How the Appendix R dent strain method works
A dent changes the curvature of the pipe wall. Appendix R of ASME B31.8 estimates the strain from three pieces you can measure from a laser scan, a profile gauge or a high-resolution geometry tool:
Edition matters
The equations changed over time. The 2003 edition used t instead of t/2. The 2007 edition fixed that. The 2018 edition introduced the combined-strain equations above, with the 2/√3 factor, and changed the sign convention for R2. DigSheet uses the 2018-and-later form. Check which edition your procedure references.
Limits
- If either radius is smaller than about 5 × wall thickness, the deformation is "sharp". The result is then a severity index, not a true strain, and DigSheet warns you.
- Strain alone does not address fatigue, rerounding, or dents with metal loss, cracks or gouges. Under 49 CFR 192.933, a top-side dent with metal loss is an immediate condition unless an engineering analysis per 192.712(c) shows otherwise.
- Curvature from ILI data depends heavily on filtering and sensor spacing. Compare like with like.
FAQ
What is R0?
The undeformed pipe surface radius, taken as OD/2 by default. You can override it, for example to use the mid-wall radius.
Where does the 6% limit come from?
ASME B31.8 para. 851.4.1 allows dents with strain up to 6% to remain in service, subject to other conditions. Your procedure may use a different limit, so it is editable.