The Design of a Mechanical Damage Inspection Tool Using Dual Field Magnetic Flux Leakage Technology

[+] Author and Article Information
J. Bruce Nestleroth, Richard J. Davis

 Battelle, 505 King Ave, Columbus, OH 43201

J. Pressure Vessel Technol 127(3), 274-283 (Mar 01, 2005) (10 pages) doi:10.1115/1.1989349 History: Received December 30, 2004; Revised March 01, 2005

This paper describes the design of a new magnetic flux leakage (MFL) inspection tool that performs an inline inspection to detect and characterize both metal loss and mechanical damage defects. An inspection tool that couples mechanical damage assessment as part of a routine corrosion inspection is expected to have considerably better prospects for application in the pipeline industry than a tool that complicates existing procedures. The design is based on study results that show it is feasible to detect and assess mechanical damage by applying a low magnetic field level in addition to the high magnetic field employed by most inspection tools. Nearly all commercially available MFL tools use high magnetic fields to detect and size metal loss such as corrosion. A lower field than is commonly applied for detecting metal loss is appropriate for detecting mechanical damage, such as the metallurgical changes caused by impacts from excavation equipment. The lower field is needed to counter the saturation effect of the high magnetic field, which masks and diminishes important components of the signal associated with mechanical damage. Finite element modeling was used in the design effort and the results have shown that a single magnetizer with three poles is the most effective design. Furthermore, it was found that for the three-pole system the high magnetization pole must be in the center, which was an unexpected result. The three-pole design has mechanical advantages, including a magnetic null in the backing bar, which enables installation of a pivot point for articulation of the tool through bends and restrictions. This design was prototyped and tested at Battelle’s Pipeline Simulation Facility (West Jefferson, OH). The signals were nearly identical to results acquired with a single magnetizer reconfigured between tests to attain the appropriate high and low field levels.

Copyright © 2005 by American Society of Mechanical Engineers
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Figure 1

Two-magnetizer implementation of mechanical damage inspection technology

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Figure 2

Model results for a two-magnetizer implementation of mechanical damage inspection technology

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Figure 3

Model results for a three-pole magnetizer implementation of mechanical damage inspection technology

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Figure 4

Center pole position configurations to tune magnetization levels for three-pole magnetizer

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Figure 5

Alternative magnet polarity configuration that provides field levels inappropriate for dual field inspection

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Figure 6

Modeling results for different pole pitch of a three-pole magnetizer

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Figure 7

Velocity changes the magnetic flux most significantly where the flux enters the pipe and where flux is diverted at pipeline anomalies

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Figure 8

Magnetic field at the outer surface of a pipe with a wall thickness of 7.6 mm (0.300 in.) at a velocity of 4 and 8km∕h (2.5 and 5.0 mph)

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Figure 9

Magnetic null and pivot point in backing bar

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Figure 10

Magnet bar split at the null point with a ball joint coupling for passing pipeline bends

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Figure 11

Maximum collapse of a typical MFL magnetizer tool for the passage of obstructions such as a 12% diameter restriction

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Figure 12

A split low field backing bar that attains appropriate field levels while increasing tool flexibility

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Figure 13

Split backing bar collapse of the passage of obstructions such as a 30% diameter restriction as compared to a traditional MFL magnetizer capable of a 12% collapse

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Figure 14

Dual magnetization MFL tool prototype

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Figure 15

Comparison between a single magnetizer configured for low magnetic field inspection and the low field signal for the three-pole dual-field magnetizer. A photograph of the gouge is inset.




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