Detection of corrosion fatigue cracks in membrane boiler tubes

• US 5,526,691 A
• Filed: 08/03/1995
• Issued: 06/18/1996
• Est. Priority Date: 07/12/1993
• Status: Expired due to Term

• Current Assignee: McDermott Technology, Inc.
• Original Assignee: Babcock & Wilcox Company (Babcock International Group PLC)
• Inventors: Maclauchlan, Daniel T., Latimer, Paul J.
• Primary Examiner(s): Chilcot, Richard
• Assistant Examiner(s): NOORI, MAX H
• Application Number:
  US08/511,049
• Time in Patent Office: 320 Days
• Field of Search: 73/592, 73/598, 73/599
• US Class Current: 73/592
• CPC Class Codes: G01N 2291/0258 Structural degradation, e.g. fatigue of composites, ageing of oils
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/044 Internal reflections (echoes), e.g. on walls or defects
• CPC Class Codes: G01N 2291/102 one emitter, one receiver
• CPC Class Codes: G01N 2291/2634 cylindrical from outside
• CPC Class Codes: G01N 2291/2695 Bottles, containers
• CPC Class Codes: G01N 29/2412 using the magnetostrictive properties of the material to be examined, e.g. electromagnetic acoustic transducers [EMAT]; investigating the presence of flaws using eddy currents G01N27/90, magnetostrictive transducers B06B1/08, measuring magnetostrictive properties G01R33/18

Abstract

A method for detecting corrosion fatigue cracks in membraned boiler tubes in a membrane tube panel employs at least one EMAT coil generating ultrasonic SH shear waves at a predetermined beam angle. The method of the present invention provides better signal-to-noise ratios than conventional ultrasonic techniques as well as not requiring any couplant.

Claims

1. A method for detecting corrosion fatigue cracks in boiler tubes, comprising the steps of:

positioning two electromagnetic acoustic transducers externally on a boiler tube of a boiler tube membrane tube panel, one electromagnetic acoustic transducer being positioned at approximately 120° and

the other at approximately 170°

adjacent the boiler tube;

generating ultrasonic SH shear waves with at least one of said electromagnetic acoustic transducers;

propagating the ultrasonic SH shear waves into the boiler tube at a predetermined beam angle with an adjustable frequency with the ultrasonic SH shear waves undergoing no mode conversion upon successive bounces as the ultrasonic SH shear waves propagate around the boiler tube;

measuring the returned ultrasonic SH shear waves from the boiler tube with at least one of said electromagnetic acoustic transducers; and

determining from the returned ultrasonic SH shear waves any corrosion fatigue cracks with data acquisition means.

2. A method as recited in claim 1, further comprising the step of scanning the boiler tube for corrosion fatigue cracks.

3. A method as recited in claim 1, wherein the ultrasonic SH shear waves are generated in a frequency ranging from 1 MHz to 4 MHz depending upon the tube diameter and wall thickness.

4. A method as recited in claim 3, wherein the frequency of the ultrasonic SH shear waves is adjustable with a range of 1 MHz to 4 MHz as required.

5. A method as recited in claim 4, wherein the predetermined beam angle is determined by the tube size and obtained by adjusting the frequency to obtain the desired beam angle by a relationship of sin ⊖

=c/2Df

where;

f=frequency

c=shear wave velocity

D=separation between adjacent conductors in an EMAT coil.