Phased Array UT Application For Boiler Tube Inspection in

Phased Array UT Application For Boiler Tube Inspection in Manufacturing And In-Service

Anandamurugan S1, Siva Sankar Y2

1GE Inspection Technologies, JFWTC, Bangalore, India

2GE Inspection Technologies, JFWTC, Bangalore, India

[email protected],

[email protected]

Keywords: Phased Array UT, Boiler Tube, Sector Scan, Weld Inspection, Tube Inspection, Low Profile Tube Inspection Abstract.

In Power industry, boiler tube weld quality and integrity are critical to safety in wide range of structures. Typical boiler tubes thickness range varies between 4 mm to 12 mm and diameter will be between 38 mm to 118 mm. Welding will be carried out on the boiler tubes in order to achieve the required shape and necessity. The major failure in these welded regions is Tube leakage. Typical thermal power plant need to inspect minimum 10% of overall weld joints per maintenance shutdown. For example, 38,000 (approx.) weld joints will exist for a typical 500 MW unit. Radiographic Inspection (RT) is one of the major NDT techniques which are used for such application. But RT has its own limitations like radiation hazard, film processing with chemical, poor accessibility due to close proximity, poor detection of planar defects, time consuming. Conventional UT (A-scan) is an alternative for this application without radiation hazard but it doesn’t record image or archival.

Phased Array UT (PAUT) is another best option to inspect the tube weld. It has an advantage of good Probability of Detection (POD), image recording with 3-dimensional data on Top, Side and End Views with overlay for easy decision making. Though the technique is powerful and efficient to inspect welds, there is a challenge on mechanics to fulfill the fit for need of low profile dimensions ranges from 30 mm to 90 mm of outer diameter with the space between tubes < 12.7 mm. Complexity in preparing the PAUT scan plan to ensure the weld coverage for this application is another challenge. In this paper it will be discussed about the complete solution of boiler tube weld inspection using Phased Array UT & mechanics. How the design of low profile probes & wedges will provide the greater accessibility to inspect the tubes lying closer to each other. Study has been made on 10 different types of weld defects and compared the results with conventional and digital radiography against PAUT to assess the POD.

1. Introduction

Welding is an essential manufacturing process performed in almost every major industry. Hence, the weld quality and integrity are critical to safety. In order to make sure about the quality of welds, it is important to inspect the welds during manufacturing which helps in reducing the cost of rework and extending the life of components by detecting and rectifying the flaws. Several NDT techniques can identify different kind of defects such as porosity, incomplete penetration, inclusion, cracks and lack of fusion which compromise the weld strength.

Mor

e In

fo a

t Ope

n A

cces

s D

atab

ase

ww

w.n

dt.n

et/?

id=

1515

6

mailto:[email protected]

Boiler tubes are one of the significant components in boilers for heat transfer. Welding will be carried out on boilers in order to achieve the required shape and necessity. There are number of NDT techniques for the inspection of boiler tubes [1]. Major inspection of such kind of welds will be carried out using Radiography [2]. Besides the general radiographic constraints such the radiation hazard, chemical usage etc., the main constraint of this technique on boiler tube inspection is difficult to inspect because of their close proximity and poor detection of planar defects. Another technique is conventional UT which will not have space constraint issue, but doesn’t give the image or storing of recorded data [3].

Recent advancements in electronics from analog to digital made Phased Array Ultrasonic Testing [4] as one of best option for inspecting the tube welds. This technique has the advantage of good image recording capability, probability of detection and most interestingly the data can be demonstrated in 3-dimensional planes of Top, Side and End Views.

2. Circular Palm Scanner

Though Phased array ultrasonic testing is powerful and efficient to inspect welds, there is a challenge on mechanics to fulfill the fit for need of low profile dimensions ranges from 30 mm to 90 mm of outer diameter. Alstom Inspection Robotics’ innovations have come up with different kind of scanners for the inspection of circumferential welds on pipes and tubes. The so-called Handheld Circular Palm Scanner [Fig. 1 and 2] allows the inspector a faster and more precise flaw detection using phased array ultrasonic technology with encoder feedback. As a consequence, butt weld inspections can be handled in a concurrent and repeatable manner with a comparable documentation of the inspection result. Its major value is that areas no longer have to be evacuated due to harmful radiation.

Fig1: Circular Palm Scanner on the field

The Circular Palm Scanner is used for tube weld inspection on confined spaces employing

phased array ultrasonic technology. The various types of scanners in the Handheld Scanner Series are geared to specific geometric constraints, and thus provide targeted support appropriate to the specific inspection situation. The Handheld Scanner Series features a range of manual scanners with integrated position sensors that provide reliable position feedback of the manual movements. As a consequence, manual inspections can be carried out with significantly higher precision and reproducibility compared to classical manual inspection procedures. Alstom scanner is compact, rigid and lightweight scanner. This scanner has easy access between tube gaps as narrow as 12.7 mm (0.5”).

Corutesy: Alstom

Fig2: Skeleton of Circular Palm Scanner (Left); Dual Side Scanner (Right)

The Circular Palm Scanner allows a 360° inspection with faster, more precise and reliable

flaw detection and sizing. Butt weld inspections are exercised in a concurrent and repeatable manner. They are continuously documented to fulfill quality assurance needs and to allow for a comparison of flaw detection over a determined period of time. By replacing classic radiography technology by phased array ultrasonic technique, the area of inspection has no longer to be evacuated prior to the inspection process; butt weld inspections can now be processed parallel to other activities. Thus, scheduled downtimes during maintenance work may be kept as brief as possible.

3. Setup

Before making the setup, the inspection plan was created by Level III operator using desktop software called ‘Inspection Plan Creator’, which helps the user to make sure about the full coverage of the weld [Fig. 3]

Fig3: Make sure of weld coverage using ray tracing

Phased array ultrasonic Inspection has been conducted on different specimens with ten different indications which were induced artificially. Four tubes of 48.5 mm OD x 5 mm thickness mild steel weld specimens were considered for the experimentation. The following table gives the list of defects induced in the four tube specimens.

Corutesy: Alstom

Table1: Tube Specimen with known defects

All the four specimens were manufactured by GTAW process and defect lengths are defined as per ASME rejection criteria. Calibration blocks [Fig.4] were defined with same dimensions as that of specimen and EDM notches were introduced on inner and outer radius of the block.

Fig4: Typical calibration block diagram with notches

By taking the thickness of the specimen into consideration we have selected the low profile probe of frequency 10 MHz with 32 elements and pitch as 0.3 mm and specially designed radial wedges were used for the experimentation [Fig. 5]. GE Inspection Technologies has developed wide range of probes and wedges to inspect various dimensional tubes.

Fig5: Low profile probe and wedge setup

Sonaspection # GE # Flaw # Flaw Type Length Height

4825-01 GE-8

1 Centerline Crack - 2 20 3

2 Slag 20 3

3 Root Crack -2 20 3

4825-02 GE-5

1 Toe Crack 15 2

2 Porosity 25 3

3 Inc. Root Pen. 15 1

4825-03 GE-6

1 L.O.S.W.F 20 3

2 Root Over Pen. 15 3

3 Centerline Crack - 1 20 3

4825-04 GE-7

1 Root Crack -1 20 2

2 S.C.C 20 4

3 Burn Through 10 N.A.

Inspection plan is created and validated using desktop application called ‘Inspection Plan Creator’, which helps in positioning the probe at right distance from the weld center line for entire coverage of the weld. Validated inspection plan is imported onto instrument. USM Vision along with Alstom scanner is used for data acquisition [Fig.6]. As we are using low profile wedges for boiler tube continuous water supply is provided to have proper contact with the specimen throughout the inspection. Scan position of the probe from weld center line helps the operator to place the probe at right position to cover the entire weld region [Fig.6]. Acquired data is reviewed through Rhythm UT software which helps in data interpretation.

Fig6: Experimental setup of USM Vision connected to Alstom scanner

4. Results 4.1 Experimental Results

The study has been done with Conventional A-scan UT, Phased Array UT, and Radiography using Digital X-ray. Digital X-ray is captured with 2 shots of 90° views. During In-service 90° is highly impossible due to the accessibility issue of source & film placement. All the three methods (Conventional, PAUT and digital radiography) results were compared. 4.1.1 PAUT Results

Ten different types of defects were introduced into four tubes and the experiments were carried out low profile probe of 10 MHz frequency. USM Vision is used for performing phased array ultrasonic testing which provides the volume corrected images along with weld overlay that helps in exact location of the defects. For PAUT in order to have high probability of detection phased array sector scan is taken into consideration and the sector range is from 40° to 70°. Tube 1 (4825_01) has three different induced defects such as Center Line crack, Slag and Root Crack at different locations on the specimen [Fig. 7].

Fig7: Schematic diagram of 4825_01 (Top) and the respective phased array image represented

by Top, Side and Frame views (Bottom)

All the three defects were easily captured because of the nature and orientation of these defects. Defect sizing was done using 6dB drop method and the defect lengths are approximately in the same order of induced defect lengths. Experiments and defect analysis is continued on all other three tube specimens also.

1 2

3

Tube Name: 4825_01 1 – Center Line Crack 2 – Slag 3 – Root Crack

Tube Name: 4825_02 1 – Toe Crack 2 – Porosity 3 – Inc. Root Penetration

Center Line Crack Top & Bottom tips





1 2

3

Tube Name: 4825_03 1 – L.O.S.W.F 2 – Root Over Pen. 3 – Center line crack

1

3

Inc. Root Penetration

Center Line Crack


by Top, Side and Frame views (Bottom) Display of overlay on the frame view will help the operator to locate and predict the

defect type. Frame view is displayed as angle corrected view, where all the rays displayed in straight lines, so that the reflectors will get repeated. The volume corrected top view overlaps all the reflections from the frame view and display as single reflector.

PAUT inspection and the images evaluated by ASNT UT Level III. PAUT has shown a

great POD of all the defects were detected. Below graph[Fig. 11] will depict the overall comparison chart about the POD of different type of weld defects. The sensitivity is calibrated using the tube sample of same diameter and thickness ID & OD notch of 10 mm wide x 0.5 mm height. Couplant should contain no ingredients which will harm the test piece. Because of low profile wedges continuous water supply is provided to have proper contact with the specimen throughout the inspection.

Phased array inspection of tubes missed few defects like root over penetration in tube 3(4825_03) because of not much variation in thickness and S.C.C in tube 4(4825_04) due to branching and Porosity is detectable with low sensitivity in tube 2(4825_02) because of beam scattering. But it helped in detecting the critical defects like incomplete root penetration, lack of side wall of fusion and cracks with high sensitivity.

Tube Name: 4825_04 1 – Root Crack 2 – Stress Corrosion Crack 3 – Burn Through

1 2

3

Root Crack

Fig11: PAUT results (Defect Type on X-axis versus Defect Length on Y-axis)

RT Results The radiography inspection is carried out by ASNT RT Level II inspector and the images

evaluated by ASNT RT Level III. Defects are rated based on the ASME VIII Div.2 acceptance/rejection criteria. Based on the results it’s surprisingly came out that: Detection of 50% indications on Ir-192 Detection of 70% on Film X-ray Detection of 60% on Digital X-ray

Fig12: RT results (Defect Type on X-axis versus Defect Length on Y-axis)

4.2 In-Situ Boiler Tube Weld Inspection Results Field trails were done at various industries worldwide. Trails were done on lots of in-site

boiler tube weld joints and the results were compared with Radiographic results. It’s been observed that TCG corrected 6dB drop method gave better results of length sizing. Few defects like Toe-crack, Centre line crack and lack of side wall fusion were noted as oversized because of beam spread.

Fig13: Field trails of Boiler tube Inspection

5. Conclusions

This paper concludes that Phased Array Ultrasonic Testing (PAUT) will provide better detectability of boiler tube defects. Volumetric inspection of single weld after setup can be completed in 20 sec approximately. Inspection can be done during welding. Alstom scanner is compact, rigid and light weight which makes the easy access between gaps as narrow as 12 mm. Minor weld defects can be detected early enough and repairs may be made quickly and more efficiently than with radiographic testing.

Detection of defects like porosity is a challenge because of the defect nature. Porosity reflections are low sensitive in nature because of beam scattering. One another challenge is the inspection of water wall panels because of 180° access. This solution is best suited for the tubes with 360° access and proximity > 12.7 mm. R&D work is going on both probes and scanners to efficiently inspect the water wall panels.

References

[1] H M Sadek, NDE Technologies for the examination of heat exchangers and boiler tubes – principles, advantages and limitations, Insight, Vol. 48, 2006, pp. 181-183. [2] R.J.Pardikar, Digital radiography and Computed radiography for Enhancing the Quality and Productivity of Weldments in Boiler components, WCNDT-2008. [3] J. Krautkramer, H. Krautkramer, Ultrasonic Testing of Materials, 1983. [4] Laurent Enenkel, Johannes Buechler, Jerome Poirier, David Jervis, A Portable Solution to Enable Guided Ultrasonic Inspection, WCNDT-2012.

Documents

Phased Array UT Application For Boiler Tube Inspection in