GB2088064A - Method and apparatus for measuring structural fatigue cracking - Google Patents

Abstract

Austenitic pipes installed in pyrolysis or reforming ovens are tested and conclusions drawn as to whether they may be further used by use of two measurement sensors (5, 6) for testing for permeability and cracks respectively, which are disposed adjacent to one another and may be moved on or partially about the pipe (1) by means of a movable sensing head (2, 4) driven by a motor (3), thereby to enable structural changes and crack formation to be localised and recorded simultaneously by a graphic recorder (8) operating in synchronism with the drive motor (3). The extent of the damage may be detected and evaluated by assessment of the superposed diagrams and by their comparison with a zero measurement and/or with an earlier recording. <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring structural fatigue cracking The invention relates to a partially automatic apparatus and method for measuring structural fatigue cracking for monitoring austenitic materials, in particular austenitic pipes which are installed in a vertical position in pyrolysis or reforming ovens and which are to be tested for further usability within the framework of preventive maintenance.

In the operating condition the pipes above referred to undergo a local structural change which is caused by oxidation, carburization, nitration and sulphurization and which leads according to the operating loads and conditions of use applicable in each case to the early failure of the pipes as a result of complex structural fatigue cracking. The complex damage of the material begins on the inner side of the pipe and spreads over its cross-section until a critical material condition has been exceeded and consequently cracks occur. This phenomenon overlaps with the appearance of cracking due to creep, which cracks begin on the exterior of the pipe and lead to complex damage as a result of structural fatigue cracking.

At present only a few test methods are known which are suitable for monitoring austenitic pipes which are installed in pyrolysis or reforming ovens.

These known test methods generally only enable a point-like or very localised monitoring of cracks in the material. Such monitoring does not provide information relating to the cause of these cracks.

Only surface cracks on the external wall of the pipe may be readily detected and localised by means of penetration methods. X-ray and gammaray testing methods only provide information relating to macroscopic defects which lie transverse to the direction of radiation. Ultrasonic testing cannot at present be used on account of the surface condition of the above-mentioned pipes in the operating position. Although indirect information concerning structural changes may be possible from point-like carburization measurements, complex structural changes may not at present be directly detected.

A combination of penetration methods, gamma-ray testing and carburization measurements is very time and material intensive.

The cracking tested in this way may only be localised with difficulty during evaluation or may only be identified with a high degree of technical complexity in terms of measurement. This is caused by the different physical bases of the individual test methods. More especially, it is not possible to obtain information concerning the depth and extent of complex structural fatigue cracks. This situation is unsatisfactory as it necessitates relatively long dead times for checking the pipes, is complex in terms of manual operations, apparatus, testing techniques and the maintenance of these operations, as for example described in the Offenlegungsschrift No.

2,741,016, and does not provide reliable information concerning the depth of structural fatigue cracks which may be present, so that in cases of doubt destructive material testing must be carried out.

An object of the invention is to counter the above described disadvantages of known testing methods and to enable localised information relating to the type, size and extent of damage to austenitic pipes, which are installed in pyrolysis or reforming ovens, to be obtained, detected and documented during an inspection cycle without the need for destructive material tests and with savings in time and apparatus, so that a comparison with inspections carried out subsequently shows the increase of the extent of the damage or the appearance of fresh damage.

A further object of the invention is to provide a partially automatic structural fatigue cracking measurement method which enables various information relating to the condition of the material, in particular the location, time, size and extent of material changes of austenitic pipes, to be obtained and detected in one measuring process along a predetermined measuring distance both in the vertical and horizontal directions, in practice over a predetermined measuring area, and enables reliable information to be obtained with respect to the remaining service life of the pipe.

This object is solved in accordance with the invention in that two special measurement sensors are carried by a sensing unit adjacent to one another or above one another, this unit being moved in the vertical and horizontal directions over a selected measuring area of an austenitic pipe to be tested by remote control.

One of the sensors, which is connected to a commercially available permeability measuring device to effect corresponding measurements, enables the detection of complex structural changes caused by carburization, oxidation and/or suiphurization, as the shape of a magnetic field released by a permanent magnet changes if the para-magnetic properties of the austenitic material have been changed due to the causes given above.

The second sensor, which is connected to a commercially available crack testing device operating in accordance with the eddy current principle, is used to detect cracks, wherein a coil having a high frequency a.c. current flowing through it is guided over and in contact with the austenitic pipe to be tested. If a crack is present, the eddy current field produced is modified and has a feedback effect on the coil impedance. The size of this change in impedance forms a measurement of the depth of the crack.

The measurements provided by both devices are supplied via an amplifier to an external recorder and are recorded in a superposed manner. The diagram recorded during the tracing of the measurement area enables a very rapid and sufficiently accurate localization of damage which has been discovered, as its length extends in synchronous relationship to the measuring distance. It is therefore in the first instance preferred to trace the measuring distance vertically and only test it further by horizontal and vertical tracing at those points where structural fatigue cracking or cracks are initially found. In addition, the diagrams obtained are compared with a zero measurement which has been previously obtained from an austenitic pipe of the same type which has not been used and are possibly correlated with diagrams from earlier measurements.

These comparisons enable the person skilled in the art immediately to recognise the extent of the structural fatigue cracking and of the formation of cracks and enable a sufficiently accurate picture of the service life to be expected under normal conditions or of the need for immediate replacement.

These comparisons also enable the operator of the plant to obtain further information relating to the operating regime, the burner setting and the planning of maintenance operations. Improved use of the remaining service life of the pipes is thereby rendered possible.

The invention is exemplified in the following description, making reference to the accompanying drawing, in which: the single figure (Figure 1) shows apparatus for measuring.structural fatigue in accordance with the method of the invention.

A sensing unit 2 is applied to a pipe section 1 to be tested, which unit includes a drive motor 3 which can be remotely controlled to drive a slide 4 of said sensing unit around or along the pipe for an appropriate measuring distance. Two measurement sensors 5, 6 are carried by the slide 4. Outside of the oven in which the pipes to be tested are disposed, there is provided a movable test unit 7 which consists of two commercially available universal measuring devices for the measurement of permeability and for crack testing and to which a recorder 8 is connected.

A commercially available permeability measuring device is used as the measuring device for the measurement of permeability, this device having a measuring accuracy which is sufficient to detect changes in permeability which are met in practice. An eddy current crack depth measuring device produced by VEB-Maass (DDR) can be used for the surface crack testing device.

For testing purposes, both devices are connected through a test cable 9 and via an amplifier 10 with the sensors 5, 6. Measurement losses arising from the cable extension 9 are compensated by the amplifier 1 0.

A TSS 101 rapid recorder produced by VEB Messgeraetewerk Zwoenitz can be used as the recorder 8.

The unit 2 including the slide 4 are caused to move from a marked point of the pipe by remote control via a remote control cable 12 providing a drive signal synchronised with the recorder drive.

The measuring distance covered by the sensors 5, 6 is therefore in direct proportion to the advance of the recorder 8. In this respect the selected measuring distance is in the first instance preferably in an upward vertical direction along the pipe.

The two sensors 5, 6 signal the condition of the material via the amplifier 10 and the test cable 9 to the devices of the test unit 7, where the condition is displayed and supplied to the recorder 8, which records overlapping diagrams with its writing eiements.

During evaluation of diagrams obtained in one test programme, in particular by comparison with a zero measurement obtained from an unused pipe, it could be seen that heavy structural fatigue cracking had occurred, but specific points of cracking could not be identified. The position of these points could readily be determined by accurate transfer of the recording strip onto the pipe. This showed that structural fatigue cracking was located at the points at which the burners were directed immediately onto the pipes.

These areas were more accurateiy tested by longitudinal and transverse movements of the sensors. In this respect, by comparison with the zero measurement and with the earlier measurement, it could be seen that the beginnings of cracks were extended, which showed that this pipe could be retained for at least a further quarter, bearing in mind the stress caused by its service life and its through-put, if it were only subjected to abnormai temperature variations for short periods, and therefore that immediate replacement of the pipe as a result of crack formation was not necessary.

Claims (5)

1. A partially automatic method for measuring structural fatigue cracking for the monitoring of austenitic materials, more especially pipes, which are installed in pyrolysis or reforming ovens, by means of the measurement of permeability and crack testing, according to which two measurement sensors disposed adjacent to one another on a movable sensing head are moved by means of a remote control unit from a start position on the workpiece along a predetermined measurement distance in the vertical and/or horizontal direction and the sensed outputs are supplied via an amplifier and a display unit to a graphic recorder which records the outputs in a superposed manner, the location, type, size and extent of material changes being shown perse by the diagrams and being capable of evaluation by comparison with a predetermined zero measurement and/or with an earlier measurement.

2. A method as claimed in claim 1 , wherein the measurement sensors are moved firstly along the measuring distance in the vertical direction to produce an initial record and are then moved both in the vertical and horizontal directions only at those points at which material changes are visible from the initial record.

3. A method of measuring structural fatigue substantially as hereinbefore described.

4. Apparatus for measuring structural fatigue in a workpiece of austenitic material, comprising a sensing unit mounted for movement along and/or around a workpiece by a drive motor which is remotely controlled, a sensor for permeability testing and a sensor for crack testing carried by the sensing unit, a remote graphic recorder receiving the outputs of the two sensors to produce correlated records of said two outputs, and a control cable coupling the remote recorder drive with the drive motor for synchronised operation thereof.

5. Apparatus for measuring structural fatigue substantially as hereinbefore described with reference to the accompanying drawing.