GB2098732A - Detection of defects in tubes by ultrasonic techniques - Google Patents

Abstract

Longitudinally extending defects D such as "galling" in tubes, for example, heat exchange tubes, are detected by directing ultrasonic beam energy from an internally disposed transducer 2 into the tube wall along a chordal path to induce circumferentially extending shear waves. Return echoes from defects follow the same path. The ultrasonic probe 1 is rotatable about an axis 4 and may have separate transmitting and receiving transducers arranged one above the other. <IMAGE>

Description

SPECIFICATION Detection of defects in tubes by ultrasonic techniques This invention relates to methods and apparatus for the detection of defects in tubes by ultrasonic techniques.

One known method of inspecting a tube using an ultrasonic technique comprises generating ultrasonic beam energy in the tube, directing it radially into the wall by means of an inclined mirror and detecting the echoes from the inner and outer wall surfaces; the time internal between the echoes is indicative of the wall thickness. This method is suitable for detecting the extent of corrosion of a tube, for example a heat exchange tube, but it is an object of the present invention to provide a method and apparatus for detecting defects, such as cracks, grooves and pitting corrosion extending longitudinally of the tube.

According to the invention in a method of detecting longitudinally extending defects in a metal tube ultrasonic beam energy is directed along a chordal path into the inner wall surface of the tube to induce circumferentially extending shear waves in the wall thereof. The beam energy is refracted at the inner and outer wall surfaces causing it to propagate round the tube wall by internal reflection and on encountering a longitudinally extending defect a return echo is transmitted along an identical path.

The time interval between the initial pulse and the echo may be used to determine whether a defect occurs at an inside or outside surface of the tube.

The invention finds use in the on-site detection of longitudinally extending defects in the heat exchange tubes of a sodium/water heat exchanger used in the nuclear reactor art.

Invention will also reside in apparatus for detecting longitudinally extending defects in a metal tube, the apparatus comprising a rotatable ultrasonic transmitter/receiver probe for insertion along the longitudinal axis of a tube and having an ultrasonic crystal disposed to direct ultrasonic beam energy along a chordal path in a plain normal to the rotational axis.

A method and apparatus for detecting longitudinally extending defects in a tube and embodying the invention are described by way of example with reference to the sole accompanying drawing which is a cross-sectional view of an ultrasonic probe.

In the drawing there is shown a water-filled metal tube 'T' housing a coaxially disposed rotatable ultrasonic transmitter/receiver probe 1. The probe has an ultrasonic crystal 2 displaced from the rotational axis and disposed to direct an ultrasonic beam along a chordal path 3 in a plane which is normal to the rotational axis 4. In use, the ultrasonic beam energy is refracted into the wall thickness of the tube to propagate round the tube wall by internal reflection. On encountering a longitudinally extending defect 'D' in the tube wall a return echo is transmitted along an identical path to the transmitter/receiver so that by rotation and longitudinal displacement of the transducer probe the length of the tube can be scanned for longitudinally extending defects such as cracks, pitting corrosion and wear grooves.

In an alternative apparatus (not shown) there are two ultrasonic crystals disposed one above the other relative to the longitudinal axis of the tube, one crystal being a transmitter whilst the other is a receiver of ultrasonic energy; the arrangement avoids difficulty which may be caused with a single crystal transducer whereby the crystal is still ringing from an initial pulse when the echo is received.

The preferred angle subtended by the chordal path and the radius of the tube designated a and is between 1 and 28" because with an angle of less than 15 a compression wave is also produced in the steel and with an angle greater than 28" total internal reflection occurs and no ultrasound is refracted into the steel.

The method and apparatus illustrated by the drawing find application in the detection of longitudinal defects in heat exchanger tubes. In one example of tube-in-shell heat exchanger wherein heat is exchanged between water flowing through the tubes and sodium within the shell, it is found that defects occur in the external surfaces of the tubes where they engage grid supports. Frictional contact with the grids causes longitudinal scratches or gouges in the tubes, a phenomenum known in the art as "galling". The described apparatus is suitable for on-site insertion in the heat exchange tubes for detecting the extent of galling which has taken place.

1. A method of detecting longitudinally extending defects in a metal tube comprising directing ultrasonic beam energy along a chordal path into the inner wall surface of the tube to induce circumferen- tially extending shear waves in the wall thereof and detecting echo signals deflected from defects therein.

2. Apparatus for detecting longitudinally extending defects in a metal tube, the apparatus comprising a rotatable ultrasonic transmitter receiver probe for insertion along the longitudinal axis of the tube and having an ultrasonic crystal disposed to direct ultrasonic beam energy along a chordal path in a plane normal to the rotational axis.

3. Apparatus according to claim 2 wherein the ultrasonic transmitter/receiver probe has two ultrasonic crystals disposed one above the other relative to the longitudinal axis of the probe, one crystal being a transmitter whilst the other is a receiver of ultrasonic energy.

4. A method of detecting longitudinally extending defects in a metal tube substantially as hereinbfore described with reference to the accompanying drawings.

5. Apparatus for detecting longitudinally extending defects in a metal tube substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Detection of defects in tubes by ultrasonic techniques This invention relates to methods and apparatus for the detection of defects in tubes by ultrasonic techniques. One known method of inspecting a tube using an ultrasonic technique comprises generating ultrasonic beam energy in the tube, directing it radially into the wall by means of an inclined mirror and detecting the echoes from the inner and outer wall surfaces; the time internal between the echoes is indicative of the wall thickness. This method is suitable for detecting the extent of corrosion of a tube, for example a heat exchange tube, but it is an object of the present invention to provide a method and apparatus for detecting defects, such as cracks, grooves and pitting corrosion extending longitudinally of the tube. According to the invention in a method of detecting longitudinally extending defects in a metal tube ultrasonic beam energy is directed along a chordal path into the inner wall surface of the tube to induce circumferentially extending shear waves in the wall thereof. The beam energy is refracted at the inner and outer wall surfaces causing it to propagate round the tube wall by internal reflection and on encountering a longitudinally extending defect a return echo is transmitted along an identical path. The time interval between the initial pulse and the echo may be used to determine whether a defect occurs at an inside or outside surface of the tube. The invention finds use in the on-site detection of longitudinally extending defects in the heat exchange tubes of a sodium/water heat exchanger used in the nuclear reactor art. Invention will also reside in apparatus for detecting longitudinally extending defects in a metal tube, the apparatus comprising a rotatable ultrasonic transmitter/receiver probe for insertion along the longitudinal axis of a tube and having an ultrasonic crystal disposed to direct ultrasonic beam energy along a chordal path in a plain normal to the rotational axis. A method and apparatus for detecting longitudinally extending defects in a tube and embodying the invention are described by way of example with reference to the sole accompanying drawing which is a cross-sectional view of an ultrasonic probe. In the drawing there is shown a water-filled metal tube 'T' housing a coaxially disposed rotatable ultrasonic transmitter/receiver probe 1. The probe has an ultrasonic crystal 2 displaced from the rotational axis and disposed to direct an ultrasonic beam along a chordal path 3 in a plane which is normal to the rotational axis 4. In use, the ultrasonic beam energy is refracted into the wall thickness of the tube to propagate round the tube wall by internal reflection. On encountering a longitudinally extending defect 'D' in the tube wall a return echo is transmitted along an identical path to the transmitter/receiver so that by rotation and longitudinal displacement of the transducer probe the length of the tube can be scanned for longitudinally extending defects such as cracks, pitting corrosion and wear grooves. In an alternative apparatus (not shown) there are two ultrasonic crystals disposed one above the other relative to the longitudinal axis of the tube, one crystal being a transmitter whilst the other is a receiver of ultrasonic energy; the arrangement avoids difficulty which may be caused with a single crystal transducer whereby the crystal is still ringing from an initial pulse when the echo is received. The preferred angle subtended by the chordal path and the radius of the tube designated a and is between 1 and 28" because with an angle of less than 15 a compression wave is also produced in the steel and with an angle greater than 28" total internal reflection occurs and no ultrasound is refracted into the steel. The method and apparatus illustrated by the drawing find application in the detection of longitudinal defects in heat exchanger tubes. In one example of tube-in-shell heat exchanger wherein heat is exchanged between water flowing through the tubes and sodium within the shell, it is found that defects occur in the external surfaces of the tubes where they engage grid supports. Frictional contact with the grids causes longitudinal scratches or gouges in the tubes, a phenomenum known in the art as "galling". The described apparatus is suitable for on-site insertion in the heat exchange tubes for detecting the extent of galling which has taken place. CLAIMS

1. A method of detecting longitudinally extending defects in a metal tube comprising directing ultrasonic beam energy along a chordal path into the inner wall surface of the tube to induce circumferen- tially extending shear waves in the wall thereof and detecting echo signals deflected from defects therein.

2. Apparatus for detecting longitudinally extending defects in a metal tube, the apparatus comprising a rotatable ultrasonic transmitter receiver probe for insertion along the longitudinal axis of the tube and having an ultrasonic crystal disposed to direct ultrasonic beam energy along a chordal path in a plane normal to the rotational axis.

3. Apparatus according to claim 2 wherein the ultrasonic transmitter/receiver probe has two ultrasonic crystals disposed one above the other relative to the longitudinal axis of the probe, one crystal being a transmitter whilst the other is a receiver of ultrasonic energy.

4. A method of detecting longitudinally extending defects in a metal tube substantially as hereinbfore described with reference to the accompanying drawings.

5. Apparatus for detecting longitudinally extending defects in a metal tube substantially as hereinbefore described with reference to the accompanying drawings.