What is Thermowell and How to install

Thermowell are threaded, socket weld, and flanged. Thermowells are classified according to their connection to a process. For example, a threaded thermowell is screwed into the process; a socket weld thermowell is welded into a weldalet and a weld-in thermowell is welded directly into the process pipe or vessel. A flanged thermowell has a flange collar which is attached to a mating flange on the process vessel or pipe.

Thermowell Types

Thermowells are most often constructed from machined bar stock in a variety of materials and may be coated with other materials for erosive or corrosive  protection. They are available with threaded, welded or flanged connections. The stem or shank that extends into the process may be straight with constant diameter, tapered all the way from entry point to the tip, partially tapered, or stepped. A variety of performance criteria and process conditions must be considered when selecting the best design for an application. In the following sections we will discuss detailed aspects of thermowell design and their application.

Thermowell Materials

Carbon Steels

Low cost materials with little corrosion resistance. Used in low temperature and stress applications where the measured medium protects it from corrosion, oils, petroleum, tars etc.

Maximum temperature: 530oC.

Chrome/Moly Steels

High strength steels used in pressure vessels and industrial boiler plant. Resistant to chloroform, cleaning agents, food products and carbon disulphide.

Maximum temperature: 600oC.

304 Stainless Steel

Low cost corrosion resistant material, used extensively used in food, beverage and chemical processing where good corrosion resistance is required. A low carbon grade, 304L is available which can be welded without impairing it’s corrosion resistance.

Maximum temperature: 900 oC.

321 Stainless Steel

Similar properties to 304SS above, except that this grade is titanium stabilized to prevent intergranular corrosion when welded.

316 Stainless Steel

Best corrosion resistance of the austenitic stainless steels due to the addition of molybdenum, widely used in chemical processing, offers useful resistance to H2S. As with 304, a low carbon grade, 316L is available for welded applications.

Maximum temperature: 900 oC.

310 Stainless Steel

Heat resistant material, which can be used up to 1150oC with useful resistance in sulphur bearing atmospheres. Corrosion resistance is slightly better than 304SS, but not as good as 316SS. Can be welded with caution.

446 Stainless Steel

Ferritic stainless steel with excellent resistance to sulphurous atmospheres at high temperature, however due to its low strength at high temperature, thermowells made from this material should be mounted vertically. Used in heat treatment processes, iron and steel furnaces, gas production plant and it has some useful resistance to molten lead. Good corrosion resistance to nitric acid, sulphuric acid and most alkalis gives it some limited use in chemical plant.

Maximum temperature: 1150oC.

Duplex Stainless Steel

These grades combine high strength with excellent corrosion resistance, especially to chloride stress corrosion cracking, however a tendency to brittleness limits their use to approx 300 to 315oC maximum, sub-zero use is also restricted to approx 50oC because of brittleness due to the ferrite content. Main uses include offshore

Inconel® 600

A very widely used nickel-chromium-ion alloy with excellent high temperature strength and oxidation resistance, however it is very vulnerable to attack in sulphurous atmospheres above 500oC. Good resistance to chloride-ion stress corrosion cracking and nitriding environments. Use extensively in chemical industries for its strength and corrosion resistance. Easily welded, can normally be used without post weld heat treatment.

Maximum temperature: 1212oC.

Inconel® 625

A nickel-chromium alloy with excellent resistance to pitting and crevice corrosion, unaffected by radiation embrittlement, widely used in aerospace applications and in marine environments. Good weldability, can be used in the as-welded condition.

Maximum temperature: 1093 oC.

Incoloy® 800

Superior to alloy 600 in sulphur, cyanide salts and neutral salts. Extensively used in steam/hydrocarbon reforming plants for pigtail piping, manifolds and waste heat boilers and in the internal components of secondary reformers. Widely used in heat treatment equipment and as a heater sheath material.

Maximum temperature: 1093oC.

Incoloy® 825

A nickel-iron-chromium alloy with exceptional resistance to many corrosive environments, it is superior to almost all metallic materials for handling sulphuric acid and shows excellent resistance to phosphoric acid. It is widely used in chemical processing, oil and gas recovery, acid production, pickling operations, nuclear fuel reprocessing and handling of radioactive wastes.

Maximum temperature: 540oC.

Incoloy® MA 956

An iron-chromium-aluminium alloy produced by mechanical alloying, it features great strength at high temperatures with excellent resistance to oxidation, carburization, and hot corrosion. These properties make it especially suitable for gas-turbine combustion chambers and other aggressive environments containing sulphur and chloride salts.

Maximum temperature: 1350oC.

Monel® 400

Nickel-copper alloy with very good corrosion resistance, commonly used to handle sea water, hydrofluoric acid, sulphuric acid, hydrochloric acid and most alkalis. Typical applications include marine fixtures, chemical processing equipment, gasoline and water tanks, process vessels and piping and boiler feedwater heaters.

Maximum temperature: 538oC.

Hastelloy® C276

Nickel-molybdenum-chromium alloy with excellent corrosion resistance, especially in chlorinated environments. Widely used in chemical plant where it tolerates ferric and cupric chlorides, solvents, chlorine, formic acid, acetic acid, brine, wet chlorine gas and hypochlorite. Can be easily welded and maintains its properties in the as-welded condition.

Maximum temperature: 1093oC.

Haynes alloy 214

Excellent high temperature material, with the highest resistance to oxidation and carburization of almost any alloy, it is recommended for temperatures of 950oC and above, it shows useful resistance up to 1315oC but its strength is severely reduced. Applications include ceramic firing furnace parts, automotive catalytic converter internals and in the industrial heating industry furnace flame hoods and rotary calciners. Has good resistance to chlorine contaminated environments, which allows its use in hospital waste incinerators.

Maximum temperature: 1204oC.

Haynes alloy 230

Excellent high temperature strength, oxidation resistance and long term thermal stability. Used in aerospace, chemical processing and high temperature heating applications.  Recommended for use in nitriding environments.

Maximum temperature: 1149oC.

Haynes alloy 556

A multipurpose alloy which offers good resistance to sulphidising, carburising and chlorine bearing atmospheres. Common applications include waste incinerators, petroleum processes where sulphur is present, chloride salt baths, exhaust gas probes, the alloy is one of very few that can survive in molten zinc, making it ideal for galvanizing processes.

Maximum temperature: 1093oC.

Haynes alloy HR160

A high temperature alloy with outstanding resistance to high temperature corrosion, it has excellent resistance to sulphidation, and chloride attack in both reducing and oxidizing atmospheres. Its resistance to attack by the products of combustion of low-grade fuels make it particularly useful in municipal, industrial, hazardous and nuclear waste incinerators.

Maximum temperature: 1204oC.


A lightweight material with good strength in the 150 to 470oC range. Excellent resistance to oxidizing acids such as nitric or chromic, it is also resistant to inorganic chloride solutions, chlorinated organic compounds and moist chlorine gas. Its good resistance to seawater and salt spray, allows it to be used in off-shore installations. Can be welded with special precautions to protect from atmospheric contamination.

Mounting Methods

Thermowells are typically mounted by one of the following methods

  • Threaded thermowells are threaded into process piping or tank, which allows for easy

installation and removal when necessary. While this is the most commonly used method of mounting, it has the lowest pressure rating of the three options. Threaded connections are also prone to leakage and therefore are not recommended for applications with toxic, explosive or corrosive materials.

  • Welded thermowells are permanently welded to process pipes or Thus, removal is difficult and requires cutting the thermowell out of the system. Welded thermowells have the highest pressure rating and are generally used in applica- tions with high velocity flow, high temperature, or extreme high pressure. They are necessary where a leak-proof seal is required.
  •  Flanged thermowells are bolted to a mating flange that is welded onto process pipe or

They provide high pressure ratings, easy installation, and simple replacement. Flanged thermowells are used in applications with corrosive environments, high-velocity, high temperature, or high pressure.

  • Vanstone / Lap Joint thermowells are mounted between the mating flange and the lap joint These thermowells allow for the use of different materials for the thermowell coming in contact with the process and the overlaying flange which can save material and manufacturing costs. They are a good choice for corrosive applicationssince there are no welds in this design weld-joint corrosion is eliminated. As an option they can be provided as a forging.

Thermowell Installation 

Thermowell manufacturers 

Thermowell Manufacturers











































Technical Reference;-

Emerson , Thermocouple Instruments Limited, Omega 

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