Instrumentation

Thermowells

Thermowell is a cylindrical shape installed like a cantilever into the process piping. It facilitates temperature measurement while resisting process fluid forces. Thermowells are pressure tight receptacles adapted to receive a temperature sensing element. It is also provided with external threads or other means for tight pressure attachment to a vessel or pipe. Thermowells are used in measuring the temperature of a moving fluid in a conduit, where the stream exerts an appreciable force. Thermowells are generally fabricated by bore drilling bar stock.

Its Design requires thermal & stress consideration; its ability to withstand static pressure, steady-state fluid impingement, turbulence, & dynamic excitation due to Von Karman vortices.

Usage of thermowells

  • Protect temperature sensing elements installed in a tank, vessel or pipe from breaking
  • Facilitate servicing by easy removal of  temperature sensing elements without plant shutdown
  • Use as Test wells.
  • Protection tubes are used in high temperature flue gas service and duct applications.

Thermowell Materials

  • Thermowell material should match pipe and fitting material to avoid dissimilar welds unless process requires other material of construction.
  • Refer to a Material selection guide offered by ASME B31.3 & 31.1 or by the vendor. Check for Pressure-Temperature rating for that particular material from the P-T tables.
  • Recommendation of the process licensor or material specialist should be followed for special application.

Thermowell Terminology


L= Unsupported Length
B= Tip Diameter
(B-d)/2= Minimum Wall Thickness
A=Root Diameter
d= Bore Diameter
B/A= Taper Ratio
L/B= Aspect Ratio
d/B= Bore Ratio

  • Immersion Length (R): The distance between the free end/tip of the thermowell and the point of immersion in the medium, this is being measured.
  • Insertion Length (U):   The distance between the free end/tip of a thermowell and (but not including) the external threads of other means of attachment to a vessel.
  • Bore Dia. (d): Diameter of inner bore in which the temperature element is installed.
  • Lagging Extension (T): Pipe insulation or lagging extension is the extra length provided between the thermowell piping connection and the temperature element connection. This extension also acts as a heat sink, which may cause an error in the measurement.

Notes:

  1. U is always equal to or larger than R.
  2. Immersion in pipe such that sensitive area in center third of line but no less than 2”.
  3. Immersion in vessel shall be of min. 6” depth inside vessel wall.

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