Ultrasonic thickness measurement

Magnetic flux pigs can be run in liquid and gas lines, but rely upon previous wall thickness calibrations taken by direct measurement using ultrasonics, or by reference to pipe manufacturer s data sheets. The maximum wall thickness capabilities for the more sophisticated designs are typically 15 mm for 10 inch ID pipe and 30 mm for 30 inch ID pipe. 

The assessment of the material thickness in the weld area is affected by changes in the profile and the thickness of the pipe material, andaccuracy is therefore reduced at these locations. In addition, the results are also affected by external welded attachments. For new pipelines a baseline survey should he considered during the precommissioning phase to enable construction defects to be discounted in future surveys. 

Conventional compression wave ultrasonics may be used to measure the residual wall thickness in pipework and vessels handling potentially corrosive fluids. The measurement accuracy depends upon the actual wall thickness and the condition of the outer surface of the pipe or vessel in contact with the prohe, hut will typically be 0.5 mm. 

Correlations of actual pipe wall wastage can be made with data from the installed intrusive corrosive monitoring devices, but care has to be taken in deriving corrosion rates from ultrasonic wall thickness data in view of the limited accuracy of the technique. 

The precise locations on the pipe or vessel being examined should be permanently marked in order to ensure that successive ultrasonic readings are always taken at the same location. 

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The Dow Freeport in-service inspection procedures are similar to those reported earlier in this chapter. The Dow out-of-service (internal) inspection includes ultrasonic thickness measurements at all benchmark locations. Other test methods include shear wave ultrasonics, eddy current, and radiography. Engineers use ultrasonic thickness readings to project the remaining useful life of the vessel and to determine when the next internal inspection should be scheduled. 

Physical Methods Ultrasonic thickness measurement for Metal Loss Radiography... 

In general, the most widely used field technologies for inspection are ultrasonic thickness measurement, while for online methods they are corrosion test specimens, electrical resistance, and linear polarization probes. Both of the inspection methods and the first two online methods measure metal loss. The last method measures corrosion rate, but only in a sufficiendy conductive process environment, normally water. 

Ultrasonic Measurements—The ultrasonic thickness measurement method is quite popular for in-service corrosion testing. The major advantage of this method is that the equipment is portable and very easy to use. The major disadvantage is that a bare metal surface is required for accurate measurements. The presence of coatings and/or corrosion products can introduce errors into the thickness measurements [69]. Curved surfaces such as piping bends and small tubing diameters require special attention. 

Interior corrosion is best evaluated by a hydrostatic test combined with careful visual inspection. Ultrasonic thickness-measuring and flaw-detection devices may be used to evaluate specific conditions. Corrosion limits for both low and high pressure steel cylinders were dis-... 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

The second example concerns a thickness measurement problem. Normally this is an easy task for ultrasonic testing However,... 

In general a thickness measurement using ultrasound is done by measuring the time of flight of the ultrasonic pulse and calculating the thickness of the objeet multiplying the time and the known constant sound velocity in the material. 

Based upon a piezoelectric 1-3-composite material, air-bome ultrasonic probes for frequencies up to 2 MHz were developped. These probes are characterized by a bandwidth larger than 50 % as well as a signal-to-noise ratio higher than 100 dB. Applications are the thickness measurement of thin powder layers, the inspection of sandwich structures, the detection of surface near cracks in metals or ceramics by generation/reception of Rayleigh waves and the inspection of plates by Lamb waves. 

In wide sectors of industry there is a growing need of inspection methods which go without liquid coupling media. The excitation of bulk and surface waves by means of air-coupled ultrasonic probes is therefore an attractive tool for NDE. This is tme e.g. for the rapid scanning of large composite structures in the aerospace industry [1]. In other cases, the use of liquid couplants is prohibitive like the thickness measurement of powder layers. 

Table 1. Three different ultrasonic measuring techniques for thickness measurement.

In addition, NDT plays an important part in industrial maintenance. During plant shutdowns for instance, many thousands of ultrasonic wall thickness measurements are taken on piping, vessels, furnace tubes etc. All these thickness readings have to go into extensive data bases, and this process is, thanks to modem computers and data loggers, ever more automated. 

Studying modem approaches for such schemes, one can see that knowledge of operational conditions and potential degradation mechanisms play a prominent role. Surprisingly, the role of NDT is often limited to tlie use of conventional methods such as ultrasonic wall thickness measurements, ultrasonic inspection, radiography, and last but not least visual inspection. 

MFL is currently being used for the inspection of hundreds of kilometres of piping in the Oman desert, see figure 5., whereby an inspection rate of one kilometre per day is easily achieved. Try to realise what effort would have been needed if this would have been done with conventional ultrasonic wall thickness measurements. 

After a satisfactory verification film is produced, an assembly may be fabricated specifically for destructive inspection to validate that the verification film was accurate. This correlation allows the use of verification film rather than more expensive destructive inspection for future changes such as duplicate tool fabrication and tool or detail modification. Simple assemblies are usually not destructively inspected because of high confidence that the verification film is entirely representative of the expected bondlines. Complex or large parts may not be destructively inspected because of the cost of the details and assembly time. In these cases other means of validating the verification film are used. Meticulous pre-bond detail and post-bond assembly thickness measurements may be sufficient to prove bondline thickness control. Ultrasonic inspection and X-ray photography (discussed previously) may be sufficient to prove that details are in the correct places and bonds are good. 

Pressure waves of the same nature as sound waves but of greater frequency, i.e., shorter wavelength, and therefore inaudible. Ultrasonic waves have been used for the detection of ply separations and other voids in rubber-textile composites such as tyres, and for thickness measurement of coatings, etc., where access is possible from only one side. 

Tsukahara, Y., Takeuchi, E Hayashi, E., and Tani, Y. (1984). A new method of measuring surface layer-thickness using dips in angular dependence of reflection coefficients. IEEE 1984 Ultrasonics Symposium, pp. 992-6. IEEE, New York. [214] Tsukahara, Y Nakaso, N., Kushibiki, J., and Chubachi, N. (1989a). An acoustic micrometer and its application to layer thickness measurements. IEEE Trans. UFFC 36, 326-31. [213-215]... 

Many organizations rely heavily upon the use of a D-Meter. A D-Meter is a relatively inexpensive handheld digital display ultrasonic thickness gauge for determining the remaining thickness of vessel walls and piping. Ultrasonic wall-thickness measurements resemble radar or sonar in technique. A burst of ultrasound is emitted via a probe into a material and bounces off the rear wall. The time interval measured for this reflection to return is a measure of wall thickness. 

Some applications of ultrasonics are inspection for flaws in forgings, flat-rolled products such as strips, sheets and plates, castings, extrusion billets, rolling blooms or slabs, bar stock, pipes, welded joints, bonded joints, monitoring cracks, measurement of thickness, measurement of fluid levels, microstructural features and monitoring corrosion. 

Corrosion may be monitored by measuring changes in thickness with time using ultrasonic thickness gages. An automated ultrasonic inspection system has been devised and used in monitoring corrosion in nuclear waste containers and typical variations in ultrasonic indications in corroded and uncorroded areas have been recorded successfully. Major types of equipment inspection by ultrasonic technique has been done in mill components, power equipment, jet engine parts, aircraft components, railway materials, automotive components and other machinery components. 

If the vessel is not insulated or does not have a liner, the wall thickness may be tested from either the outside or from within using a D-Meter. This tool can quickly and easily measure the thickness of piping or a vessel wall, D-Meters are generally the inspector s favorite tool. The name D-Meter is the technician s lingo for the simple handheld digital ultrasonic thickness meter. Often corrosion patterns on vessels and equipment are... 

TABLE 39.2. Relative index values for corneal thickness measured by ultrasonic pachymetry over time ... 

Ultrasonic thickness gauges. To measure the thickness of concrete walls or steel structure for design verification. 

In areas where general corrosion is the expected form, a simple ultrasonic thickness gage can be utilized to determine the extent of corrosion, based on baseline readings made at installation or previous inspections. The entire unit need not be examined. Attention can be focused on those areas most likely to corrode, such as liquid levels, mixing zones, or areas of high turbulence. Corrosion probes, which can be placed in process equipment or pipelines, can monitor corrosion conditions by measuring an actual corrosion current, or other process parameters known to be related to general corrosion rates. These data can be constantly monitored and recorded to predict equipment wear, or as an alert to upset conditions. 

M. Krause, Ch. Maierhofer, and H. Wiggenhauser, Thickness measurement of concrete elements using radar and ultrasonic impulse echo techniques, in Proc. 6th International Conf. on Structural Faults and Repair, Engineering Technics Press, London, 1995, pp. 17-24. 

Fig. 25.2 Ultrasonic transmission measurement results for a sample of two aluminum plates 4 mm thick bonded together by an adhesive epoxy layer of 30 p,m thickness showing the transmitted strain amplitude s- of the fundamental frequency versus the input strain amplitude and the linear fit of the first 12 measuring points.

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