When fluid flows past a thermowell inserted into a pipeĀ or duct, vorticies form at both sides of the well. 
TheseĀ vorticies detach, first from one side, and then from theĀ other. This phenomenon is known as the Von Karmann effect. The frequency of shedding of theseĀ vortices is a function of the diameter of the thermowell, the fluid velocity and, to a lesser extent, the ReynoldsĀ number.The vortex shedding subjects the thermowell to a periodic transverse force. As the vortex sheddingĀ frequency approaches the natural frequency of the thermowell, the thermowell will oscillate, and is liable toĀ snap off. (The natural frequency of the thermowell is aĀ function of its shape and material of manufacture.)
The Von Karmann effect must be taken into accountĀ when designing thermowells of sufficient strength to withstand service conditions, and generallyĀ thermowells are chosen such that the sheddingĀ frequency is always less than or equal to 80% of theĀ natural frequency.
Thermowell points to be taken care-
- Bore, ādā shall be 0.26 inch maximum to pass plug gauge of 0.254 inch OD.
- Bore /OD eccentricity shall be +/- 10 percent of minimum wall thickness.
- Bore depth shall be +/- 1/32 inch for depths to 30 inches.
- Overall length, āLā shall be +/- 1/32 inch for lengths to 30 inches.
- Insertion Length, āUā shall be +/- 1/16 inch for lengths to 12 inches and +/- 1/8 inch forĀ lengths over 12 inches.
- Head diameter shall be +/- 0.01 inch
- Shank diameter shall be +/- 0.01 inch
- Tip diameter shall be +/- 1/32 inch
- Tip thickness +/- 1/32 inch
- Thermowells shall have the tag number and material on the well head. For testĀ thermowells, provide 1/2-inch NPT brass plug with brass chain attached to head of wellĀ by non-welding technique. Bore drilling may be performed either by twist drill or gunĀ drill.
Thermowells purchased for installation on piping and vessels areĀ categorized into four types:
- Machine Thread (āMTā Type) recommended for wall thicknesses more than 3/4Ā inch;
- 1-inch NPT Threaded (āTā Type) used for line sizes 6 inches and smaller, withĀ wall thicknesses less than or equal to 3/4-inch;
- 1-1/4 inch NPT Threaded (āTā Type) used for line sizes 8 inches and larger, withĀ wall thicknesses less than or equal to 3/4-inch; and
- 3-inch 150# Flanged (āFā Type) used for lined piping, atmospheric tanks andĀ vessels.
Van Stone Thermowells
It is advisable to use full penetration welds for flanged thermowells to achieve the highest fatigue strength rating. Forged thermowells accomplish the same goal without welds, but at a much higher cost. Van Stone style thermowells accomplish this without the cost.
Van Stone termowells are machined from solid bar and are connected using a separate and reusable backing flange. Van Stone termowells mounted between mating flanges for process insertion.
This design offers two advantages:
- Any weld stress is eliminated.
- The slip on flange not being exposed to the process conditions can be of a less expensive material.
Applications:
- Chemical, process, apparatus and plantengineering; petrochemical; on/offshore
- High chemical loads
- High process loads
Also Read: Thermowell Insertion & ImmersionLength
Please suggest on Welded thermowell with Flange .How much tiled accept with flange referance according to length wise. If possible ,please share all details.
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