Wall Thinning Evaluation
SIB-96-123
In the design of nuclear power plants, it is assumed that a uniform pipe wall thickness exists. In service, piping components may experience wall thinning due to cavitation, general corrosion, microbiologically influenced corrosion (MIC), pitting, or flow assisted corrosion (FAC). As a result, non-standard evaluation techniques are required for the justification of continued operation, especially in cases where the wall thickness does not meet Code minimum requirements.
Soon after a 1988 nuclear power plant feedwater piping component failed due to FAC, SI started the development of acceptance criteria for locally thinned piping components. These criteria were published in Electric Power Research Institute (EPRI) Report NP-5911. This report later became the standard for the evaluation of thinned piping components, which led to the development of ASME Code Case N-480. SI continues to be involved in Section XI Code activities in this area.
In response to an industry issue regarding localized wall thinning in the outer bend radii of as-built elbows, SI performed an evaluation and industry survey to show that the wall thinning did not affect component integrity. This evaluation included finite element stress analysis and a statistical evaluation for a large sampling of wall thickness measurements. Based on this work, a strong technical basis was made that showed acceptability of the elbows, thereby eliminating the need for thousands of unnecessary inspections.
SI has also played a significant role in the understanding of water chemistry conditions conducive to wall thinning and MIC. In 1988, SI developed the MIC Sourcebook for EPRI in EPRI Report NP-55801. An additional sourcebook on detection and control of MIC was issued in 1990 in EPRI Report NP-6815D.
SI's patented BIoGEORGE™ monitoring probe provides on-line monitoring of biofilm activity to assure that MIC attack is prevented. This probe has assisted numerous facilities in the management of pipe wall thinning due to MIC.
To evaluate the effects of pitting on piping structural integrity, SI has performed both finite element stress analyses and evaluations in accordance with NRC Generic Letter 90-05 to show acceptability for continued service.
For one utility, SI prepared a wall thinning evaluation handbook to help assure that consistent approaches were used for the evaluation of thinned piping components. This handbook included various fracture mechanics techniques applied to thinned piping components and provided justification for continued operation at a large number of plant sites.
To demonstrate the acceptability of thinned piping components for further service, the wall thickness at the next inspection period must be predicted. Then, a screening and evaluation methodology must be employed, as shown below.
For piping components such as elbows, reducers, branch connections, valves, etc., the simplified methods shown above may not be appropriate. For more complex piping components, alternate criteria must be utilized. In some cases, detailed finite element stress analysis can show that stresses in a piping component meet the requirements of the original code of construction. With its expertise in finite element stress analysis and ASME Code criteria, SI can perform a rapid acceptability assessment on all types of piping components subjected to wall thinning and in many cases justify continued operation for these components.
If you would like additional information regarding SI's capabilities in wall thinning evaluation, please contact SI.
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