A COMPUTERIZED SYSTEM FOR THE ADVANCED INSPECTION OF REACTOR VESSEL STUDS AND NUTS BY COMBINED MULTI-FREQUENCY EDDY CURRENT AND

ULTRASONIC TECHNIQUE

W. Kelb, KontrollTechnik, Germany J. M. Sarteel, CapTech, France

R. Leriche, CEGELEC NDT, France

Introduction

Studs and nuts of reactor vessels have to be examined periodically by non-destructive techniques. The requirements according to ENIQ or RSEM lead to improved demands regarding examination and consideration of influent parameters, accuracy in determination of the defect lengths, the axial and circumferential positions of indications as well as the reproducibility of the inspection result. With the newly developed computerized scanning system both eddy current as well as ultrasonic technique can be applied to the inspection of both studs and nuts. All scanning procedures are pre-programmed ensuring a maximum possible accuracy for determination of the indication position and a maximum in reproducibility of the inspection result especially for testing of complex geometric areas like the transition zones from the threads to the shaft. Due to their long lifetime, bolts and nuts show in many cases a high quantity of mechanical imperfections like dents, scratches, missing material, corrosion, etc. leading to extensive signal analysis and documentation work. Particularly for the eddy current testing the probe guiding was heavily improved in order to achieve a high signal to noise ratio and preventing multiple indications from imperfections allowing automatic signal evaluation techniques. Additionally the automatic data analysis software allows the fast, safe and uncomplicated evaluation of indications and direct result storage including all relevant signal and position data to the indication list. The overall concept and operation of the system and examples of results from qualification and practical inspections are presented.

Requirements to stud and nut inspection systems

The requirements of e.g. ENIQ, RSEM, KTA certifications regarding detection capacity, defect length determination, accuracy, etc. led to improvements of the entire testing systems for studs and nuts. Figure 1 shows the main requirements and the influenced part of the testing system.

Fig. 1:Main requirements to the testing system by ENIQ, RSEM, KTA certifications

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Evolution of stud and nut testing systems

Kontrolltechnik GmbH is working in the development of mechanized inspection systems for reactor stud and nut inspections as well as in the execution of inspection services for several years. Within this period the functionality and size of the manipulator systems, the sensor technique as well as the signal analysis software were subject to permanent improvements. Figure 2 shows the evolution of the testing system representative for the manipulators.

Type RS 150 L max. stud length1500

mm max. stud diameter 160 mm

Type RS 210 L max. stud

length 2100 mm

max. stud diameter 250 mm

Type RS 110 L max. stud length 1100 mm max. stud diameter 120 mm

Design 2004 Design 2008 Design 2012

Design 2004 Design 2008 Design 2012

Fig. 2: Evolution of manipulator systems for the inspection of reactor studs and nuts

Function and Features of the Manipulator Type RS 210 L

The stud and nut testing manipulator Type RS 210 L is the latest design. From its dimensions the manipulator is constructed to test studs of the EPR reactor, which are currently the longest in place. Figure 3 shows the main functions and features of the manipulator type RS 210 L. The concept of the manipulator took into consideration that besides the requirements in accuracy and reproducibility, the studs can be installed in an easy, time effective and safe manner. By the safety cabinet and several emergency stop switches all requirements concerning human safety are ensured. Additionally the functionality of the RS 210 L type manipulator took into consideration the minimization of human errors in operation of the unit.

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upper stud fixture front view

safety ring

used for stud installation

laser pointer

for 0-point adjustment

safety cabinet will be closed for operation

eddy current probe holder 1x for shaft inspection 1x for thread inspection

computer-controlled

3-axis motion for probe manipulation

control panel for stud installation functions and adjustment of 0-position

turning motor

control rack including eddy current unit

side view

computer-controlled linear axis and flexible shaft for

ultrasonic probe insertion

Fig. 3: Function and Features of the Manipulator Type RS 210 L

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While the previous manipulator types are designed for eddy current testing only, the type RS 210 L includes an additional ultrasonic testing facility through the bore of the stud. The ultrasonic probe is connected to a flexible shaft positioned at the rear side of the manipulator. The flexible shaft will be pushed into the bore of the stud via a roller guiding as shown in figure 4.

computer-controlled linear axis connected to the flexible shaft for ultrasonic probe insertion

ultrasonic probe body

with angle probe

roller guiding for flexible shaft

Fig. 4: Connection of the ultrasonic probe in the manipulator type RS 210 L

Installation and adjustment of studs

In general the studs are provided in upright position placed in a transport cradle. For installations into the manipulator the studs are lifted out of the cradle and can be directly placed into the adapter piece (figure 5 and figure 6).

Fig. 5:stud lifted out of the transport cradle

Fig. 6:stud placed into the adapter piece

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Once the stud is placed in the adapter piece it has to be secured against falling over. Therefore the safety ring will be closed, and the eye bolt at the top of the stud can be unscrewed. Afterwards the upper bolt fixture will be driven down to fix the stud (figure 7).

Fig. 7:stud placed in the anipulator secured by the safety ring

The stud is now ready to be turned into the 0-position. Depending on definition, the 0-position can be either a special marking on the stud or the start of the thread. For 0-position alignment, a laser pointer is used, which can be fixed either to the sensor arm or to the upper bolt fixture (figure 8). The bolt will be turned using the control panel until the laser is aligned with the defined 0-position and the encoder is set to zero.

Fig. 8: Application of the laser pointer for 0-position alignment

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The eddyMax – RotoMax data acquisition software

The eddyMax – RotoMax data acquisition software combines the signal data acquisition, signal display in different modes and control of the manipulator. Figure 9 shows the screen display and functions.

signal display selectable up to 16 impedance windows

signal display selectable up to 16

chart windows

color scan display signal amplitude is

converted into colors

parameter window selectable up to 8 differential

channels and 8 absolute channels

both at up to 4 frequencies

toggle between color scan display and

manipulator control window

manipulator status window

shows the actual position and

status of the drive axes

Fig. 9: Screenshot of the eddyMax RotoMax data acquisition software, with display in scan mode

The testing multi-frequency mode is necessary for signal phase discrimination of defect signals from noise signal caused e.g. by mechanical damage or corrosion. All signal data a re stored inclusive the entire parameter setting. By the toggle the operator can switch between the signal chart display and the manipulator control window. The manipulator control window shows the active scanning procedure and allows manual manipulator control as well as generation of scanning procedures by teach-in or keyboard input. Figure 10 shows the screenshot of the RotoMax software toggled to the manipulator control window.

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manual manipulator operation window for manual control of the

manipulator

manipulator status window shows the actual position and status of the

drive axes

active scanning procedure can be generated either by teach in or input

of calculated values

Fig. 10:Screenshot of the eddyMax RotoMax data acquisition software, with display in manipulator control mode

Computer-controlled scanning procedure

Figure 11 shows an example for a scanning procedure of the transition zone from the shaft to the thread. The scanning procedure determines the axial and radial position of the probe, the inclination of the probe to the vertical axis as well as the rotating speed of the stud and the trace feed of the probe. For each section to be tested a scanning procedure will be established, ensuring a reliable, reproducible and fast coverage of the area to be tested.

Fig. 11: Example for a computer-controlled scanning procedure

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Eddy current probe and probe guiding

Due to their long lifetime bolts and nuts show in many cases a high quantity of mechanical imperfections like, dents, scratches, missing material, corrosion, etc. leading to extensive signal analysis and documentation work. Particularly for the eddy current testing an important point was made to the improvement of the probe guiding in order to achieve a high signal to noise ratio and preventing multiple indications from imperfections allowing automatic signal evaluation techniques. Therefore the following requirements are valid for the performance of eddy current sensors and probe guiding.

Requirements to the performance of the eddy current sensors the eddy current sensor has to work in absolute mode to allow the evaluation of the defect

length and defect profile display. the eddy current sensor should be focused. Especially for the testing of threads the

use of compensation coils in the neighbor turns should be prevented - avoiding double indications from one source.

the eddy current sensor for threat testing shall have the highest sensitivity in the root of the thread.

Requirements to the performance of the probe guidances the probe guidance shall follow the contour caused by unroundness and bending of the bolts

in order to minimize signals caused by sensor lift-off. the probe guidance shall have a minimal contact area not to transmit mechanical

imperfections from the neighbor area and turns to the probe. the probe guidance shall cause minimal lift-off change in transition zones with

changing diameter. Extensive tests on different probe configurations led to the result, that sensors with two separated ferrite cores in differential and absolute mode show the best results according to the above requirements. Special attention was turned to the fact to have a sensitive absolute mode, because all signal evaluations are using the absolute signal, whereas the differential signals are used for confirmation or extended signal analysis purposes only. Figure 12 show the eddy current probes and the wiring of the sensors.

thread probe

shaft probe

Fig. 12 : eddy current sensors for shaft and thread testing

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Both the thread and the shaft probe have the same configuration and wiring, using internal compensation coils (Rc1 and Rc2) to get a better matching of the absolute coils. The tip and the ferrite cores of the thread sensors are tapered to fit into the thread contour, whereas for the shaft sensor the ferrite cores in the tip remain round. Figure 13 shows by the example of a long notch the necessity for sensitive working of the absolute channel. Figure 13 shows the signal response from a 174 mm long notch in the threaded section of the bolt for the differential channel, absolute channel without high-pass filtering and absolute channel with 0.2 Hz high-pass filtering. It can clearly be seen, that only the absolute channel without high- pass filter shows the real contour of the notch and thus allow the evaluation of the notch length.

differential channel, HP-filter off

absolute channel, HP-filter off

absolute channel, HP-filter 0,2 Hz

Fig. 13: signal response from a 174 mm long notch in the threaded section of the bolt for the differential channel, absolute channel without high-pass filtering and absolute channel with 0.2 Hz high-pass filtering

The probe guidance for the shaft and transition has 2 grades of freedom and the guidance for threads has 3 grades of freedom in order to compensate out-of-roundness and bending of the studs. Both guidances are using rollers to ensure a minimal contact area and therefore minimize noise and phantom signals due to superimposed lift-off at mechanical imperfections. The optimized probe guidance is an essential factor for the application of computerized signal evaluation, defect length determination and reproducibility of the testing result. Figure 14 shows the design of the probe guidance for the shaft and thread testing and figure 15 shows the guidances in operation placed to the stud.

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Probe guidance

for shaft and transition zone

Probe guidance

for threaded area

Fig. 14: probe guidances for shaft and thread testing

Fig. 15: probe guidances for shaft and thread testing in operation placed to the

stud

To give an example of the signal quality reached by the optimized probe guidance, Figure 16 shows the signal response of a 1.0 mm deep notch recorded with a former model of probe guidance compared to signal response of the same notch recorded with a improved probe guidance. The former guidance shows a relative high lift-off signal in horizontal direction superimposed to the signal from the notch. In such a case the determination of the signal phase is unreliable. With the new probe guidance lift-off changes are completely suppressed resulting in a clear and defined notch signal.

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Signal response from a 1.0 mm deep notch with the former probe

guidance used up to year 2011

Signal response from a 1.0 mm deep notch with the new probe guidance used from year 2012

Fig. 16: signal response of a 1.0 mm deep notch got with a former model of probe guidance compared to signal response of the same notch got with a improved probe guidance

The eddyMax data analysis software

Due to their long lifetime studs and nuts show in many cases a high quantity of mechanical imperfections like, dents, scratches, missing material, corrosion, etc. leading to extensive signal analysis and documentation work. Additionally reproducibility is an important factor for follow-up inspections which shall indicate differences to previous inspections. The eddyMax data analysis software is developed under consideration of these factors. Figure 17 shows a screenshot and a description of the main features. The testing result for each tested area is shown in a colored scan display. With the cursor a sector can be selected and the impedance signal from this sector is shown. The analysis function automatically evaluates the signal phase, signal amplitude and defect length as well as defect position of the signal in the sector. All data can be exported for reporting e.g. to Excel. This feature ensures an operator independent, reliable, fast and comprehensive signal evaluation and reporting.

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cursor selecting the sector in the scan to be

evaluated

menue functions color scan display selectable from

16 channels

evaluated signal parameter

amplitude, phase , position, length, ..The parameter can be exported for reporting

selection of the evaluation channels

y-chart display of the selected channels and

automatic length evaluation

color palette editable for up to 20

colours

Impedance display of the selected channel

Fig. 17: screenshot and a description of the main features of the eddyMax data analysis software

Another feature of the analysis software was made for the evaluation of signal data from the shaft and transition zone. Whereas for testing of threaded areas a notch will only be passed one time by the eddy current sensor and therefore will only generate a single signal, for testing of unthreaded areas the signal display is different. For testing the shaft or transition zone, the feeding path is approximately 0.5 mm per turn, meaning that a notch will pass the influence zone of the sensor on several turns and generate a signal on each turn. For length evaluation only the turn with the maximal signal amplitude has to be evaluated. In the eddyMax data analysis software the signal of all relevant turns can be selected and will be automatically evaluated. The turn with the highest signal amplitude can easily be selected and the result exported for reporting (see figure 18).

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cursor selecting the sector in

the scan to be evaluated

y-chart display of the tracks selected by the cursor

Selection of the track with the highest amplitude for documentation

Fig. 18: evaluation of indications in the shaft and transition zone by the eddyMax data analysis software

Summarization

The main features and advantages of the inspection system can be summarized as follows:

The manipulator is designed for safe, easy and fast installation and zero alignment of bolts and nuts

The manipulator fulfills all necessary safety regulations Easy teach-in or input of precise and reproducible scanning procedures Advanced design of probes and guidances to reach a high signal-to-noise ratio Powerful multi-frequency / multi-channel data acquisition and data analysis software

ENIQ qualification of the system for swiss nuclear plants RSEM qualification in process

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