GB2046911A - Method and apparatus for measuring the wall thickness of tubes - Google Patents

Abstract

A technique for measuring the wall thickness of a tube 2 using ultrasonic signals emitted in succession from transducers 6 in a head 4, in which the transducers are acoustically coupled with an inner surface of the tube by liquid occupying a chamber 12 between seals 8 and 10 mounted on the head and in fluid-tight sliding contact with the said inner surface so that the head, transducers, seals and liquid in chamber 12 can be moved along the tube to different positions where measurement is desired. Liquid leaking from chamber 12 can be trapped by seals 18 mounted on the head through a universal joint allowing the arrangement to pass around bends, and the chamber may be replenished through a pipe 14. <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring the wall thickness of tubes This invention concerns a method and appparatus for measuring the wall thickness of tubes.

It is known to measure the wall thickness of a tube using pulses of ultrasonic signals transmitted from an ultrasonic signal transducer or probe. In principle a pulse of an ultrasonic signal travelling at known speed is emitted from the probe, located inside the tube, towards the inner surface of the tube. Some of the signal is reflected back to the probe from the inner surface and some is reflected back from the outer surface of the tube wall and reaches the probe at a later time after passing through substantially twice the thickness of the tube wall. By measuring the time difference between the two received signals, dividing it by two and multiplying the quotient by the speed, the wall thickness is derived.

Wall thickness is sometimes measured at different positions along a tube to discover if the tube has become flawed by reason of its wall thickness becoming reduced below an acceptable value by corrosion. Such flawing may occur for example in steel pipes at the point where they emerge rom the sea, or in boiler pipes, heat exchanger pipes, reactor and reformer pipes.

The probe is frequently used in conjunction with a flaw detector being an electronic apparatus which provides the electrical driving signal for the probe, receives from the probe electrical signals representing the reflected signals, computes the wall thickness, and provides an output signal which is a function of the wall thickness.

So that all wall thickness may be measured, it is necessary to acoustically couple the probe to the inner surface of the tube so that the ultrasonic signals are transmitted therebetween. This may be done by applying the probe against the inner surface by spring pressure but roughness of the inner surface can spoil the contact causing transmission to break down. As an alternative, it is known to provide the coupling by water occupying the distance between the probe and the inner surface of the tube. To do this, a tube being monitored must usually be detached at both ends from the installation to which is is normally connected, and the tube disposed vertically. The probe is lowered into the tube through the open upper end and the tube filled with water.Either the bottom end of the tube is blocked to support a static water column or, the bottom is left open and water poured into the open end in such large volume as to keep the tube filled with a moving column.

One object of the invention is to provide a method of measuring the wall thickness of a tube using ultrasound, which merely requires access to be had from the outside to one end of a tube and does not require the use of large amounts of water or other liquid.

According to one aspect of the invention a method of measuring the wall thickness of a tube using ultrasonic signals comprises introducing an ultrasonic transducer into the tube interior and providing a body of liquid which occupies the space between the transducer and the inner surface of the tube, using fluid tight sealing means applied against the inner surface of the tube so as to prevent or impede escape of the liquid past the sealing means so as to maintain the liquid between the transducer and inner surface, and advancing said body of liquid along the tube by sliding the sealing means therealong when it is desired to measure the wall thickness at different locations along the tube to which the transducer is moved.

According to another aspect of the invention, an apparatus for measuring the wall thickness of a tube using ultrasonic signals comprises an ultrasonic transducer, sealing means associated with said transducer, said sealing means being arranged to make sliding but substantially fluid tight sealing contact with the inner surface of the tube so that said sealing means can maintain a body of liquid in the space between the transducer and inner surface, and said sealing means being moveable along the tube with the transducer to advance said body of liquid with said transducer.

The relationship between a transducer and at least one seal is such that when the transducer is introduced into a tube which has one end higher than the other, the seal is below the transducer. The seal makes sealing contact with the inner surface of the tube so that the seal supports a body of liquid, for example water or oil, above the seal. This liquid occupies the space between the transducer and inner surface of the tube. The seal is connected with the transducer in such manner that as the latter is moved up or down the tube, the seal slides along the inner surface and moves the body of liquid with the transducer.

A pair of spaced seals may be provided with the transducer therebetween. The body of liquid is trapped between the seals which are moveable simultaneously with the transducer so that the liquid also moves therewith to maintain the acoustic connection between wall and transducer. In this case the transducer can be moved along one arm of a Utube, round the bend and up the other arm without loss or substantial loss of liquid provided the seals are close enough together to remain simultaneously in sealing contact with the tube bend. This form of the apparatus can cope with other forms of rounded bends be sides U-bends, and can also be used in horizontal tubes.

The transducer can be connected to cable supplying high frequency electrical driving signals to the transducer and conveying electrical signals, corresponding to received relfected signals from the transducer. This cable may be connected to a flaw detector or electronic thickness meter. If desired a pipe passed along the tube may open through a valve adjacent the transducer so that liquid may be supplied through the pipe to replace any liquid lost from the body of liquid between the transducer and inner surface of the tube wall.

The transducer with at least the two said seals may be propelled by compressed gas along a tube, for example a horizontal tube. In this case the apparatus may carry the means to provide the transducer driving pulse, and measure the time delay between the same pulse reflected from the inner and outer surface of the tube wall, and may carry a distance measurer operated, for example, by a wheel running along the inside of the tube to correlate the measured time delays with different positions along the pipe length. The aforesaid means may record the measured time delays or calculate and record the wall thickness, at different positions, for subsequent analysis.

If desired, a plurality of transducers disposed in radial array may be used, to observe different portions of the tube wall at the same position along the tube. Each transducer may be operated in turn and the wall thickness measurements derived therefrom recorded in sequence.

The transducers may be detachably mounted in a common probe housing. A plurality of housings of differing diameters may be provided for use in tubes of differing diameters. The larger the tube diameter, the larger the number of transducers the relevant probe housing accommodates so that effective observation around the tube wall can be carried out. Naturally the seal or seals used in conjunction with the different housings are of such size as to be in sealing engagement with the inner surface of the tube wall. Some care must be taken in designing the housing so that each transducer therein is positioned relative to the tube wall so that the ultrasonic signal emitted by the transducer is focussed to a narrow beam by the time it reaches the inner surface of the tube.

The invention will now be further described, by way of example, with reference to the accompanying drawing in which: Figure 1 is a diagrammatic and fragmentary view of apparatus for carrying out the invenlion, and Figure 2 is a block diagram of an electrical control for the apparatus.

In Fig. 1 there is shown at 2 a tube whereof its wall thickness is to be measured.

Disposed inside the tube is a probe housing 4 carrying a plurality of detachable ultrasonic transducers 6 in radial array. These transducers are located between radially extending flexible seals 8 and 10 in fluid tight engagement with the housing and in fluid tight contact with the inner surface of the tube 2 along which latter the seals can slide. The space 1 2 between seals 8 and 10 is filled with a body liquid occupying the space between the transducers and the inner surface of the tube. A cable 14 conveys high frequency electrical driving signals to the transducers and conveys therefrom the electric signals corresponding to the reflected ultrasonic signals received by the transducers. This cable can be enclosed in a pipe 1 6 attached to the housing.The pipe may be of plastics material and is relatively stiff or rigid over a short length but flexible over a longer length. The housing 4 can be moved along the tube by pushing on the pipe 1 6. This movement of the housing also moves the seals 8 and 10 so that the liquid in the space 1 2 is maintained in position relative to the transducers. To replenish the space 1 2 in case of liquid leakage, liquid passed down the pipe 1 6 is conveyed through the housing 4 to the space 12.

The supply of liquid may be controlled by a valve in the housing or adjacent thereto.

To maintain the pipe 1 6 rigid so it may push the housing long distances, a plurality of bracing means formed by discs or washers 20 are firmly mounted on the pipe at spaced intervals, albng each of which intervals the relatively short length portion of pipe is relatively rigid. The discs, which can slide along the tube 2, have approximately the same diameter as the inner diameter of the tube and are braced against its inner surface when the pipe is pushed. By spacing the discs 20 at known intervals and marking each in units of length from the transducers, an operator by reading the uppermost disc can see how far the transducers are into the tube.

The housing 6 is sized and the spacing between the seals 8 and 10 is such that the housing can pass around bends in the tube whilst the seals continue in sealing contact with inner surface of the tube.

To trap liquid emerging from space 1 2 passed the seal 10, a trap formed by seals 1 8 may be provided. These seals, which are in fluid tight sliding contact with the inner surface of the tube 2, are mounted on pivotable arm 22 of a universal joint within a rubber bellows 24 from which emerges the other arm 26 of the joint mounted on the housing. The relative pivoting between arms 22 and 26 allows the apparatus to turn corners whilst maintaining the sealing effects.

In very long tubes a separate trap which may have spherical form may be located at the bottom of the length of tube under investi gation.

Although the apparatus in Fig. 1 is moved manually along the tube 2 from one location to another at which wall thickness measurement is desired, the operation of the probes and the recording of the measurements, is carried out using the control of Fig. 2. The probe housing 4 is moved to a desired location and then hand switch 30 is operated to switch on the control. Under the control of multiplexer 32, the flaw detector 34 produces a high frequency electrical signal, supplied as a single short pulse via cable 14 to each transducer 6 in turn in a predetermined sequence in response to operation of decoder 36 in accordance with the multiplexer signal.

Therefore each transducer 6 gives in turn an ultransonic emission and receives the two reflections thereof which are converted to electrical signals by the transducer and through the decoder are supplied to the flaw detector before the next transducer is pulsed.

On receiving the first of the two reflected signals a clock in the flaw detector is started and is stopped by reception of second reflection. The time delay is thus measured and used by the flaw detector to compute the wall thickness observed by the transducer. An analog output signal from the flaw detector corresponding to the wall thickness is converted to a digital signal by digitiser 37. This signal is supplied to interface 38 giving an output to printer 40 providing a printout of the measured values in the sequence in which the transducers are energised.

Thus for each position along the tube at which the measurements are made, the values measured may be printed as a block associated with which may be a printout of the length position along the tube to which that block relates or a printout of the numerical order in which that block of measurements was derived in relation to the other blocks to correlate the block with a particular position along the tube length, it being either predetermined or known how far head 4 is moved along the tube by'the operator between each successive measuring position.

The flaw detector may include an adjustable control and visual display settable by the operator to a minimum numerical value which the wall thickness may have. Each time a thickness less than that minimum is measured a signal from the flaw detector to the printer causes the printout of that thickness to be distinctively presented and/or alarm or warning means may be provided to respond to the signal.

To measure the wall thickness of narrow bore tubes the probe housing 4 is made very slender and an ultrasonic transducer is mounted in the housing so the transducer faces along the axis of the tube. The housing also comprises a reflector, for example an outer surface of a cone of stainless steel, onto which the ultra-sonic signal from the transducer is incident and turned through 90 onto the tube wall from which it is reflected back onto the reflector and thence onto the probe.

Claims (16)

1. A method of measuring the wall thickness of a tube using ultrasonic signals comprising introducing an ultrasonic transducer into the tube interior and providing a body of liquid which occupies the space between the transducer and the inner surface of the tube, using fluid tight sealing means applied against the inner surface of the tube so as to prevent or impede escape of the liquid past the sealing means so as to maintain the liquid between the transducer and inner surface, and advancing said body of liquid along the tube by sliding the sealing means therealong when it is desired to measure the wall thickness at different locations along the tube to which the transducer is moved.

2. A method as claimed in claim 1, in which the sealing means comprises first and second spaced seals, the transducer is disposed between the seals in a chamber containing the liquid, and walls of said chamber are defined by the seals and said inner surface.

3. A method as claimed in claim 1, in which the transducer and sealing means are mounted on an elongate member which is flexible along its length but relatively rigid over a shorter portion of its length, the transducer and sealing means are propelled along the tube by pushing effort exerted on the member along its length direction towards said transducer and sealing means, the member being maintained relatively rigid within the tube (to transmit said effort) by bracing means extending between the member and the inner surface of the tube and spaced along the member by intervals sufficiently small that the length portion of the member between adjacent bracing means is relatively rigid.

4. A method as claimed in any one preceding claim, in which the tube extends upwardly and a trap is provided below the sealing means to catch liquid which may escape past the sealing means.

5. A method as claimed in claim 4, in which the trap ib moved along the tube with the sealing means.

6. An apparatus for measuring the wall thickness of a tube using ultrasonic signals comprising an ultrasonic transducer, sealing means associated with said transducer, said sealing means being arranged to make sliding but substantially fluid tight sealing contact with the inner surface of the tube so that said sealing means can maintain a body of liquid in the space between the transducer and inner surface, and said sealing means being moveable along the tube with the transducer to advance said body of liquid with said transducer.

7. An apparatus as claimed in claim 6, in which the sealing means comprises first and second spaced seals to form in use, in conjunction with the inner surface of the tube, walls of a chamber for the liquid, and the transducer is disposed between the seals.

8. An apparatus as claimed in claim 6 or claim 7, further comprising second sealing means spaced from first mentioned sealing means, said second sealing means being moveable with the transducer and first sealing means and arranged to make fluid tight sliding contact with the inner surface of the tube such that when the apparatus is in use with the second sealing means below the first sealing means a trap is provided by the second sealing means to catch liquid which may escape downwardly past the first sealing means.

9. An apparatus as claimed in claim 8 in which the first sealing means and transducer are connected to the second sealing means by a universal joint.

10. An apparatus as claimed in claim 6 or claim 7, in which the transducer and sealing means are mounted on a pipe to move along the tube with said sealing means and transducer.

11. An apparatus as claimed in claim 10 in which the pipe is arranged for supplying liquid to said space.

12. An apparatus as claimed in claim 10 or claim 11, in which the pipe contains an electrical cable connected to the transducer.

13. An apparatus as claimed in any one of claims 10 to 12, in which the pipe is flexible along its length but relatively rigid along shorter length portions, bracing means for the pipe are mounted on the pipe at spaced intervals, and the length of the portion of the pipe between adjacent bracing means is relatively rigid, the arrangement being such that in use each bracing means extends from different sides of the pipe to or proximate to the inner surface of the tube over which said bracing means can slide.

14. An apparatus as claimed in claim 13 in which each bracing means is a disc through which the pipe passes.

1 5. An apparatus as claimed in claim 1 3 or claim 14 in which each bracing means is marked to indicate the distance it is from the transducer along the pipe.

16. A method of measuring the wall thickness of a tube using ultrasonic signals substantially as hereinbefore described with reference to Fig. 1 or with reference to Figs. 1 and 2 of the accompanying drawing.

1 7. An apparatus for measuring the wall thickness of a tube using ultrasonic signals substantially as hereinbefore described with reference to Fig. 1 or with reference to Figs. 1 and 2 of the accompanying drawing.