Thermowell fouling

If the exterior of the thermowell becomes fouled, the indicated temperature, generated by the thermocouple, will drop. The problem is that the external cap of the thermowell assembly radiates a small amount of heat to the atmosphere. Normally, this has a negligible effect on the indicated temperature. However, when the process temperature is 600 to 800°F, the thermowell is in a vapor phase, and the thermowell becomes coated with coke, I have seen the indicated tem-... 

When distilling heat-sensitive materials, a correct temperature indication near the bottom is critical. Under these conditions, thermowells are also most likely to foul, and consequently, read low. The low reading can mislead the operator or automatic controller into adding heat at a time when heat input needs cutting. In one peroxide service accident (97), this led to an explosion. In such services, additional temperature connections should be positioned a short distance above the bottom to permit cross-checks. 

Incorrect bottom (or reboiler) temperature indication This can be due to a faulty thermocouple, but a more likely cause is fouling of the thermowell. Ironically, bottom or reboiler thermocouple fouling tends to occur in services that are most vulnerable to a malfunction of this instrument, i.e., when heat-sensitive materials are distilled. A fouled thermowell will read low this in turn will enhance heat input into the column bottom, either automatically or by operator action. The greater heat input will accelerate thermal degradation in one peroxide service incident (97), it caused overheating and an explosion. 

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