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Thermowell Designs and Best Practices: Understanding the Importance of Wake Frequency and ASME PTC 19.3

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Introduction In process industries, accurate temperature measurement is essential for safety, efficiency, and product quality. Thermowells are a critical component in achieving this, providing a protective barrier between the temperature sensor and the process media. This blog explores the history of thermowells, their design best practices, and the importance of wake frequency calculations per ASME PTC 19.3.

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A Brief History of Thermowells Thermowells have been used since the early 20th century to protect temperature sensors from high pressure, flow-induced forces, corrosion, and mechanical damage. Originally machined from solid bar stock, early designs were simple and often customized for specific applications. As industries evolved, so did the need for standardized thermowell designs that could reliably perform under increasingly harsh conditions. This led to the development of engineering guidelines, culminating in the ASME PTC 19.3 standard.

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Thermowell Design Basics Thermowells come in several forms—threaded, flanged, welded, or Van Stone—each with design considerations such as:

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• Stem Length and Insertion Depth: Impacts response time and measurement accuracy.

• Shank Profile: Tapered, stepped, or straight—affects strength and vibration resistance.

• Material Selection: Must match process temperature, pressure, and corrosion resistance requirements.

• Process Conditions: Flow velocity, fluid density, and pressure all influence thermowell integrity.

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JMS "Swiftywell" is approved by ASME PTC19.3 as an approved flanged thermowell as it gets extra support by threading in as well.  Based on the Van stone design.

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A well-designed thermowell balances durability with responsiveness, ensuring the temperature sensor operates safely and effectively.

Why Wake Frequency Calculations Matter One of the most critical considerations in thermowell design is mitigating the risk of mechanical failure due to flow-induced vibration, known as vortex shedding. When fluid flows past a thermowell, it creates alternating low-pressure zones that cause the well to oscillate. If the natural frequency of the thermowell matches the vortex shedding frequency, resonance can occur—leading to catastrophic fatigue failure.

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This is where Wake Frequency Calculations come in.

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ASME PTC 19.3-TW (Thermowell) provides a methodology to calculate the frequency at which vortex shedding occurs and ensures that the thermowell’s natural frequency is at least 2.2 times higher—a safety factor to prevent resonance.

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Key factors in wake frequency calculations include:

• Fluid velocity and density

• Thermowell dimensions and material properties

• Mounting style and boundary conditions​

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Helical thermowells shift turbulence but risk buildup and erosion—and are not ASME PTC 19.3-TW approved.

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Best Practices for Thermowell Design and Selection

1. Always Perform Wake Frequency Calculations: Even for low-velocity or seemingly benign process conditions.

2. Use Tapered Designs When Possible: They provide better vibration resistance than straight or stepped profiles.

3. Limit Insertion Length: Shorter insertion reduces risk of resonance. Use velocity collars to improve support.

4. Choose the Right Material: Consider temperature, pressure, corrosion, and mechanical stresses.

5. Document and Review: Use tools like JMS SwiftyCalc to maintain records of thermowell design and ensure ASME compliance.

 

Tools That Help: JMS SwiftyCalc JMS SwiftyCalc simplifies and automates wake frequency calculations according to ASME PTC 19.3-TW. It allows engineers to:

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• Generate instant pass/fail reports

• Compare multiple thermowell designs

• Save configurations for later revisions

• Calculate theoretical maximum insertion lengths and flow rates

• Get pricing from JMS with the push of a button

 

​Conclusion Thermowells may seem like a small part of the process system, but their proper design and evaluation are critical to operational safety and accuracy. By understanding their history, adhering to best practices, and following ASME PTC 19.3-TW guidelines, engineers can ensure robust and compliant temperature measurement systems. Tools like JMS SwiftyCalc make it easier than ever to get it right the first time.

For more information or to try SwiftyCalc, visit: https://www.jms-se.com/swiftycalc/​​​​​