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Introduction To Thermowell:
‘Thermowell’ as the name indicate is a kind of well for thermal measuring element. This thermal measuring element may be any RTD, TC or TG. Thermowells are used as a protective covering for any temperature element used in the processes and industries. These thermowells may be of solid barstock construction or it may be made from a hollow pipe or sometimes it may be of combination (barstock and pipe both).
Helical Strake Thermowell:
Helical strake thermowells are made from solid barstock rod of suitable metallurgy selected as per the application. Helical strake thermowells are specially designed thermowells used in the critical process applications specifically in petrochemical industries against high velocity conditions. The construction of a helical design thermowell is similar to an ordinary thermowell but the helical or screw based construction at the immersed bottom part make it suitable to sustain against high velocity critical process applications.
Types Of Helical Strake Thermowell:
Helical strake are designed at the bottom or tip part which neutralizes the serve impact of high vibrations caused due to high velocity of process medium. The length of helical strake depends on the nozzle standout of the site where these thermowells are installed. We can design helical strake either by machining on the thermowell or by welding of same material wire on the thermowell. Based on this criterion, helical design will be of following two types:
- Solid machined helical design thermowell
- Wire weld helical designed thermowell
Figure 1 :Helical design thermowell (Solid machined)
Figure 2 :Helical design thermowell (Wire Weld)
Figure-1 is solid machined helical construction made from single barstock rod without any weld of wire while Figure-2 is a wire weld helical construction in which a wire of same material is welded in a symmetry in order to form the helical stracks. This construction is based on the ASME STS-1. For credibility point of view, solid machined construction is better than the wire weld construction but wire weld thermowells are easy to manufacture and cost effective.
Need Of Helical Design Thermowell:
Before manufacturing of any barstock thermowell, an analysis is performed in order to ensure sustainability of this thermowell against the process conditions. This analysis is termed as ‘Wake Frequency Calculation’ or commonly abbreviated as WFC. This calculation is based on the standard ASME PTC19.3-2016. There is a set of input process parameters (Design temperature, Design pressure, Medium velocity, Medium density, Viscosity, Insertion length of thermowell and its other dimensional details) which need to be entered in WFC tool and accordingly we get the result which ensures whether the designed thermowell will be suitable to work or not.
If performed WFC result of any thermowell fails, then some corrective actions need to be done in thermowell dimensions which includes Increase in the diameter of thermowell based on the nozzle ID, reduction in length and use of velocity collar. If thermowell fails even after these corrections then helical design thermowell is the solution. So, helical design is used only when an ordinary thermowell fails in WFC. The helix calculation is done in accordance with ASME STS-1
How Helical Design Works?
In critical process applications, an ordinary thermowell fails and may break inside the process due to induced vibrations affecting on its bottom, hub and surface. As the flow of process medium enhances, vortex is generated which create a fluid area where flow rotates in axial or round shape. Once a vortex move, it create linear and angular momentum, energy and mass and results in structural difficulties.
Formed helical strakes at the immersed part of thermowell prevent the forces and vibrations which are caused due to vortex shedding hence helical strakes acts as vibration stabilizers. The vortices against helical thermowell are much smaller than in case of a non-helical thermowell. So, this special design break the excess vibrations and allows the thermowell to sustain against the process conditions.
Advantages Of Helical Design Thermowell:
- Helical strakes thermowell reduces the vortex induced vibrations to a great extent hence sustainability of thermowell increases significantly against the process.
- Installation inside nozzle is easy in comparison to a thermowell with collar.
- Suitable to install against high flow rate and in thin and uneven nozzle sizes.
- No need to increase the diameter of thermowell which results in better response time as compared to large diameter thermowell.
Helical Calculation:
Dimensions of helical strake thermowells are in accordance with ASME STS-1. Three individual wires of same material of construction as thermowell may be attached or welded on the circumference of thermowell. The alternate and better method allows the complete helical with a single rod without any wire welding.
Strakes are 120 degree apart from each-other. Helix strake pitch should be 5 times of thermowell’s tip diameter. Height of strake should be maintained 10% of the thermowell’s tip diameter. After calculating length of helix for 1 revolution as per STS-1 standard, the helix angle to be maintained at 58 degree for effective VIV reductions. This calculation can be performed step-by-step as follows by considering a suitable example:
| Step-1: | It is assumed that the ‘Tip Diameter’ of thermowell is abbreviated as ‘B’ which is 28mm (Assumption) |
| Step-2: | Pitch of strake is denoted by ‘P’ which is 5times of B so P is 140mm. |
| Step-3: | Height of strake is denoted by ‘H’ which is 10% of B so H is 2.8mm. |
| Step-4: | Length of Helix for 1 revolution is calculated by √[(Pitch)2+(πxB)2] as follows:
√[(Pitch)2+(πxB)2] = √[(140)2+(3.14159×28)2] = √[19600+7737.36] = √[27337.36] = 165.34mm |
| Step-5: | Angle of Helix is calculated by Sin−1(Pitch of strake / Strake length) as follows:
Sin−1(Pitch / Strake length) = Sin−1( 140 / 165.34) = Sin−1(0.84674) = 57.95 (Equivalent to 58 Deg.) |
Hence, Helix angle should be maintained to 58 Deg. For effective strake formation. This can be shown as follows in Figure-3:
Figure-3: Height of Strake, Pitch of strake, Length of Helix and Angle of Helix
CONCLUSION:
Installation of helical strake thermowells are credible and sustainable solution against the high velocity critical process applications specifically in petrochemical industries. Reduction in Vortex induced vibrations (VIV) enables these thermowells to be better and strengthened choice as compared to an ordinary barstock thermowell. Tempsens offer Thermowell with collar and helical design thermowell.
Abbreviations:
OD1: Outer Diameter-1 or Root Diameter of Thermowell
OD2: Outer Diameter-2 or Tip Diameter of Thermowell
ID: Inner Diameter of Thermowell
IL: Total Insertion length below process connection (Including Nozzle)
L1: Immersion Length or Helical length (Excluding Nozzle)
For More Details-https://tempsens.com/catalog/contact-temperature-sensors/thermowells.html
