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Wake Frequency Calculation

What Has Changed with the 2016 Revision of the ASME PTC 19.3 TW Standard?

| Author / Editor: Kai Grabenauer / Dr. Jörg Kempf

Angled installation of a thermowell into the pipeline
Angled installation of a thermowell into the pipeline (Source: Wika)

It stands as the benchmark: Those who operate with thermowell strength calculations in accordance with the globally recognised standard ASME PTC 19.3, will protect themselves against all eventualities. With the additional mark of “TW-2016”, the standard specifications are now available in a revised version. The revision provides some clearer and more-detailed definitions.

It was 42 years ago that the American Society of Mechanical Engineers (ASME) published a standard for temperature measurement — PTC (Performance Test Code) 19.3-1974. The provisions that it contained for the wake frequency calculation were based on J. W. Murdock’s fundamentals from 1959.

This standard remained unchanged for more than 20 years. In 1995 there was a serious accident at the Japanese nuclear power plant in Monju. It was caused by the rupture of a thermowell due to turbulence: The vibration direction of the tube was parallel to the flow direction of the medium in the pipe. Such an in-line resonance was not considered in the original calculation basis from 1974. This edition of the standard only considered the more frequently occurring vibrations at right angles to the flow direction.

As a consequence of the Monju incident, the ASME standard from 1974 was completely revised and the result was published in 2010. Its most important improvement in comparison to the original issue — apart from a variable Strouhal number — was the inclusion of the in-line resonance (which had become the problem in Monju) in the calculation. Since then, ASME PTC 19.3 TW-2010 has been applied successfully worldwide in all industries whose processes are subject to high loads such as those arising from high flow rates. This applies, for example, to companies in the oil, gas and chemical industries.

Now ASME PTC 19.3 TW-2016 — an updated version — is available. The amendments contained within it relate to the following three aspects.

Requirements for Passing through the In-line Resonance

In chapter 6-8.5 the requirements for passing through the in-line resonance with a ratio of the Strouhal or excitation frequency to natural frequency fs/fn = 0.5 are more clearly defined (see picture gallery, diagram 3).

As a result, passing through the in-line resonance is possible, if the process medium is gaseous and the range of the in-line resonance does not fall within the continuous operation of the plant. As expected passing through this in-line resonance is the responsibility of the plant owner. Here, the permissible flexural fatigue stress must not be exceeded during the cumulative number of vibrations of 1011 cycles. How this translates into figures is shown in the table (see picture gallery). The calculations refer to a solid-machined thermowell from stainless steel with a tapered stem, 25/19 mm diameter and 6.6 mm bore.

Furthermore, the process medium must not adversely affect the fatigue strength of the thermowell material. For the case, which cannot be completely eliminated, of a thermowell rupture resulting from in-line resonance, the standard demands safety provisions to prevent severe injury to personnel or damage to the plant.

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