A thermowell is typically machined from a solid bar, although longer designs may use several lengths butt-welded together. If from a single piece, it will have a blind bore to accept the sensor, which is inserted into the process vessel, and a flange, cap or fitting at the other end that is used to fix the thermowell in place. The flange can also be welded if required, but this will introduce the potential for increased corrosion through the deterioration of properties in the heat-affected zone as well as crevices or other physical imperfections. The simplest designs will be straight-sided, but having a smaller tip size can reduce thermal mass and therefore increase response rates, whilst a fully tapered design can reduce the weight and allow longer probe lengths. Longer
A wide selection of materials can be utilised in the manufacture of thermowells, depending upon the operating environment, configuration, lifetime requirements and cost limitations. Ceramics are used where the operating temperatures are very high but the atmosphere is not particularly aggressive. Carbon steels can be used for slightly lower temperatures, and provide a low-cost solution where corrosion is minimal, such as in reheating or annealing furnaces. However, most applications will combine a degree of corrosivity and elevated temperatures, requiring materials with resistance to oxidation and corrosion. Therefore, stainless steels and nickel alloys are widely used in applications such as power generation, chemical process industries, incinerators, pulp and paper manufacture.
Duplex stainless steels and super duplex stainless steels might not seem an obvious choice for thermowells where they are limited to a long-term operating temperature limit of 250degC only. However, their outstanding combination of high strength, high corrosion resistance and relatively low cost makes them an ideal choice for processes with mid-range temperatures only. In addition, they are readily available ex-stock in a multitude of sizes. Langley Alloys carries several hundred tonnes of duplex stainless steel (S32205, F51, 1.4462) and super duplex stainless steels S32750 (F53, 1.4410, SAF2507), S32760 (F55, 1.4501, Zeron 100) and Ferralium 255 (S32550, F61, 1.4507) respectively.
Nickel alloys can be used at far higher temperatures due to their ability to withstand oxidation. The high nickel content ensures that such alloys form a dense protective oxide layer upon exposure, which serves to protect them from further degradation.
Alloy 825 (Incoloy 825, UNS N08825, 2.4858) is the lowest cost nickel alloy widely available, reflecting the fact that it was developed at a time when expensive nickel was in short supply. Although its mechanical strength is limited in comparison with duplex and super duplex stainless steels, it retains much of strength up to 540degC. Therefore, it can be used in applications where these intermediate temperatures are experienced, with a reasonable degree of corrosion resistance. In addition, like austenitic stainless steels, but unlike duplex and super duplex stainless steels, it can also be used safely down to cryogenic temperatures (-196degC) whilst retaining some impact toughness.
Where higher temperatures will be seen, a more highly-alloyed grade such as Alloy 625 (Inconel 625, UNS N06625, 2.4856) can be used. With >58% nickel content, it will retain most of its higher mechanical properties up to 650degC but can be selectively used at temperatures approaching 1000degC.
Applications in the chemical process industry can require quite specific alloys in order to resist corrosion. One such example is the use of Alloy K-500 (Monel K-500, 2.4375) or Alloy 400 (Monel 400, UNS N04400, 2.4360). These alloys are composed almost entirely of nickel and copper, which ensures superior resistance to a wide range of acids, alkalis and moving seawater. Therefore, they are commonly used for thermowells in the production of acetate, acetic acid, brines and seawater, sodium fluoride, ammonia phosphate, barium chloride, bromine, magnesium chloride, oxalic acid and sodium chloride.