Inspection of Welds in Small Pipes by NDT Ultrasonic Phased-Array techniques. Detection, positioning, sizing and classification of defects.

Transcription

1 More info about this article: Inspection of Welds in Small Pipes by NDT Ultrasonic Phased-Array techniques. Detection, positioning, sizing and classification of defects. J. Amador Sillero 1, Francisco J. Fernández 1 and Francisco A. Godínez 1 1 Tecnatom S.A., Spain, jasillero@tecnatom.es Abstract The ASME Case Code N-659-2, related to Section III, permit to replace the radiographic inspection of welds by an ultrasonic inspection under some specific conditions. For the case of circumferential welds in small diameter pipes, Tecnatom has developed an inspection technique that uses the phased-array technology that allows to save the time of inspection and the dose to the personnel that perform this work in nuclear facilities. The aim of this paper is to show the development carried out for the detection of parallel and perpendicular defects in circumferential welds of pipes using Phased-Array Ultrasonic Testing (PAUT). To this end, different inspection techniques have been developed in a limited series of reference blocks with realistic defects, while qualification of the inspection procedure and personnel have been performed in a larger sample of blocks with the postulated defectology. The scope of the project covers the inspection of welds in carbon steel pipes with diameters ranging from 50 to 100 mm, wall thickness between 5 and 8 mm, access to welding from one or two sides and severe restriction on the free height between components. The postulated defects are lack of fusion, incomplete penetration, cracks and porosity. Tecnatom have defined the mechanical system and the electronic equipment suitable for the execution of the inspection, making the necessary improvements and adaptations to meet the requirements. During the project, several methodologies for axial and circumferential inspection based on pulse-echo and emission-reception techniques have been developed, configuring the acquisition electronics with different types of sectorial scans and transmitting the ultrasonic beams to the material through specific developed wedges. The inspection procedure has been qualified using the ENIQ type methodology, based on a technical justification by means of theoretical and experimental evidence together with the practical demonstration to meet the validation criteria defined in the reference normative.

2 t t 1. Introduction The ASME Code publishes the Case Code N that describes the necessary conditions for the ultrasonic volumetric inspection of the circumferential welds of pipes previously required by radiographs by the departments NB-5200, NC-5200, ND-5200, WB and WC This solution has multiple advantages such as the reduction of the inspection time, the reduction of the dose received by the personnel, the obtaining of records to be evaluated by different personnel, the greater sensitivity of the inspection technique... To apply the volumetric inspection by ultrasounds, the main requirements of the Case Code N are: The inspection must be carried out with an automatic data acquisition system. An ultrasonic record must be generated. 100% of the weld volume + 13 mm must be covered on each side of the weld. Inspections must be carried out in the four directions (2 perpendicular + 2 parallel). Nominal angles of 45º and 60º or differences of more than 10º must be used. A written procedure is required and demonstrated in blocks. Calibration blocks must be of equivalent material (composition and metallurgical structure). The blocks for the technique validation must contain flat and volumetric defects, superficial and embedded, parallel and perpendicular to the welding line. The personnel for the acquisition and analysis of the results must be trained and qualified. To solve this inspection in the case of pipes of small diameter and thickness, Tecnatom has developed an own procedure using Phased-Array Ultrasound technology in different configurations. 2. Specimen geometry The scope of development of Tecnatom covers the inspection of the weld plus the 13 mm close to it, which includes the Heat Affected Zone (HAZ), in carbon steel pipes with diameters between 50 and 100 mm and thickness of wall between 5 and 8 mm. It has been considered circumferential butt welds with a generic profile in "U" shape and without removing the over-thickness of the weld bead (crown and root). Inspection Volume Heat Affected Zone 12, ,7 Inspection Volume ID OD Heat Affected Zone OD ID Figure 1.- General configuration of the welding with access from one or two sides 2

3 It has been considered the possibility of having only one side of the weld accessible or both, and the necessary free space around the pipe for the installation of the equipment has been reduced to a minimum, with only 38 mm of free height being necessary. 3. Defectology The postulated defectology is summarized in Table 1: Table 1.- Main characteristics of the postulated defectology. Orientation Type of defect Position Defectology Circumferential (Parallel to WCL) Axial (Normal to WCL) Lack of fusion Incomplete root penetration Cracks Porosity Open to the inner surface (ID) Open to the outter surface (OD) Embedded In the weld zone and HAZ. Example For the demonstration of the procedure, a set of 47 blocks with realistic defects representative of the postulated defectology with a minimum size of 1.5x5 mm have been designed and manufactured. Figure 2 shows some examples of the defects implanted: Embedded lack of fusion ID crack ID incomplete root penetration Embedded crack 3

4 Embedded porosity ID crack in HAZ OD lackl of fusion 4. Inspection system Transversal ID crack Figure 2.- Example of defectology introduced in the demonstration blocks. The inspection system consists mainly in a phased-array ultrasonic electronics, a semiautomatic mechanical equipment for the movement and registration of the position of the probes and a PC with the acquisition and evaluation SW. A general outline of it is presented in Figure 3. Linear phased-array probes of 7.5 MHz and 16 elements have been used, with a lowprofile housing to avoid possible access restrictions and shoes designed and manufactured ex profeso for the techniques developed with the shaped contact surface to adapt them to the diameter of the pipes to be inspected. Figure 3.- Lay out of the inspection system and mechanical equipment used 5. Inspection technique Two different inspection techniques based on pulse-echo have been developed based in the generation of sectorial scans. For perpendicular inspection, an ultrasonic beam is generated perpendicular to the welding line, while an oblique beam is generated for parallel inspection. 4

5 Both techniques were developed based on SW of ultrasound simulation, being optimized over reference block sets with artificial reflectors type notch and holes, and demonstrated on blocks with realistic defects representative of the postulated defectology. This process is shown in Figure 4: Perpendicular inspection Parallel inspection Demonstration with realistic defects Adjusting with artificial reflectors Simulation Technique definition 6. Acceptance criteria Figure 4.- Development of the inspection techniques The following acceptance criteria for the demonstration of the procedure are defined according to the normative: 5

6 Detection: the procedure demonstration must show successfully that all indications of defects produce an amplitude response greater than 20% FSH of the reference level. Sizing in length: the mean square error (RMS) of the lengths of the indications measured by PAUT, in comparison with the true lengths, must not exceed 6.4 mm. In this case, the RMS is defined as: Where: mi = length of the measured defect. ti = defect length in plane n = number of defects. = The characterization of defects was performed according to the ISO The classification of the defects is made through the successive application of the following discrimination criteria: Echo amplitude: an indication with an echo amplitude that is at least equal to the reference level plus 6 db is characterized as flat. Directional reflectivity: an indication whose difference in echo amplitudes from two different directions is at least 9 db is characterized as flat whenever the greater amplitude is greater than the reference level minus 6 db. Static pattern of the echo: the static pattern of the indication is compared with that obtained in the reference reflector, if the static pattern of the echo is simple and smooth, the indication is classified as not flat. Dynamic echo pattern: if a single echo occurs but irregular that moving the probe produces significant amplitude fluctuations or if the signal has a set of signals to move the probe traveling within the pulse envelope up to its own maximum towards the center of the envelope and then decrease, the indication is classified as a large and rough reflector. 7. Results Through the experimental evidences obtained with the inspection of the demonstration blocks with realistic defects, it has been demonstrated the adequacy of the techniques developed to the initial requirements: Detection: all defects included in the blocks are detected from both sides of the weld. Positioning: the position in X (perimeter), Y (welding side) and Z (depth) of the indications is determined appropriately. Dimensioning: the length of the indications is determined in an appropriate way. Classifications: all defects were classified adequately according to the criteria. 6

7 In this way, the inspection techniques developed by Tecnatom with the described characteristics demonstrate its capacity to meet the objectives established by the regulations. In the Figure 5 some representative examples of the results obtained in the inspection of blocks with the different postulated defectology are shown. Figure 5.- Example of detection of defects in reference blocks The values of the medium deviations in positioning and sizing the defects in the reference blocks are shown in Table 2: Table 2.- Result of the technique validation Parameter Perpendicular inspection Paralell inspection Positioning in X direction RMS: 3,2 mm RMS: 6,2 mm Positioning in Y direction RMS: 1,4 mm RMS: 1,0 mm Positioning in depth RMS: 0,6 mm RMS: 0,5 mm Sizing in length RMS: 2,1 mm RMS: 0,9 mm Minimun Amplitude FSH: 23,3% FSH: 55,6% 8. Conclusions Tecnatom has defined an inspection system and has developed different techniques based on phased-array ultrasonic technology for the inspection of circumferential welds in small diameter pipes according to the requirements of the ASME Code Case N It has been detected and characterized the representative defects in the demonstration blocks meeting the initial requirements and the validation criteria for the technique: porosity, lack of fusion, lack of penetration and cracks oriented in a perpendicular and parallel direction to the weld center line. These indications have been positioned along 7

8 the weld and in depth, also determining the length of the defects and the side of the weld center line in which they are within the tolerances required by the regulations. This development allows the substitution of the inspection established by X-ray in the ASME Code for one based on ultrasound, complying with the requirements set out in the Code Case N-659-2, allowing the improvement in the detection of defects and reducing inspection time and the dose to the personnel that carries it out. Acknowledgements The inspection techniques shown in this works was supported and co-developed by Nucleoeléctrica Argentina S.A.-Servicios para Centrales (NASA-SPC). In special, we would like to thank next people for their participation during the project: - Mauro Mattioli. - Sebastián Gómez Loberza. - Mariano Ferrucci. - Germán Alvarez References 1. ASME BPVC, Section V, Article ASME BPVC, Section XI, Appendix VIII. 3. ASME BPVC, Case N (08): Use of Ultrasonic Examination in Lieu of Radiography for Weld Examination (Section III, Divisions 1 and 3). 4. ISO-23279: Non-destructive testing of welds -- Ultrasonic testing -- Characterization of indications in welds. 8