Knowledge Center · Thermowell pressure ratings

Thermowell pressure ratings — load diagrams and flow limits

A thermowell pressure rating gives the maximum allowable pressure at a given temperature. The load diagrams on this page show ratings for screw-in and weld-in thermowells, and for the protection tubes of complete temperature sensors, according to material, dimensions and immersion length. Flow limits and maximum tightening torques are included for mechanical selection.

Screw-in thermowells — stainless steel

Find the operating temperature on the horizontal axis, select the curve for the relevant immersion length, and read the maximum allowable pressure from the vertical axis. The boiling-point curve separates water from superheated steam up to the critical point.

The first three diagrams cover stainless-steel screw-in thermowells with a G 1/2″ thread. Diagram 1 covers 1.4541 (X6CrNiTi18-10) with a 1 mm wall. Diagrams 2 and 3 cover 1.4571 (X6CrNiMoTi17-12-2) in Ø8×1, Ø9×1 and Ø11×2 mm. Allowable pressure decreases with increasing immersion length; curves are provided for lengths up to 160, 250 and 400 mm.

The ratings apply to both separate thermowells and the protection tubes of complete sensors such as type RT-B / TC-B. The same dimensions are used for both configurations.

Load diagram 1Load diagram 1 — maximum allowable pressure by temperature for screw-in thermowells in 1.4541 with a 1 mm wall, at immersion lengths up to 160, 250 and 400 mm. Load diagram 1 Material: 1.4541 / X6CrNiTi18-10 0 10 20 30 40 50 60 0 100 200 300 400 50 36.5 18 Immersion length L ≤ 160 mm Immersion length L ≤ 250 mm Immersion length L ≤ 400 mm Temperature [°C] Maximum allowable pressure [bar g] Protection tube: wall thickness 1 mm · immersion lengths up to 160, 250 and 400 mm Flow limits: air and superheated steam 25 m/s · water 3 m/s Max. tightening torque: 50 N·m
Load diagram 2Load diagram 2 — maximum allowable pressure by temperature for screw-in thermowells in 1.4571 in Ø8 × 1, Ø9 × 1 and Ø11 × 2 mm, at immersion lengths up to 160, 250 and 400 mm. Load diagram 2Material: 1.4571 / X6CrNiMoTi17-12-2 0501001502002503003504000102030405060Immersion length L ≤ 250 mmImmersion length L ≤ 400 mm5036.522.5 Temperature [°C] Maximum allowable pressure [bar g] Protection tube dimensions: Ø8 × 1 mm · Ø9 × 1 mm · Ø11 × 2 mm
Load diagram 3Load diagram 3 — maximum allowable pressure by temperature for screw-in thermowells in 1.4571, Ø11 × 2 mm, at immersion lengths of 160 and 250 mm, with the boiling-point curve separating water from superheated steam. Load diagram 3Material: 1.4571 / X6CrNiMoTi17-12-2 0501001502002503003504000102030405060708090100Immersion length L = 160 mmImmersion length L = 250 mmBoiling-point curveSuperheated steamWater916530 Temperature [°C] Maximum allowable pressure [bar g] Protection tube: Ø11 × 2 mmFlow limits: air and superheated steam 40 m/s · water 5 m/sMax. tightening torque: 100 N·m

Weld-in thermowells — 13CrMo4-5 and 10CrMo9-10

The weld-in thermowells on this page are rated for several hundred bar at temperatures up to 600 °C. Diagram 4 covers 1.7335 (13CrMo4-5) and diagram 5 covers 1.7380 (10CrMo9-10). The curves are given by thermowell type: D1 and D4 use the short reinforced neck; D2 and D5 use the long neck. Dimensions are listed below. Separate curves apply to air and steam. For water, the ratings apply up to 450 bar at flow velocities up to 5 m/s. The table covers MA-PW D/DS/DDS weld-in thermowells and the corresponding RT D/DS/DDS and TC D/DS/DDS temperature sensors.

Load diagram 4Load diagram 4 — maximum allowable pressure by temperature for weld-in thermowells in 1.7335 (13CrMo4-5), types D1, D2, D4 and D5 in air and in steam, with the boiling-point curve and the critical point. Load diagram 4Material: 1.7335 / 13CrMo4-5 0501001502002503003504004505005506000100200300400500600700D1, D4 — airD2, D5 — airD1, D4 — steamD2, D5 — steamBoiling-point curveSuperheated steamWaterCritical point663440475386348220 Temperature [°C] Maximum allowable pressure [bar g] Weld-in thermowell · flow limit for air and superheated steam: 60 m/sWater: 5 m/s up to 450 bar
Load diagram 5Load diagram 5 — maximum allowable pressure by temperature for weld-in thermowells in 1.7380 (10CrMo9-10), types D1, D2, D4 and D5 in air and in steam, with the boiling-point curve and the critical point. Load diagram 5Material: 1.7380 / 10CrMo9-10 0501001502002503003504004505005506000100200300400500600D1, D4 — airD2, D5 — airD1, D4 — steamD2, D5 — steamBoiling-point curveSuperheated steamWaterCritical point597396475117570 °C Temperature [°C] Maximum allowable pressure [bar g] Weld-in thermowell · flow limit for air and superheated steam: 60 m/sWater: 5 m/s up to 450 bar
Dimensions of weld-in thermowell types covered by the diagrams (mm)
TypeL1Kd3 Ød1 Ød2 ØThread
D1140652412,57,0M18x1.5
D22001252412,57,0M18x1.5
D4200652412,57,0M18x1.5
D52601252412,57,0M18x1.5

Flow limits and tightening torques

Pressure, flow and tightening torque all contribute to the mechanical load on a thermowell. Flow creates a bending load, while tightening torque loads the threaded connection.

The load diagrams cover pressure under the stated flow conditions. Vibration introduces dynamic loading that depends on the flow conditions and the immersion length. Where vibration is relevant, dimensioning requires a separate assessment alongside the pressure rating.

Flow limits and maximum tightening torques
ThermowellAir / superheated steamWaterMax. tightening torque
Screw-in, Ø8×1 / Ø9×1 (diagrams 1–2)25 m/s3 m/s50 N·m (G 1/2″)
Screw-in, Ø11×2 (diagram 3)40 m/s5 m/s100 N·m
Weld-in, types D1–D5 (diagrams 4–5)60 m/s5 m/s up to 450 bar—

The materials behind the curves

The diagrams cover four materials. Screw-in thermowells use 1.4541 (X6CrNiTi18-10) and 1.4571 (X6CrNiMoTi17-12-2). Weld-in thermowells use the heat-resistant steels 1.7335 (13CrMo4-5) and 1.7380 (10CrMo9-10) for the pressure and temperature ranges shown in diagrams 4 and 5.

Material selection also depends on the medium, corrosion conditions, environment and temperature. See Thermowell materials for the material guide.

Frequently asked questions

What is a thermowell pressure rating?

The maximum allowable pressure a thermowell can withstand at a given temperature. The rating depends on the material, dimensions, immersion length and medium, so it is given as a load diagram rather than a single value.

How do I read a thermowell load diagram?

Find the operating temperature on the horizontal axis, select the curve for the relevant immersion length or thermowell type, and read the maximum allowable pressure from the vertical axis. The boiling-point curve separates water from superheated steam up to the critical point.

What limits the pressure rating of a thermowell?

Temperature, material, wall thickness and immersion length. Increasing temperature reduces material strength, and a longer unsupported length reduces allowable pressure. The curves combine these effects.

What are the flow limits for thermowells?

For the screw-in thermowells on this page, the stated limits are 25–40 m/s in air and superheated steam and 3–5 m/s in water. For the weld-in types, they are 60 m/s in air and steam and 5 m/s in water at pressures up to 450 bar.

What is the difference between a screw-in and a weld-in thermowell?

A screw-in thermowell is mounted in a threaded boss; a weld-in thermowell is welded into the process. The screw-in thermowells shown here are rated up to about 90 bar, depending on dimensions and immersion length, and the weld-in thermowells reach several hundred bar at temperatures up to 600 °C.

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