Wall Thickness Verification on Seamless Pipe
An inspector is verifying the wall thickness of a lot of 200 seamless carbon steel pipes (OD 168.3 mm, nominal wall 7.11 mm, specification allows -12.5% tolerance). The inspector uses an ultrasonic thickness gauge with a dual-element transducer, calibrated on a step wedge with known thicknesses. The gauge reads 6.85 mm on a pipe. The inspector must decide if the pipe is acceptable. The specification requires that thickness be measured at four quadrants around the circumference at each end. The inspector has measurements: 6.85, 6.90, 6.88, 6.92 mm at one end and 6.80, 6.85, 6.82, 6.88 mm at the other end. The gauge calibration is current and the transducer is coupled properly. The pipe surface is slightly pitted but within allowable limits.
1. Determine whether the pipe meets the wall thickness acceptance criteria. Show all calculations and state the final decision.
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The minimum allowable wall thickness is calculated from the nominal wall and the tolerance: 7.11 mm × (1 - 0.125) = 7.11 × 0.875 = 6.221 mm. The lowest measured thickness is 6.80 mm, which is greater than 6.221 mm. Therefore, the pipe meets the wall thickness requirement. The average thickness is not specified as a criterion, but for completeness, the average of all eight readings is (6.85+6.90+6.88+6.92+6.80+6.85+6.82+6.88)/8 = 6.8625 mm, also above the minimum. The decision is acceptable.
2. If the gauge had been calibrated with a flat plate instead of a step wedge, how might that affect the decision? Explain the potential error and its impact.
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Calibrating with a flat plate instead of a step wedge could introduce a systematic error because the step wedge better simulates the curvature and geometry of the pipe. A flat plate calibration might not account for the curvature effect on sound velocity and coupling, potentially causing readings to be offset. If the offset is positive (readings higher than actual), the true thickness might be lower than measured, possibly below the minimum. If negative, readings might be lower than actual, leading to unnecessary rejection. The inspector should use a calibration standard that matches the material and geometry as closely as possible. In this case, since the readings are comfortably above the minimum, a small error might not change the decision, but it could if the margin were tighter.