WO1992021965A1

WO1992021965A1 - A method and an apparatus for the dectection of corrosion in pipes

Info

Publication number: WO1992021965A1
Application number: PCT/DK1992/000174
Authority: WO
Inventors: Allan Northeved; Michael Svan
Original assignee: Force Institutterne
Priority date: 1991-05-31
Filing date: 1992-05-29
Publication date: 1992-12-10
Also published as: DK169900B1; DK104791D0; AU1901292A; DK104791A

Abstract

A method of inspecting tubular members, especially of measuring the wall thickness of the tubular member, said method comprising the steps of inserting a measuring probe with an ultrasonic transducer (12) for transmission of an ultrasonic pulse to the wall of the tubular member and for detection of the ultrasonic pulses reflected by the inner and outer side of the tubular wall. The measuring probe comprises an inclined mirror guiding the ultra-sound transmitted by the transducer in form of a beam in order to scan the tubular member along a substantially helical path during the insertion of the probe, an ultra-sound-transmitting fluid being fed to the annular space between the measuring probe and the tubular member during the scanning. According to the invention, the rotating mirror (22) is placed in a preferably closed chamber (20) filled with fluid, said chamber (20) being rotated together with the mirror (22) and comprising an ultrasonic window for the transmitted ultrasonic beam. Accordingly, the turbulence is reduced in the fed water. As a result, the risk of air bubbles interfering with the measurement has been considerably reduced, and accordingly it is possible to scan at a higher speed than previously.

Description

Title: A Method and an Apparatus for the Detection of Corrosion in Pipes

Technical Field

The invention relates to a method of inspecting tubular members, especially of measuring the wall thickness of a tubular member, said method comprising the steps of in¬ serting a measuring probe with an ultrasonic transducer for transmission of an ultrasonic pulse to the wall of the tubular member and for detection of the ultrasonic pulses reflected by the inner and outer side of the tu¬ bular wall, said measuring probe comprising a rotatable, inclined mirror guiding the ultra-sound transmitted by the transducer in form of a beam in order to scan the tubular member along a substantially helical path during the insertion of the probe, where the inclined mirror is arranged in a chamber and caused to rotate by means of an electric motor, an ultra—sound—transmitting fluid be¬ ing fed to the space between the measuring probe and the tubular member during the scanning.

Background Art

US-PS No. 4,361,044 discloses a way of rotating an in¬ clined mirror placed together with an ultrasonic trans¬ ducer in a chamber of a measuring probe, said measuring probe being insertable in a pipe to be examined. The mirror is caused to rotate by means of a motor shaft in the chamber apparently filled with nothing but air. This measuring probe is encumbered with the drawback that the majority of the ultrasonic energy from the transducer is reflected due to the lacking acoustic coupling.

Shell International Research has produced a measuring probe, cf. EP Patent Application No. 12,474, where the fluid is fed to the space between the measuring probe and the tubular member during the scanning. The fluid is also used for rotating an inclined mirror reflecting ultrasonic energy from an ultrasonic transducer, said mirror being supported by a water—driven turbine rotat¬ ing about a shaft parallel to the longitudinal axis of the probe. In this manner an improved acoustic coupling is achieved between the ultrasonic transducer and the fluid surrounding the probe. Such a water—driven turbine is, however, encumbered with the drawback that the ac¬ curacy involved is not very high because the rotary speed is not particularly accurate. In addition, the rotation involves some turbulence with the result that air bubbles, if any, can interfere with the measuring. Both drawbacks limit the scanning speed.

Description of the Invention

The object of the invention is to provide a method of the above type for scanning tubular members, and which allows a higher scanning speed than previously known.

The method according to the invention is characterised by the inspection of the tubular member being carried out by placing the rotating mirror in a preferably closed chamber filled with fluid, said chamber being rotated together with the mirror and comprising an ul¬ trasonic window for the transmitted ultrasonic beam. In this manner turbulence is avoided in the fed water. As a result the risk of possible air bubbles interfering with the ultrasonic transmission and consequently the mea¬ suring has been considerably reduced and it is possible to scan at a higher speed than previously.

The outer side of the rotatable chamber filled with fluid may furthermore be shaped substantially as a rota- tionally symmetrical body with a generator curve in form of a monotonous function such that air bubbles, if any, automatically move towards the narrowed end during the rotation of the chamber so as to be drained off through outlet openings. In this manner the risk of possible air bubbles interfering with the measuring has been further reduced.

According to the invention the mirror may be rotated by means of at least one DC motor arranged in a fluid-tight cavity inside the measuring probe. The rotary speed of such a motor can be controlled very accurately with the result that an improved resolution is obtained and a higher scanning speed is rendered possible.

The driving shaft of the DC motor may according to the invention be sealed relative to the cavity housing the motor by means of a bearing with an iron—containing lu¬ bricant in connection with one or more strong annular magnets, whereby a fluid-tight bearing is obtained.

The measuring probe with the chamber filled with fluid may according to the invention be centered by means of one or more external resilient portions, said portions being circumferential and partially blocking for water. Such a resilient portion may for instance be a^' resilient metal tape with a lengthwise bending and a suitable num¬ ber of transverse slits.

The invention relates furthermore to an apparatus for carrying out the method of inspecting tubular members, especially of measuring the wall thickness of a tubular member, said apparatus comprising a measuring probe for transmission of an ultrasonic pulse to the wall of the tubular member and for detection of the ultrasonic pulses reflected by the inner and outer side of the tu¬ bular wall, said measuring probe comprising a rotatable, inclined mirror guiding the ultra-sound transmitted by the transducer in form of a beam in order to scan the tubular member along a substantially helical path during the insertion of the probe, where the inclined mirror is arranged in a chamber and caused to rotate by means of an electric motor, an ultra—sound—transmitting fluid being fed to the space between the measuring probe and the tubular member during the scanning. According to the invention, the rotatable, inclined mirror is arranged in a chamber filled with fluid and is caused to rotate to¬ gether with the mirror, said chamber comprising an ul¬ trasonic window for the transmitted ultrasonic beam. The resulting apparatus is particularly suited for inspec¬ tion of tubular members.

According to the invention the rotation of the chamber Is preferably caused by means of at least one DC motor arranged in a closed cavity of the measuring probe. A DC motor is capable of rotating at a very accurate rotary speed. As a result an improved resolution is obtained during the scanning procedure.

The driving shaft of the motor may advantageously be sealed relative to the closed cavity by means of a bearing with an iron—containing lubricant in connection with one or more strong annular magnets . The magnets retain the lubricant and provide a fluid—tight sealing.

Brief Description of the Drawings

The invention is described in greater detail below with reference to the accompanying drawings , in which

Fig. 1A and IB illustrate a measuring probe to be in— serted in tubular members to be inspected,

Fig. 2 Illustrates a centering means for the measuring probe, and

Fig. 3 illustrates an alternative embodiment of the mea¬ suring probe.

Best Mode for Carrying Out the Invention The apparatus shown in Figs. 1A and IB for inspection of tubular members, especially for measuring of the wall thickness of said tubular members, comprises a measuring probe to be inserted in said tubular members. The mea- suring probe comprises a tubular housing 2 including one or more electric motors 4 in response to the necessary torque. When several motors 4 are used, the driving shafts of said motors are interconnected through non- rigid couplings. The projecting driving shaft of the front motor 4 extends by means of a shaft coupling 6 through a fixed primary winding of a transformer and is fixedly connected to the secondary winding 8 capable of rotating relative to said primary winding 7. The secon¬ dary winding 8 of the transformer is secured to the end of a hollow shaft 10 housing conduits to an ultrasonic^' transducer 12 which is mounted at the opposite end of the shaft 10. Adjacent the secondary winding 8, the hol¬ low shaft 10 is mounted in a bearing, preferably a ball bearing 13 fixedly arranged inside the tubular housing 2. The ball bearing 13 is followed by a magnetic sealing 16 in form of a bearing with iron-containing lubricant in connection with strong annular magnets. The magnets retain the iron-containing lubricant although the bear¬ ing is subjected to a fluid pressure. As a result, a fluid-tight sealing is provided. The magnetic sealing 15 is followed by another fixedly arranged ball bearing 17 housing the hollow shaft 10. The hollow shaft 10 pro¬ jecting from the bearing 17 is secured to a cylindrical and axially mounted ultrasonic transducer 12. The ultra- sonic transducer 12 communicates with the conduits in the shaft 10 and is activated by means of the signal fed through the transformer 7, 8 with the rotatable secon¬ dary winding 8. A signal of a frequency of about 5 to 15 MHz is fed to the primary winding 7 from a preamplifier 19 mounted on a circuit card behind the electric motor 4 in a fluid-tight cavity. A control circuit for the elec¬ tric motor 4 is also mounted on said circuit card. The control circuit ensures that the motor 4 rotates at the desired speed of rotation which must be constant. The motor 4 is a DC motor because such a motor can be pro¬ duced with very small dimensions.

A chamber 20 filled with fluid is mounted in front of the cylindrical and axially arranged ultrasonic trans¬ ducer 12, said chamber 20 housing an inclined ultra¬ sound-reflecting plate 22. The plate 22 forms preferably an angle of about 45° with the axis of the tubular hous¬ ing 2. The ultrasonic beam reflected by the plate 22 through an ultrasonic window in the chamber 20 follows thereby a helical path if the transducer 12 and the mir¬ ror 22 rotate at a constant speed and the measuring probe is moved at a constant speed in for instance a pipe .

Centering means 23 on the outer side of the tubular housing 2 of the measuring probe are adapted to center said measuring probe inside the pipe to be inspected. The centering means 23 are formed by resilient portions which are circum erential and water—permeable , cf . Fig. 2. Each resilient portion is formed by a tape of resi¬ lient metal which is bent lengthweise and slit trans¬ versely at suitable intervals. The tape is placed about the tubular housing 2. One edge of the tape is secured to the tubular housing 2 whereas the other edge is dis— placeable in response to the diameter of the pipe to be inspected. The slitting ensures both the desired cen¬ tering and a partial blocking for water. The centering means 23 need not necessarily be made of a resilient metal sheet, but may also be made of another resilient sheet material.

In order to obtain the best possible ultrasonic coupl¬ ing, water has been fed to the space between the ultra¬ sonic probe and the pipe to be inspected. The water is fed through a hose 25 connected with the probe and is discharged through two outlets 26 from the probe to the space between said probe and the pipe to be inspected.

A stop disc 27 is mounted at the end of the chamber 20 filled with fluid and housing the inclined mirror 22. The stop disc 27 stops the flow of water to a predeter- mined degree and ensures that the space between the pipe and the chamber 20 is filled with water.

Fig. 3 illustrates a particularly preferred embodiment, where the chamber filled with fluid is shaped as a sub¬ stantially rotationally symmetrical body with a mono- tonous generator curve and preferably being conical in such a manner that air bubbles, if any, are automatical¬ ly collected due to the centrifugal forces at the nar¬ rowed end of the conical surface provided with the stop disc 27. If the stop disc 27 is provided with openings 28 for the discharging of air bubbles, said bubbles can¬ not influence the ultrasonic transmission during the measuring procedure.

The method used for measuring the wall thickness is based on the pulse-echo-technique. The measuring probe is moved axially inside the pipe. The transducer 12 transmits an ultrasonic pulse towards the shaft of the tubular housing 2.. The pulse is deflected about 90° by the rotating mirror 22 and reaches the pipe to be in¬ spected. The rotation of the mirror 22 has the effect that the inner side of the pipe is scanned along a heli¬ cal path. Some of the ultrasonic energy is reflected by the inner side of the pipe wall. The remaining ultrason¬ ic energy penetrates the wall and reaches the other sur¬ face, i.e. the outer side of the pipe wall, where the majority of the ultrasonic energy is reflected. As a result, a series of echoes arises, said echoes indicat¬ ing the wall thickness which is optionally displayed on a screen of a cathode-ray oscillograph. Changes in the wall thickness indicate that a degree of corrosion has occurred. At a rotary speed of about 14,000 rpm and a resolution of 64 points per revolution and a repetition frequency of 10 kHz it is possible to scan at a speed of about 160 mm/sec, which is considerably faster than previously known.

The method according to the invention may for instance be used for inspecting pipes in heat exchangers compris¬ ing hundreds of pipes. A corrosion in the pipes may in¬ volve serious accidents and high losses of profits in case it is necessary to repair a heat exchanger. Accord¬ ingly, a strong demand exists for a method of quickly inspecting the condition of the pipes.

A hydraulic or pneumatic motor can be used instead of an electric motor, where a separate in- and outlet for fluid or air Is provided. In order to ensure a constant rotary speed, a code disc is also provided which trans¬ fers the angular turning to a computer through electric conduits .

Claims

Claims .

1. A method of inspecting tubular members, especially of measuring the wall thickness of a tubular member, said method comprising the steps of inserting a mea¬ suring probe with an ultrasonic transducer for transmis¬ sion of an ultrasonic pulse to the wall of the tubular member and for detection of the ultrasonic pulses re— fleeted by the inner and outer side of the tubular wall, said measuring probe comprising a rotatable, inclined mirror (22) guiding the ultra-sound transmitted by the transducer in form of a beam in order to scan the tubu¬ lar member along a substantially helical path during the insertion of the probe, where the inclined mirror (22) is arranged in a chamber and caused to rotate by means of an electric motor (4) , an ultra-sound-transmitting fluid being fed to the space between the measuring probe and the tubular member during the scanning, c h a r - a c t e r i s e d by the inspection of the tubular mem¬ ber being carried out by placing the rotating mirror (22) in a preferably closed chamber (20) filled with fluid, said chamber (20) being rotated together with the mirror (22) and comprising an ultrasonic window for the transmitted ultrasonic beam.

2. A method as claimed in claim 1, c h a r a c t e r ¬ i s e d by the used ultrasonic transducer (12) being arranged in the chamber (20) filled with fluid.

3. A method as claimed in claim 1 or 2 , c h a r a c — t e r i s e d by the ultrasonic transducer (12) commu¬ nicating with the remaining portion of the measuring probe through a transformer with a secondary winding (8) capable of rotating relative to the primary winding (7) .

4. A method as claimed in the claims 1 to 3 , c h a r- a c t e r i s e d by the mirror (22) being rotated by means of at least one DC motor (4) arranged in a fluid- tight cavity inside the measuring probe.

5. A method as claimed In claim 4, c h a r a c t e r¬ i s e d by the driving shaft (10) of the motor (4) be¬ ing sealed relative to the cavity housing the motor (4) by means of a bearing (15) with an iron-containing lu¬ bricant in connection with one or more strong annular magnets .

6. A method as claimed in the claims 1 to 5 , c h a r ¬ a c t e r i s e d by the measuring probe with the cham— ber (20) filled with fluid being centered by means of one or more external resilient portions (23), said por¬ tions being circumferential and partially blocking for water.

7. A method as claimed in claim 1, c h a r a c t e r— i s e d by the outer side of the rotatable chamber (20) filled with fluid being shaped substantially as a rota— tionally symmetrical body with a generator curve in form of a monotonous function such that air bubbles, if any, automatically move towards the narrowed end during the rotation of the chamber so as to be drained off through outlet openings (28).

8. An apparatus for carrying out the method of inspect¬ ing tubular members, especially of measuring the wall thickness of a tubular member, as claimed in one or more of the preceding claims 1 to 7 , said apparatus compris¬ ing a measuring probe for transmission of an ultrasonic pulse to the wall of the tubular member and for detec¬ tion of the ultrasonic pulses reflected by the inner and outer side of the tubular wall, said measuring probe comprising a rotatable, inclined mirror (22) guiding the ultra—sound transmitted by the transducer in form of a beam In order to scan the tubular member along a sub¬ stantially helical path during the insertion of the probe, where the inclined mirror (22) is arranged in a 11 chamber and caused to rotate by means of an electric motor (4), an ultra-sound—transmitting fluid being fed to the space between the measuring probe and the tubular member during the scanning, c h a r a c t e r i s e d in that the rotating inclined mirror (22) is arranged in a chamber (20) filled with fluid and is caused rotate together with the mirror (22) , said chamber (20) com¬ prising an ultrasonic window for the transmitted ultra¬ sonic beam.

9. An apparatus as claimed in claim 8, c h a r a c ¬ t e r i s e d by the rotation of the chamber (20) being caused by means of at least one DC motor (4) arranged in a closed cavity of the measuring probe.

10. An apparatus as claimed in claim 9, c h a r a c - t e r i s e d by the driving shaft (10) of the DC motor

(4) being sealed relative to the closed cavity by means of a bearing (15) with an iron-containing lubricant in connection with one or more strong annular magnets.

11. An apparatus as claimed in claims 8 to 10, c h a r— a c t e r i s e d by some outer resilient portions (23) centering the measuring probe in the tubular member, said portions being circumferential and partially blocking for water.

AMENDED CLAIMS

[received by the International Bureau on 19 October 1992(19.10.92); original claims 1-11 replaced by amended claims 1-13

(2 pages)]

1. A method of inspecting tubular members, especially of measuring the wall thickness of a tubular member, said method comprising the steps of inserting a mea¬ suring probe with an ultrasonic transducer for transmis¬ sion of an ultrasonic pulse to the wall of the tubular member and for detection of the ultrasonic pulses re- fleeted by the inner and outer side of the tubular wall, said measuring probe comprising a rotatable, inclined mirror (22) guiding the ultra-sound transmitted by the transducer in form of a beam in order to scan the tubu¬ lar member along a substantially helical path during the insertion of the probe, where the inclined mirror (22) is arranged in a chamber and caused to rotate by means of an electric motor (4) , an ultra-sound—transmitting fluid being fed to the space between the measuring probe and the tubular member during the scanning, c h a r - a c t e r i s e d by the inspection of the tubular mem¬ ber being carried out by placing the rotating mirror (22) in a preferably closed chamber (20) filled with fluid, said chamber (20) being rotated together with the mirror (22) and comprising an ultrasonic window for the transmitted ultrasonic beam, said ultrasonic transducer (12) being placed in the chamber (20) filled with fluid, and said mirror (22) being rotated by means of at least one DC motor (4) arranged in a fluid tight cavity inside the measuring probe.

2. A method as claimed in claim 1, c h a r a c t e r- I s e d by the outer side of the rotatable chamber (20) filled with fluid being shaped substantially as a rota- tionally symmetrical body with a generator curve in form of a monotonous function such that air bubbles, if any, automatically move towards the narrowed end during the rotation of the chamber so as to be drained off through outlet openings (28). 3. An apparatus for carrying out the method of inspect¬ ing tubular members, especially of measuring the wall thickness of a tubular member, as claimed in one or more of the preceding claims 1 to 7 , said apparatus compris- ing a measuring probe for transmission of an ultrasonic pulse to the wall of the tubular member and for detec¬ tion of the ultrasonic pulses reflected by the inner and outer side of the tubular wall, said measuring probe comprising a rotatable, inclined mirror (22) guiding the ultra-sound transmitted by the transducer in form of a beam in order to scan the tubular member along a sub¬ stantially helical path during the insertion of the probe, where the inclined mirror (22) is arranged in a chamber and caused to rotate by means of an electric motor (4) , an ultra-sound-transmitting fluid being fed to the space between the measuring probe and the tubular member during the scanning, c h a r a c t e r i s e d in that the rotating inclined mirror (22) is arranged in a chamber (20) filled with fluid and is caused rotate together with the mirror (22), said chamber (20) com¬ prising an ultrasonic window for the transmitted ultra¬ sonic beam, the rotation of the chamber (20) being caused by means of at least one DC motor (4) arranged in a closed cavity of the measuring probe.