Page 558 - Corrosion Engineering Principles and Practice

520 C h a p t e r 1 2 C o r r o s i o n a s a R i s k 521

Detected
Flux Leakage

N S

Magnetic Field Sensor

FIGURE 12.20 Pipe inspection using MFL.

wall and directly measures its thickness. Since this technology
requires a clean pipe wall, it is generally not used for certain
pipelines such as crude lines with a paraffin build-up. There are
also wall-thickness limitations with the UT tool. It works well
with heavy-wall pipe, but not as well with thin-wall pipe, and
it is not as widely used as the MFL tool. This particular tool of-
fers the following advantages in a pipeline environment:
• Direct measurement and high accuracy of wall thickness
and defect depth;
• Precise distinction between internal and external defects
with the exception of defects adjacent to welds;
• UT tools can also be used to approximate the remaining
strength of affected pipe area;
• UT tools provide a more precise description of anomalies
than MFL tools.
Crack Detection Tools
Crack detection tools are typically designed for longitudinal crack
detection, but they can also be adapted to circumferential crack
detection.
1. UT crack detection tool generates an ultrasonic signal into the
pipe wall that is reflected off the internal and outer surfaces
of the pipe. If a crack is detected, the signal reflects back along
the same path of the tool. Since a liquid couplant is required
between the sensors and the pipe wall, this tool works only
with liquid pipelines.
2. Transverse MFL tools magnetize the pipe wall around its
circumference to detect cracks, such as longitudinal seams
cracks and longitudinal seam corrosion. This tool is similar to