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1、 Approach to TOFD inspection 1 of 6 Introduction The high performance of the Time of Flight Diffraction technique (TOFD) with regard to the detection capabilities of weld defects such as cracks, slag, lack of fusion et cetera has led to a rapidly increasing acceptance of the technique as a pre-servi
2、ce inspection tool. Since the early 1990s TOFD has been applied to several international round robin exercises, where it was validated against the more traditional radiographic and pulse echo testing. The use of TOFD has lead to major time savings during new build and replacement projects. Simultane
3、ously, the use of Tofd also gives an excellent base line inspection for future inspections. The use of Tofd has now been recognised internationally with codes and standards available for not only its use but also the availability of acceptance criteria Advantages of the Sonovation approach Because n
4、o ionising radiation is used, the inspection does not interfere with the production process. TOFD inspections can therefore in general take place in parallel to production work during day shift. There is no need for transportation of pipe-spools, vessels or other parts to an X-Ray facility; thus imp
5、roving the logistical problems caused by Radiograhy. Thickness variations do not greatly affect the inspection speed. Also very thick materials can be examined in the same fast and efficient manner TOFD examinations are possible to temperatures of more than 400C. Partly welded thick sections can be
6、examined at pre-heat temperature to avoid costly root repairs from the outside surface after completion of welding TOFD data is very accurate and is presented in an easily understood format. All data is stored for future reference and can be made available to the client in a format compatible to tha
7、t used in data bases for inspection and maintenance management. Any differences between the situation after a period in service and the newly build situation can be easily established. This forms a good basis for Life Time and Inspection Interval Extension programs. Insensitivity to defect orientati
8、on, together with encoded positioning leads to improved inspection sensitivity and accuracy, which results in optimised reproducibility. Sonovation TOFD pre-service inspections are based upon procedures developed by Sonovation in conformance with the ISO 9001 quality system. All Sonovation staff are
9、 trained in accordance with the Sonovation Written Practice, which conforms to ASNT SNT- TC-1a, ISO 9000 and where necessary to ASME XI ed. 1992 appendix 8 or ASME I and VIII, division 1 whereby several probes are moved in parallel to the weld at different distances to the weld centre line delivers
10、a reliability comparable with the Radiographic techniques. Two techniques however deliver a much higher probability of detection and reliability: TOFD and mechanised Pulse Echo Meander scanning. For the examinations performed by Sonovation the Microplus system was used. It was proven that the Mechan
11、ised Pulse Echo Meander Technique results in the highest probability of detection and TOFD delivers the lowest False Call Rate. The reliability derived from these figures is therefore only 1% different. One thing to bear in mind is, that the time required for the examination with Pulse Echo meander
12、scanning, including the subsequent analysis of the data and the reporting thereof, takes much more time than the same for the TOFD technique. As a result the costs for this type of examination will in most cases be a factor 10 higher then for Time of Flight Diffraction. The main reason for the good
13、score of the Pulse Echo Meander scan is, that the whole weld volume is examined with several angle probes (not only directed at the weld preparation) such that defects of many different orientations are detected, this in contrast with line scanning, whereby a compromise is sought between the number
14、of probes that would have to be used to look at all depths with all different angles of incidence and the number of probes that can be used within the physical and Approach to TOFD inspection 4 of 6 economical constraints. For TOFD of course, the main reason for the excellent score is the fact that
15、the existence of diffraction signals is almost independent of the orientation of the defect, such that in one line scan the whole weld volume could be examined. Price and Cost The inspection speed of TOFD is extremely high, and is hardly affected by the weld thickness. Whether a weld is 6 mm thick o
16、r 50 mm does significantly alter the time spent on one weld., because the weld can still be examined in one single pass of the probes. The manufacturer of piping welds during a project in 1995/1996 calculated that to have each weld inspected TOFD was actually cheaper than radiography for 25 mm wall thickness and over as illustrated in the following graph, comparing relative inspection prices fo
