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The Advancement of NDT Technology and its Application in the Safety Assessment of Pressure VesselsChen Guo-hua
Institute of safety engineering,
Department of industrial equipment and control engineering,
South China University of Technology, Guangzhou 510640, China
The advancement of NDT technology is interviewed; its useful application for assessing the safety of pressure vessels is discussed. The hot problems of NDT technology are put forward: the reliability, the intelligent and the quantitative technology. Finally, the development tendency of NDT technology used in the safety assessment of pressure vessels is put forward in four aspects.
Keywords: NDT, reliability, quantitative technology, pressure vessels, safety assessment
Pressure vessels are special equipment loaded pressure. There are about 22,500 thousand pressure vessels serviced in China up to now. Because of the historical, technological and managing reasons, most of them contain very serious defects, even serviced beyond their designed life, which have led to a lot of exploded disasters. According to the statistics, during ten years from 1977 to 1987 there are 2254 exploding accidents of boiling and pressure vessels, which resulted in 1,200 people died and 4,900 injured, the estimated economic loss reached 80 to 90 hundreds million yuan RMB. Mean exploding accident rate per ten thousand boiling and pressure vessels is 10 times more than that of the developed country, the momentous accidents are 100 times as that of the developed country (Jia Yong-tai,1995). With fracture mechanics put forward and developed, it is possible to make safety assessment for welded structures (such as boiler, pressure vessels and pipeline) with defects. During assessing and analyzing process, the key problem of structure containing defects is how accurately to decide the defects parameter (including to determine orientation, size, and shape), because defects are important for the integrity of welded structure (Nichols,1979).
Welding is one of technology that is adopted to manufacture pressure vessels, but it still exists shortcomings that can't be neglected, it is difficult to avoid defects introduced during welding process. On the other hand, some defects can be introduced for the equipment in service because of the load fluctuation, material performance degradation and stress corrosion. In order to ensure security, NDT methods are commonly utilized to inspect the equipment and then carrying out safety assessment. It is obvious that NDT inspection and control technology are the basic foundation for the application of safety assessment and explosion prevented technology of boiling and pressure vessels.
The safety assessment codes for pressure vessels with defects mainly contain two types as follows:
First: the assessment codes based on KIC and COD criteria: its basic theory is KIC criterion KI=KIC corresponding fracture mechanics, which based on stress strength factor KI, and COD criterion d=dc, which based on crack tip open displacement d. During elastic range or little yield range both KIC and COD theory are approximate. With NDT technology advanced, it become possible for determining the defect size and it sets up a good foundation for the engineering application of fracture mechanics.
Secondly: assessment codes based on JIC criterion. Owing to the limitation of KIC criterion and the imprecision of COD criterion, with engineering knowledge developed, the elastic and plastic fracture mechanics theory is put forward and become perfected. Particularly with the publication of EPRI J integral engineering method, the application of defect assessment is spread and popular by using elastic and plastic fracture mechanics. Up to now, the theory of elastic and plastic is adopted in almost all of the assessment criteria and engineering application, which based on J integral theory. It reflects the idea "fitness for purpose" which leads structure integrity assessment technology to a new era.
Nowadays, all assessing criteria or codes, most of which based on the principle of "fitness for purpose". That is to say, for a certain weld of a given structure, if there are some defects, carrying out the assessment for the existent defects in term of strict theory, after considering the security to some extent it can't reach failure state, the weld is taken as "fitness for purpose".
A group of the Chinese eighth-five years project of science and technology, has investigated the safety assessment of boiler and pressure vessels in-service and the technology of explosion forecasting, compiled a safety assessment codes (third draft) for boiling and pressure vessels in-service based on many experiments and engineering safety assessment. Which referred Swedish safety assessment code and some of which adopted British PD6493:1991. It was a scientific and applied codes which follows similarity international codes and absorbed the last science and technology achievements and reflected our scientific research level and engineering experience, the J integral engineering assessment theory and two-criterion failure assessment diagram were also adopted. It can be known from which that how to determine the defect size is the basic premise of safety assessment. The detection methods include UT, Eddy current, radiographic inspection and so on.
Up to now, over the world the NDT technology of welded structure is going through a rapid change owing to the effect of technology, regulations and economic factors. The power to push the NDT technology developing is that the introduction of new materials, the advancement of inspection technology itself, the new method of welding techniques, the personnel qualification for inspection, the increasing pressure of economic benefit and manufacturing quality, and the urgent demand for prolonging structure aging life. The NDT technology related the safety assessment of boiler and pressure vessels is simply reviewed as follows (Ditchburn,1996; Cowfer,1993; Zheng Shicai,1996; Chen Jimao, 1994):
Before UT technology has been set up, X-ray imaging was the only feasible way to find buried defects in welds. The current acceptance codes of welds has been developed after discovering the inherent advantage and limitation of radiographic inspection. There are several helpful ways that can be used to record the X-ray transmitted intensity without using film. The main advantage of X-ray inspection is that it fits to automate and directly get digital images of checked structure parts. Therefore, it is easily to connect defects automatic imaging processor and analysis software with inspection assessment system.
The main limitation of X-ray imaging mainly lies in detection and measure of crack size. The other limitation lies in the safety use of X-ray and gamma radiation.
3.2 Ultrasonic testing(UT)
Ultrasonic testing has already become an important NDT technology up to now, which can be used to inspect and judge a lot of welded structures in-service such as off-shore structures, nuclear industries, pressure vessels industries and so on. During last few decades, UT technology has been developed from a purely manual operation technique to a manual operation with computer-aided, even to use automatic scanners. The automatic system can connect many piezoelectricity sensors for assessing weld. For example, in nuclear industries and pressure vessels industries the Program for Inspection Steel Components (PISC) I,II,and III have aimed to get optimal application of UT. At present, people are eager to reduce the cost of weld inspection. In modern ultrasonic system, the inspection cost has reduced with the appearance of automatic scanner, usage of multi-sensors, and the computer-aided processing, which improves the speed and safety of inspection but increases the cost of equipment and calibration. Acceptance/rejection criteria can be used in the pre-research about automatic application by artificial neural network.
3.3 Theory of Alternating Current Potential Dorp (ACPD)
ACPD theory is almost specially used to measure crack dimension, which only fits for the opening crack in the surface. When using ACPD technology, the alternating current is put on the conducting specimen and a uniform electric field is set up on the specimen surface with contacting probes. Surface opening cracks disturb the electrical current and the potential drop across the crack. Through crack and near crack using point contact technology which can measure this potential drop then calculate the depth of crack in the unflawed area, but if in which exists obvious crack close or phenomena of crack surface-bridged, the result maybe imprecise.
3.4 Technology of Infrared NDT
Technology of infrared NDT has been developed and become a sensitive detecting method, which can reduce time and workload. By using this technology we can make stress analysis and a full-field nondestructive detection, then the dangerous point of pressure vessels can be given out.
Although there are different character and advancement for the applications of NDT technology and methods, the following common problems exist and has become the focus. Mainly including reliability of NDT technology, determining the location, quantity and nature of defect exactly, and exact assessment (accept/ reject). For pressure vessels those problems are universal and representative.
4.1 Reliability of NDT
Research on the reliability of NDT was begun with the research of the relationship between defects of plane parts and fracture rule in 1960's. From the middle of 1970's, the theory of analysis NDT reliability has developed, during 1980's theory of probability of detection was put forward by using distributing parameters based on assessment of reliability data. Up to now, no theory can completely detect any defects whose size is larger than a given small value.
According to statistics, the ability of NDT system can be defined as the probability of detection, it means that the defect size can be detected is larger than a given value. According to reference (Provan, 1989) Packman was the first person to research the reliability or sensitivity of NDT and made analysis based on the theory of binomial distribution. From 1980's to now, many countries have spent a lot of money to assess the ability of different NDT system by many experiments.
Silk (Silk, 1991) obtained the curve of probability of detection, which lies in structure or on surface by analyzing and comparing with Marshall's(Marshall, 1976).
Dimitrijevic (Dimitrijevic, 1993) gained the function between POD and defect size. Three issues were used during NDT process:
After these procedures occupied, the POD value was remarkably improved and reached 0.933 even if the defect size was only 4 mm.
The researches of POD on NDT system have started in China and have made a lot of progress(Jia Yongtai, 1995; Zheng Shicai,1995; He Zheyun,1988; Liu Xiuli, 1995,1996; Chen Guohua, 1996~1999). Liu xiu-li has researched the POD curve by detecting surface crack using MT and gained function of POD and defect size under the confident level 95%, and carried out regressing analysis by two-parameter weibull, exponential and power function, respectively. At the same time, the statistic analysis method of POD was pointed out.
After completed the national eighth-five years science and technology project: "Study on UT reliability of hazardous defects of boiling and pressure vessels in service". Jia yong-tai has obtained the result that the detect ability of ultrasonic system reached 90% under 95% confident level whose length is larger than 10 mm and the depth of defect is larger than 1.0 mm. The POD/CL-h probability curve was plotted with the change of depth of defect. During the research the most important is that it was found it wrong that the defect whose depth is less than 2mm can't be detected by UT.
It was also pointed out in reference  that NDT, inspection and supervision technology began later in China. There are many problems to be solved now. For example the reliability of detection, ultrasonic measure of defect's high. So it is very urgent to research and improve the level of ultrasonic detection and supervision technology; ensure reliable safety assessment.
Although there is widely research on the exam ability of detect for NDT system and the distribution law of the defect size, the problems of the following several aspects haven't been solved yet. For instance, how to assess the availability of the NDT result in quantity, how to deal with the uncertainty of the defect parameter correctly when assessing the reliability of structures with different thickness, how to treat the defect uncertainty of structures with different shape and weld direction, and how to consider the effect of POD when determining the distribution law of defect size and so on. The author has ever tried to analyze and discuss these questions above and carried out preliminary study on the application of the fuzzy theory on the defect quality grade assessment.
4.2 Application study on the defect quantitative and intelligent technology
Defect quantitative technology is the base of safety estimation and management of boiling and pressure vessels. Only we have accurate quantity, can we accurately evaluate the safety about the structures with defects and make reasonable evaluation about vessels if it is safe to be used, maintained or rejected. The application research of defect quantitative technology has been the hot problems of NDT, and which can be focused on the following aspects: the application of quantitative digital imaging technology, the method of artificial neural network, and the analyzing theory of wavelet.
The study on the defect quantitative technology and intelligent assessing system has just begun, but the artificial intelligent has improved quite great with the development of computer technology. Pattern recognizing, neural network, and expert system have become more and more mature, the wavelet theory has also become vigorous. All of the above will bring a considerable developing potentiality to NDT technology and intelligent assessment.
4.3 Application study on the online inspection and intelligent assessment
In this field, the main studying purpose is the online technology including methods quantifying and dealing with the uncertainty of defect parameters in engineering. The direction of system safety assessment can be changed from certain to reliable, fuzzy, and intelligent assessment. And combined with new probe technology, inspection technology (especially online fault diagnose technology), and computer-aided technology, therefore a new safety assessment system of inspection, supervision can be formed and become more and more popular and applicable. So the research in the field has potential engineering application values and exciting engineering application prospect.
The reliability and quantified technology of NDT technology plays a more and more important role in the safety management of boiling and pressure vessels. Because of the increasing number of boiling and pressure vessels, it is very necessary for their safe inspection. The author tries to point out the developing tendency as follows (Lei Yinzhao, 1994; Masanta, 1996; Shong Tailei, 1997; Shi Keren. 1996; Chen Yanhua, 1998):
The reliability of NDT technology will be increased and the online quantified technology will become possible. Especially, the integrity assessing system including inspecting, quantifying, fracture mechanics analyzing, and experts knowledge application and so on will be developed also.
The digital products of NDT will become more and more important for its convenience. Because the computer hardware and software, artificial intelligent and imaging technology have provided a beneficial condition.
The application fields of NDT technology will become more and more widely in the safety of boiling and pressure vessels than it is before. It can be applied not only in defect inspection but also in creep injury and crack opening supervision.
With the development of NDT technology, it is possible to carry out the material microcosmic assessment and nondestructive assessment. In these fields its application will bring great social benefits and economic benefits.
The development of intelligent NDT technology has made it possible to establish safety evaluation methods for the pressure vessels with defect that has the knowledge-based and engineering-oriented merits. It is a very important direction to carry out the online intelligent assessment of structure reliability.
Overall, there are a lot of difficult problems about weld NDT especially in reducing the cost of inspection without injuring the integrity of structure. These difficulties can be solved by the cooperation among welding engineer and NDT experts. With the knowledge of NDT, weld producing, and fracture mechanics are the basic backgrounds to get optimal results of safety assessment.
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