Inspectioneering Journal

Utilities' High Energy Piping Systems Utility Industry's Application of Accoustic Emission (AE) Yields Measurable Improvement in Inspection Program Cost and Overall System Intergrity

This article appears in the November/December 2008 issue of Inspectioneering Journal.

High energy piping (HEP) systems, main stream lines and hot reheat lines (typically low chrome molybdenum steels), are susceptible to creep damage can lead to leaks, and in extreme cases, catastrophic rupture. To ensure safe and reliable operation as plants age, utilities periodically inspect critical components, conventional inspection methods for HEP systems are radiographic (RT), ultrasonic (UT), field metallography and replication, and magnetic particle (MT) testing.

These methods are labor intensive, require extensive scaffolding for access and removal of insulation. Costs are estimated @ $2.6 million per inspection cycle for the 5,000 of hot reheat piping in 5 fossil units. Conservative estimates show at least a $190K, net savings, using AE, at one site, alone.

Use of AE was studies s a global screening techniques due to potential benefits. Material defect when stressed by operating conditions emit acoustic energy (elastic strain waves). AE piezoelectric transducers can detect and locate the source of this energy. Centrally located data acquisition equipment collects and analyzes incoming emissions. Follow-up, localized UT or RT inspections, are conducted only at these identified AE source locations, Less labor intensive than conventional inspection practices, AE requires the installation of Waveguides, at 15’ intervals, to handle the 950 to 1,000°F surface temperatures. Sensors are attached to the Waveguides. A 1/4” diameter hole is cut in the insulation at each attachment point where the 1/4” diameter rods are stud welded to the exterior pipe surface.

Waveguides and sensors can be attached on-stream, eliminating the need for an outage to install the AE equipment. Once installed, the AE monitoring is conducted during plant load cycling and plant cooldowns.

AE applications guidelines, developed by EPRI, can be used as a foundation for AE inspection of seam-welded hot reheat piping in power plants. Normal plant operation provides the stressing mechanism for the test. A floating threshold is used to compensate for the fluctuating background noise caused by stream flow.

Results have shown good cross-correlation to conventional inspection techniques. Additionally, AE indicated activity at a spool piece, which upon further investigation may required replacement.

Tests have shown that significant AE activity is generated during normal plant load cycling. This work justifies future development as a continuous, on-line monitoring tool for high energy piping (HEP) systems to defect flaws activated during plant operation and warn of critical growth. EPRI continues development with AE technology to achieve greater effectiveness and efficiency.

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