PAUT based Techniques Developed for Power Industries and Refinery in

Taiwan

Yung-How Wu

1, Chu-Chung Hsiao

2

MCL, Industrial Technology Research Institute Rm 827, Bldg. 52, No. 195, Sec. 4, Chung Hsing Rd., Chutung, Hsinchu, Taiwan 31040, R.O.C.

886-(0)933792939; yhwu01@itri.org.tw 886- (0)922956668; cch01@itri.org.tw,

ABSTRACT

Although conventional ultrasonic method is still widely used elsewhere, PAUT technique is applied increasingly

for improving the ISI of utility, refinery and as well the emerging wind power in Taiwan. PAUT together with

scanner is one of the major advantages to precede conventional UT by enhancing the defect characterization or

inspection speed. In this study, special scanners were designed and employed with PAUT for the inspection of

composite blades, bolts of wind turbine and anchor bolts of nuclear RPV skirt. Results showed that delamination

of composite and fatigue crack of bolt may be detected and sized in more confidence and convenience for in-situ

inspections. Moreover, PAUT was also used to characterize carefully the inner crack of pressure vessel for FFS

assessment. Crack was found in safe margin and hence remained to operate. In conclusions, PAUT together with

manipulator designed for specific purpose may facilitate the applications of PAUT technique widely in ISI of

various industrial sectors.

Keywords: Phased Array, Ultrasonic, Wind turbine, Blade, Bolt, Refinery, FFS

INTRODUCTION

Though phased array ultrasonic technique (PAUT) has been well known and commonly used in medical field, it

is just used increasingly in industrial sectors as energy, transportation and materials. Most NDE technicians now

a day are still using conventional single element ultrasonic method in manufacturing or in service inspection due

to the higher cost and skill required by PAUT technique. However, the demand of fast transportation and low

energy consumption vehicles and structures is driving the application and evolution of advanced digital NDT

methods such as PAUT. Better inspection result may be achieved to meet the more stringent quantitative

requirements for better O&M by FFS for example. It therefore generates a cycle for the continuing evolution

and challenge of various NDT disciplines as shown in Figure 1. To meet more stringent application demands,

materials of higher strength and thinner section are pursued by the mechanical engineers that smaller defects are

then required to find and measure by NDT technicians with advanced systems or techniques. Inspectors with

better training, experience and education is necessary and in turns, to assure faster transportation and as well

lighter or safer structures.

Figure 1: The correlation and challenge of NDT related disciplines

As known by NDE professionals, automation or mechanized NDT is a good way to minimize human error and

to increase accuracy of the inspection. Thanks to the fast growing of microelectronic and processing techniques,

NDE methods especially as UT and ET are used together with manipulator by inspectors more than ever today.

To develop automated digital NDT methods for improving the inspection performance, various disciplines

should be considered as shown in Figure 2.

Figure 2: Various disciplines to consider in automated digital NDT methods.

AUTOMATED TECHNIQUES FOR ABWR RIP WELD

The Taipower advanced BWR under construction was developed by working group of GE, Nippon Tokyo

Electric, Hitachi and Toshiba. One of major differences between ABWR and conventional BWR is the ten units

of RIP (Reactor Internal Pump) designed for independent cooling water circulation. Each RIP was connected to

the RPV by a circumferential weld located deep inside narrow gap of 18mm as in Figure 3. Moreover to

alleviate radiation exposure during ISI, remote operation NDE method and scanner is required for the inspection

of RIP weld right under RPV. Figure 4 shows a remote controlled inspection system including a special designed

scanner, PAUT probes and EC FGI method together with camera monitoring was developed to inspect

circumferential dissimilar weld in the gap.

Figure 3: ABWR RIP location and the weld inside the gap of 18mm

Figure 4: Remote operation PAUT/ECFGI system and accessories for RIP weld inspection

The PAUT or array EC probes designed for this application were limited in dimension by the narrow gap and of

17mm in maximum thickness. And for inspecting nickel based dissimilar metal weld, frequency less than

1.5MHz was chosen. Both 1D 16 elt and 2D 30elt PAUT probes were made for detecting and sizing

circumferential or axial cracks on RIP reference sample. Two probes were set on probe module to cover the area

generally specified by ASME code. Moreover, EC FGI technique based on current perturbation principle was

also incorporated in this RIP inspection method for detecting and sizing open crack on outer surface of weld and

HAZ. Both techniques proved satisfactory in crack detection and sizing on sample block as shown in Figure 5.

(a) (b)

(c) (d)

Figure 5: (a) RIP sample block with artificial cracks, (b) PAUT result of circ. crack-F4 and axial crack-F2,

(c) arrangement of EC FGI or PAUT probe relative to the weld and defects and (d) EC FGI result of

surface cracks F2, F3 and F4.

Probe module & RIP sample block

ECFGI system & array probes

PAUT Probe

Axial cracks Circ. crack

ECFGI coils

WIND TURBINE BLADE INSPECTION

Composite blade, tower and turbine are three main structural components of wind turbines. Statistics showed

that downtime ratio due to wind turbine blade damage was generally more than 30 percent and the impact to the

operating cost of wind power had been great. Even worse, huge wind blades, once broken, may drop a few

hundred meters around endangering life and property safety. Failure of wind turbines and even bearing, anchor

bolts of wind tower did occur in several locations in Taiwan as shown in Figure 6.

Figure 6: Failure of wind turbine blades, bearing and anchor bolts in different locations in Taiwan.

Wind turbine blades are normally made with GFRP or CFRP and fixed to the wind turbine hub with bolts of

annular distribution. Internal delamination or damage of blades could be ignored during service and extend

further to cause unscheduled shutdown. In this study, PAUT method with special manipulator was developed

mainly for the inspection of delamination at trailing edge of turbine blade. Considering the rope access condition,

the light weighted manipulator designed was capable of holding the probe to contact the blade surface and to

scan along the edge steadily as in Figure 7. Though ultrasonic frequency lower than 5MHz was generally used

for inspecting resin matrix composites, the PAUT probe employed in our case was of 10MHz, 128 elements.

Results shown in Figure 8 clearly presented gas hole, interface and back wall echoes in trailing edge region

suggesting that the composite blade had been manufactured with good matrix quality.

Figure 7: Manipulator designed the inspection of trailing edge of wind turbine blade.

Figure 8: PAUT result by using manipulator to scan along the trailing edge of turbine blade

PAUT METHOD USED WITH FFS ASSESSMENT

During scheduled shutdown, inner crack was detected by UT on the inner surface of a hydrogen absorber which

is filled catalyst pearl powder inside the pressure vessel. Since it may cost a lot to open the tank for repair,

PAUT was then used to locate and size the crack in more details for FFS assessment. As shown in Figure 9,

PAUT result was combined with the local structure of vessel and weld and then to generate a FEM model with

cracks in measured dimension of 5.4mmD 35mmL.

Figure 9: PAUT result of inner crack integrated with local structure of vessel and weld

Based on the conditions assumed in Table 1, a safety factor (SF = KIC/Keff) was defined and calculated, where

KIC is the material constant and Keff is the effective stress intensity under various dimensions of crack and weld

geometry. Considering various crack depth a and weld crown height d, the distribution of maximum Keff in MPa

and SF was shown in Figure 10. The stress at crack tip was not high compared to other area such as the vessel

plate or weld crown corners. And the greater the SF, the safer the structure is with crack present.

Table 1: Parameters used in FFS assessment

Parameters Value

Young’s modulus* (E) 200 GPa

Poison ratio ( ) 0.3

Operation pressure (P0) 10 MPa

Weld crown height (d) 0.10 mm ~ 10 mm

Crack depth (a) 0.10 mm ~ 15 mm

Fracture toughness* (KIc) 50 MPa*m0.5

(a) (b)

Figure 10: (a) Von Mises stress distribution while crack depth is of 4mm and weld crown height 4mm assumed,

(b) distribution of SF and Keff vary with parameters a and d.

RPV ANCHOR BOLT INSPECTION

Bolting degradation and failure is not rare in nuclear power plants and it was also found on RPV anchor bolts

due to stress corrosion cracking. Though conventional ultrasonic PE method are still used as a common practice

in pre service or ISI of bolt, PAUT based technique with manipulator was proposed and under development for

better crack detection and sizing mainly. And, various notches and FBH were made on bolts of different length

for defect characterizing as shown in Figure 11. To cover the whole inspection area of bolt in dimension of 76

mmD 660mmL, a 128 elt of 10 MHz probe was used. Results as shown in Figure 12 indicate that notch size

down to 1mm deep may be clearly shown with A, B and C scans. Notches of various length in circumferential of

radial direction may be differentiated directly by the images but should be correlated carefully further.

CONCLUSIONS The continuing demand for faster transportation and lighter, safer structures has been driven the NDT to achieve

better inspection performance, efficiency and reliability. Portable PAUT system together with mechanical

scanner is a representative option among NDT methods to achieve the goal particularly in harsh environments

with radiation or high altitude for instance. Despite this, human error is always a crucial concern for reliable

inspection and hence performance demonstration is necessary.

REFERENCES 1. “GE Nuclear Energy-ABWR Design Control Document”, Rev. 4, March 1997.

2. Wu, Yung-How and Hsiao, Chu-Chung, “Development of remote operation RIP inspection method using

PAUT technique”, contract research, Taipower/ITRI, Dec. 2009.

3. Wu, Yung-How and Hsiao, Chu-Chung, “Development of Non-destructive Inspection Techniques for Wind

Turbine Components”, 2013 Taiwan Wind Energy Symposium, National Taiwan Ocean University/Taiwan

Wind Energy Association, Dec. 5, 2013.

4. Wu, Yung-How and Hsiao, Chu-Chung, “Inspection and FFS evaluation of crucial structures in refinery”,

contract ISI, CPC/Universal Inspection Technologies/ITRI, Mar. 2010.

5. Wu, Yung-How and Hsiao, Chu-Chung, “Development and Feasibility Study of PAUT technique for

Anchor Bolts”, Invited talk in technical forum of anchor bolt inspection and safety control, Taipower, May,

2013.

σmax

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

a (m)

0

0.002

0.004

0.006

0.008

0.01

d (

m)

2<SF<5

SF>5unit: MPa*m0.5

(a) (b)

Figure 11: (a) Reference sample of RPV skirt anchor bolt with notches and FBH, (b) manipulator seated

on bolt end for PAUT probe scanning.

(a)

(b) (c)

Figure 12: (a) Circumferential notches of 2.5 mmD along the bolt were detected. (b) notches of different

circumferential length may be compared directly and (c) notch down to 1mmD may be all clearly shown.