Thermowells

A thermowell is a cover that shields a temperature sensor from the process fluid. It allows for sensor insertion/removal in a safe manner while ensuring process integrity and pressure sealing.

Selecting the right Resistance Temperature Detector (RTD) depends on several factors, including process pressure, temperature range, flow velocity, insertion length, tip profile, and compatibility between the sensor and process connection.

Protection tubes shield RTDs or thermocouples from corrosion, pressure, and mechanical stress. They are installed in the process medium with the sensor remaining replaceable.

Metallic sheaths (e.g., SS, Inconel) provide high strength and corrosion protection. Non-metallic sheaths such as ceramics are used for high-temp and chemically aggressive media.

Ceramic sheaths withstand extreme temperatures and chemical attack, suitably being used for molten metals, glass, and furnace applications.

• Metallic tubes are fabricated or machined from SS, Inconel, or Monel and offer strong protection in high-velocity or high-pressure fluid systems.

• Threaded, flanged, socket weld, and Van Stone are popular, with varying advantages in installation, maintenance, and strength.

• Fabricated thermowells consist of several pieces that are welded together, ideal for low to moderate process conditions, and provide economical protection.

• Barstock thermowells are machine-turned from solid metal bars, offering exceptional mechanical strength and toughness for high-stress applications.

• Van Stone thermowells are made from a single bar with a slip-on flange, with a leak-free seal without having to weld the flange.

• Typical tip profiles are straight, tapered, stepped, and helical—each one suited to optimize response time, strength, and flow resistance.

• Thermowells are composed of a stem (shank), tip (sensing end), and process connection. They’re built to house sensors while withstanding process conditions.

• Shank construction is the shape of the stem (straight, stepped, or tapered), which influences strength and response time of the sensor.

• Types of Flange are raised face (RF), flat face (FF), and ring-type joint (RTJ), depending on pressure rating and sealing surface.

• Welding types are full penetration welds, fillet welds, and socket welds, each of which is qualified for strength and leak-tight performance.

• WPS specifies the way welding is done; PQR checks it through testing. Both ensure welds are safe and of quality.

• Special coatings such as PTFE, ceramic, or carbide resist corrosion, scaling, and abrasion in tough environments.

• Normal tests involve hydrostatic pressure tests, dye penetrant examination, radiography, material testing, and dimensional inspection.

• This checks chemical composition and mechanical properties to guarantee ASTM or ASME conformity.

• It verifies all important dimensions such as insertion length, bore, and flange alignment according to drawing specifications.

• This test places high fluid pressure on the thermowell to verify its seal and structural strength.

• DPI is a non-destructive inspection that identifies surface cracks or welding flaws by using a fluorescent or visible dye.

• Radiographic examination employs X-rays or gamma radiation to identify internal flaws or discontinuities in welds and wall thickness.

• Frequency limits, as specified by ASME PTC 19.3 TW-2010, forestall resonance and vibration-induced failure due to flow-induced turbulence.
  

• Stress on the thermowell is minimized at low velocity, enabling longer insert lengths or weaker profiles.
  

• Select the thermowell material according to the temperature range and the environment (corrosive, oxidizing etc.) in which it is to be used.
  • These wells can be made from different materials like SS304, SS316, HRS446, Inconel, Monel, Ceramic, etc.
• According to the construction of Thermowell (Steeped Shank, Straight Shank, Tapered Shank)
  • Steeped Shank- Provide faster response time and lower drag force.
  • Straight Shank- Extremely strong, but response time is slower and drag force on the fluid flow is high.
  • Tapered Shank- Provide good response time and strength.
• Thermowell Insertion Length
  • For best temperature measurement accuracy, the “U” dimension should be long enough to permit the entire temperature-sensitive part of the measuring instrument to project into the medium being measured.
    Liquid temperature measurement: One inch or greater.
    Gas temperature measurement:- three inches or greater.
• Resistance to vibration.
  • Fluid flowing past the well forms a turbulent wake (the Von Karman Trail), which has a definite frequency based on the diameter of the well and the velocity of the fluid.
  • The thermowell must have sufficient stiffness so that the wake frequency will never equal the natural frequency of the thermowell itself. If the natural frequency of the well were to coincide with the wake frequency, the well would vibrate to destruction and break off.
• To avoid the Thermowell failures caused by excessive pressure, drag forces, high temperature, corrosion, vibrations, it is recommended to run thermowell calculations based on your applications:
  • Maximum or operating temperature
  • Maximum or operating pressure
  • Fluid(gas or liquid) velocity
  • Fluid Density.

• Accessories comprise compression fittings, bushings, thermocouple connectors, gaskets, and support collars for installation and sealing.