Floor scanning has problems not evident in the

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Floor scanning has problems not evident in the testing of tubular goods, where certain parameters can be closely controlled. Probably the greatest problem is that tank floors are never flat whereas tubes are always round. The unevenness of tank floors makes it hard to get reasonably consistent quantitative information. The application of rigid accept/reject criteria based on signal amplitude thresholds is also very unreliable for quantitative information. A realistic approach is required in the application of this test technique and in the design of the test equipment to ensure that fewer significant discontinuities are missed. Test Conditions To optimize the test, it is necessary to consider the environment and address the physical restrictions imposed by the actual conditions found when testing most tank floors. Climate. The range of temperature and humidity conditions varies enormously during the year and around the world. The effect on both operator and equipment must be taken into consideration. Cleanliness. Most aboveground storage tanks are dirty and sometimes dusty places to work. The conditions vary widely and depend on how much the tank operator cleans the floors in preparation for magnetic flux leakage scanning. As an absolute minimum, a good water blast is necessary and all loose debris and scale must be removed from the test surface. The surface does not have to be dry but puddles of standing water need to be removed. The cleaner the floor, the better the test. Surface Condition. Significant top surface corrosion and buckling of the floor plates represent serious limitations to both the achievable coverage in the areas concerned and also the achievable sensitivity. Although very little can be done to improve this situation before testing, it must be considered in the design of the equipment. The effect of corrosion and buckling on the sensitivity of the test must be appreciated by both the tank operator and the inspector. Any physical disturbance of the scanning system as it traverses the floor will result in the generation of noise. The rougher the surface, the greater the noise and therefore the more difficult it is to detect small indications. Equipment It is important that magnetic flux leakage equipment produced for this particular application be designed to handle the environmental and practical problems always present. Figure 9b shows a mobile floor scanning unit. Powerful rare earth magnets are well suited for introducing the required flux levels into the material under test. Electromagnets by comparison are excessively bulky and heavy. They do have an advantage in that the magnetic flux levels can be easily adjusted and turned off if necessary for cleaning. Permanent magnet heights can be adjusted to alter flux levels but the bridge requires regular cleaning to remove ferritic debris. The buildup of debris can impair system sensitivity significantly.
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  • Fall '19
  • Nondestructive testing, electromagnetic testing, Magnetic flux leakage

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