Thermowell Wake Frequency: Avoiding Catastrophic Vibration Failure

November 10, 2025 Design Calculators Engineering Team 18 min read Mechanical Design

A broken thermowell in a high-pressure steam pipe is not just a leak; it's a projectile. Most failures are not caused by simple pressure, but by an invisible enemy: Vortex Shedding. We break down the ASME PTC 19.3 TW-2016 standard, explain the Strouhal Number, and show you how to design wells that won't snap off.

Imagine a solid steel bar, 20mm thick, snapping cleanly in half like a dry twig. This happens more often than you think in industrial piping systems. The culprit is rarely the static pressure of the fluid. A standard thermowell can easily handle 100 bar of pressure.

The real killer is Flow-Induced Vibration. When fluid rushes past a cylindrical object (like a thermowell shank), it doesn't flow smoothly. It creates turbulence. Specifically, it creates alternating low-pressure vortices on the downstream side of the well. This is known as the Von Karman Vortex Street.

These vortices exert oscillating forces on the thermowell—pushing it up, then down, then up, then down. If the frequency of these push-pull forces matches the Natural Frequency of the thermowell, you hit Resonance. The amplitude of vibration multiplies by 10 or 20 times, and steel undergoes fatigue failure in minutes.

The Flagpole Analogy

Think of a flag flapping in the wind. The pole is the thermowell, and the wind is the fluid. Even a steady wind causes the flag to flap back and forth rhythmically. This is Vortex Shedding.

Now, imagine if the rhythm of the flapping matched the natural "wobble" of the pole exactly. The pole would start swinging violently until it snapped at the base. That is what happens inside your pipe.

ASME PTC 19.3 TW-2016: The Rule Book

For decades, engineers relied on the old 1974 standard, which was simplistic and often unsafe. In 2010 (updated in 2016), ASME released PTC 19.3 TW, a comprehensive mathematical standard that every mechanical engineer must follow.

The standard mandates four specific checks:

  1. Frequency Limit: Is the wake frequency far enough away from the natural frequency?
  2. Dynamic Stress: Can the well withstand the cyclic forces at the root?
  3. Static Stress: Can it withstand the steady drag force of the flow?
  4. Pressure Limit: Can the tip withstand the hydrostatic pressure?

Of these, the Frequency Limit is the one that fails 90% of designs.

The Science: Strouhal Number & Frequency

The frequency at which vortices shed (fs) is calculated using the Strouhal Number (St), a dimensionless constant (roughly 0.22 for cylinders).

fs = (St × V) / B

Where:

  • V: Fluid Velocity (m/s). Faster flow = Higher frequency.
  • B: Tip Diameter (m). Thinner well = Higher frequency.

The Golden Rule: To avoid resonance, the Wake Frequency (fs) must be less than 80% of the Thermowell's Natural Frequency (fn).

Ratio (r) = fs / fn < 0.8

Check Your Wake Frequency

How to Fix a Failing Design

So, you ran the calculation and your Wake Frequency Ratio is 1.0 (Deadly Resonance). What do you do? You cannot change the flow rate of the process. You have to modify the thermowell.

1. Shorten the Immersion Length (L)

This is the most effective fix. The natural frequency of a cantilever beam is inversely proportional to the square of its length (fn ∝ 1/L2).

Reducing the length from 250mm to 200mm significantly increases the Natural Frequency, moving it safely away from the Wake Frequency.
Trade-off: Shorter wells might not reach the "middle third" of the pipe, potentially affecting temperature accuracy. (Though typically, just getting past the boundary layer is enough).

2. Increase the Root Diameter (A)

Making the thermowell thicker at the base increases its stiffness, which increases its Natural Frequency. A tapered design is stronger than a straight design for this reason.

3. The ScrutonWell (Helical Strakes)

If you cannot shorten the well (e.g., in a very large pipe) and cannot make it thicker, you can change the physics of the fluid. A ScrutonWell has helical ridges (strakes) machined onto the stem.

These ridges disrupt the formation of the vortex street. Instead of one big coherent vortex shedding at a specific frequency, you get many small, chaotic vortices. This effectively suppresses the flow-induced vibration, allowing safe operation even in high-velocity steam.

Velocity Collars: The "Bad" Idea

In the past, engineers would weld a "support collar" on the thermowell to brace it against the nozzle wall.
Do not do this.

  • It creates a rigid constraint that is hard to model.
  • It requires a perfect fit; any gap creates hammering damage.
  • It creates a stress concentration point.
  • ASME PTC 19.3 generally discourages reliance on collars for vibration suppression.

Conclusion: Respect the Velocity

Temperature measurement looks static, but inside the pipe, it is a violent dynamic environment. A broken thermowell can cause a containment breach, leading to fires, explosions, or steam release.

Never install a thermowell based on "rule of thumb" sizing. Always perform a Wake Frequency Calculation using the specific velocity, density, and viscosity of your process fluid. If the velocity is high (>5 m/s for liquids, >15 m/s for gases), be paranoid. Check the math.

Validate Your Thermowells

We provide tools compliant with ASME PTC 19.3 TW-2016 to keep your piping safe: