Thermowell - Compliance with Temperature Class T3
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Question

Thermowell - Compliance with Temperature Class T3

patchlam (Chemical)

(OP)

16 Oct 15 02:38

Hi there,

I am currently trying to employ a thermowell into my process to measure the flue gas temperature which has the range of 490-521 degree Celsius. However, the specification requires all the instruments to be in compliance with temperature class T3 which is 200 degree Celsius. Noted that the process pipe that I am trying to insert the thermowell is insulated internally. Therefore, I am wondering is that any possibility that the heat within the flue gas will be transferred to thermowell and eventually to the connection head via conduction?
If yes, does it mean that the surface temperature of the thermowell connection head will be more than 200 degree Celsius and it is not in compliance with that temperature class anymore?

Thanks for all your time!

danw2 (Industrial) · Answer 1 · 2015-10-16 03:43:17

The graph below is from the manual for the Rosemount 148 RTD transmitter and well assembly.
Link

The temperature rise effect due to process temperature for a 540 Deg C process temp is a temperature rise above ambient of only 32 Deg C with 75mm of extension length.

I guess a little lag will do you.

To get to 200 Deg C at the head would require a very high ambient temperature and a design quite different than the 148.

It appears that the model 148 appears includes a standard extension (according to assembly diagrams in the same manual) that acts as a lag, but it isn't clear whether the chart is for the 'standard' extension length or standard plus the additional.

Anecdotally, my wife bought one of those solid stainless frying pans with an uninsulated, polished solid stainless steel handle. The handle conducts heat surprisingly poorly; with no danger of burning one's hand.

patchlam (Chemical) · Answer 2 · 2015-10-16 06:00:54

This is super helpful! Just that I am wondering, what are the factors that actually minimize the heat conduction to the connection head? The temperature rise remains the same even the transmitter is in continuous operation and exposed to 540 deg C for months?

Michael2009 (Petroleum) · Answer 3 · 2015-11-16 10:58:48

This topics is interesting.
What is fuel gas flashing point? Can fuel gas heat up to 490-521 degC? is it normal process?
Sorry. I am not process engineer. but I am curiously on the application.

btw the Rosemount heat transfer graph only showing heater transfer from element tip to RTD casing head. How about thermowell surface? in this case, the thermowell flange face should insulate also?

patchlam (Chemical) · Answer 4 · 2015-11-17 00:42:33

Hi Michael,
I am not sure about the flashing point. Basically the fuel gas is a natural gas. And the flue gas produced can reach the temperature until 600 degC. I guess it is normal in the gas turbine operation.

vichoo84 (Electrical) · Answer 5 · 2015-11-19 12:57:56

Hi,

Please tell me what is Internally insulated.
As u mentioned of high temperature it will definitely affect the transmitter.
Normally for this kind of high temperature service the transmitter shall be remote mounted.

patchlam (Chemical) · Answer 6 · 2015-11-20 00:45:16

Basically, internally insulation means the insulation layer is adhered at the internal surface of the process pipe or ducting. May I know what is the idea of remote mounted transmitter then?

danw2 (Industrial) · Answer 7 · 2015-11-20 17:32:57

>remote mounted transmitter?

The thermowell mounts in the process with the sensor (RTD or thermocouple) and a terminal block in the head:

The temperature transmitter is mounted remotely, sometimes using pipe mount brackets,

sometimes flange mount for wall mounting,

sometimes DIN rail mount if there's a panel in the vicinity:

Cabling - copper wire for RTDs, thermocouple wire for T/Cs - connects the sensor to the transmitter

Assemblies with integral cabling are available, but I rarely see them:

The point being that the transmitter is removed from heat generated by the process.

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Thermowell - Compliance with Temperature Class T3
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