Choosing Safe Static Control FIBC by Dr. Paul Holdstock, Texene LLC.

Static electricity is created whenever materials come into contact, rub together and separate. This is a process known as tribocharging. If one or other of the materials is insulating, the charges generated can remain in place indefinitely and can lead to hazardous electrostatic discharges. With powder and granular materials, the surface area available for charging is greatly increased compared to solid materials. When such materials are conveyed through ducts and chutes, there is plenty of opportunity for tribocharging to occur leaving the materials highly charged.

With the introduction of flexible intermediate bulk containers (FIBC) in the late 1960's, the need for managing static electricity became a prime concern. Polypropylene, which became the material of choice for FIBC, is highly insulating and will not naturally dissipate static electricity. Any charge generated when filling a standard polypropylene FIBC cannot be dissipated. Furthermore, when the FIBC is emptied, even more charge will be generated as the contents contact and separate from the sides and discharge spout of the FIBC. Highly charged materials will inevitably give rise to electrostatic discharges that can ignite combustible dusts and any solvent vapour that may be present when FIBC are emptied.

Evolution of Static Protective FIBC
In the 1970's the first generation of static protective FIBC started to appear. The original idea was to try to make FIBC more like metal containers. Some early designs used aluminised film laminated to polypropylene fabric to make flexible "metal" containers. However, these designs soon gave way to the now common practice of weaving a grid of conductive yarns into polypropylene fabric. The conductive grid in each panel of the FIBC, i.e. sides, top, base, spouts, etc., need to be connected together and to a common grounding point, which must then be securely connected to ground. These conductive FIBC for all practical purposes behave just like metal containers in that they collect charge on the conductive grid and transport it quickly to ground.

Introduction of Type C FIBC
By the 1980's, the use of conductive FIBC was becoming widespread and industrial accidents caused by incendiary discharges from FIBC were indeed reduced. However, incidents did still occur even with conductive FIBC. A group of process safety engineers working in large chemical companies in Germany and Switzerland helped to establish the requirements for safe conductive FIBC and introduced a classification system [1]. In their system, conductive FIBC were called Type C and they had to meet certain requirements with regard to the spacing and interconnection of conductive yarns and the electrical resistance between the conductive grid and ground. The classification and design requirements were later incorporated into International Standards [2 - 4].

Dangers of Type C FIBC
When designed and built to the right requirements and when properly and securely grounded by personnel handling the FIBC, Type C FIBC offer adequate protection against hazardous electrostatic discharges. However, even with established design requirements, there were still incidents of fires and explosions being caused by incendiary discharges from Type C FIBC. During the early 1990's, Dr. L.G. Britton reported case histories on several explosion incidents caused by failure to ground Type C FIBC in the publication Process Safety Progress [5]. The grounding of Type C FIBC is their main mechanism for dissipating static electricity, but it is also their major weakness.

1) Type C FIBC Must be Grounded to Work Safely
If Type C FIBC are not grounded, or if there is a break in the electrical connections between the various parts of the FIBC, charge will still accumulate on the conductive grid but it will not be able to find its way to ground. The result is that all the charge built up on the grid is available to form a spark that will release all the stored energy at once. Such sparks are more than capable of igniting combustible dusts and solvent vapours.

2) Manufacturing & Assembly is Critical to Performance
Careful attention needs to be paid to the manufacturing and operation of Type C FIBC to ensure that all parts are properly interconnected and that the FIBC is properly and securely grounded before all filling and emptying operations. In referring to the dangers of Type C FIBC, Dr. Britton stated "In particular it should be appreciated that the potential for operator grounding error and sudden nemesis can be very high".
One approach to the potential hazards associated with Type C FIBC, is to tighten up controls during manufacture and use. Reputable and safety conscious manufacturers will ensure their production staff are fully trained and informed of the necessity to ensure electrical interconnection throughout the entire FIBC, and they will undertake 100% inspection and testing of their Type C FIBC. Unfortunately, this is not always the case as they rely on single unit, manual manufacturing processes which can vary from FIBC to FIBC. Controls have been introduced by companies using Type C FIBC in the form of interlocks that prevent filling or emptying unless there is a secure ground connection in place on the FIBC. With rigorous manufacturing control, thorough operator training, and the use of special handling equipment, Type C FIBC can be used safely. The human factor is involved in all of these controls and so there is still a finite risk of human error.

3) Human Error
Dr. Britton's report raised serious concerns within a number of large companies that shipped their products in FIBC. They realised that even if they invested in all the equipment needed to handle Type C FIBC safely and required their FIBC suppliers to carryout 100% inspection and testing, they had no control over how their customers would handle the FIBC. Many of their customers would be small companies that would not invest in expensive handling systems and may not even train their operators correctly. FIBC were needed that were inherently safe and did not need the intervention of an operator. In other words, FIBC that provide full static protection without being grounded.

CROHMIQ® Static Protective Type D FIBC - A Better Solution
CROHMIQ® Static Protective Type D FIBC were the first to provide full control over static electricity without the need for grounding. CROHMIQ® FIBC dissipate electrostatic charge safely into the atmosphere via low energy corona discharging. The components used to dissipate charge are an integral part of the woven polypropylene fabric used to construct the FIBC. Unlike Type C FIBC, there is no need to electrically interconnect the different panels in CROHMIQ® FIBC, and so manufacturing errors are automatically eliminated. There is also no need to ground CROHMIQ® FIBC, and so the need for complex and expensive grounding systems is eliminated, as is the risk of human error. CROHMIQ® FIBC are intrinsically safe and require no input from operators when filling or when emptying.

CROHMIQ® FIBC are designed and engineered to provide total electrostatic safety across a wide range of industries without the need for grounding. This unique static protective technology was pioneered by CROHMIQ's engineers in 1995. Since then, CROHMIQ® FIBC have been used to package over 50 billion pounds of products for the world's most demanding industries, including pharmaceuticals, fine chemicals, pigments, and food products. In light of the safety record established by CROHMIQ®, the concept of a static protective FIBC that does not require grounding was formally recognised in 2003 by the inclusion of Type D within the classification system specified in International Standards [2 - 4].
In addition to meeting all electrostatic safety standards, CROHMIQ® FIBC comply with FDA and EU regulations for food contact and pharmaceutical applications.
CROHMIQ® FIBC are used throughout the world by major companies as an essential part of their regulation of combustible dust 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

References
[1] Mauer, B., Glor, M., Lüttgens, G. and Post, L., "Hazards associated with propagating brush discharges on flexible intermediate bulk containers, compounds and coated materials", Inst. Phys. Conf. Series No. 85, pp. 217-222, IOP Publishing Ltd 1987.
[2] CLC/TR 50404, Electrostatics - Code of practice for the avoidance of hazards due to static electricity.
[3] NFPA 654, Standard for the Prevention of Fires and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids.
[4] IEC 61340-4-4, Electrostatics - Part 4-4: Standard test methods for specific applications - Electrostatic classification of flexible intermediate bulk containers (FIBC).
[5] Britton, Laurence G., "Static hazards using flexible intermediate bulk containers for powder handling", Process Safety Progress, Vol. 12. No. 4, AIChE 1993.
CROHMIQ, CROHMIQ FIBC, CROHMIQ blue, CROHMIQ white & the blue fabric color are registered trademarks of Texene LLC. CROHMIQ technology is owned by Texene LLC and is protected by US Patents Nos. 5,478,154, 5,679,449, and 6,112,772, and additional foreign patents and patent pending.