Standardization of Acoustic Emission Testing of Fossil Power Plant Seam-Welded High Energy Piping

The structural integrity of seamed fossil high energy piping has become a major safety and O&M issue again with eight recent failures of seam-welded piping since 1992. These include failure of six superheat link piping segments, two of them catastrophic, and the failures of two long-seamed bends in hot reheat lines. Advanced methods of inspecting piping welds with ultrasonic techniques, such as Time Of Flight Diffraction and Focused/Phased Arrays, is pushing back the envelop of detection to earlier stages of creep damage. But these are still very expensive and involve considerable logistics planning and downtime to perform. EPRI has sponsored development activities since 1986 to mature the utilization of a real-time online evaluation method for seam-welded piping: Acoustic Emission (AE) Guidelines were published in 1995, and over 90 full-scale tests have been performed from 1996–2003 to develop a database and correlate results with other established evaluation methods. Tests to date have shown high sensitivity to early stage creep damage, which is evidenced by development of cavities (cavitation) around nonmetallic inclusions and carbides in the grain boundaries of the weld heat affected zone and fusion zone. Successful double-blind testing with advanced ultrasonsic inspection methods, and additional confirmation with advanced cryo-cracking metallography, have proven both the reliability and sensitivity of the AE technique. The economics of the method are highly favorable. Only small areas of insulation need to be removed every 4.6–6.1 m (15–20 ft) to weld “waveguides” to the piping surface. These form a linear location array along the length of piping, providing global coverage of the piping system. Testing is performed online with normal peak loading and load cycling. No outage schedule is required to perform the AE examination. The ASTM E07.04 Subcommittee on Acoustic Emission is currently developing a standard based on the EPRI testing database: Document WK658, “Standard Test Method for Acoustic Emission Examination of Seam-Welded High Energy Piping.” Results will be presented showing that the AE method has become a reliable and economical field evaluation tool for seamed high energy piping.