Method and apparatus for detecting external cracks from within a metal tube
AssigneeSandia Corporation
InventorThurlow W. H. Caffey
Life of this patent
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Abstract
A method and tool using a continuous electromagnetic wave from a transverse magnetic-dipole source with a coaxial electric-dipole receiver is described for the detection of external sidewall cracks and other anomalies in boiler tubes and other enclosures. The invention utilizes the concept of radar backscatter rather than eddy-currents or ultrasound, which are sometimes used in prior art crack-detection methods. A numerical study of the distribution of the fields shows that the direct transmission from the source to the receiver is reduced from that in free space. Further, if the diameter of the receiver dipole is made sufficiently small, it should be possible to detect cracks with a scattering loss of up to −40 dB in thin-walled boiler tubes.
Claims26
An apparatus for detecting structural anomalies in a tube, a portion of said 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 said tube, said electric field being parallel to said z-axis, and having a null, said null located in said z-axis;
- and
- a receiver located in said null receiving a backscattered signal from said tube, wherein anomalies in said tube cause a detectable change in said backscattered signal.
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 said z-axis;
- generating a backscattered signal from said electric field off said tube;
- and
- receiving said backscattered signal with a receiver being located at a null.
A method for detecting structural anomalies in a right circular cylindrical 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 said z-axis said continuous-wave electric field being generated by a transmitter, said transmitter comprising a pair of orthogonal magnetic dipoles centered at a coordinate origin of said Cartesian coordinate system and having a first moment oriented along said x-axis and a second moment oriented along said y-axis, wherein an operating frequency of said electric field is below a cutoff frequency of a cylindrical waveguide formed by said tube and wavelength of said operating frequency is smaller than a dimension of an anomaly in said tube, said dimension being parallel to said z-axis;
- generating a backscattered signal from said electric field off said tube;
- and
- receiving said backscattered signal with an receiver, said receiver comprising a short electric dipole aligned with and along said z-axis, wherein a cross-talk constituting a self-clutter between said transmitter and said receiver sets a lower bound to a signal-to-noise ratio at 6 dB.
An apparatus for detecting structural anomalies in a right circular cylindrical tube, a portion of said 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 pair of orthogonal magnetic dipoles centered at a coordinate origin of said Cartesian coordinate system and having a first moment oriented along said x-axis and a second moment oriented along said y-axis, said pair of orthogonal magnetic dipoles generating a continuous-wave electric field within said tube, said electric field being parallel to said z-axis, and having a null, said null located in said z-axis of said electric field;
- and
- a short electric dipole receiver aligned with and along said z-axis, said short electric dipole receiver located in said null and receiving a backscattered signal from said tube, wherein anomalies in said tube cause a detectable change in said backscattered signal.
Dependencies are inferred from the claim text.