A thermowell is a closed-end tube inserted permanently into a process pipe or vessel, into which a temperature sensor (thermocouple or RTD) is inserted — the thermowell isolates the sensor from the process fluid, allows sensor removal without process shutdown, and provides mechanical protection from flow and pressure. Non-contact temperature sensors measure emitted infrared (IR) radiation from the target surface without physical contact — IR thermometers cover −50 to +3,500°C on moving, inaccessible, or molten targets; two-colour pyrometers extend range to +4,000°C for furnace and metalworking applications.
Challenges
Thermowell Resonance (Strouhal Vortex Shedding) Causing Thermowell Fatigue Fracture
High-velocity flow past the thermowell creates vortex shedding at a frequency that may match thermowell natural frequency — resonance fractures the well.
Thermowell Lag Error from Thermal Resistance Between Well Wall and Sensor
Air gap between sensor tip and well bore retards heat conduction — temperature response time is 30–120 seconds; step changes missed.
IR Thermometer Emissivity Error on Shiny Metal Surfaces (ε = 0.05–0.30)
Polished metal reflects ambient radiation as well as emitting its own — apparent temperature reads much lower than true surface temperature.
IR Measurement Error from Airborne Dust, Steam, or CO₂ Absorbing IR Signal
Particulate dust, water vapour, and CO₂ absorb infrared in certain wavelength bands — attenuated signal reads lower temperature.
Thermowell Insertion Length Too Short (Minimum 10× Bore Diameter Rule)
Short insertion leaves sensor tip in conductive boundary layer near vessel wall — reads ambient wall temperature, not process temperature.
Solutions
- Thermowell Wake Frequency Analysis (ASME PTC 19.3 TW) Before Installation: Calculate vortex shedding frequency vs. thermowell natural frequency — avoid resonance by changing shank diameter, length, or adding Vortab.
- Spring-Loaded Sensor with Conductive Thermal Paste + Minimum 0.1 mm Diametral Clearance: Spring ensures tip contact; thermal paste fills air gap — reduces thermal lag from 90 s to <15 s for equivalent well size.
- Emissivity Tables + Two-Colour (Ratio) Pyrometer for Metallic Surfaces: Two-colour pyrometer uses ratio of two wavelengths — ratio cancels emissivity effects; accurate on metal surfaces regardless of emissivity.
- Narrow-Band IR Detector at 5 µm (CO₂-transparent) or 3.9 µm (H₂O-transparent) Wavelengths: Select wavelength band outside CO₂ and H₂O absorption lines — transparent atmosphere; accurate reading in combustion and steam environments.
- Minimum Insertion Length = 10× Thermowell Bore Diameter (or ⅔ of Pipe Inner Diameter): Standard rule ensures sensor tip is fully immersed in process fluid — eliminates conducted heat error from vessel wall.
Applications
- All Process Industry: Thermowell standard for all pipe and vessel temperature measurement — isolates sensor from process; allows in-service replacement.
- Steel & Metals: Optical pyrometer on molten metal — 800 to 3,500°C; non-contact on inaccessible target.
- Glass Manufacturing: Radiation pyrometer through glass furnace viewport — 1,000 to 2,000°C; non-contact.
- Food Process Lines: IR thermometer on moving conveyor product — no contact; hygienic; fast response.
- Electrical / Mechanical: IR thermometer on electric motors, bearings, and switchgear — predictive maintenance; no contact required.
Models & Capacities
Thermowells and Non-Contact Temperature — Models & Reference | |||||||||
Type | Shank Form | Process Connection | Insertion Length [mm] | Max Velocity [m/s (liq / gas)] | Max Pressure [bar] | Max Temp [°C] | Material Options | Wake Freq. Analysis Required | Response Time (approximate) |
Thermowell — straight shank | Straight (uniform Ø) | Threaded NPT/BSP; flanged | 50–1,500 | <3 m/s liq; <30 m/s gas | 300 | 500 | SS 316L; Hastelloy; Inconel; Monel | >1 m/s — ASME PTC 19.3 TW required | 30–90 s (depends on annular gap) |
Thermowell — tapered shank | Tapered (small tip) | Threaded NPT/BSP; flanged | 50–600 | <10 m/s liq; <80 m/s gas | 200 | 500 | SS 316L; Hastelloy; Duplex | Preferred for high velocity — better vibration resistance | 20–60 s |
Thermowell — stepped shank | Stepped (large root, small tip) | Flanged PN16–PN150 | 100–1,500 | <12 m/s liq; <100 m/s gas | 400 | 600 | SS 316L; Alloy 625; Inconel 625 | Yes for all high-velocity applications | 15–45 s |
Thermowell — Van Stone | Lap joint flange | Lap joint flange (removable) | 150–600 | <8 m/s liq | 100 | 300 | SS 316L; SS 304 | For lower velocity; removable without breaking flange | 30–80 s |
Thermowell — sanitary | Straight (thin wall, Ra <0.8 µm) | Tri-Clamp; DIN 11851 | 50–300 | <3 m/s liq | 16 | 200 | SS 316L; Ra <0.8 µm | Low velocity — not normally required | 10–40 s (short insertion) |
IR thermometer (handheld) | Non-contact (portable) | N/A | N/A | N/A | N/A | −50 to +2,500 | Housing plastic/aluminium | N/A | <1 s (instantaneous) |
IR thermometer (fixed mount) | Non-contact (fixed) | Mounting bracket | N/A | N/A | N/A | −50 to +3,500 | SS housing; water-cooled option | N/A | <1 s; 4–20 mA output + relay |
Two-colour pyrometer (ratio) | Non-contact (optical) | Sight pipe or window | N/A | N/A | N/A | +500 to +4,000 | Fibre optic + electronics | N/A | < 10 ms; 4–20 mA output |