INTRODUCTION

Thermowells permit the removal of thermal elements for calibration, replacement, or repairs and also allow the use of portable sensors. The well or protection tube is permanently inserted into the pipe or vessel and is secured by threads, flanges, or welds, allowing the thermal element to be inserted into the well without causing any stress (Figure 4.14a). THERMOWELL TYPES Thermowells are usually metallic and may be coated with other materials to provide additional corrosion protection. Integral flanges or threaded sections enable the wells to be secured to the pipe or vessel (Figure 4.14b). Internal threads secure nipples which extend the head beyond any insulation. It is important to avoid contamination of temperature sensors by oil (such as cutting oils) left on the inner surface of drilled wells or accumulations of foreign materials within wells.Protection Tubes Protection tubes can also be ceramic when used to protect noble metal thermocouples (TCs) or as sighting tubes for radiation pyrometers. While not as strong as metallic thermowells, they do not droop and can withstand higher temperatures; in addition, they are free of contamination which can cause thermocouple drift due to vapor deposition of elements at higher temperatures. Mullite and high-purity alumina are commonly used as ceramic thermowell materials. In addition to being corrosion-resistant, mullite can operate up to 3200 ° F (1750 ° C) and alumina up to 3540 ° F (1950 ° C). When platinum TCs are used at temperatures exceeding 2200 ° F (1200 ° C), mullite should not be used, because it contains impurities that can contaminate platinum. For such applications, high-purity alumina is the proper choice. Dual protection tubes may be used where the outer tube provides mechanical protection and the inner tube provides corrosion or permeation protection. See Figure 4.14c for an example of protective layers used on a high-temperature TC.