·Home ·Table of Contents ·Methods and Instrumentation

Development of Automatic Ultrasonic testing Method Using Super High Speed Zigzag Scanning Technique Ikuji HOSHINO and Riichi MURAYAMA Corporate Research and Development Laboratories, Sumitomo Metal Industries Ltd. 1-8 Fuso-cho, Amagasaki 660-0891, Japan Phone +81-6-6489-5772 Fax +81-6-6401-9463 Kazuo FUJISAWA Technology Division Junichi NISHIO Engineering Division Sumitomo Metal Industries Ltd. 4-5-33 Kitahama Tyuuou-ku, Oosaka 541-0041, Japan Masumi SAKA Department of Mechanical Engineering, Tohoku University 01 Aza Aoba Aramaki Aoba-ku, Sendai 980-8579, Japan Contact

Abstract

We developed a high speed automatic ultrasonic inspection system for gas pipeline girth welds where newly developed angle beam array probes were used. The system consists of the array probes, a light weight mechanical scanner, a flaw detector and the signal processor. The array probe in which 32 transducer elements are arrayed 2mm apart from each other scans electrically ultrasonic beam in the direction parallel to the weld. In combination with mechanical scanning, the system with two array probes inspects the whole volume of weld of a pipe 762mm in diameter and 19 mm in thickness within 2 minutes. The inspection time was 10 times shorter than that of the conventional probe system. The array probe system had the same detectability as that of conventional angle beam probe system.

1. Introduction

X-ray radiographic testing technique has been applied to the final inspection of gas pipeline girth welds in Japan. However, it is not easy to handle this technique because controlled area must be set for the safety, and the examiner must insert the X-ray source in the pipeline to the position of the weld in the case of internal source technique. It also takes a long time for the preparations described above, taking radiography and processing films.

Therefore, many automatic ultrasonic inspection systems for the pipeline girth weld have been developed, which improved the recordability of the examination data compared with manual inspections.^[1] The conventional automatic ultrasonic system scans the angle beam ultrasonic probes in directions parallel and normal to the weld so that whole volume of weld may be inspected. However, it takes nearly 20 minutes to scan the inspected area of a pipe of 762mm in diameter and 19 mm in thickness, when 2mm scan interval is adopted.

First we developed a ultrasonic angle beam array probe which has the same performance (such as detectability and beam profiles) as the conventional angle beam probe that was usually used in the weld inspection in Japan. And then, we made a super high speed automatic scanning system with the array probes.^[2] The present paper describes performances of the array probe in comparison with the conventional probe and detectability of flaws in the pipeline girth weld by using the super high speed automatic scanning system.

2. Concept of high speed zigzag scanning inspection method

The concept of the high-speed zigzag scanning inspection method is shown in Fig.1. The probe has small transducer elements of N in number which are arrayed in the direction parallel to the weld (circumferential direction). First, elements of No.1 to I are electronically selected for transmitting and receiving ultrasonic beam. Next No.2 to I+1, then No3 to I+2 are electronically selected. At last, No(N-I+1) to N are selected. When an interval of element is s mm, the whole width scanned electrically in the sequence is (N-I+1) x s mm. While the sequences are made repeatedly, the array probe is mechanically scanned in the direction normal to the weld (axis direction). One axial scan of the array probe corresponds to N-I+1 axial scans of s mm intervals made by the conventional probe of I x s mm in width. Therefore mechanical scan intervals may be set as (N-I+1) x s mm. Next axial scan is made in reverse direction of previous axial scan. This zigzag scanning makes it possible to inspect whole volume of weld much faster than the scanning with conventional probe.

Fig 1: Concept of high speed zigzag scanning

3. High speed zigzag scan ultrasonic inspection system

3.1 Development of high speed zigzag scan ultrasonic array probe
In order to develop a high speed zigzag scanning array probe, transducer material and shape, an interval between elements and a method of dividing elements acoustically are optimized. Since the data sampling intervals were determined as 2mm in both directions from other investigations, it was determined that 1.9mm wide elements were arrayed with 0.1mm spaces. Lead Titanate PbTiO₃ is selected as transducer material. The transducer height and frequency were determined to be 10mm and 5MHz respectively. Refraction angle was 65 degrees. These were determined since the usually used conventional probe is 10mm x 10mm in transducer size, 5MHz in frequency and 65 degrees in refraction angels.

As for the method of dividing elements acoustically, three methods were compared. In the first method, both transducer and electorode were cut to elements. In the second method, electrode and a part of transducer were cut. In the third method, only electrode was cut. Pulsers simultaneously drive five elements of the probe and the signals simultaneously received by the same elements were summed to form echo signal. Echoes from an artificial vertical drilled hole of 4mm in diameter and 4mm in depth of a 15mm thick Japanese Industrial Standard Test Block A2 were investigated. Echo heights and signal to noise ratios from the drilled holes at 1 skip distance are shown in Fig.2. Ultrasonic beam profiles are shown in Fig.3.

Fig 2: Influence of dividing method of elements

Fig 3: Ultrasonic beam profiles in three dividing methods

The method of cutting electrode shows 3.5dB higher echo height and 5dB higher signal to noise ratio than the other two methods. The beam profiles in three methods are almost same. Therefore, we finally selected the method of cutting only electrode to use.

In order to increase the scanning speed, it is better to increase the number of elements, as known from the explanation in Chapter 2. However the increase of elements makes the probe width large, which induces the difficulty in acoustic coupling between the probe and the pipe. Therefore we decide to make 32ch elements probes. The surfaces of angle beam wedge were machined to the same radius of the pipe of 762mm in diameter in order to make acoustic coupling stable. The curved transducer elements was bonded to another curved surface of wedge so that each transducer element had the same beam path distance in the wedge.

3.2 Transmitting and receiving condition in array probe
Finally we decided to use 6 elements for transmitting and receiving ultrasonic beam because the ultrasonic beam profile could be adjusted the same as that of the conventional angle beam probe of 10mm x 10mm in transducer size.

Echoes from vertical drilled holes of 4mm in diameter and 4mm in depth machined at outer and inner surfaces of the pipe were investigated by changing focus distance of the probe. Figure 4 shows the measured echo height from the outer and inner drilled holes, when we changed the focus distance by changing delays among elements. The transmitter focus distance was set as same as the receiver one. The echo height from the inner flaw became the highest when focus distance was set to be 66mm. The echo height decreased gradually when focus distance was set longer, and is only 4 dB lower when it was set to be 200mm. The echo height from the outer defect showed almost same value when focus distance was set longer than 100mm. Therefore we set focus distance to be 100mm or more.

Fig 4: Relation between focusing distance and echo height

Ultrasonic beam profiles were measured by scanning the probe in axial and circumferential directions at focus distances of 200mm The results are shown in Fig.5 with the results obtained by using the conventional probe. The array probes have almost same beam profiles as conventional one. Therefore, we confirmed that we could use the array probe instead of the conventional probe.

Fig 5: Ultrasonic beam profiles of phased array linear transducer and conventional transducer

3.3 Outline of the new automatic inspection system
The block diagram of the high-speed zigzag scanning ultrasonic inspection system is shown in Fig. 6. A light weight scanning device with the array probes is easily set to a guide rail installed around the pipe. The guide rail which can be separated into two semicircular parts is also easily set to the pipe. The weld is inspected simultaneously from both sides by scanning the two array probes in zigzag pattern described in Chapter 2.

The whole wave forms of logarithmic amplified signals within monitor gate are sampled at 2mm intervals in circumferential and axial directions. The wave forms are stored with the probes positions in a hard disc to produce B and C scope images .

Fig 6: Block diagram of automatic ultrasonic inspection system using high speed zigzag scanning

4. Experimental results

Detectability of the developed array probes was compared with that of the conventional probes by using the high-speed zigzag scanning ultrasonic inspection system. In the case of the conventional probes, one channel receiver and transmitter for the array probe of the system was used. Vertical and side drilled holes and notches were machined in the weld of a pipe. The comparison results in the ultrasonic flaw lengths and heights are shown in Fig.7. The abscissa shows the artificial flaw type and threshold level used in the measurement of length and height. The differences between the measured values were less than 2mm. Therefore it was confirmed that the array probe has the same detectability as the conventional one. The array probe system required only two minutes to inspect a whole weld of the 762mm diameter pipe, which is 10 times faster than that with the conventional probe system.

(a)Flaw length	(b)Flaw height
Fig 7: Detectability of artificial flaws

5. Conclusions

1. The high speed zigzag scanning ultrasonic inspection system which used 32ch array probes was developed for the girth weld inspection of the gas pipe line of 762mm in diameter.
2. The developed probe has curved transducer elements bonded to a curved wedge which make the probe scanning smooth with stable acoustic coupling with the pipe.
3. By using combinations of electrical scans in the circumferential direction made by the array probe and mechanical scans in the circumferential direction by the light weight scanner, the system inspects the whole volume of weld within two minutes. The inspection speed is 10 times faster than that with the conventional probe system.
4. The beam patterns and flaw evaluations of the array probe were almost same as those of the conventional probe.

Acknowledgement

This development was made as a part of "the trunk line pipe technique development project" in the Japan Gas Society which was supported by the Ministry of International Ttrade and Industries of Japan. The authors acknowledge the members related to the project who gave us valuable opinions and advice for the project.

References

1. T.Masuda, K.Ito, H.Miyata, K.Tsuchiya and T.Yahata "Development of Automatic Ultrasonic Inspection System for Large Diameter Pipeline" Symposium of Ultrasonic Non-destructive Evaluation of Japanese Society for Non-destructive Inspection, pp.183-190,1995
2. T.Masuda and K.Itou "Study on the Ultrasonic Testing Techniques Using Phased Array Method" Proceedings of Japanese Society for Non-destructive Inspection Fall Conference 1995 pp. 385-392

© AIPnD , created by NDT.net

|Home|    |Top|