Are your temperature sensors safe?
12 October 2009
Chris Chant, business development manager at Okazaki Manufacturing Company (OMC) claims that companies are making mistakes when installing temperature sensors in hazardous areas. He outlines how they can ensure their temperature sensors are safe.
“When it comes to installing temperature sensors such as thermocouples or resistance thermometers [RTDs] in hazardous areas, many companies are still making mistakes with regard to equipment certification requirements. Some of our customers, for example, do not fully understand or are not aware of product certification requirements based on their Zone and classification choice. Also they are unaware of the new ATEX harmonised standards and how these standards affect equipment being sourced,” says Chant. He continues: “It is crucial that these companies understand the ATEX harmonised standards, and which supplier equipment is properly certified for the selected hazardous area and protection concept that they have selected, otherwise safety of installations cannot be guaranteed.”
According to Chant, companies that use temperature measurement products such as thermocouples and RTDs installed in potentially explosive atmospheres need to ensure that the sensor assembly (sensor, enclosure and terminal glands) actually meets the overall area classification requirements for each specific installation. In Europe, this is covered by the ATEX Directives, including the new ATEX harmonised standards.
Directive 94/9/EC covers equipment that is intended for use in potentially explosive atmospheres. The CE mark on equipment such as this represents the manufacturer’s declaration of compliance to this directive and to the EN standards that are harmonised to it. All equipment should be supplied with a declaration of conformity stating compliance with these standards and issued with product certification provided by a notified body such as BASEEFA or SIRA here in the UK.
If the chosen certification classification is Exia for use in zone 0, 1 or 2 then there is no need for product certification in the case of thermocouples or RTD sensors, but care must be taken to ensure the correct installation, with selection of an appropriate barrier, and to ensure that the IP rating of the termination is correct and that there is appropriate terminal clearance.
Of more concern is when thermocouples or RTD sensors are designated Exd (flameproof). In this case product certification is 100 per cent required. Care should be taken when using a terminal housing – the sensor should be terminated in a suitable Exd-certified enclosure. As Chant explains: “A standard temperature assembly such as this normally requires a spring-loaded sensor to ensure good contact is made with the tip of the thermowell. The base of the housing is therefore fitted with a flame path-controlled bore or collar that provides a flame path seal into the base of the housing, enabling free movement of the sensor via spring loading.”
However, Chant warns that this can lead to certification problems. “Although the enclosure rating is not affected, the inclusion of the sensor introduces a possible fault condition, which requires additional certification.”
OMC has overcome these challenges by having the complete assembly (sensor and enclosure) certified as Exde IIC T6, which provides Exd certification for the enclosure and Exe (increased safety) certification for the sensor. This means that the user can install the assembly in a Zone 1 hazardous area without requiring additional thermocouple or RTD IS safety barriers. If, for example, a head mounted, hockey puck style transmitter is installed, no extra certification is needed for the electronics module.
For many years, the EN50014 series of standards has been the backbone of ATEX compliance. However, by the end of 2008, most of the series was superseded by standards from the EN60079 series and has been withdrawn. This presents a potential problem for holders of ATEX EC type examination certificates to these withdrawn standards. To help customers, all OMC products have now been upgraded and are fully certified to the latest ATEX harmonised standards.
Other scenarios also require careful consideration. For example, if companies install the latest Hart and digital fieldbus transmitters (such as a Yokogawa YTA 110 and YTA 320 or an Emerson 3144P), care should be taken to ensure that approval of the complete assembly is certified, not just the transmitter. First, the integral transmitter and housing should carry its own certification, then the temperature sensor coupled to this transmitter should also carry its own certification or be glanded to the unit using an approved terminal gland.
To simplify installation, OMC has developed its FPN (flameproof nipple) range of thermocouple or RTD sensors. These units, which are pressure tested to 1,000psi, can be fitted to other manufacturers’ temperature transmitters, while still maintaining the spring loading feature and an IP66/67 rating, without requiring any compression glands to be fitted that would restrict movement of the sensor. Again, by working with BASEEFA, OMC has achieved Exde IIC T6 certification for the FPN range.
When terminating mineral-insulated (MI) thermocouples and RTD sensors into suitably certified Exd or Exe enclosures, engineers also need to ensure that the terminal glands carry the appropriate certification. The glands themselves need to be approved for use with sensors rather than just for cables. As Chant explains: “This is because, unlike cables, a temperature sensor is not terminated at each end and so can have different fault conditions. We offer a comprehensive range of thermocouples and RTDs that are certified as combined units with their own range of termination glands, giving an overall certification of Exde IIC T6. This greatly simplifies the whole compliance process for the customer.”
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