Structural Integrity recently had the opportunity to support a client’s emergent needs when their Standby Service Water (SSW) piping system experienced a pinhole leak just downstream of a valve. Concerned about other locations in the piping system with similar configurations, the site asked SI to assist with the expedited development of assessment and disposition plans for these other components. In response, SI was able to lean on our core competencies in failure analysis, advanced NDE inspection, and flaw evaluation to develop and deploy a comprehensive solution that met our client’s expedited timeline and helped them to mitigate the threat of future unplanned outages. The following sections outline how SI utilized our in-depth knowledge, cutting-edge technology, and world-class engineering to meet our client’s needs.
Following discovery of a pinhole leak in a 10-inch diameter SSW pipe, downstream from a valve, SI’s client removed the section of pipe from service and sent the damaged section to SI’s Material Science Center where a comprehensive failure analysis was completed. As part of the failure analysis, SI conducted detailed visual examinations, chemical analysis, high-resolution 3D digital microscopy, metallography, and deposit analysis on the received sample. Based on the detailed analyses completed, SI was able to confirm that the observed damage was caused by cavitation that resulted from the turbulent flow and abrupt pressure changes at the throttled upstream valve.
Cavitation happens when the local pressure is reduced to that of the vapor pressure, which commonly occurs in pumps and throttled control valves. At this low pressure, bubbles of water vapor or steam form. The bubbles flow downstream with the water, become unstable, and implode. The bubble collapse releases a small but intense shock wave or microjets that break away the protective oxide layer. The newly exposed surface corrodes and the process repeats. Bubble formation and collapse may occur in just a small fraction of a second. Each bubble collapse produces a relatively small amount of damage, with significant damage accumulating during thousands of bubble formation and collapse cycles. Once surface irregularities are formed, bubble formation/attachment will tend to concentrate at damage sites, eventually producing deep, localized attack such as that observed on the pipe section.
The failure analysis completed by SI further determined that the cavitation was occurring intermittently and was likely associated with specific operating conditions of the valve and pipe, suggesting that the cavitation could potentially be minimized through operational changes or controls. The failure analysis process also confirmed that the pipe material had a normal microstructure and that there were no anomalies that contributed to the loss of material.
In preparation for the field inspection of the other at-risk components, SI was also tasked with developing an NDE approach for efficient high-resolution wall thickness characterization and demonstrating the approach on the 10-inch sample that was removed from service. Obtaining accurate, high-resolution thickness information was paramount as the data would potentially be used as input for Finite Element Analysis (FEA) if any identified material loss exceeded a critical threshold.
To meet this demanding requirement, SI employed a dual-matrix phased array probe for fast high-resolution coverage and our patent-pending non-mechanized position encoding technology, LATITUDE™. LATITUDE enables the NDE operator to manipulate the inspection probe by hand, with no mechanical hardware, while still gathering multi-axis encoded data. This approach reduces both the amount and complexity of equipment required on-site and eliminates a significant amount VIEWS of data post-processing time by removing the need to manually merge individual line scans. This enables the solution to be deployed more quickly and more cost-effectively than other more complicated scanning systems.
The encoded PAUT corrosion mapping approach was demonstrated on the 10-inch diameter sample with the pinhole leak, prior to destructive testing. The results from this examination are shown in Figure 2. Analysis and measurement results from the destructive testing confirmed the accuracy of the developed NDE examination approach.
In preparation for the examination of the in-service components, SI developed evaluation templates such that detailed evaluations of any identified material loss could be conducted rapidly. The evaluation templates met the structural requirements of the Code of Construction and, therefore, represent a more accurate description of the required section geometry relative to hand calculated tmin values. Any inspection findings that meet the requirements of an analysis completed with the prepared templates can be considered acceptable for continued operation until the end of the inspection interval.
If thinning was discovered that was near or below tmin, SI was on-call to conduct specific FEA modeling using thickness measurements obtained from NDE examinations and location specific loading. In this situation, finite element models are utilized to calculate the stress field associated with non-uniform pipe thickness obtained from the NDE examinations. Guidance from ASME Section III, NB-3200, Design by Analysis, is taken to determine how component stresses are combined as well as the specific stress allowables. A typical finite element model used in this type of analysis is shown in Figure 3.
A detailed FEA model (developed to easily accept inspection grid data) and calculation template were created for our client as a contingency. If the observed wall thinning profile is found to meet the Code of Construction stress limits, additional uniform wall loss can be applied to determine the amount of allowable thinning. Integration between the NDE and engineering teams, along with the preparation of a template calculation and FEA model, allowed SI to guarantee rapid support. Thus, if an evaluation was required, the analysis results would be available within 24 hours.
In addition to performing corrosion mapping on the 10-inch pipe sample that was removed from service, SI deployed a team of two NDE professionals to conduct on-site examinations of one 10-inch and two 18-inch components with similar configurations as the component that had experienced the pinhole leak. On each component, corrosion mapping was completed over a 360° area from the valve flange girth weld, continuing downstream for approximately 12 to 18 inches, to capture the area that was determined to be susceptible to cavitation. Each examination took several hours, including set-up, primary and supplemental data acquisition, and teardown.
Some amount of cavitation damage was identified in each component examined; however, none of the damage exceeded the allowable tmin and the FEA modeling contingency was not implemented.
SI provides the individual services described in this article on a regular basis for many of our clients but it is when the SI team gets to collaborate, bringing together our combined areas of expertise, that we are able to maximize the value of the solutions that we provide and to deliver on a tight timeline. For more information about the work summarized here, please contact Eric Houston, firstname.lastname@example.org.