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SAFETY IN THE PLANT


Static grounding protection for Type C FIBC located in potentially flammable/ combustible gas or dust atmospheres


Type C bags are designed to dissipate static electricity through static dissipative threads that are interwoven through the bag’s material.


Grounding tabs located on the bags are points where grounding systems can be connected to ensure static electricity does not accumulate on the bag. To ensure bags destined for use in hazardous areas will not accumulate static electricity to hazardous levels there are several standards that provide guidance on the key parameters to which Type C bags must comply.


The primary standard IEC 61340-4-4 states that the resistance through the 8 bag should be less than 1 x 10 ohms (100 meg-ohm). This means that the resistance from a point on the bag to a grounding tab should never be higher than 100 meg-ohm. The latest edition of NFPA 77 recommends an upper value 7 resistance of 1 x 10 ohms.


contacts are interlocked with the process then the material cannot flow without the permission of the operator.


Are the Type C FIBCs 10 meg-ohm or 100 meg-ohm bags?


is to ensure that after many cycles of repeated use, the static dissipative threads are functioning correctly and, more importantly, to ensure that bags not of Type C construction are not permitted to be used in the hazardous area. Additional benefits with grounding systems are that they can control the movement of the powder through output contacts interlocked with valves or PLCs.


This standard has superseded the recommendations contained in the 2003 CENELEC technical report stipulates 8 a maximum value of 1 x 10 ohms .The latest edition of 7 TRGS 727 recommends 10 ohms value of resistance.


Checking and grounding Type C bags When a company wishes to utilise Type C bags they must provide a means for grounding the bag. This can be achieved with either passive (single pole clamp and cable) or active means (monitoring systems). However, given the scale of the charge that can build up on bags and the resulting energies that can be reached by static sparks, in combination with the presence of a combustible dust cloud, an active grounding system is the better choice. The primary benefit of checking the resistance through the bag


MARCH/APRIL 2020


The illustration highlights how a bag can be checked for its static dissipative capability in combination with providing active grounding of the bag. Following the connection of two quick release clamps, the Earth-Rite® FIBC system will identify if the bag is operating in accordance with the relevant standard. This is achieved by sending an Intrinsically Safe (Hazloc approved) signal (red line in the illustration) through the bag. If the green ground status indicators pulse continuously, the operators know the bag is grounded. The system verifies the grounding of the bag by ensuring the signal returns via a verified true earth ground. If there is any charge on the bag it will leave the bag via the static dissipative threads to the verified ground. If the output


The primary question to address when selecting a Type C FIBC grounding system is to determine which standard the bags in use are constructed to. Although bags manufactured in accordance with the 10 meg-ohm requirement are growing in number, there is a significant percentage of Type C FIBCs manufactured in accordance with the 100 meg-ohm requirement.


Ensure the grounding system you select is specific to the type of bag in use on site


If a grounding system is selected that monitors a narrow range of resistance, for example, monitors from 0 ohms up to 50 meg-ohms, this creates a problem and this problem could have two outcomes.


The second outcome is if 100 meg-ohm bags are specified for the site. As the grounding system has a cut-off resistance of 50 megohms, it will fail any bag that is operating between 50 meg-ohms and 100 meg-ohms. A direct consequence of this problem is that the system could reject a bag that is perfectly adequate and result in delayed operations while the operators are replacing the bag.


It is, therefore, of paramount importance to determine what types of Type C bags the site will be using. On that basis the site can select a system that will monitor the full range of 10 meg-ohm bags or a select a system that will monitor the full range of 100 meg- ohm bags.


For more information please contact: sophie.deronde@hoerbiger.com www.newson-gale.co.uk


Enquiry No. 31


www.reviewonline.uk.com


31


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