important International Standard specifies test procedures for FIBC that must be grounded (Type C) and those for which ground connection is not necessary (Type D). The test procedure specified for Type C FIBC is based on resistance measurements, and ignition testing is specified for Type D FIBC. In addition, to these tests, the fabric used in the construction of both Type C and Type D FIBC must be tested to determine breakdown voltage. Breakdown voltage of below 6 kV ensures that FIBC cannot generate highly energetic propagating brush discharges.
Testing for Type C
The resistance measurement procedure for Type C FIBC requires the FIBC to be suspended from its lifting loops as it would be in normal operation. Measurements are made between conducting yarns and each of the designated ground bonding points. Testing in this way ensures not only that the fabric is sufficiently conducting, but also that there is electrical continuity throughout the entire FIBC. Measurements are made at both low humidity (20 ± 5 % RH) and high humidity (60 ± 10 % RH). Type C FIBC can only be qualified under IEC 61340-4-4:2005 if all the measured resistance values are less than 10^8, and the breakdown voltage is less than 6 kV.
Testing for Type D
The two main parameters that define the challenge of ignition testing is the charging current, i.e. how much charge is delivered to the FIBC under test, and the minimum ignition energy (MIE) of the challenge gas, i.e. how easy it is to ignite. Prior to the introduction of IEC 61340-4-4:2005, charging currents used by some test laboratories were quite low, often less than 1 µA (microamp). The reason behind this choice of charging current was that it was sufficient to cause ignition from a plain non-static protective FIBC (i.e. Type A). However, testing with such low charging currents can only show that an FIBC is somewhat better than a Type A FIBC. In order to be qualified as safe, FIBC must be challenged with a charging current that is representative of the worst case likely to be found in practice. Data from end users confirm that 3 µA is a better representation of the highest charging current found in FIBC handling operations. A similar argument was used to define the MIE of the challenge gas. The most easily ignitable solvent vapour likely to be used in FIBC handling operations is methanol heated above ambient temperature, which has a MIE of about 0.14 mJ. This is the value specified for ignition testing in IEC 61340-4-4:2005, although it is achieved by using a 5.4% mixture of ethylene in air. The gas mixture, flow rate and means of control are all specified in the IEC standard to ensure the accuracy, reproducibility of test results, and simulation of the range of environments to which the FIBC is exposed.
The procedure for ignition testing is to fill the FIBC under test with polypropylene pellets of a specified size range into which charge is injected at a rate of -3 µA. Negative polarity is used because research data predicts that electrostatic discharges from negatively charged materials are more likely to cause ignition than those from positively charged surfaces. For Type D FIBC, the testing is carried out with the FIBC completely isolated from ground. Charging in this way is a good simulation of actual FIBC handling operations. As the FIBC is being filled with charged pellets, a gas probe is brought up to the side of the FIBC. The gas probe contains a grounded, spherical electrode surrounded by a shroud that directs a flow of flammable gas in front of the electrode. Any electrostatic discharge that is generated when the probe approaches the FIBC will pass through the localised flammable atmosphere and if it contains sufficient energy it will cause the ignition. If the FIBC under test is a correctly designed Type D, then no ignition will occur. However, to prove this the gas probe must be brought up to different areas covering all parts of the FIBC in multiple approaches. IEC 61340-4-4:2005 specifies that at least 50 approaches must be made to each side of the FIBC, and at least 10 additional approaches to all other panels, spouts and other accessories on the FIBC. As with resistance measurements, ignition testing must be done at both low and high humidity. It is also a requirement of the IEC standard for ignition testing to be carried out on the largest and smallest size of FIBC of a particular design. On a typical FIBC, about 400 to 500 gas probe approaches must be made. If any one of these approaches results in an ignition then the FIBC is considered to have failed. Type D FIBC can only be qualified under IEC 61340-4-4:2005 if no ignition occurs during ignition testing, and the breakdown voltage is less than 6 kV.
In addition to specifying test procedures and performance requirements, IEC 61340-4-4:2005 also specifies how FIBC that have been qualified in accordance with the standard should be labelled.
Only FIBC that have been qualified exactly in accordance with the procedures specified in IEC 61340-4-4:2005 can legitimately be labelled as such. FIBC that have not been tested in strict accordance with the IEC standard cannot be labelled in this way. Qualification to IEC 61340-4-4:2005 must be supported by test reports that fulfil all the requirements of the standard. Older test reports, even if the test procedures are similar to those specified in the standard, cannot be legitimately used to support qualification to IEC 61340-4-4:2005. This is because older test procedures may not involve the rigorous control and calibration requirements specified in the IEC standard, and may not produce equivalent results. It is, therefore, important when checking test reports to ensure that all the information is reported in accordance with the IEC standard. That includes specific details of the test parameters and conditions, which for ignition testing should include, temperature and relative humidity, filling rate, charging current, gas composition, gas flow rate, MIE and number and location of gas probe approaches. A summary of the test parameters and conditions specified in IEC 61340-4-4:2005 is shown below:
Temperature / Humidity: a) 23 ± 2 °C and 20 ± 5 %RH; b) 23 ± 2 °C and 60 ± 10 %RH
Filling rate: 1.1 ± 0.1 kg/s
Charging current: 3.0 ± 0.1 µA (negative polarity)
Gas composition: 5.4 ± 0.1 % ethylene (balance air)
Gas flow rate: 0.21 ± 0.04 l/s
Minimum ignition energy: 0.14 ± 0.01 mJ
Number of gas probe approaches: At least 200 at each humidity level
There exists on the market today FIBC that are sold as Type D FIBC but do not comply with the requirements of IEC 61340-4-4:2005. Some of these FIBC existed before the standard was published and their manufacturers have not improved their designs in order to meet the standard. Others have been designed in an attempt to get round the standard. The purpose of any standard is to set minimum acceptable levels of performance without placing an undue burden on manufacturers. In the case of IEC 61340-4-4:2005, the purpose is to establish a minimum level of safety to ensure that FIBC tested using the specified methods and meeting the specified performance requirements are qualified as safe for use in a broad range of industries. The test methods and associated limit values were established by an international committee of experts encompassing FIBC manufacturers, test laboratories, process safety engineers and FIBC end users. It is fair to say that IEC 61340-4-4:2005 represents the current state of knowledge as to what constitutes a safe static protective FIBC. A great deal of caution, therefore, needs to be exercised when considering FIBC that do not comply with the IEC standard.
Some FIBC manufacturers have tried to invent their own classification and label their FIBC as D+, Dplus, CD, etc. They make claims to the effect that their FIBC are Type C that do not need to be grounded, or are groundable Type D. For a start, none of these designations are recognised in any national or international standard. The requirements for Type C FIBC are very clearly defined in IEC 61340-4-4:2005. For a Type C FIBC to meet the required safety standard, the resistance to ground must be less than 10^8 and the FIBC must be grounded during normal operations. If the resistance to ground is greater than 10^8, the FIBC cannot be legitimately described as Type C. If a Type C FIBC is used without a proper ground connection, incendiary sparks will be produced. The requirements for Type D FIBC are also clearly defined in the IEC; they must pass the specified ignition test without being grounded. If an FIBC needs to be grounded in order to pass the IEC standard ignition test, then it cannot be legitimately described as Type D. Logically, if a FIBC cannot meet the basic safety requirements of Type D, it is nonsense to describe it as D+. The use of non-standard classification on FIBC is a transparent marketing gimmick for FIBC that do not comply with recognised safety standards.
How to Identify FIBC Qualified to IEC 61340-4-4:2005
There are three components to establishing whether or not an FIBC has been qualified according to IEC 61340-4-4:2005:-
1) A label on the FIBC containing at least the information specified in IEC 61340-4-4:2005;
2) A test report showing results of breakdown voltage measurements (Type C & Type D), and resistance measurements (Type C) or ignition testing (Type D);
3) A test certificate confirming that the FIBC meets all the requirements of IEC 61340-4-4:2005. This may be a separate certificate or incorporated as part of the test report.
A valid test report must contain details of all the relevant test parameters and conditions specified in IEC 61340-4-4:2005 and a full description of the FIBC under test. If a separate test certificate is provided, the test report on which it is based must be available for inspection. There are examples of test certificates that imply testing was done according to IEC 61340-4-4:2005 but when test reports are inspected it is clear that there a significant deviations from specified procedures that render the test report invalid as a means of supporting qualification to IEC 61340-4-4:2005.
Testing & Certifying CROHMIQ® Static Protective Type D FIBC
Texene LLC is the only company in the FIBC industry to own and operate its own state-of-the-art FIBC testing facility that is designed and built to meet the exact requirements of IEC 61340-4-4:2005. The testing facility is used to qualify FIBC made from CROHMIQ® Static Protective FIBC Fabrics, for quality control testing, and to support the Texene Continuous Safety Certification Programme™ (CSC). The CSC Programme is a unique, complimentary service provided by Texene to users of CROHMIQ® Static Protective Type D FIBC. The first stage of CSC is the initial safety qualification. Each design of CROHMIQ® FIBC is tested according to IEC 61340-4-4:2005 prior to despatch of the first shipment to the customer. Initial qualification is achieved by testing 3 samples of each FIBC design; each one must pass. The customer is provided with a full test report and a summary test certificate. The second stage of CSC is the continuous re-qualification of CROHMIQ® FIBC. At regular intervals agreed with the customer, e.g. every six months, or every 10,000 FIBC delivered, the customer sends one sample of each CROHMIQ® FIBC design to Texene for re-qualification. Testing is again conducted according to IEC 61340-4-4:2005 and the customer is provided with a summary certificate and full test report for each re-qualification test.
CROHMIQ® FIBC are used throughout the world by major companies as an essential part of their regulation of electrostatic hazards. Thanks to their outstanding safety record, CROHMIQ® FIBC are widely acknowledged as the safest and most cost effective solution for controlling static electricity in FIBC handling operations.
For more information about CROHMIQ® FIBC fabrics, visit www.crohmiq.com.
TEXENE LLC is a leading manufacturer of technical textiles focused on delivering precision engineered, safety performance products worldwide.