Metals for Thermowells

The most common designs are sized to fit thermometers, thermocouples or resistance temperature detectors. The process fluid transfers heat to the thermowell wall, which in turn transfers it to the sensor. If the sensor fails, it can be replaced without emptying the vessel or piping.

Construction

A thermowell is typically machined from a solid bar, although longer designs may use several lengths butt-welded together. A single-piece well has a blind bore to accept the sensor, which is inserted into the process vessel, and a flange, cap or fitting at the other end to fix it in place. The flange can be welded if required, but this introduces 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 are straight-sided; a smaller tip size reduces thermal mass and increases response rate, while a fully tapered design reduces weight and allows longer probe lengths.

Choosing a material

A wide selection of materials can be used, depending on the operating environment, configuration, lifetime requirements and cost limitations. Ceramics are used where 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. Most applications, however, combine a degree of corrosivity with elevated temperatures, requiring materials with resistance to oxidation and corrosion - so stainless steels and nickel alloys are widely used in power generation, chemical process industries, incinerators, and pulp and paper manufacture.

Duplex and super duplex stainless steels

Duplex and super duplex stainless steels might not seem an obvious choice for thermowells, as they are limited to a long-term operating temperature of 250degC. However, their outstanding combination of high strength, high corrosion resistance and relatively low cost makes them ideal for processes with mid-range temperatures, and they are readily available ex-stock in a multitude of sizes. We carry 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).

Nickel alloys

Nickel alloys can be used at far higher temperatures due to their ability to withstand oxidation. The high nickel content ensures that they form a dense protective oxide layer upon exposure, which protects 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 compared with duplex and super duplex stainless steels, it retains much of its strength up to 540degC, so it can be used in applications at these intermediate temperatures with a reasonable degree of corrosion resistance. In addition, like austenitic stainless steels, but unlike duplex and super duplex, it can be used safely down to cryogenic temperatures (-196degC) while 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 more than 58% nickel content, it retains 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 to resist corrosion. One 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. 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.

We stock a wide range of duplex stainless steels, super duplex steels and nickel alloys, in sizes from 1/2" (12.7mm) to 16" (406.4mm) diameter bars. In addition, we can provide a range of first-stage machining, including deep-hole boring, milling and turning, together with in-house inspection and testing.