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Akzo Tank 40
Nondestructive Inspection of an FRP Composite Tank
By: Dr. Michael Werner
Supported by: Materials Technology Institute (MTI) Project 126-98
Location: Tank T-40, Akzo Nobel Functional Chemicals LLC
Gallipolis Ferry, WV November 27-28, 2001
The objective of this effort is to demonstrate nondestructive capabilities for quantitive inspection of Fiber Reinforced Polymeric (FRP) vessels using proprietary microwave sensors.
Encouraging results were obtained in an on-site tank inspection performed to test the sensitivity of novel microwave resonator sensors to known liquid-filled blister defects in FRP tanks that have been in hydrochloric acid (HCl) service. At issue was whether the sensor can distinguish, clearly and repeatably without false alarms, a blistered area from a known good area of the tank.
The sensor is sensitive to the effective microwave loss factor (e'') of whatever admixture of test materials are placed in contact with the sensor. In general, liquid-filled blisters on the tank internal surface stand out because aqueous liquids are very lossy (i.e., large e''). However, before the inspection there was concern whether Tank T-40's blisters could be detected because, although the blisters are quite large, they are located inside the tank wall that is nominally 1½ inch thick, as specified in T-40's mechanical drawing. The electromagnetic field excited by the sensor is evanescent (i.e. decays exponentially as it penetrates) into the test material; consequently, the more distant an anomaly, the less the anomaly would stand out from the background.
Actually, T-40's blisters proved to be quite visible to the sensor with excellent (40%) signal contrast, in spite of the thick wall and competing variables; among them, an unknown chemical residue on the tank exterior.
Since all the blisters in this tank were quite large it was not possible to determine the lowest threshold area of blister visibility for the sensor used in the inspection. To learn more, a future inspection should encounter a range of blister sizes to determine the threshold size for a given wall thickness. Further, an ideal test would combine destructive testing with NDT to compare the actual blister distribution with the mapping representation derived from sensor images, and to observe a blister in cross-section.
The T-40 test results are in contradistinction to KDC's first on-site inspection (DuPont, Belle WV) where all the blisters were sub-threshold, being quite small in area and had apparently been drained of liquid content while the tank was kept empty.
This project is ongoing. Another inspection of this type was conducted during June, 2004. The report will be available at this site in August.
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