·Table of Contents
·Workshop - Guided Wave
Rapid Long range Inspection of Chemical Plant Pipework Using Guided WavesD.N. Alleyne, B. Pavlakovic,
Guided Ultrasonics Ltd, 17 Doverbeck Close, Nottingham, NG15 9ER, UK
M.J.S. Lowe, P. Cawley
Dept Mechanical Engineering, Imperial College, Exhibition Road, London, SW7 2BX, UK
The use of cylindrical guided waves propagating along the pipe wall is potentially a very attractive solution to this problem since they can propagate a long distance under insulation and may be excited and received using transducers positioned at a location where a small section of insulation has been removed. There has been a considerable amount of work on the use of guided waves for the inspection of pipes and tubes, most of which has been on small (typically 1 inch) diameter heat exchanger tubing (see, for example, [1-4]).
The authors have developed a guided wave technique designed for the screening of long lengths of pipes for corrosion. It seeks to detect corrosion defects removing of the order of 5-10% of the cross sectional area of the pipe at a particular axial location. It is designed to operate on pipes in the 2-24 inch diameter range, though in principle, it could be used on both smaller and larger pipes.
Fig 1 shows the group velocity dispersion curves for a 6 inch, schedule 40 steel pipe. There are about 50 modes below 100 kHz and in order to obtain signals that can reliably be interpreted, it is essential that only one of them be excited. In most guided wave testing, the sensitivity of the test is a function of the signal to coherent noise ratio, the coherent noise being caused by the excitation of unwanted modes. This coherent noise cannot be removed by averaging, whereas if low signal levels cause a poor signal to random noise ratio, significant improvements can be obtained by averaging. The original implementation of the technique used the L(0,2) mode (the terminology used to describe cylindrical guided modes is discussed in ) at frequencies around 70 kHz. This mode is very attractive to use for long range testing [5-7] since it is practically non-dispersive in this frequency range. Its mode shape is similar to that of the s0 mode in plates at low frequency-thickness products, the particle motion being predominantly axial and the strain being roughly uniform through the pipe wall. It is therefore well suited to the detection of corrosion which may initiate at either surface of the pipe.
|Fig 1: Dispersion curves for 6 inch, schedule 40 steel pipe.|
Alleyne and Cawley  reported the development of a dry coupled piezoelectric transducer system for the excitation of the axially symmetric L(0,m) modes in pipes. It comprises a ring of piezoelectric elements which are clamped individually to the pipe surface; no coupling fluid is required at the low ultrasonic frequencies used here. The number of elements in the ring should be greater than n where F(n,1) is the highest order flexural mode whose cut off frequency is within the bandwidth of the excitation signal. In the initial configuration, rings of 16 elements were used on 3 inch pipes, while 32 element rings were employed on 6 and 8 inch pipes. This gave the possibility of operating at frequencies up to around 100 kHz; in practice, most testing is done at 50 kHz and below, so it has been possible to reduce the number of transducers in a ring.
Initial site trials of the technique carried out in the research phase have been reported previously [5,7]. Propagation distances of approaching 50 m were obtained and by using two rings of transducers it was shown to be possible to obtain uni-directional propagation. Signal to coherent noise ratios of better than 40 dB were obtained on site, approaching 50 dB being obtained on clean pipe in the laboratory.
The axial length of a defect such as a corrosion patch does influence the reflectivity to some extent; in some exceptional cases, a resonance phenomenon occurs in which the reflections from the start and end of the defect cancel each other, so a greatly reduced reflection is seen back at the transducers. This occurs at discrete frequencies for defects with a sharp beginning and end so this danger can be overcome by testing at at least two frequencies. The effect is much more severe with machined calibration defects than with real corrosion patches that tend to be much less regular.
The initial site trials reported in [5,7] showed that corrosion defects of the target size could reliably be identified. However, echoes were also seen from butt welds since the weld caps are not generally removed so the weld presents a change in acoustic impedance. This makes it difficult to identify defects at welds and also introduces the possibility of a weld being incorrectly identified as a defect. This problem can be overcome by measuring the extent of mode conversion produced by a reflector.
If an axially symmetric mode is incident on an axially symmetric feature in the pipe such as a flange, square end or uniform weld, then only axially symmetric modes are reflected. However, if the feature is non axially symmetric such as a corrosion patch, some non axially symmetric waves will be generated. These propagate back to the transducer rings and can be detected. If the L(0,2) mode is incident, the most important mode conversion is to the F(1,3) and F(2,3) modes which have similar velocities to the L(0,2) mode in the operating frequency range (see Fig 1). The amount of mode conversion obtained depends on the degree of asymmetry, and hence on the circumferential extent of the defect. Fig 2 shows the direct reflection and the mode converted reflections from a full wall thickness notch as a function of circumferential extent for an L(0,2) mode input. At low circumferential extent (which is the region of interest for the detection of critical corrosion in practical situations) the mode converted F(1,3) reflection is almost as large as the direct reflection so if these two reflections are of similar size, it can be concluded that the feature is localised to a small region of the circumference. The results of Fig 2 are for a 3 inch pipe, but similar results are obtained with other sizes. Further details can be found in .
|Fig 2: Measured and predicted reflection coefficients for a through-thickness notch in a 3 inch, schedule 40 steel pipe at 70 kHz as a function of the circumferential extent of the notch. (L(0,2) mode input. After )|
The use of three or four rings adds to the cost of the system and also to the mass, which becomes significant when larger pipe sizes are being tested. An alternative to the longitudinal, L(0,2), mode is to use the torsional, T(0,1) mode. This has the advantage of being non-dispersive across the whole frequency range (see Fig 1) and there is no other axially symmetric torsional mode in the frequency range, so axially symmetric torsional excitation will only excite the T(0,1) mode. This means that only two rings of transducers are required in order to obtain single mode, unidirectional excitation. Torsional forcing can be achieved by simply rotating the same transducers used for the L(0,2) mode through 900 so that they apply force in the circumferential, rather than axial direction. When the T(0,1) mode is reflected from a non-axially symmetric feature, mode conversion is primarily to the F(1,2) mode, rather than to F(1,3) as in the L(0,2) case shown in Fig 2. As in the case of an incident L(0,2) mode, at low circumferential defect extents, the amplitudes of the reflected incident and mode converted modes are similar. The torsional mode also has the advantage that it will detect longitudinal cracks, whereas the longitudinal modes are essentially insensitive to thin defects parallel to the pipe axis. However, a disadvantage of this sensitivity to axial features is that the torsional mode reflects relatively strongly from support brackets that are welded axially along the pipe. Large reflections from these features reduces the range of the test and also make it more difficult to detect corrosion at the brackets. In this relatively unusual case, the longitudinal mode may be preferable. In practice, the more convenient, torsional mode is most commonly used, but in occasional special applications the longitudinal mode is employed.
|Fig 3a: Guided Ultrasonics Ltd Wavemaker 16 instrument and transducer rings for 3 inch pipe;|
The sensitivity of guided waves to defects in the pipe wall is a function of frequency. In general, the sensitivity of the test decreases as the frequency is reduced, but the effect is not always severe; this issue will be discussed in a future paper. The frequency used does affect the spatial resolution and the range. The speed of the torsional (T(0,1)) wave is about 3.2 km/s, while that of the extensional (L(0,2)) wave is about 5.4 km/s. Therefore at 50 kHz, the wavelengths are 64 mm and 108 mm respectively. In order to limit the frequency bandwidth of the excitation and so to ensure that only the desired modes are generated, Wavemaker 16 uses windowed toneburst excitation  which limits the bandwidth. A 5 cycle toneburst is often used; the bandwidth can be further reduced by increasing the number of cycles. This increases the power input and helps to increase the range. However, there is a cost in spatial resolution, though Wavemaker 16 employs a special signal processing technique to minimise this effect.
|Fig 4: Typical signals from (a) axisymmetric feature e.g. weld; (b) corrosion|
|Fig 5: Wavemaker 16 report from test adjacent to road crossing.|
Fig 4 shows typical reflections from symmetric and asymmetric features; the increase in the mode converted signal can clearly be seen in the asymmetric case and this is a key element of the defect identification scheme. Fig 5 shows an example report generated by the Wavemaker 16 software for an epoxy painted, 4 inch pipe at a test position adjacent to a road crossing. The test range extends over more than 20m on either side of the rings which are located in the middle of the plot. The software identifies welds and computes a distance-amplitude correction (DAC) curve for the welds. It then calculates the defect call level by comparison with the weld echo level and the calculated output amplitude, knowing that a typical site weld is a -14 dB reflector. The echo identified as +F2 is the only one where the red (mode converted) signal is significant compared to the black (reflection of incident mode) signal and this indicates possible corrosion at the entry point to a road crossing.
If the pipe is coated by a viscous substance such as bitumen, or a solid such as epoxy or GRP, then both shear and longitudinal modes can couple between the pipe and the coating. The attenuation is then controlled by the properties and thickness of the coating, the displacement of the pipe surface in all directions (radial, circumferential and axial) and the frequency. Typical loss rates at a test frequency of 60 kHz in the L(0,2) mode are less than 0.5 dB/m for thin (less than 1 mm) paint or epoxy coatings, around 0.5 dB/m for a few mm of GRP or spun epoxy and 3-10 dB/m for bitumen. This means that the test range on bitumen coated pipes is severely reduced. However, it can be increased by switching to lower test frequencies and this is exploited in Wavemaker 16, though at a cost of reduced resolution and sensitivity. If the pipe is buried in soil, the attenuation rate depends on the properties of the soil and how well it is compacted onto the pipe. The losses when the pipe is buried in concrete can be even more severe. However, the lengths involved are often quite short and the concrete is often not well bonded to the pipe wall, which greatly reduces the attenuation. Again, the range can be increased by reducing the frequency.
Fig 1 shows that in a 6 inch diameter pipe, the L(0,2) mode becomes dispersive below about 20 kHz as it drops towards its cut-off frequency at about 10 kHz. This cut-off frequency is dependent on the pipe diameter so in a 2 inch pipe, the L(0,2) cut-off is at about 30 kHz and testing is not feasible much below 50 kHz. This limits the improvements that can be gained by reducing the frequency in lossy systems. However, the symmetric torsional mode, T(0,1), does not have a cut-off frequency so this mode can be used at lower frequencies on pipes of any diameter.
Fig 6 shows an example of the success of reducing the frequency of operation. A 10 inch pipe was paritally wrapped as it passed through a short earth wall. Fig 6a shows the result obtained at 29 kHz. The attenuation through the buried section is higher than expected (as evidenced by the very small weld signal marked -F4.) Since it was determined that the buried section was not adequately inspected, the frequency was lowered and the test was perfomed again. At 21kHz (Fig 6b), better penetration of the earth wall was obtained. Small echoes can now be seen at the point where the pipe exits the wall. The somewhat non-symmetric nature of these echoes (as well as their shape) indicates that there is likely to be corrosion at this location (beginning at the location marked -F3). Fortunately, access could be gained to the other side of the earth wall for more testing. From this side, the corrosion at the beginning of the earth wall is very evident as the large echo with large asymmetric (red) component marked +F2 in Fig 6c. A rough approximation of the size of the corrosion can be obtained by using the DAC curves. This technique reveals that the average cross sectional loss is about 10-15 percent; a photograph of the corrosion is shown in Fig 6d.
Inspection of 10 inch pipe passing through earth wall. (a) 29 kHz; (b) 21 kHz); (c) from other side of wall; (d) photograph of corrosion.
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