Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
1999-02-08
2001-08-07
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
C324S644000
Reexamination Certificate
active
06271670
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains generally to detecting cracks and other anomalies in tubes such as steam tubes used in boiler applications. A method and apparatus are disclosed in which, in the preferred embodiment, a transverse magnetic-dipole source positioned inside a tube generates a continuous electromagnetic wave and electric field parallel to the axis of the tube. The wave reflects off the external wall of the tube and returns to a coaxial electric-dipole receiver. The transmitting and receiving antennas are designed and positioned to minimize crosstalk and at the same time distinguish cracks and other anomalies in the tube based on the return signal.
2. Description of the Related Art
Historically, and even to the present day, the major cause of boiler outages is the degradation of the steam tubes by which heat is extracted from the boiler. Although there is deposition and corrosion upon the internal surfaces of the tubes, the principal cause of failure is by the development of exterior cracks. Of course, when a boiler is otherwise down for repair, the exteriors of all apparently satisfactory tubes are inspected for cracking. Unfortunately, some cracks may not be discovered because the exterior surfaces are more or less covered by combustion or corrosion products. An inspection tool that could be passed through a boiler tube, and detect an unseen sidewall crack from within, would increase the intervals between boiler outage and provide substantial cost savings.
Other approaches for detecting cracks in metal and other structures have been described which utilize electromagnetic waves. Unlike the invention disclosed here, though, those techniques do not utilize radar backscatter. Crack and anomaly detectors that employ the use of eddy currents, for example, include the waste storage tank probe/scanner produced by ZETEC™ and the logging and survey tools marketed by TUBOSCOPE PIPELINE SERVICES™ (See: ZETEC™ INC., “Waste Storage Tank Probe/Scanner—Remote Field Testing (RFT) Probes”, and “Introduction to Pipeline Services”, “Survey Tool Operation” and “TruRes”). Eddy current and ultrasonic Lamb-wave techniques have been studied, and various combinations of pancake-coil arrays have been investigated and described in reports from Argonne National Laboratory. (See: D. R. Diercks, et al., “Overview of Steam Generator Tube Degradation and Integrity Issues”, Argonne National Laboratory, Argonne, Ill.; S. Bakhtiari, et al., “Modeling of Eddy Current Probe Response for Steam Generator Tubes”, Energy Technology Division, Argonne National Laboratory, Argonne, Ill.; and D. S. Kupperman, et al., “Characterization of Flaws in a Tube Bundle Mock-up for Reliability Studies”, Argonne National Laboratory, Argonne, Ill.)
The concept of the present invention, by contrast, offers a technically different approach. Radar backscatter represents an improvement over existing technology because it offers the same or better resolution with respect to crack location and mapping than current methods while not requiring external components such as magnets (some eddy-current systems), immersion in water (some ultrasonic systems), or mechanical steering (pancake-coil systems). Likewise, the radar backscatter method of the present invention operates where a pulse-type radar system cannot. The dimensions of boiler tubes and oil/gas pipelines, for example, would require such a high-frequency system that the thickness of the metal wall would completely absorb the electromagnetic field before the field could reach the exterior.
BRIEF SUMMARY OF THE INVENTION
The present invention utilizes the concept of radar backscatter to detect cracks and other anomalies in tubes by generating from within a tube a continuous electromagnetic wave from a transverse magnetic-dipole antenna, reflecting the wave off the external surface of the tube, receiving the reflected signal using a coaxial electric-dipole receiver designed and positioned so that crosstalk between the transmitting and receiving antennas is minimized, and analyzing the return signal for signs of a tube anomaly.
Accordingly, an advantage of the present invention is that it provides a method for detecting anomalies in a tube or other enclosure which method utilizes the concept of radar backscatter.
Another advantage of the present invention is that it provides an apparatus for detecting anomalies in a tube or other enclosure which apparatus comprises a transverse magnetic dipole source and a coaxial electric-dipole receiver.
Yet another advantage of the present invention is that it provides an apparatus for detecting structural anomalies in a tube, a portion of the tube defining a longitudinal axis coincident with a z-axis of a Cartesian coordinate system comprising an x-axis, a y-axis and a z-axis, the apparatus comprising a transmitter generating a continuous-wave electric field within the tube, the electric field being parallel to the z-axis, the null of the electric field being located in the z-axis; and a receiver being located in the null receiving a backscattered signal from the tube, wherein anomalies in the tube cause a detectable change in said backscattered signal.
Another advantage of the present invention is that it provides a method for detecting structural anomalies in a tube, a portion of which defines a longitudinal axis coincident with a z-axis of a Cartesian coordinate system comprising an x-axis, a y-axis and a z-axis, the steps comprising transmitting a continuous-wave electric field parallel to the z-axis; generating a backscattered signal from the electric field off the tube; and receiving the backscattered signal with an internal receiver.
These and other advantages and objects of the present invention are fulfilled by the claimed invention in which the receiving antenna is a short electric dipole (ED) aligned with the axis of a tube which is also the z-axis of a Cartesian coordinate system. In the preferred embodiment, the transmitting antennas are a pair of orthogonal magnetic dipoles (MDs) which are centered at the coordinate origin and with moments along the x-axis and y-axis, respectively. (Throughout this disclosure, the term “short” in short electric dipole means much less than a wavelength in the surrounding medium.) The E
z-
field from a MD whose moment is along the x-axis is applicable to both dielectric or conductive host media. The ED is located in the null of the E
z-
field of the MD to minimize crosstalk, and the ED may also be offset along the z-axis as a matter of practical construction. The currents in the orthogonal MDs can be weighted so that a bi-lobed, combined E
z
-beam can be steered in the azimuth to illuminate the interior of the tube wall in uniform angular increments. A target having a diameter of only 254 microns is detectable. This is not necessarily a lower limit to the minimum size, but merely an example.
REFERENCES:
Zetec, Inc., 1370 NW Mall St., Issaquah, WA 98027-0140; “Waste Storage Tank Probe/Scanner—Remote Field Testing(RFT)Probes” (undated) No month/year available.
Tuboscope Pipeline Services, “Introduction to Pipeline Services—Survey Tool Operation; Trures” (undated), pp. 2, 16-18 No month/year available.
D. R. Diercks, W. J. Shack, and J. Muscara, “Overview of Steam Generator Tube Degradation and Integrity Issues,” Argonne National Laboratory (undated) No month/year available.
D. S. Kupperman and S. Bakhtiari, “Characterization of Flaws in a Tube Bundle Mock-Up for Reliability Studies,” Argonne National Laboratory (undated) No month/year available.
S. Bakhtiari and D. S. Kupperman,Modeling of Eddy current Probe Response for Steam Generator Tubes, Argonne National Laboratory (undated) No month/year available.
Thurlow W. H. Caffey, “A Tool to Detect External Cracks from within a Metal Tube,” SAND 97-0170 (Printed Jan. 1997, Published Feb. 20, 1997).
Thurlow W. H. Caffey, “Elements of a Continuous-Wave Borehole Radar,” SAND 97-1995 (Printed Aug. 1997).
Elliott Russell D.
Metjahic Safet
Nguyen Vincent Q.
Sandia Corporation
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