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

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

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Installation in Pipe Elbows and Angled Installation in Pipelines

With the mounting of thermo­wells in pipe elbows, “TW-2016” distinguishes between two fundamental cases (see picture gallery, Fig. 1): In case 1, the thermowell tip points in the direction of the flow. This installation is calculated as if the thermowell is subjected to flow along its entire insertion length (L).

In case 2, the thermowell is subjected to a flow from its tip; a position which is preferable for many applications. Here, the calculation is also carried out over the entire insertion length, however, the calculation of the static bending moment is outside of the scope of ASME PTC 19.3 TW-2016. In this case, the determination of the bending moment and the Strouhal number should be made on the basis of computational fluid dynamics (CFD) or experimental measurements.

Gallery

Gallery with 8 images

For an angled installation of a thermowell into the pipeline (see Fig. 2), the standard has been expanded by a chapter (6-10.8). With the calculation, the direction of flow does not matter — the thermowell will be designed in accordance with a conservative method, as though it was installed perpendicular to the process flow.

Requirements for the Minimum Tip Thickness of the Stem

The tip thickness of the stem (t) within the dimensional limits in accordance with ASME PTC 19.3 TW-2016 has now been defined as at least 3.0 mm (chapter 4.1). This change has enormous relevance, primarily for the optimisation of the thermowell in the event of a failed calculation. The requirements contained in “TW-2010” state the tip thickness must at least correspond to the minimum wall thickness of the thermowell at the tip. This led to large problems in the everyday application.

To understand this, one must bear in mind which possibilities for optimising the thermowell design are actually PTC 19.3 conformant (see Fig. 3). Plant operators can either shorten the insertion length (L) or strengthen the root diameter (A) and the tip diameter (B) of the stem that is subjected to the flow. However, for these diameters and the taper ratio B/A resulting from it, the specifications of table 4-1-1 in chapter 4.1 set limits.

For example, the taper ration B/A must be within 0.58 (= max. taper) and 1 (= straight stem). In practise however, these regulations are not without problems: Thus a strengthening of the root diameter is limited by the inner diameter of the flange nozzle. An increase in the tip diameter, on the other hand, has a negative effect on the response time of the thermometer.

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