ASME B31.3 requires that the combined stress of piping should be calculated by an equation including bending stress, torsional stress and axial stress terms.

One of our customers requested that the combined stress of small bore piping around the weld in the field should be actually measured and assessed; correct measurement of the combined stress enables the determination on if any repair work is needed for the piping or not. In performing the repair work for the piping, it is essential to evaluate how much stresses are applied to which direction. To enable measurement of stresses at several tens of locations, an easy and simple measurement method was required.

In this respect, we developed  “strain gauge holder” as a tool for measuring combined stress of small bore piping around the weld in the field. The strain gauge holder is equipped with 4 sets of 3-element strain gauge rosettes located every 90 degrees in the circumferential direction. With use of this strain gauge holder, bending stresses in 2 directions, axial and torsional stresses can be measured synchronously. It has been confirmed that the strain gauge holder is able to give correct measurements equivalent to those measured by the strain gauges attached by adhesive to the piping surface. It takes only about 10 minutes to install the strain gauge holder in the field and it can be repeatably used. As a result of trial use on 200℃ piping, there was no damage to the strain gauge holder.

Measured bending stresses in 2 directions can be plotted on x-y graph like Lissajous figure, which show the bending direction of pipe vibrations and therefore providing useful information for the design of pipe reinforcing supports.

The torsional stress against the sum of bending stress and axial stress absolute values can be also plotted on x-y graph. The combined stress mentioned above can be calculated as the distance between each point and the original point in the x-y graph . The degree of influence of torsional stress on the combined stress can be identified in the x-y graph. Waveforms of the combined stress on a time axis can be also plotted.

By the measurements taken using the strain gauge holder, the frequency of each term of stress constituting the combined stress can be obtained. Resultant data are helpful in determining if the calculated results in the design and analysis are valid or not.

It is planned to utilize the measurements taken by the strain gauge holder in the fatigue life assessment of the small bore piping around the weld in future.


A Method for Measuring Combined Stress of Small Bore Piping Around Weld in Field Using Strain Gauge Holder

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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