Thermowells are cylindrical pressure-tight fittings used to protect the temperature sensors which are inserted into a pipe or vessel in industrial applications. A thermowell is basically a tube closed at one end and mounted in the process stream. The thermowell acts as a barrier between the sensing element and the process medium. It protects the sensing element against corrosive process media and fluid pressure and velocity.\n\n\n\nImportance of Thermowells\n\n\n\nThermowells play a crucial role in the successful measurement of temperature in industrial processes.\n\n\n\nThey Protect the sensor.Thermowells ensure that process temperature is passed to the sensor (proper heat transfer).Improved heat transfer, results in better accuracy.Allow the removal of the sensing element while maintaining a closed system.\n\n\n\nTypes of thermowells\n\n\n\nDepending on the stem design shape four types of thermowell are available:\n\n\n\nStraight ThermowellStepped thermowell Tapered Thermowell, andBuilt-up thermowells\n\n\n\nStraight thermowell has the same diameter for the entire insertion length. They are simple to fabricate and possess good rigidity. \n\n\n\nStepped thermowell has stepped diameters; Normally at the tip, they have a smaller (typically 1\/2") diameter while a larger diameter (Typically 3\/4") at the top. Due to decreased thermal inertia at the process end, these thermowells respond more rapidly to temperature changes than their straight counterparts.\n\n\n\nTapered thermowell has varying diameters (smooth continuous taper) over its full insertion length. Tapered thermowell is suitable for high-velocity heavy-duty applications and possesses a fast response time.\n\n\n\nBuilt-up thermowells are suitable for very long process insertion lengths. They are available in all the above types but a length of pipe is welded between the tip and process connection to give the long insertion length.\n\n\n\nFig. 1: Types of Thermowells\n\n\n\nDepending on material types Two types of Thermowells can be found:\n\n\n\nPipe Fabricated.Bar stock thermowells.\n\n\n\nDepending on End connections, five types of thermowells are available. they are\n\n\n\nThreaded thermowellsWeld-In thermowellsSocket Weld thermowellsVan Stone or Lap Joint flanged thermowellsFlanged thermowells\n\n\n\nThe threaded type is generally the least costly and most versatile.\n\n\n\nFig. 2: Thermowell types based on End Connection\n\n\n\nMaterials of Thermowells\n\n\n\nRight material increases the longevity of a thermowell. Material selection of thermowell will depend on the chemical, temperature and flow rate of the process fluid. With increase in temperature and fluid concentration, the corrosive effects normally increases. At the same time, suspended particles of the fluid will cause erosion. So all these parameters need to be addressed while selecting thermowell material. Some of the most frequently used thermowell materials are listed below:\n\n\n\nCarbon SteelBrassSS 316 \/ SS 304SS316 with Teflon \/ Zirconium Coated or Tantalum \/ Titanium steathing. GraphiteChromium\/molybdenum steelsIncoloyInconelMonelHastelloyHaynes AlloyTitaniumAluminum\n\n\n\nHowever, the most widely used thermowell material is stainless steel as it is cost-effective and highly resistant to heat and corrosion. For pressurized vessels, Chromium\/molybdenum steel is used. Cobalt, nickel, chromium and tungsten constitute the Haynes alloy that is widely used for sulphidising, carburizing, and chlorine-containing environments. The use of carbon steel thermowell is only limited to low-temperature pressure applications due to its very low resistance to corrosion.\n\n\n\nThermowell Insertion Length\n\n\n\nThe length from the connection point to the thermowell tip is known as the insertion length of the thermowell (U dimension in Fig. 4). For better accuracy, the thermowell insertion length should be long enough. This will allow the entire temperature sensitive portion of the measurement device to extend into the medium being measured.\n\n\n\nThermowell Installation\n\n\n\nLagging is used when the thermowell is installed through an insulation media on pipeline or equipment.\n\n\n\nFig. 3: Examples of Thermowells\n\n\n\nAn improperly specified thermowells, will result in:\n\n\n\nFailure due to poor welding practices.Poor compatibility with the temperature and media.Inadequate temperature transfer.Incompatibility with the process velocity leading to failure due to vibration.The gap between the OD of the thermocouple sheath and the ID of thermowell must be very close.The bore of the thermowell must be uniform and linear.\n\n\n\nDesign of Thermowells\n\n\n\nThe design of thermowell should cater to process media, pressures, temperatures, velocity, specific gravity, etc.Proper MOC (Material of construction).Wall thickness Vs response time.Bore diameter, Insertion length, Taper requirements, Overall length, etc.Calculate the Vibration using Wake Frequency methods.\n\n\n\nFig. 4: Details of Thermowell\n\n\n\nFig. 5: Various dimensions of Thermowells.\n\n\n\nThermowell Installation Requirements\n\n\n\nRefer to Fig. 6 and Fig. 7\n\n\n\nFig. 6: Requirements during installation\n\n\n\nFig. 7: Installation Requirements\n\n\n\nSelection of a thermowell\n\n\n\nThe key parameters to consider while selecting a thermowell are:\n\n\n\nProcess connection size and type.Process insertion lengthLagging lengthExtension lengthSensor length Interior diameter (bore) for the sensor or thermometerInternal threads for the sensor or thermometerShape of thermowell (straight, stepped, tapered, built-up)Material of construction\n\n\n\nThermowell Design Code\n\n\n\nPiping Designers follow ASME PTC 19.3 for guidelines regarding thermowells.\n\n\n\nThermowell Vibrations\n\n\n\nAs thermowells are immersed in the process flow, a bending force will be experienced by them. Additionally, depending on the fluid velocity and thermowell diameter, a certain vibrational frequency will be generated leading to thermowell vibration. In general, such vibrations are of small magnitude and considered negligible. But, the thermowell must have sufficient stiffness and rigidity to absorb those vibrations. In such a scenario, tapered thermowells are preferred as they have more stiffness. However, if the generated frequency approaches the natural frequency of the thermowell, it can cause severe unacceptable vibrations. ASME PTC 19.3 provides formulas to determine if a thermowell is acceptable for a given application. Many thermowell manufacturers have generated a velocity rating table that recommends the maximum fluid velocity for their thermowells to avoid resonance.\n\n\n\nInformation required for Purchasing Thermowell\n\n\n\nThe following information must be supplied to the thermowell manufacturer\/vendor while placing an order.\n\n\n\nProcess Connection SizeThermowell Insertion LengthLagging extensionShank configurationProcess Connection typeNominal BoreThermowell MaterialProcess Design temperature and pressure\n\n\n\nDisadvantages of thermowell\n\n\n\nThe main disadvantages of a thermowell are:\n\n\n\nCompared to naked sensor, slightly slower response to temperature changes.Resistance to flow medium.Increased cost for purchasing a thermowell.