Helical Strake Thermowell

What is Helical Thermowell?

The Shape, dimensions, material, and mounting connections are identical as normal thermowell. The helical thermowell is having a helical strake at the tip which reduces high vibrations. The construction of the thermowell is based on the parameters mentioned in the standard ASME STS- 1. The strake can be machined directly or can be welded on the thermowell with a wire of the same material.

What is Helical Strake and How its Reduce Vibrations?

Helical strakes are aerodynamic stabilizers that reduce forces and deflections experienced on the stack due to vortex shedding. Strakes consist of 3 vanes which can be wrapped in a helical pattern on the upper one-third part of the stack. The vortices generated by the wake of the helical strake are much smaller and non-uniform as compared to the normal thermowell, are considered negligible when evaluating vortex-induced vibrations.

What is the need for helical thermowell?

  • Helical Strake Thermowell is a groundbreaking concept design to break down the excess vibrations induced by the vortices in the flow.
  • Reduces the amplitude of oscillations by 90%.
  • Simple and quick installation.
  • Suitable for high flow rates and small nozzle sizes.
  • Better response time than conventional methods.
  • Eliminates use of support collars.

Types of helical thermowells based on manufacturing:

  1. Wire Welded
    Strakes are made by welding 3 wires, which can be arranged in a helical pattern on the upper one-third of the stack. Easy to manufacture. An economic and quick way to make helical thermowells, but strength and life are less.
  2. Solid Machined
    In this type, Helical Strake is made by a machining process. As compared to other types, the strength and life of solid machined helical thermowell are more desirable. The finishing of this process is better than the welded counterpart.

Helical Strake Calculations:

The design and dimensions of the helical stakes are based from ASME standard STS-1. Factors for designing a helical strake thermowell are:

Tip Diameter of thermowell B
Pitch of strake (P) 5XB
Height of strak 10%(B)
Length of helix for 1 revolution √(Pitch2+(ΠXB)^2)
Position of strake sin−1 (Pitch / strake length)
Angel of helix 3 stakes at 120° from each other