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Sending Temperature Signal of Furnaces in Olefin to DCS
4-20 signal or mv to marshaling for transferring signals of temperature in furnaces to DCS.

In our plant thermocouple sensor are transferring mv to marshaling cabinet. Then a transmitter barrier change it to 4-20 and goes to control station. My question is it possible to change mv signal of sensor to 4-20 via transmitter in site near furnaces in Olefin plant to than transfer 4-20 to marshaling cabinet? This cause that we do not use thermocouple cable and instead use is cable which decreases cost but what about standards? Is there any standard that tell us to apply this method instead of 4-20?

Appreciate your help.

Use of local temperature transmitters suitable to the electrical class is common for the reasons you refer to.

Head mounted transmitters are not beneficial in severe services involving vibration or thermowell resonance.

>In our plant thermocouple sensor are transferring mv to marshaling cabinet.

Answer:
Thermocouples generate a mV signal. I assume this means that the raw thermocouple mV signal is sent to the marshalling cabinet. Is this correct?

>Then a transmitter barrier change it to 4-20 and goes to control station.

Answer:
A 'barrier' is normally an Intrinsic Safety barrier that provides safety, but does not provide conversion from mV to 4-20mA.
A 'transmitter' normally converts from mV to 4-20mA.

>My question is it possible to change mv signal of sensor to 4-20 via transmitter in site near furnaces in Olefin plant to than transfer 4-20 to marshaling cabinet?

Answer:
Yes, assuming there's a place to mount it where it won't be physically destroyed or cooked to death from ambient heat. 2 wire, loop powered transmitters are designed specifically for this task.

>This cause that we do not use thermocouple cable and instead use is cable which decreases cost but what about standards?

Answer:
If your plant has copper cable between the thermocouple and the 4-20mA transmitter, then you have a cold junction error of unknown magnitude
If your plant uses copper cable from the 4-20mA transmitter to the DCS/control room, that's standard practice.

>Is there any standard that tell us to apply this method instead of 4-20?

Answer:
Just about any web page on thermocouple fundamentals/basics will tell you that you need thermocouple extension wire between the thermocouple and the 4-20mA transmitter.

Answer:

yes this is correct and in marshaling there is a Universal Temperature Converter type KFD2-UT2-Ex2 that convert mv to 4-20. Then a transmitter barrier change it to 4-20 and goes to control station.

What about speed of transfer? I think mv goes faster than 4-20, and it is important in furnaces that we know instantly temperature which is very high about 850 degc. Maybe this is reason that designer bare the cost of laying a multicore thermocouple from site to marshaling to maintain high speed transfer. do you think so?

In furnaces is there any code standard for transferring temperature to c.c.r?

I've got one spec sheet on my desk for a DIN rail mount temperature transmitter that has a 135mS update rate, and another spec sheet for head mount TT has a 500mS update rate.

If update rate is important, then you'll have to shop for a transmitter that meets your requirements.