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

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

WIKA Alexander Wiegand SE & Co. KG

Diagram 4 (see picture gallery) illustrates the effectiveness of a change of root and tip diameters with the example of a straight thermowell with a stem of 16 mm diameter:

First the root diameter is increased to 27 mm (= max. taper). In the example case, this measure would improve the ratio of the Strouhal frequency to the natural frequency (fs/fn) by 51 percent, without considerably diminishing the response time.

Gallery

Gallery with 8 images

An increase in the tip diameter, carried out in a second step, to 27 mm (= straight design) would yield an additional optimisation of the frequency ratio by 13 percent, however at the price of a clearly longer response time.

Using the previous standard, its value would deteriorate further, since the tip thickness would accordingly have to be increased to 10 mm.

In addition, the sensor length would need to be adapted to match the changed bore depth. In accordance with the current ASME PTC 19.3 TW-2016, however, the tip thickness could remain unchanged so long as its thickness is at least 3 mm.

Further changes

The revision of the TW section of PTC 19.3 has also been used by ASME to simplify the understanding and readability of the standard through a modern style of illustration as well as to clean up the rounding errors in the calculation examples.

Summary

The ASME PTC 19.3 TW-2016 standard offers users improvements in key details. But as with the writing of computer programs, also with wake frequency calculations the motto “garbage in, garbage out” always applies. This means that the results of the strength calculation can only be as good as the process data on which they are based as input parameters.

If, for example, the flow rate of the medium in the process varies by ±20 percent, the relevant results, such as the frequency ratio, will show the same level of variation. This variation, in turn, makes a serious evaluation of how the thermowell must be designed impossible.

* The author is Product Manager, CoE Europe Electrical, Temperature Measurement, Wika Alexander Wiegand SE & Co. KG, Klingenberg/Germany.

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