Acoustic Emission Testing of Seam-welded High Energy Piping Systems in Fossil Power Plants

The structural integrity of seam-welded fossil high-energy piping has remained a major safety and operations-maintenance issue for US utility companies. Several failures of seamwelded superheat and hot reheat piping segments have occurred since 1992, two of them catastrophic. Advanced methods of inspecting piping welds with ultrasonic testing (UT), such as time-of-flight diffraction and focused/phased arrays, are pushing back the envelope of detection to earlier stages of creep damage, but these are still very expensive, and involve considerable logistical planning and downtime to perform. The Electric Power Research Institute (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) testing guidelines were published by EPRI in 1995, and over 100 full-scale tests have been performed to develop a database and correlate results with other established evaluation methods. An effort was begun in 2002 to standardize the testing method within ASTM (AE Subcommittee of E07.04) utilizing the developed database. Tests to date have shown high sensitivity to early stage creep damage, which is evidenced by development of cavities around inclusions in the grain boundaries. Successful double-blind testing with advanced ultrasonic inspection methods has 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 m 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. Results will be presented showing that the AE method has become a reliable and economical field evaluation tool for seam-welded high energy piping.