Electricity: measuring and testing – Magnetic – Magnetic sensor within material
Reexamination Certificate
1998-09-30
2002-03-19
Snow, Walter (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetic sensor within material
C324S240000, C702S038000
Reexamination Certificate
active
06359434
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to ferrous pipelines, and more particularly, to systems for determining the location of defects in ferrous pipelines, such as water pipelines or sewage lines, based on inputs of non-destructive evaluation (NDE) remote field technique (RFT) data. (also known as remote field eddy current, RFEC, data).
BACKGROUND OF THE INVENTION
Pipelines, such as water mains and sewers, are vital to the quality of life of individual citizens and to the economic productivity of society. Over time, water pipelines will deteriorate, and eventually, they will fail entirely. Keeping these lines operable is a challenge faced by every community, both in terms of maintenance and repair costs and in terms of engineered capacity. In meeting these challenges, it is essential to have accurate information on the condition of the pipeline. Traditionally, communities have relied on indirect methods of deterioration detection, e.g., visible leaks, soil corrosion potential, statistical pipe break frequencies, pressure drops, soil settlement, etc., or by manually exhuming a portion of the pipeline in order to extrapolate the condition of the entire pipeline.
More recently, a technology has emerged that measures the condition of water pipelines in a more accurate and non-destructive manner. This technology borrows aspects of knowledge available from current small-bore ferromagnetic tube analysis. In particular, a remote field technique (RFT) measurement device is used to evaluate the wall thickness of the larger ferromagnetic water pipes, Such as those used in municipal water systems, using remote field technique.
Small-Bore Ferromagnetic Tube Analysis
As background information and referring to
FIG. 1
, a typical small-bore RFT measurement device includes an exciter coil and a sensor coil, separated by a distance of ~2 or more pipe diameters. A probe is inserted into and pulled through a ferrous tube. The exciter coil is energized with a low frequency (100-300 HZ typical) alternating current that creates an alternating magnetic field, a portion of which travels within the tube and a portion of which travels outwardly through the tube walls. The alternating magnetic field traveling within the tube is rapidly attenuated due to eddy currents induced in the tube wall. The portion that passes outward through the tube walls propagates along the tube exterior and is attenuated less rapidly. At some point, the inside field is weaker than the outside field (usually past ~2 or more pipe diameters), and part of the outside field propagates back into the pipe, and can be measured by the detector coil.
Thus, the measured electromagnetic field has passed through the tube wall twice, once propagating outward at the exciter coil, and once propagating inward at the detector coil. With each passage through the tube walls, the signal is reduced in strength and delayed (i.e., signal transit time is increased.) Changes in wall thickness will affect the amount of transit time taken for the signal to go from the exciter coil to the detector coil, which can be measured as a phase shift in the returning electromagnetic field. In addition, the strength of the field will also be altered, which is measured as a signal amplitude change. As the measurement device travels along a body of tube, the detector signal's phase and amplitude are determined by the RFT instrument and are digitally recorded and/or displayed in strip chart form.
FIG. 2
is an example of such strip chart data. The information of
FIG. 2
is from the article Remote Field Eddy Current Analysis in Small-Bore Ferromagnetic Tubes, by David D. Mackintosh, David L. Atherton, and Sean P. Sullivan, in Materials Evaluation, Vol. 51, No. 4, April 1993, pp. 492-495, 500.
In small-bore ferromagnetic data analysis, an analyst will first obtain signal calibration information from a sample tube, preferably identical in size and material to the tube in question. By machining various known defects in the sample tube, pulling an RFT measurement tool through the sample tube, and then measuring the resulting RFT data signals, the analyst will develop a baseline or library of data values for known defects.
After obtaining calibration data, the analyst passes the measurement device through the tube of interest. The measurement data is visually displayed and/or printed in strip chart form. The analyst determines nominal signal values for phase and amplitude by inspection of the strip chart data. Any recurring, relatively flat, data segments are likely tube segments without defects, and therefore representative of nominal signal values.
The analyst next identifies potential defect signals that will require further analysis. At each potential defect signal location, evaluation of its type is made, including a determination of whether the defect is circumferential or one-sided pitting and a determination of the defect length (long or short) relative to the spacing between the exciter and the detector which affects the method as calculation in RFT analysis. Often this evaluation is made using a polar type plot of the values of phase and amplitude for a select portion of the measurement data. The polar plot of
FIG. 3
illustrates one type of polar plot in a voltage plane. In the voltage plane, the data is displayed in polar form, with the axis scaled and rotated so that the Cartesian coordinate (1,0), or 1∠0° in polar form, represents the nominal data signal. The positive X-axis represents the 0° degree change of signal phase from nominal signal and counter-clockwise rotation direction indicates an increase in phase, such as would occur with a decrease in wall thickness. Because there are phase and amplitude values for each total circumferential wall thickness, there exists a theoretical Reference Curve, labeled “Reference Curve” in
FIG. 3
that defines the theoretical signal values for decreasing uniform circumferential wall thickness.
Based on the change in the phase and amplitude at the defect with respect to the nominal signals, it is possible to determine the metal loss and circumferential defect extent using the calibration information, the skin-depth equations, and the mathematical theory of RFT analysis. The amount of metal loss can then be correlated to actual physical dimensional changes in the tube wall thickness.
To ease the task of plotting RFT measurement data for small bore tubes, a software tool (termed ADEPT) was invented in the early 1990's. The ADEPT tool is capable of displaying the small-bore ferromagnetic tube RFT data on a computer monitor in various forms. The ADEPT tool requires the analyst to select a particular portion of the data stream, to enter a calibration amount, and to supply a nominal signal value. Using this information, the ADEPT tool plots the defect trace in voltage plane polar form. The defect trace of the polar plot in
FIG. 3
is shown in
FIG. 4
as it would appear in ADEPT on the voltage plane. The voltage plane data in the ADEPT display has been rotated and normalized so that the nominal signal occurs at a position (1, 0) Cartesian or 1∠0° polar in the voltage plane polar plot. The Reference Curve is displayed to indicate the theoretical trace a uniform change in wall thickness would cause, which aids in defect pitting evaluation.
The analyst plots a select portion of the signal data using ADEPT and then visually searches for shapes that are similar to known defect shapes. Various characteristic shapes represent certain types of defects or other anomalies. For examples of specific shapes, see the article Remote Field Eddy Current for Examination of Ferromagnetic Tubes, by David D. Mackintosh, David L. Atherton, Thomas R. Schmidt, and David E. Russell, in Materials Evaluation, Vol. 54, No. 6, June 1996, pp. 652-657. In
FIG. 4
, the plot of the phase and amplitude values of
FIG. 3
results in an elongated loop shape representing one-sided pitting.
At each selection data portion, the analyst can request ADEPT to calculate the remaining wall thickness based on information about the d
Rajan Varagur S. V.
Ridley Rodney K.
Staples Lawrence B.
Winslow Jens
Yu Yuwu
Christensen O'Conner Johnson Kindness PLLC
Hydroscope Cananda Inc.
Snow Walter
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