Detection requirements are a function of the overall goal of an inspection. Some operators use inspections to locate future problem areas. Here, the inspection is intended to find imperfections that can grow into defects. Other operators are more interested in ensuring that their lines have no defects or critical defects that threaten integrity. In this case, an inspection must uncover larger defects only.

Imperfection and Defect Sizes
(36-inch-diameter x 0.375-inch wall thickness X60)

The defect detection requirements for an MFL inspection can be estimated from the curve shown above. The yield pressure was used in the figure because it is a common design and maintenance value. Also, it is the dividing line between an acceptable imperfection and a defect as defined here and in the ASME B31G acceptance criterion for corrosion defects ^{[ASME B31G]}.

Design codes and regulations generally assume a pipeline can withstand a pressure equal to the yield pressure ^{[U.S. Department of Transportation]}. Thus, the curved line shown in the figure can be considered a minimum threshold of detection for MFL inspection tools. If an MFL tool finds metal-loss regions that are smaller than shown in the curved line, the tool can detect imperfections as well as defects.

Defects lie above and to the right of the curve shown above. The required probability that an MFL tool can detect these defects must be high. An acceptable probability of detection is hard to define, and there are very few reports of the required probabilities of detection used by pipeline operators.

Some operators have specified that inspections "detect and report all general corrosion ... greater than 20 percent ... and all pitting corrosion ... greater than 40 percent" with a confidence level of 80 percent. This implies that 20 percent of defects greater than a certain size could be missed. Others have specified repeatability rates, which are related to probabilities of detection; as an example, one operator has specified a repeatability rate of 95 percent. Repeatability is a difficult measure of detection reliability if the defect is repeatedly missed.