Twisted Sensor Tube Reduces Vortex Shedding Fatigue
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Solution Overview
Problem
Thermowells used in process fluid temperature measurement are prone to fatigue stress failure due to vortex shedding, which can lead to breakage and potential damage to downstream components, especially when vortex frequencies approach the natural frequency of the thermowell.
Innovation Solution
A twisted sensor tube configuration, such as a thermowell, is used, where the tube is twisted in a pitch of 1 turn per 8-16 widths to form a spiral shape, reducing vortex shedding forces. This configuration is easier to manufacture and more robust than traditional methods, utilizing a square or polygonal cross-section that is twisted to create a rugged surface exposed to fluid flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional cylindrical thermowell is used, then the manufacturing is simple, but vortex shedding causes high fatigue stress and potential failure
Solution Approach 1:
The patent applies asymmetry by transitioning from a symmetric cylindrical cross-section to an asymmetric twisted configuration. The thermowell is twisted along its length at a specific pitch (1 turn per 8-16 widths), creating an asymmetric surface that disrupts the symmetric vortex shedding pattern. This asymmetric geometry prevents the formation of regular vortex streets, thereby reducing fatigue stress while maintaining structural integrity.
Solution Approach 2:
The patent employs curvature by twisting the thermowell along its length, creating a spiral or helical surface geometry. This curved configuration changes the flow path of the fluid around the thermowell, preventing direct impingement and reducing the intensity of vortex shedding. The curved surface distributes stress more evenly throughout the structure, enhancing durability.
2Reliability
If helical strakes are attached to reduce vortex shedding, then fatigue stress is reduced, but manufacturing complexity increases
Solution Approach 1:
The patent merges the vortex shedding reduction function into the thermowell structure itself rather than adding separate helical strakes as attachments. The twisting is integrated into the thermowell fabrication process, creating a unified structure that eliminates the need for additional components. This integration simplifies manufacturing by combining form and function into a single fabrication step.
Solution Approach 2:
The patent extracts the vortex shedding reduction mechanism from the separate helical strake attachment and incorporates it directly into the thermowell geometry. By taking out the need for external attachments and embedding the anti-vortex feature within the thermowell structure itself, the design achieves the same protective effect with simpler manufacturing.
3Reliability
If the thermowell is twisted to reduce vortex shedding, then fatigue stress is minimized, but manufacturing precision requirements increase
Solution Approach 1:
The patent specifies a parameter range for the twist pitch (1 turn per 8-16 widths) rather than requiring a single precise value. This parameter change approach provides manufacturing flexibility, allowing tolerance ranges that are easier to achieve in practice. The effectiveness of vortex shedding reduction is maintained across this range, reducing the stringency of precision requirements while ensuring reliable performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The twisted thermowell significantly reduces vortex shedding forces, enhancing the durability and longevity of the sensor tube by eliminating or minimizing fatigue stress, thus preventing breakage and ensuring reliable pressure containment.
Implementation Method 1
A twisted sensor tube configuration, such as a thermowell, is used, where the tube is twisted in a pitch of 1 turn per 8-16 widths to form a spiral shape, reducing vortex shedding forces
Data Source
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AI summary
A sensor tube (12, 120) for protecting a sensor (13) inserted into a moving process fluid is provided. The sensor tube (12, 120) includes a process interface section (16) for mounting to a process vessel and an extended section extending from the process interface section (16) to a sealed end (22, 404). The extended section includes a twisted section (20) having a longitudinal axis. The process interface section (16) and the extended section define a sensor bore (36) configured to receive a sensor (13) therein. The twisted section (20) has a cross section that includes at least three equally sized walls and wherein the walls form helixes along the longitudinal axis of the twisted section.