TOFD厚壁压力容器焊接检测超声成像计算机技术.docx

1、TOFD厚壁压力容器焊接检测超声成像计算机技术TOFD - the emerging ultrasonic computerized technique, for heavy wall Pressure Vessel welds examinationTOFD 厚壁压力容器焊接检测超声成像计算机技术F. Betti, A. Guidi, B. Raffarta - NUM PIGNONE - Massa (Italy)G.Nardoni, P. Nardoni, D. Nardoni - I&T Brescia (Italy)L. Nottingham - Structural Integri

2、ty - U.S.AIndex Abstract Fundamentals of the TOFD technique TOFD norms TOFD Applications Physical principle of TOFD Configuration of system Scanning plan Scanning direction Real time acquisition and records Image evaluation Experiences on examination of welds Conclusions Bibliography AbstractThe pap

3、er presents TOFD, the Time of Flight Diffraction Technique, currently the most promising ultrasonic technique for examination of heavy wall thickness welds on Pressure Vessels TOFD, is a computerized ultrasonic system, able to scan, store, and evaluate indications in terms of height (through thickne

4、ss weld), length, position, with a degree of accuracy never achieved with other ultrasonic techniques. A record of examined sections is available in unprocessed form on floppy disk or CD and readable in Word for Windows 95. The ASME Code, with Code Case no. 2235 has accepted this method in lieu of r

5、adiography examination for thicknesses over 4 (1). Experiences on 75 mm up to 300 mm thick welds of 2.5 / 3 % Q 1 Mo, 0.25 V and Carbon steel are discussed. The technique has been applied on longitudinal, circumferential and nozzle-shell/bead welds. Scanning Plan Data and its validation are reported

6、 with used test blocks. Cladded layers in TOFD images are presented. Based on experience to date, prospects for this new technique are of great interest. Fundamentals of the TOFD techniqueThe TOFD technique was first applied in 1985 at the Harwell Center (UK) in response to insistent requests to siz

7、e cracks in nuclear reactor welds (2). The TOFD technique is a fully computerized system able to scan, store, and evaluate indications in terms of height, length, and position with a grade of accuracy never achieved by other ultrasonic techniques. The TOFD technique is based on diffraction of ultras

8、onic waves on tips of discontinuities, instead of geometrical reflection on the interface of the discontinuities. This phenomena makes TOFD ideal for identifying cracks, lack of fusion located along the vertical axis of the weld (in particular for narrow gap preparation) or with any other orientatio

9、ns, because detection is not affected by the negative consequence of ultrasonic beam deviation from the receiver due to unfavourable orientation of the discontinuity. These features have extended the use of TOFD to replace Radiography and complex Ultrasonic inspection by tandem technique wherever pl

10、anar defects (cracks, lack of fusion) are the main object of examination. The main disadvantages to both the aforementioned techniques are expensive manipulation for Radiography on big parts and the amount of time required to perform the tandem technique due to the accuracy necessary for reliable te

11、sting. TOFD overcomes both techniques in terms of speed of examination and higher accuracy.TOFD normsExperience acquired in application of TOFD has fully demonstrated its reliability and detection capacity with respect to other methods (3). This has induced potential users to establish procedures fo

12、r coding TOFD in recognized standardized norms. The British Standard Institute has issued the first draft: BS No 7706 (4) ASME Code has included in Section V, art. 4, Appendix E, the computerized ultrasonic system and TOFD technique for ultrasonic examination of welds (5). An ad hoc Workgroup has be

13、en nominated from within the ASME Technical Committee to prepare a document establishing the conditions for using computerized systems, such as TOFD in replacing Radiography examinations of welds, in the over 4 thickness range (see Code Case). CEN (comite Europeen de Normalisation) has issued the do

14、cument CEN/138/WG on practical application of TOFD for ultrasonic examination of welds (6).TOFD ApplicationsOne of the major applications of TOFD is the ultrasonic examination of welds after final heat treatment and/or hydraulic testing, to verify the absence of cracks not detectable by Radiography

15、and to prove conformity with prior ultrasonic manual examination carried out during construction. In addition to higher sensitivity, the TOFD technique allows the full examination data to be recorded on hard disk, displaying all discontinuities in C/scan images. This enables off line evaluation of i

16、ndications by computer using dedicated software. Very accurate sizing of defects can be achieved and printed for documentation. Another advantage and current application of TOFD is its use in monitoring welds during the service life of components. Stored data acquired from initial examinations, made

17、 during the final stage of construction, can be compared with new data obtained from in-service inspection. This allows the stability of existing indications to be determined with high accuracy and reliability. The more accuracy guaranteed by TOFD in sizing thruwall extention of flaws allows more re

18、liable fracture mechanic calculation for the residual life evaluation. Companies like Exxon, Shell, Fluor Daniel, Texaco, Chevron etc. are using TOFD to replace radiography in examination of welds after PVMT or hydraulic testing. The main inspection agencies have approved procedures of the TOFD Tech

19、nique and its validation.Physical principle of TOFD Fig 1: TOFD waves pattern and corresponding A-Scan presentation Fig 2: Compressional waves (left) crossing the tip of a discontinuity generate a wide beam of spherical waves (right)Fig. 3: TOFD map of the calibration on ASME test block of 185 mm (7

20、.28 inch) with three holes of 7.5 nun (1/3 inch.) diameter. The TOFD map shows the three holes with symmetrical mode conversion indication. The straight lines at left and right of the TOFD map are respectively the later wave (scanning surface) and the back-wall waves (opposite surface). The space be

21、tween these two line represents the thickness of the weld. Typical pattern of waves involved in TOFD techniqueFour different types of waves are involved in the construction of a TOFD image: longitudinal wave generated by the transmitter and partially transformed in spherical wave when the beam cross

22、 the tip of defect the lateral wave that flows on the surface between the two transducers the longitudinal wave reflected by the backwall the shear waves generated by the mode conversion L/T on the interface of discontinuities In Figure 1 TOFD waves pattern and the corresponding A-SCAN image are rep

23、resented. The Figure 2 shows how spherical waves are generated on the tips of the cracks when incident longitudinal waves cross the tip. The Figure 3 shows a typical TOFD image obtained during calibration of equipment on two cylindrical holes. The Figure 4 represents the mathematical and geometrical

24、 model on which TOFD software is based. MATHEMATIC MODEL OF TOFD Fig. 4 - On Pitagora principle is based the software tomeasure the trough thickness dimension in TOFD Technique.Configuration of system Fig 5: Block diagram of TOFD ultrasonic computerized system.Fig 5a: The TOFD system in its practica

25、l aspect. The Figure 5 represents the block diagram of TOFD system for examination of weld. The system is composed by: computer with dedicated boards (P/R, A/D) and TOFD software probes assembled on a special holder to allow scanning parallel and transversal to the welds axis Printer to print the re

26、sult Probes need special characteristics to emit short pulses to improve accuracy m Time Of Flight measurement. In case of small thicknesses high frequency can be used, 4-8 MHz. Computers featuresPortable computer , Pentium , with minimum 3 slot free for installed the boards, color screen active mat

27、rix TFT, Windows 95. Scanning planParameters affecting the application of TOFD technique are the following: PCS (Probe Center Spacing) DZ (Depth Zone) for each range of thickness an appropriate value shall be established and validated on test block Probe centerline Probe frequency Probe angle Crysta

28、l diameter Test block All the mentioned parameters, after validation, are reported in the relevant SCANNING PLAN, see an example on Figure 6. SCAN PLAN TAB.1Fig. 6 - Scan Plan indicating the main parameters value to use in relation of the thickness examined. BlockIdent Thicknessof weld Thicknesscali

29、brationblock Calibrationhole (mm) CalibrationgroveAngleBeam DZDepth Zone PCS- Probe CenterSpacingBC: BeamCenterline mm mm 0 Location depth degree mm 1 85 90 2.5 1/4 T1/2 T3/4 T 3,28 mm 52 0-90 BC = 57 mmPCS = 184 mm 2 164 170 2.5 1/4 T1/2 T3/4 T 3,28 mm 5235 0-9090-170 BC = 57 mmPCS = 184 mmBC = 125

30、 mmPCS = 175 mm Scanning directionFig 7a: - Scanning direction - parallel to the weld axis. Fig: 7b - Scanning direction - perpendicular to the weld axis to improve sizing accuracy of indication. The scanning directions for examination of welded joint has carried out moving the holder parallel to th

31、e weld axis, see Figure 7a. In case of flaws, in order to better characterize the shape and its dimension an additional scanning perpendicular to the weld axis shall be performed (Figure 7b). When transversal cracks have to be investigated a scan in 90 orientation with respect to the primary scanning shall be applied Real time acquisition and recordsFig 8: B-Scan image in real time create and visualize on PC during the scanning of 1 meter length weld and 135 mm (5.3 inch.) of thickness. All data of examination are collected in real time by the