Thermowell Wake Frequency & Stress Calculator (ASME PTC 19.3 TW-2016)

This industrial-grade calculator validates Thermowell designs against the rigorous ASME PTC 19.3 TW-2016 standard. It performs a complete check of the Frequency Ratio Limit ($r$), Steady-State Stress, Dynamic Stress (Fatigue), and Pressure Limit. It accounts for temperature derating of material strength and utilizes the Scruton Number for damping analysis in liquid applications.

Quick Load Scenarios:

1. Process Conditions

Flow & Pressure

Velocity ($v$) [m/s]

Pressure [bar]

Temperature

Temp [°C]

Density [kg/m³]

Viscosity

Dynamic Viscosity [cP]

2. Thermowell Geometry & Material

Dimensions

Insertion ($U$) [mm]

Bore ($d$) [mm]

Diameters

Root ($A$) [mm]

Tip ($B$) [mm]

Material

Fillet Radius [mm]

Calculation Results

Frequency & Stress Analysis

Check	Value	Limit	Status

Assessment:

Vortex Shedding Simulation:

Frequency Ratio Plot

● Current Design -- ASME Limit

Step-by-Step Engineering Calculation

Evaluations based on ASME PTC 19.3 TW-2016. Material properties approximated for web demonstration (use official ASME II-D for certification).

Engineering Insights: The 4 Pillars of ASME PTC 19.3 TW-2016

1. Frequency Limit (Resonance)

The primary cause of thermowell failure is Von Karman Vortex Shedding. As fluid flows past the well, vortices shed alternatingly from the sides. This creates oscillating lift and drag forces.

If the shedding frequency ($f_s$) approaches the thermowell's natural frequency ($f_n$), resonance occurs. The vibration amplitude magnifies, leading to rapid fatigue failure.

Transverse Resonance: Occurs perpendicular to flow at $f_s$. Limit: $r < 0.8$.
In-Line Resonance: Occurs parallel to flow at $2 \cdot f_s$. Limit: $r < 0.4$ (unless damping is high).

If $N_{Sc}$ (Scruton Number) > 2.5, fluid damping suppresses the In-Line resonance, allowing operation up to $r < 0.8$.

2. Steady-State Stress

The fluid velocity exerts a constant drag force on the well, bending it downstream. The hydrostatic pressure also exerts a radial force. The combined Von Mises stress at the root (the weakest point) must not exceed the material's allowable stress limits ($1.5 \times S$) at the operating temperature.

$$ \sigma_{max} = \sqrt{\sigma_{bending}^2 + 3\tau_{shear}^2} < 1.5 S $$

High velocity steam lines are particularly prone to failing this check due to the high drag force exerted on long wells.

3. Dynamic Stress (Fatigue)

Even if the well doesn't snap immediately (Steady State Pass), the oscillating forces from vortex shedding cause cyclic stress. Over millions of cycles, this leads to fatigue cracking.

The calculation estimates the oscillating lift and drag forces and ensures the resulting cyclic stress is below the Endurance Limit ($S_f$) of the material. This is crucial for steam applications where velocities are high.

4. External Pressure Limit

The thermowell shank behaves like a thick-walled cylinder under external pressure. The wall thickness (Tip Dia - Bore Dia / 2) must be sufficient to prevent collapse or crushing under the process static pressure. This is standard ASME BPVC calculation.

5. How to Fix a Failure

If your design fails:

Shorten the U-Length: Natural frequency increases with $1/L^2$. A small reduction in length dramatically improves stiffness.
Increase Root Diameter: Thickening the root ($A$) increases the Moment of Inertia ($I$), boosting stiffness.
Use a Velocity Collar? NO. ASME PTC 19.3 TW-2016 explicitly discourages velocity collars due to the risk of impact damage from collar-to-nozzle rattling.
Twisted Square Wells? Helical strakes (like on chimneys) disrupt vortex formation, suppressing resonance. These are specialized solutions for difficult applications.