Field Trial Shows Potential of MsS Technology for Detection of Cracking Near High Crown Seals
A recent field trial of magnetostrictive sensor (MsS) ultrasonic guided wave technology showed that the method may have the potential for the detection and evaluation ofcracking in superheater tube high crown seals without removing the seal. The trial was conducted at the J.T. Deely power plant operated by City Public Service of San Antonio, Texas.
Results indicated the frequencies and wave modes likely to produce the best data and revealed that cleaning of ash deposits below the crown seal is necessary to avoid background noise that would prevent detection of the signal.
The results are published in the EPRI report entitled Evaluation of Magnetostrictive Sensor (MsS) Technology for Detection of Cracking near High Crown Seals (1016874). Qualified funders may click on the link below to view or download a copy of the report.
How to Apply the Results
The objective of this work was to provide a resource for power plant operators to use for the nondestructive evaluation (NDE) of cracking in superheater tube high crown seals. Because this area of the seal is not directly accessible, no established examination method exists to detect such cracking. The field trial and subsequent laboratory work were performed in order to determine the applicability of MsS for the detection and evaluation of high crown seal cracking without removing the seal.
An unbiased evaluation by EPRI brings valuable information to power plant owners concerning NDE methodology for specific applications. EPRI is in the unique position to identify the need for NDE research and development and quickly organize a collaborative effort to solve the problem.
Background
The MsS technology, a long-range guided wave examination technique developed and patented at Southwest Research Institute®(SwRI)®, can examine an inaccessible area from a remote accessible location. Guided waves refer to mechanical waves in ultrasonic and sonic frequencies that propagate in a bounded medium (such as a pipe or rod) parallel to the plane of its boundary. The wave is termed “guided” because it travels along the medium guided by the geometric boundaries of the medium.
During a guided-wave examination, a short pulse of guided waves is launched along the structure being examined. Signals reflected from geometric irregularities in the structure—such as welds and defects—are detected in the pulse-echo mode. From the occurrence time of the defect signal and the signal amplitude, the axial location and severity of the defect are determined.
Findings of the Field Trial
Recognizing that this technology might be able to evaluate the tubes from a
location some distance below the crown seal and then detect the circumferential cracking, EPRI suggested a field trial to evaluate its applicability.
The field trial was conducted on March 13, 2008. Of the frequencies and wave modes used in the trial, a torsional wave (T-wave) at 128 kHz provided the best data. However, the test data varied significantly from tube to tube, making the interpretation of data for the presence of crackingrather difficult. In addition, all tubes examined exhibited a high background noise level and a high degree of wave attenuation that were attributed to ash deposits on the tubes and variations in the detected signals.
A subsequent laboratory investigation conducted on the cut-out tube piece showed that a defect in the crown seal area with a size approximately 10% of the tube cross-sectional area was detectable using the 128-kHz T-wave. It was also learned that, in order to achieve a good examination, ash deposits must be removed—particularly in the area below the crown seal where cracking problems occur. This removal helps to ensure that the background noise level in that area is less than half of the target defect signal.
An additional field trial is recommended to establish the cleaning level required for examination, confirm the crack detection capability, and verify that the detection of simulated defects in the laboratory can be related to the detection of actual cracks in a plant environment. Findings fromthis additional field trial would be vital to the development of test procedures and tools thatutilities can use to examine crown seal areas.
For more information contact Stan Walker, 704-595-2081, .
View or download Evaluation of Magnetostrictive Sensor (MsS) Technology for Detection of Cracking near High Crown Seals (1016874).
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