Selecting the Right Sensor for Use in an Explosive Atmosphere

SENSOR FOR USE IN AN EXPLOSIVE ATMOSPHERE
10 November 2021

“Why worry about something that isn’t going to happen.”

A statement by Valery Legasov’s superiors when he questioned the design and operation of the RBMK reactors used at Chernobyl!   

That tragedy clearly demonstrates why we must minimise the risk of something going wrong when using equipment in hazardous environments.  

A hazardous area is an area in which an explosive atmosphere is present or may be expected to be present.  

Equipment that is to be used in such environments must be constructed, installed, and used according to standard AS/NZS 60079.0. 

If you are looking for a sensor for use in a hazardous area, there are several things you need to know about the environment: 

  • Is the substance a gas or liquid (Class I), or dust (Class II), or both? 
  • Will the hazardous substance be continually present, likely to occur in normal operation, or only likely to occur for a very short time (Zones)? 
  • Is the electrical energy within the equipment to be restricted to a level, which is below that what may cause an ignition or to limit the heating of the surface of the equipment, (Intrinsic Safety), or does the electronics need to be enclosed to contain the source of ignition (Flameproof)? 
  • What is the gas classification (Group) and/or minimum dust ignition energy (Cloud)? 
  • What is the auto-ignition temperature of the hazardous substance and the corresponding maximum surface temperature of the equipment (T Class)? 

In one instance ADM recently helped a customer, who required a platform scale and indicator for operation in Zone 1, together with the associated IECEx certification.  

Whilst ADM does not offer a complete system design service, we are able to offer solutions in accordance with the specifications determined by a qualified hazardous area system designer. 

The IECEx certification classifies the product according to the above criteria.  

The indicator needed to provide an analogue output to a PLC in a Safe Zone.   

In this case, no specific requirements were given about the hazardous environment. The customer had not yet engaged a qualified hazardous area engineer to classify the hazardous area, and advise the certification required of the equipment to be used.  

In such cases, ADM can offer practical solutions in the absence of the relevant information. However, customers must have a qualified hazardous area engineer ensure the products offered meet the required certification. 

The following diagram depicts the solution offered: 

IECEx rated weigh scale solution for use in explosive atmosphere
IECEx rated weigh scale solution for use in explosive atmosphere

 

The IECEx certification of the platform scale is as follows: 

  • Ex ia IIC T6:-  
    Ex ia (intrinsic safety), IIC (Gas group - Hydrogen), T6 (Temperature class - 85ᴼC max surface temperature) 

  • Ex ia IIIC T85ᴼC: 
    Ex ia (intrinsic safety), IIIC (Dust group – Fibers, Non-conductive dusts & conductive dusts), T85ᴼC (Temperature class - 85ᴼC max surface temperature) 

The IECEx certification of the flameproof indicator is as follows: 

  • Ex d [ia Ga] IIB+H2 T6 Gb:- 
    Ex d (Flameproof), [ia Ga] ia – (intrinsic safety), Ga (Equipment Protection Level two faults, gas), IIB (Gas group IIB plus hydrogen), T6 (Temperature class - 85ᴼC max surface temperature), Gb (Equipment Protection Level one fault) 

  • Ex tb [ia Da] IIIC T85°C Db IP66 (-20 °C≤Ta≤+40 °C):-  
    Ex tb (Enclosure for Zone 1 dust protection), [ia Da] ia – (intrinsic safety), Da (Equipment Protection Level two faults, dust), IIIC (Dust group – Fibres, Non-conductive dusts & conductive dusts), T85ᴼC (Temperature class - 85ᴼC max surface temperature), IP66 ingress protection, (-20 °C≤Ta≤+40 °C) certified at ambient temperatures Ta 

Inside the flameproof indicator there are 2 intrinsic safety barriers, which restrict the electrical energy for the load cell excitation and signal terminals: 

  • [Ex ia Ga] IIB, [Ex ia Ga] IIC, [Ex ia Da] IIIC (-20°C<=Ta<=+60°C) 
    [Ex ia Ga] IIB – ia (intrinsic safety), Ga (Equipment Protection Level two faults, gas), IIB (Gas group IIB); IIC (Gas group IIC); [Ex ia Da] IIIC - ia (intrinsic safety), Da (Equipment Protection Level two faults, dust), IIIC (Dust group – Fibres, Non-conductive dusts & conductive dusts); (-20 °C≤Ta≤+60 °C) certified at ambient temperatures Ta 

ADM’s primary concern is ensuring our customers have the most appropriate solution, which we are frequently able to supply in a timely manner from stock or one of our trusted suppliers.  

We are not interested in getting sales at any cost and would prefer to refer you to a supplier that we know can help, rather than offering something inappropriate.  

This is in the interest of our primary objective – Good Customer Service. For further information please contact ADM Instrument Engineering

IS THIS INFORMATION USEFUL?

If so, why not share it with your peers and colleagues. Simply click on the blue LinkedIn share icon below.