Reducing the cost of Hazardous area temperature measurement and display

Process temperature measurement in hazardous areas is usually performed by a resistance thermometer or thermocouple feeding a locally mounted flameproof or intrinsically safe 4/20mA loop powered transmitter. Local temperature indication can be provided by a separate flameproof or intrinsically safe loop powered indicator connected in series with the loop and calibrated to display the loop current in the required units of measurement.

For many years BEKA have manufactured a range of intrinsically safe, loop powered indicating temperature transmitters which convert the output from a resistance thermometer or thermocouple into a 4/20mA current and provide an accurate local temperature display as shown in Fig 1.

Although the first generation BEKA transmitters simplified installation by combining the transmitter and indicator in one enclosure, recent customer feedback suggested that following the trend to exclude operators from hazardous process areas, local temperature displays are now more frequently required in Zone 2 or Zone 22, but temperature sensors remain in the more hazardous Zone 1, or even in Zone 0 for tank or pipeline measurements.

Customers also emphasised that the local display must be large and very easy to read and they were also, not surprisingly, keen to reduce both purchase and installation costs. Some also requested HART® communication for conditioning and asset management, simplified documentation was also requested.

To satisfy these customer requirements, BEKA have introduced a range of second generation field mounting loop powered 4/20mA indicating temperature transmitters. Models employing combined hazardous area protection techniques achieve the requested performance and significantly reduce the loop cost by removing the need for a Zener barrier or galvanic isolator in many applications.

The new BA474ND indicating temperature transmitter may be installed in a Zone 2 or 22 hazardous area and the input may be directly connected to a resistance thermometer or a thermocouple located in any gas or dust hazardous area as shown in Fig 2, all without the need for a Zener barrier or galvanic isolator.

The new transmitter has a five digit 20mm high display which is supplemented by a 31 segment bargraph, both can be illuminated by an optional loop powered backlight to provide a large easy to read display in all lighting conditions. HART® communication complying with the latest revision 7 protocol is included. The transmitter also includes diagnostics that report the health of both the transmitter and the temperature sensor.

Hazardous area safety is achieved by multiple protection techniques. The transmitter employs 'Type of protection nL' which is an energy limiting technique based on the philosophy of intrinsic safety. This allows installation in Zone 2 gas hazardous areas without the need for energy limitation by a Zener barrier or a galvanic isolator in the non-hazardous safe area. To permit the transmitter to be directly connected to a temperature sensor located in a Zone 0 or Zone 1 gas hazardous area, the transmitter has an isolated intrinsically safe input. This is achieved with internal segregated optical and transformer isolation, plus duplicate voltage and current imitation, resulting in an Ex ia IIC input that may be connected to an earthed or floating sensor in any hazardous gas.

For application involving combustible dusts, the transmitter enclosure complies with the requirements for 'Protection by enclosures tD' which permits installation in Zone 22, again without the need for Zener barrier or galvanic isolator energy limitation in the non-hazardous safe area. The transmitter's isolated intrinsically safe input complies with dust requirements and may also be directly connected to a temperature sensor located in Zone 20 or 21 dust hazardous areas.

The new BA474ND indicating temperature transmitter has already been awarded ATEX and IECEx certification permitting installation throughout Europe and in many other countries that accept IECEx certification, or use an IECEx certificate as the basis for local approval. Finally, this new indicating temperature transmitter has an easy to understand safety certificate, which simplifies loop design, installation and loop documentation.

For applications requiring the display of a hazardous area temperature in a safe non-hazardous area, BEKA have extended the technique to another new indicating temperature transmitter, the BA474D. This model complies with the requirements for intrinsic safety associated apparatus having an intrinsically safe galvanically isolated input. When mounted in a safe non-hazardous area the transmitter input may be directly connected to a resistance thermometer or thermocouple located in Zones 0, 1, 2, 20, 21 or 22 as shown in Fig 3.

The inclusion of isolation and intrinsic safety voltage and current limiting within the instrument eliminates the need for a separate Zener barrier or galvanic isolator, again reducing the loop cost and simplifying installation. This model has also been issued with ATEX and IECEx certification and North American approval is anticipated.

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