Ultrasonic wall 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. 

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. 

Ultrasonic wall thickness measurement. Ultrasonic measurement devices measure actual wall thickness. This allows decisions to be made regarding the suitability of a cylinder for continued service. The ultrasonic energy is directed at 90 degrees to the entering surface from the piezoelectric crystal and is reflected back to the crystal at a parallel surface. 

DOT and TC have authorized by exemptions the use of ultrasonic wall thickness measurement for the requalification of certain compressed gas cylinders. 

Ultrasonic Wall Thickness Measurement. There are a variety of ultrasonic, electronic wall measurement devices which will mea-... 

Proper field monitoring and inspection techniques can be used to measure the real corrosion rate, to find safety risks on time and to avoid corrosion accidents. The field monitoring and inspection techniques normally include weight-loss coupons, electrical resistance probes, ultrasonic wall thickness measurements, infrared thermal imaging systems, etc. Ultrasonic wall thickness measurement is widely used to monitor the general and erosion corrosion rates of the equipment and pipelines in crude oil processing plants. 

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 minimum wall thickness found with a manual ultrasonic wall thickness meter was 4.0 mm ( 2.3 mm wall thickness loss). This is in good accordance to the estimated wall thickness loss by radiography, which shows for small spots even a higher loss up to 2.8 mm. The expected accuracy for these measurements is about 0.2 mm. 

Uniform surface corrosion, i.e. corrosion at a nearly uniform corrosion rate over the entire surface, is usually less problematic from an operational point of view. This factor can be taken into account in the structural element design in the form of an anticorrosive additive and can be controlled in many structural elements by means of regular wall thickness measurements, e.g. by ultrasonic means. Much more difficult problems result from local corrosion types such as pitting corrosion and stress corrosion cracking (SCC). The corrosion types are difficult to control and can rapidly lead to failure of structural elements after only a low level of mass loss. Damage from such corrosion types are rarely predictable and not only cause considerable losses in economic terms but also entail risks to safety and environmental protection. This applies in particular to system elements that must function under pressure. 

For the repetitive inspections the required hydrotest can only be performed for a limited number of the small cylinders, and even then the drums have to be removed from the line and the cylinders will be supported in defined distances for the weight of the water and the pressurisation. For the new and long cylinders even this is impossible, because they loose due to the additional weight of the water and the over-pressurisation their roundness and balances. Therefore the law in the most countries within and outside of the EU accept as a replacement of the hydrotest an additional application of different NDT methods, which were often done by an ultrasonic measurement of the wall thickness of the cylindrical part and a MT of the flat covers. 

These two transducer pairs are activated alternating. For this purpose an ultrasonic instrument is combined with a two channel multiplexer. Figure 8 presents a modified standard instrument USN52 which also implies a modified software. This system performs four measurements per second - alternating the velocity and the thickness are determined. The probe can be scanned over the surface and in every position both, the velocity and the wall thickness are indicated Using the serial interface of the instrument finally a two-dimensional map of velocity or thickness can be generated. 

Ultrasonic techniques are an obvious choice for measuring the wall thickness. In the pulse-echo method times between echoes from the outer and inner surface of the tube can be measured and the wall thickness may be calculated, when the ultrasonic velocity of the material is known. In the prototype a computer should capture the measuring data as well as calculate and pre.sent the results. First some fundamental questions was considered and verified by experiments concerning ultrasonic technique (Table I), equipment, transducers and demands for guidance of the tube. 

Ultrasonic techniques. Wall thickness can be measured to monitor the progress of general corrosion, cracks can be detected and hydrogen blisters identified. Certain construction materials such as cast iron cannot be examined by ultrasound. Skilled operators and specialist equipment is required. Plant can be examined in situ except when it is above 80°C. 

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... 

Ultrasonics (UT), which directly measures the thickness of the pipe wall by ultrasonic sounds to measure the amount of time it takes an echo to return to the sensor... 

ULTG Ultrasonic sensor Measurement of wall thicknesses 0.3... 

Direct assessment gives a possibility for direct mechanical or ultrasonic measurement of pit depth and wall thickness change due to the corrosion. X-ray radiography is used to inspect welds immediately after they are made. This technique can also show stress corrosion cracks and corrosion pits in the weld area, where the coating has failed to seal the metal structure. A promising method of mapping corrosion on the metal surface is the use of laser scanning equipment. 

Pipeline corrosion can be monitored by pushing "smart pig instruments through the line to measure local and general metal loss (by electromagnetic means). Related instruments have been developed that can make ultrasonic measurements of wall thickness. These methods are used to detect the presence of both external corrosion and internal corrosion. In some instances, video camera techniques can be used to evaluate the condition of internal pipe surfaces. Caliper typ>e measurements can be made in those relatively short lines that permit use of a cable to pull the instrument... 

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