Basics of Thermowells Design Notes

This article establishes minimum requirements for the use of thermowells with temperature sensors in process industries. Main keywords for this article are Basics of Thermowells Design Notes In Process Industry. Thermowells References. Thermowells Definitions. asme ptc 19.3. von karman vortices. 

Thermowells References

Reference is made in this article to the following documents. 
American Society of Mechanical Engineers (ASME)
ASME PTC 19.3 Performance Test Code- Temperature Measurement.

Thermowells Definitions

Immersion Length. The immersion length is the distance from free end of the temperature sensing element or well to the point of immersion in the medium whose temperature is being measurement.
Natural Frequency. The frequency of free (not forced) oscillations of the sensing element of a fully assembled transducer.
Wake Frequency. When a thermowell is inserted into a flowing liquid or gas, circular flow pattern is created downstream of the thermowell. These flow patterns are called Von Karman vortices. These vortices break away periodically, known as Vortex Shedding and causes a regular change in the force on the thermowell. This regular change or frequency of vortex shedding is called wake frequency.

Basics of Thermowells Design Notes In Process Industry

The following criteria shall be taken into account when selecting and specifying thermowells:

• In general, all thermowell process connection shall be of the flanged or welded design,  unless fluid conditions (i.e. pressure, temperature, corrosion, etc.), piping specifications, equipment requirements, and/or natural frequency analysis results dictate otherwise. Threaded type thermowells may be used for low pressure or line sizes 1 1/2″ and below.
• Thermowells shall be provided for all temperature sensing elements except surface temperature measurements and temperature measurements in low pressure ducts where the element does not need protection from harmful atmospheres, corrosive fluids, or mechanical damage.
• Thermowell design as defined herein shall not be applicable to those included in the scope of furnace design. The design of such wells, and requisitioning and purchase shall be done by Furnace vendor.
• Thermowell material shall be 316 SS unless otherwise required by process conditions. Where the nature of the fluid require the use of another material, it shall be suitable for the service.
• All thermowells shall be drilled barstock (not built-up type) except pipe type wells for use in fired heaters.
  • Flanged thermowells shall have flanges and weld filler of the same material as specified for the thermowell, and shall have full penetration welds.
  • The construction of standard thermowells shall be in accordance with Thermowell Types drawings in Fig. 1, 2 and 3.

    

    

    

• Test thermowells shall be furnished with threaded SS 316 plugs and chains.
• The thermowells shall be hydrostatically tested to an internal pressure of 17250 kPa  (2500 psig) for 15 minutes.
• Thermowells shall be suitable for stresses due to stream velocity conditions. The wake  frequency (commonly referred to as Strouhal or the Von Karman frequency) shall not exceed 66 percent of the natural frequency of the thermowell in accordance with ASME PTC 19.3. In applications where large lines, high velocities, or high pressures are present, these calculations shall be performed or purchased by the Vendor.
• Thermowells for dial thermometers and filled system instruments shall be purchased with the instruments to assure proper fit.
• For flanged connections in piping a 114 mm (4 1/2 inch) projection shall be utilized. For cryogenic service the nipple lengths shall be extended in order to allow the head to protrude from the insulation.
• Thermowells shall have a 11/2 in. flanged connection, and the pressure rating shall conform to the piping service or equipment specification. For high pressure service (e.g. ANSI class 900 and above) and high velocity service (e.g. 14 m/sec and higher), a 1-1/2 in. connection may be considered. For water, air line and small size pipeline of package unit such as lube oil line, threaded connection may be used. In general, 2 in. connection may be considered for equipment (columns and vessels/drums).  When the pipe diameter is 4 in, and smaller, the thermowell shall be installed against the flow in a 90 degree elbow or at 45 degree to the pipe. For high pressure services, as a rule, the themowell shall be inserted in an elbow (using a special tee).
• The thermowell immersion length ‘U’ shall be selected such that the tip falls in the middle third of the pipe diameter, considering maximum velocity rating of thermowell.
• When harmful vibrations caused by Von Karman vortices are anticipated, the immersion length shall be shortened so that the thermowell is protected from resonance and damage.
• Thermowells shall be stamped with the tag number of its corresponding temperature element.