Drain Dwell

Figure 2-14. Comparison of Adequate Dwell versus Insufficient Dwell on a Thermally Cracked Aluminum Block.

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 2-45 2.4.5.6.1.2 Drain Dwell NOTE The drain dwell mode SHALL be used unless the inspection instruction specifies immersion dwell. With drain dwell, the part is first covered with penetrant by spraying, brushing or immersion. Once coated, the part is placed on a rack or rest and allowed to drain during the dwell period. Comparison tests with aluminum crack blocks and nickel-chrome penetrant panels have demonstrated the improved performance of drain dwell mode compared to that of immersion dwell mode. This improved performance is due to the changes in penetrant composition that occurs during the dwell period. The penetrant vehicle is a mixture of heavy oils that dissolve and hold the dye materials in solution; and thin or lightweight solvents or oils that reduce the viscosity of a penetrant. During the drain dwell period, the lighter weight liquids evaporate, which increases the concentration of the dye material entrapped in discontinuities. The increased dye concentration enhances the visibility of the indication. The drain dwell mode is also more economical than immersion dwell mode since the excess penetrant drains from the part and is recovered. The savings with drain dwell are two-fold, since the drained penetrant is recovered and the remaining penetrant layer is much thinner than an immersion dwell layer. The thinner penetrant layer requires less emulsifier during the removal process. Generally the immersion is momentary, but at most it should be no longer than half the total dwell period. 2.4.5.6.2 MINIMUM PENETRANT DWELL TIMES NOTE Selection of a penetrant dwell time is complex and depends upon a large number of factors. A thorough knowledge of the penetrant capabilities and limitations of the penetrant system used for the type of discontinuity to be detected is required. Whenever possible, the decision of dwell time should be based upon experience of the cognizant engineering support. Documents governing dwell time should specify the mode and time of dwell. The number of factors influencing the entry of penetrant into a discontinuity complicates setting uniform minimum penetrant dwell times. Most dwell times are based on past experience with similar parts, materials and potential flaws. The minimum dwell time for service-induced defects SHALL NOT be less than 30 minutes. There is one exception to this requirement. When stress corrosion cracking is suspected the minimum dwell time SHALL NOT be less than 240 minutes. These established minimum dwell times are based on parts having a temperature of 60°F (15.5°C) to 100°F (37.2°C). When part temperatures are 100°F (37.2°C) to 120°F (49°C), the dwell time may be reduced by half, except a minimum of 5 minutes dwell time is required. When part temperatures are between 60°F (15.5°C) and 40°F (4°C), the minimum dwell time SHALL be doubled. Penetrant inspection SHALL NOT be performed with part temperatures below 40°F (4°C) or above 120°F (49°C). For long dwell times refer to paragraph 2.4.5.6.4 for precautions to take for a successful inspection. 2.4.5.6.3 Effects Of Insufficient Dwell. When the dwell time is too short to allow the penetrant to completely fill the discontinuity, the visibility of the resulting indication will be reduced. Figure 2-14 shows a thermally cracked, aluminum block with one half receiving an adequate dwell, and the other half an insufficient dwell. The differences in dwell times have different effects depending on the flaw size. The very small flaws are not indicated; the visibility of indications from medium size flaws is greatly reduced; and there is a slight reduction in the visibility of larger size flaw indications. If it is suspected that a part has not had an adequate dwell, the part SHALL be completely cleaned and then reprocessed through the entire inspection process.

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