This website uses cookies primarily for visitor analytics. Certain pages will ask you to fill in contact details to receive additional information. On these pages you have the option of having the site log your details for future visits. Indicating you want the site to remember your details will place a cookie on your device. To view our full cookie policy, please click here. You can also view it at any time by going to our Contact Us page.

Baseefa Ltd

Electrostatic hazards and control techniques associated with bulk bags

12 December 2014

Bulk bags constructed with woven fabric are very widely used in industry to package, store and ship powder and granular materials. The bags are prone to generating static charge during material loading and unloading operations, and, if allowed to accumulate, an electrostatic discharge could occur from the bag surface. 

This article, by Muhammad M. R. Qureshi of Chilworth Technology Inc., provides an overview of the types of static discharges that could originate from the different types of FIBCs and the measures to control the electrostatic ignition hazards of these bulk bags. The article also lists the testing requirements according to IEC 61340-4-4 Edition 2.0 requirements for each type of bulk bag and provides the recommended practice in selecting a particular type of bulk bag, depending on the sensitivity of flammable atmosphere. 

Introduction

Flexible Intermediate Bulk Containers (FIBCs) also known as bulk bags, super sacks, large totes, etc. have a very wide applications for packaging, storing and transporting of powders (fines, granular materials, pellets). Such bags are used with various types of material such as food-grade, chemical, inorganic, petrochemical, etc. These bags provide advantages of being collapsible (requiring less space for storage after use), cost-effective (disposable) and flexible in storing various shapes and sizes of materials, as well.

Due to these factors, the bags are preferred by many industries; however, they do not come without certain challenges: such as (1) having the potential to generating static electrical discharges which can result in discharges that can be potential ignition source for a flammable atmosphere that could exist within or around the bag (2) low physical/mechanical strength; (3) reduced shelf life, not reusable (unless tested) etc.

The use of bulk bags in industrial applications is increasing as the advantages of their use generally outweigh the disadvantages; however, it is very important that each application is assessed individually and that electrostatic hazards are controlled when using bulk bags for packaging and transporting combustible/explosible materials and/or using the bulk bags where flammable gas, vapour, and dust cloud atmospheres are present.

Electrostatic Discharges

Electrostatic charge generation normally occurs due to the process of contact and separation which takes place between individual powder particles and the conveying equipment upstream of the bulk bag, during filling and between the powder particles and the internal surfaces of the bulk bag, during emptying.

The types of discharges that may occur due to the buildup of electrostatic charge on the bulk bag include:

•Brush Discharges from the surfaces of standard insulating bulk bags and liners.  Brush discharges can ignite flammable gas and vapor atmospheres requiring up to approximately 4mJ for ignition.

•Propagating Brush Discharges from the surfaces of insulating bulk bags and liners, which have electrical breakdown voltage greater than 6,000 volts. Propagating brush discharges could have 1000 – 2000mJ of energy and can readily ignite flammable vapor, gas and dust cloud atmospheres.

•Spark Discharges from conductive parts of groundable bags if left ungrounded. These types of discharges can ignite gases, vapors and many combustible dusts.

What should I know before selecting a bulk bag?

Bulk bags should be selected according to the electrostatic and flammability characteristics of the contained powders and the presence of flammable gas/vapor atmospheres in the location where they are used. You should know the following:

•Is the powder material being packaged/transported a combustible/explosible material?

The dust (fine particulates) of most materials used in industrial application is explosible. More than 70% of the powder materials being used in general process industries are explosible when dispersed in air. However, if you are unsure whether your material is explosible or not, a Go/No Go explosion screening test in accordance with American Society for Testing and Materials (ASTM) E1226-12a: “Standard Test Method for Explosibility of Dust Clouds” can be conducted to determine if the material is explosible or not.

•What is the Minimum Ignition Energy (MIE) of the dust material?

The minimum ignition energy (MIE) is the lowest electrostatic spark energy capable of igniting a dust cloud.  The test (conducted in accordance with ASTM E2019) is used primarily to assess the potential vulnerability of powders and dusts to ignition by electrostatic discharges.

•Are flammable gases/vapours present inside the bulk bag or in the surrounding area?

The loading and unloading operations involving the bulk bags should be evaluated to determine if flammable gases/vapors could be present in the area. In some cases, the powder material being packaged might release off-gases over time which could be flammable or powder may be directly transferred from the bulk bag to a vessel containing a flammable liquid. Hence each case should be thoroughly evaluated to determine the presence of flammable gases and associated MIE.

•What are the electrostatic characteristics of powder materials?

For materials with MIE <25mJ, it is also important to know the ability of the powder particles to generate (Electrostatic Chargeability test conducted in general accordance with ASTM D257) and retain (Volume Resistivity and Charge Relaxation Time tests also in general accordance with ASTM D257) electrostatic charges during processing, filling and emptying conditions. This information can be utilized to determine the possibility of generating electrostatic discharges (an ignition source) from the material itself during filling and emptying of bulk bags. For example, conductive (e.g., metal) powders can generate spark discharges when handled in insulating bulk bags. 

Types of bulk bags and Selection Criteria

There are currently four types of bulk bags; namely Type A, B, C and D. Table 1 below lists the four types of bulk bags, their features and the recommended qualification tests.

Important Notes

•If left electrically ungrounded, type C bulk bags (groundable bags) can give rise to a higher explosion risk than type A and B (insulating) bulk bags.

•It is not possible to make general recommendations for the safe use of type D bulk bags under all operating conditions.  In particular:

- The existing type D bulk bags that incorporate antistatic yarns and an antistatic coating can still create a localized electrostatic field around the bulk bag. Ungrounded conductive objects that may be present in the close proximity of such charged bulk bag could become polarized and become charged by the process of induction. Such charged objects could give rise to subsequent spark discharges which could be a potential ignition source. Hence it is recommended that conductive objects (e.g., tools, portable containers, operators etc.) in close proximity of type D bulk bag need to be electrically grounded.

Table 1: Various Types of Bulk Bags
Table 1: Various Types of Bulk Bags

- Incendive sparks may occur during filling and emptying of type D bulk bags if the surface of the bulk bag is partially covered by an electrically conductive substance (e.g. a wet patch). 

•Irrespective of the type of bulk bag used, and depending on the electrostatic properties of the powder and the powder handling equipment (for example, whether the plant is conductive or insulating, grounded or ungrounded) other electrostatic ignition sources may still exist.

•Conductive powders (having volume resistivity < 106 Ohm.m) could accumulate electrostatic charges in type A, B or even D bulk bags (in the absence of direct ground connection), it is recommended to use only type C bulk bag with conductive powders to avoid spark discharges from the powder material itself.

Use of Inside Liners in bulk bags

The use of liners in any type of bulk bag can increase the risk of an electrostatically initiated flash fire or explosion. There are three types of liners that are permitted to be used in a particular type of bulk bag depending on the electrostatic properties of the liner material. Depending on the liner type, the following physical and electrostatic properties may need to be determined ensure the suitable type of liners;

•Surface Resistivity,

•Breakdown Voltage,

•Liner Thickness,

•Resistance to Groundable Point

These tests results provide valuable information that will be utilized to determine the safe use of a particular type of liner with a specific type of bulk bag.

Qualification Criteria (per IEC 61340-4-4 Ed 2.0: Sec 4.2) for Liners inside the bulk bags

Type L1 Liner (Conductive)

A type L1 Liner should meet the following criterion:

•Made from materials with Surface Resistivity on at least one surface = 1.0x107 Ohm per Square,

•If the material is multi-layered, or if the material has outer surface with Surface Resistivity >1.0x1012 Ohm per Square, Breakdown Voltage through the material shall be < 4KV

•If the material is multi-layered, or if the material has inner surface with Surface Resistivity >1.0x1012 Ohm per Square, Breakdown Voltage through the material shall be < 4KV and Thickness shall be <700 micrometer

Table 2: Permissible Combination of a Liner and Bulk Bag
Table 2: Permissible Combination of a Liner and Bulk Bag

Type L2 Liner (semi-conductive/static dissipative)

A type L2 Liner should meet the following criterion:

•Made from materials with Surface Resistivity on at least one surface between 1.0x109 and 1.0x1012 Ohm per Square.

•If the material is multi-layered, or if the material has outer surface with Surface Resistivity >1.0x1012 Ohm per Square, Breakdown Voltage  through the material shall be < 4KV

•If the material is multi-layered, or if the material has inner surface with Surface Resistivity >1.0x1012 Ohm per Square, Breakdown Voltage  through the material shall be < 4KV and Thickness shall be <700 micrometer

•Liner testing results must be reported for BOTH sides of the liner and show highest and lowest results (to determine if it is L1 or L2 liner)

Type L3 Liner (Insulating)

A type L3 Liner should meet the following criterion:

•Made from materials with Surface Resistivity greater than 1.0x1012 Ohm per Square,

•Breakdown Voltage through the material shall be less than 4KV

Selection Criteria (per IEC 61340-4-4 Ed. 2.0) for Bulk Bag and Liner Combination

Once the type of a particular liner and the bulk bag has been determined (using the tests discussed above), the following table (Table 2) can be utilized to determine the permissible combination of a liner with a specific type of bulk bag.

There are additional requirements (Ref: IEC 61340-4-4 Ed 2.0: Sec 6 - 9) for labelling and testing a particular type of bulk bag. It is recommended that a thorough risk assessment be conducted before using a bulk bag in potentially hazardous environments. Selecting and using a suitable type of bulk bag and Liner combination is one of the important steps to prevent the risk of fire and explosion.

Safe use of FIBC

The requirements and specifications that the FIBC must meet and the ways in which they are used depend on the nature and sensitivity of any explosive atmosphere present during filling and emptying. The final goal for the construction of FIBC is to exclude incendiary discharges from the FIBC fabric during their intended use. The conditions under which each type of FIBC should be used are shown in Table 3 below.

Table 3: Selection Criteria for Different Types of Bulk Bags
Table 3: Selection Criteria for Different Types of Bulk Bags

Hazards of Operators working 

The human body is a conductor and subject to charge accumulation. If an operator is isolated from ground, that person can accumulate a significant charge by touching a charged object (insulating bulk bag, ungrounded bag), by momentarily touching a grounded object in the presence of charges in the environment (e.g., those environment could exist in close proximity of insulating bags or even type D bags), walking on an insulating surface or by brushing surfaces while wearing nonconductive clothing.

During normal activity, the potential of the human body can reach 10 kV to 15 kV and the practical limit of the potential spark discharge is generally accepted to be 20 to 30 mJ.

However, the grounding of personnel is generally recommended when they may be exposed to a flammable atmosphere having a Minimum Ignition Energy (MIE) of < 50 mJ. Flammable gases and vapors could have an MIE on the order of < 1 mJ.

Therefore, it is strongly recommended that all personnel working in the manufacturing areas of the plant (including those around the bulk bags loading and unloading operations where MIE of a potential flammable atmosphere is < 50 mJ) should be grounded. Grounding of personnel is typically accomplished through the use of grounding wrist straps or the combination of anti-static footwear and anti-static flooring/grounded conductive mats. 

References

•NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids2013.

•IEC 61340-4-4 Edition 2.0 2012-01: Electrostatics –Part 4-4: Standard test methods for specific applications – Electrostatic classification of flexible intermediate bulk containers (FIBC).

•Safety Watch: “FIBC Selection and Safe Use” A focus article by Muhammad M. Rafique Qureshi, published online by Chilworth Technology, Inc. Jan 2013.

About the author:

Muhammad M. R. Qureshi is an engineering professional with over seven years of Process Safety Engineering and Project Management experience. His expertise includes assessing process safety hazards related to fire and explosion hazards at diverse manufacturing facilities.

He is currently Manager, Consulting & Training Service, at DEKRA affiliate Chilworth Technology Inc. based in Princeton, New Jersey.


Print this page | E-mail this page

CSA Sira Test