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John Rodgers  
Acoustic Emission Consulting, Inc.  
5000 San Juan Ave, Suite D  
Fair Oaks, CA 95628  

jr_aec@msn.com  


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Frequent Questions
Answers to popular questions

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AE fundamentals

How does acoustic emission monitoring work?

Acoustic Emission is a passive monitoring technique that is non-invasive to the structure. AE sensors "listen" to the structure at very high frequencies (300-400 KHz) and can sense the sounds of microstructural failures (cracking and inclusion decohesion are typical). Combined with ability to perform "source location" using time difference of arrival techniques, AE is the only method that can yield a real-time assessment of structural health under operational conditions.


Instrumentation

What are the instrumentation requirements?

The basic acquisition instrumentation consists of specialized AE sensors mounted on "waveguides" that are welded to the piping. The waveguides serve to isolate the AE sensors from the hot surface, as well as conducting the high frequency sound waves from the pipe wall to the sensor. Waveguide spacing is typically 15-20 ft along the full piping length to be monitored. Coax cable is strung from the sensors to the multichannel, computerized AE data acquisition system. AE systems range in capacity from 8 to 60 or more channels.


AE testing in-service

How is Acoustic Emission applied in service?

AE is performed during normal online conditions, with the only special requirement that peak loading is necessary to fully assess piping condition. Peak load cycles at 90% or higher of peak pressure for time periods of 4 or more hours are required for several cycles.


AE testing in-service

How is monitoring conducted if all sensor locations are not covered simultaneously by the instrumentation?

We usually use two or more 8-channel systems for the average line length (up to 500 ft). The line is monitored in segments of 3-5 days/segment. Monitoring is controlled remotely via modem, so we can see what is happening to plant operation and design the monitoring program accordingly. Onsite assistance from utility personnel is required to switch cables at the instrumentation to monitor different segments of piping, but all sensor positions are fully instrumented before the monitoring begins.


AE analysis

How is AE analysis performed?

Acoustic Emission analysis on high energy piping is performed by linear source location cluster analysis. Significant clusters of AE events are analyzed for their relationship to load factors (temperature and pressure in the piping system). Download the PDF files on Cluster Analysis and the SPIE technical paper in our files section for a more complete description.


Location accuracy

How accurate is AE source location?

Source location accuracy on piping with 15-20 ft waveguide spacing is typically ~1ft. Resolution is better in the center of the waveguide span than at the waveguide positions, where the uncertainty may be /- 2 ft. This has to do with geometry and wave propagation effects (download the SPIE paper for a better explanation). Resolution of relative position of sources along a seam weld can be on the order of 1-2 inches, however, with the use of specialized graphics analysis (download our paper on cluster analysis).


Economics

What are the economics of AE compared to other inspection methods?

AE has proven to be far more cost effective than the principal alternative--100% periodic ultrasonic inspection with multi-angle shear wave methods. Savings of 75% are typical, depending on the amount of follow-up inspection with other methods determined as necessary by the AE monitoring. Typically this has only involved a few selective areas (download the HEP results document from our files section). AE can also help prioritize the inspections by determining the location, likely source and severity of the defect condition.


Comparison with other creep evaluation methods

How does acoustic emission compare with other verification methods?

AE has been found to correlate very well with other highly-sensitive methods for creep inspection, including Time-of-Flight Diffraction (TOFD) and Focused-Array (FATS) ultrasonic methods, and cryo-cracking metallography. These methods can detect creep at the early cavitation stage, and have been used to verify AE findings in field test programs. See the PDF-format Powerpoint presentations by M&M Engineering and AEC from the June '99 EPRI Plant Maintenance Conf in our files section.







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