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. The sheaths are either extruded or woven and are used to protect TCs or other wires. Woven sheaths are similar to those used for electrical insulation and may be made of stainless steel, Inconel, tinned copper, fiberglass, or ceramics. They can withstand temperatures up to 2200 ° F (1204 ° C) continuously or 2800 ° F (1538 ° C) for short periods of time.ceramic oxides such as magnesium oxide (Figure 4.14d). After being packed with powdered oxides, the sheaths are swaged or rolled under pressure to reduce their diameter and tightly pack the powdered insulation. While this process serves to reduce intrusion of atmosphere within the sheath, it does not protect against humidity or moisture. Moisture intrusion is rapid and deteriorates insulation resistance to the point of thermocouple failure. Procedures for insulation resistance testing are to be found in the American Society for Testing and Materials documentation. Such testing is vital for mineral-insulated, metal-sheathed TCs stored in the open at construction sites or in repair shops for periods of time. Sheathing can also cause vapor transfer at elevated temperatures, which can result in thermocouple drift. This is the case with stainless steel sheaths used for nickel-bearing TCs such as types K and N. The offending element is manganese, and the problem can be eliminated by specifying low-manganese Inconel or modified Nicrosil sheathing.

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