Acoustic emission test specifics because almost every

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Acoustic Emission Test Specifics Because almost every hot reheat piping system is unique in design and operating characteristics, performing an acoustic emission test will require careful considerations and planning. Pretest evaluations must be made to establish background noise levels for optimum transducer frequencies, transducer spacing based on sound attenuation, typically 3 to 4.5 m (10 to 15 ft), and access to each transducer. Scheduling of the test is usually dictated by operations personnel and will essentially be performed before and during their routine shutdown schedules. High Temperature Considerations Hot reheat piping normally operates at about 540 °C (1000 °F). To prevent degradation of the transducer crystals, waveguides are required to couple the transducers acoustically to the piping. There are two ways to couple the waveguides to the piping surface: attaching the waveguides to their piping by stud welding (Fig. 13a) and using a spring loaded mechanical waveguide with a high temperature couplant placed between the rod tip and the pipe surface (Fig. 13b). Both techniques work but the welded waveguide demonstrates slightly more sensitivity. Signal Cable Considerations One of the most time consuming aspects of an acoustic emission test setup and teardown is the installation of signal cabling. Because of the length of some hot reheat piping systems and because of their multiple channels, about 1000 m (over 3000 ft) of radio guide (RG) 58 signal cable is required. RG 58 is a general purpose coaxial cable with 50 Ω impedance, 1.9 kV maximum operating potential and 5 mm (0.2 in.) outside diameter. Some utilities have installed the cabling permanently to reduce setup and teardown time. Temporary cable installation before an acoustic emission test must be performed with regard to changes that occur during the cool down process, such as extreme temperatures on secondary systems in contact with the cabling and physical movement of the hot reheat piping, which can cause tension and breakage of cabling secured to another structure. Cabling must also be routed so people don’t trip on it. 343 Electric Power Applications of Acoustic Emission Testing
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Data Acquisition The test equipment must be capable of acquiring, processing and storing active acoustic emission events for multiple transducers. Acoustic emission test signal processing, as a minimum, should include peak amplitude, counts, event duration, rise time and spatial event filtering (location) capabilities. The system must also be able to filter undesirable low amplitude emission, associated with steam flow noise, on an individual channel basis. Stressing Techniques Since the application of acoustic emission monitoring for hot reheat piping, the industry has been striving to determine the best technique to optimize discontinuity emission. Various stressing techniques have been explored, including cool down monitoring, load swing monitoring and steady state monitoring.
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  • Fall '19
  • Test, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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