Other Major Component Inspection I

Mechanized UT inspections on complex nozzle geometries S. Farley, R. Jansohn, Westinghouse Electric Germany, Germany; H. Ernst, Schweizerischer Verein

für technische Inspektionen, Germany; D. Kerr , Eskom Holdings Limited (Koeberg Nuclear Power

Station), Germany

ABSTRACT

In 2007 and 2008 Eskom Holdings Limited (Eskom) performed automated ultrasonic inspection of

forged austenitic branch welds in the primary piping in both units 1 and 2 of the Koeberg Nuclear

Power Station. The nozzle size varied between 4” to 14”. The examination volume was the inner 1/3

of the pipe wall thickness as defined by ASME XI 1992 edition. The inspection technique had to be

able to detect, length and depth size inside surface connected service induced planar flaws as well as

subsurface flaws, typically from manufacturing.

As part of the project the essential parameters had to be evaluated following the guidelines

posed in ENIQ recommended practices 1 and 2. Due to the complex geometrical conditions, modelling

was performed to determine optimal scanning angle and probe position. A special scanner was used

which enables 3 axis movements to follow previously calculated scanning curve and rotation. Practical

trials on a 14” full size test specimen, with natural defects, confirmed the probe orientation angles as

calculated by theoretical modelling.

This paper describes the modelling, practical demonstration and the inspection process that

were developed and successfully deployed in the field.

Nuclear components in light-water cooled plants have to be periodically inspected based on

regulations defined in industrial codes like e.g. ASME Boiler and Pressure Vessel Code [1]. Ultrasonic

examination is a frequently used technique to comply with the requirements for volumetric

inspections. Worldwide different regulations or guidelines are in place to direct and ensure the

qualification process of non-destructive examination techniques.

One guideline to be mentioned is the ENIQ Report “European Methodology for Qualification”

[2]. The major principle behind this qualification guideline is to ensure by a thorough process that the

used techniques and personnel involved are capable to detect certain sizes of flaws based on a certain

failure mechanism. Therefore numerous essential parameters with its influence on the inspection

reliability have to be considered and its variability has to be investigated. For reasons of traceability,

repeatability, documentation purpose and personnel radiation exposure more and more complex

ultrasonic examinations are performed in an automated way. With respect to geometrical influences

the inspection of nozzle to shell welds is one of the most challenging due to its complex mathematical

description needed.

Eskom Holdings Limited (Eskom) requested to perform automated ultrasonic inspection of

forged austenitic branch welds in the primary piping in both units 1 and 2 of the Koeberg Nuclear

Power Station in the years 2007 and 2008. The nozzle size varied between 4” to 14”. The inspection

was implemented as “state of the art” ultrasonic testing (UT) with a practical demonstration following

the principals of the ENIQ guideline but without formal qualification e.g. with additional blind tests.

Figure 1 shows a typical branch weld. The examination zone covers the cross sectional volume for the

full circumference of the component. The volume to be examined is the inner 1/3 of the pipe wall

thickness as defined in Ref. [1]. The objective of the examinations was to accurately detect, length size

and through wall / depth size inner surface (ID) connected service induced planar flaws. Sub-surface

flaws typically from manufacturing had also to be reported and sized. The set-in nozzle examinations

were performed from the outside diameter (OD) piping surface as far as physical limitations

(geometry, material, etc.) allowed.

For the sake of definition, the planar flaws will be described as follows:

INTRODUCTION

- Transverse flaw = transverse or perpendicular to the subject weld

- Longitudinal flaw = parallel to the subject weld

Figure 2 shows one point of a postulated transverse flaw on the inner surface within the inspection

volume. For each point within the inspection volume the ultrasonic beam shall hit the flaw under

optimal scan angle β and misorientation angle Φ. To obtain optimal specular reflection the plane of the

ultrasonic beam related to the misorientation angle Φ should hit the flaw surface as perpendicular as

possible.

Figure 1 - Examination Volume C-D-E-F

Figure 2 - Beam Angles

Inner Surface

Nozzle and Pipe

Central Beam

N Surface Normal Vector

T Tangential Vector

B Flaw Normal Vector

β Flaw Scan Angle

Φ Misorientation Angle

σ Flaw Skew Angle

As the probe head can just follow the contour of the outside diameter piping surface the criteria

perpendicularity (Φ = 0) alone would require a numerous variation of the probe position, scan angle

and probe skew angle. Therefore a maximum allowed misorientation angle will be introduced. This

maximum misorientation angle had to be proven by practical demonstration on real test blocs. For this

project a maximum misorientation angle of 20 deg has been found acceptable.

Figure 3 demonstrates the principal in case of a postulated longitudinal flaw. The intersection

between nozzle and pipe outer diameter has the form of a saddle (yellow curve). The green plane is the

normal plane to the intersection curve. The inspection of the weld has to be performed from the pipe

side. In an ideal case is the central beam part of the normal plane. It should be mentioned, that the

nozzle axis not part of the normal plane is.

The intersection curve of the normal plane with the pipe outer diameter results in an elliptical

curve (blue curve). This is the way the probe head should move.

Figure 3 - Geometrical Conditions for longitudinal Flaws

Mathematical modelling was used to reduce the numbers of practical trials. Existing simulation

software was adapted to this specific project. The numbers of simulation runs were narrowed for

practical reasons. Simulation was performed for the examination zone limits E and F. The probe scan

angles were defined with 45 deg, 55 deg and 60 deg. The probe skew angle was defined with 38 deg,

47 deg and 55 deg and its corresponding negative values. With this input data it was possible to

calculate the actual probe position on the outside diameter piping surface and the misorientation angle

towards the flaw plane. Figure 4 shows an example for a transversal flaw on a 6 inch nozzle.

This calculation needed then to be iterated to get a complete coverage of the inspection

volume with the foreseen inspection technique.

MODELLING

inspection point

theta [°]

inspection

point radius

[mm]

approximate

angle of

misorientation

examination

zone limit

probe

angle

[°]

probe

skew

[°]

probe

theta

[°]

probe

radius R

[mm]

67,5 185,00 20 E 55 55 91 256 Figure 4 - Beam modelling example for 6 inch nozzle geometry

In compliance with the theoretically proven approach, a suitable scanner had to be made available

which can follow the scan paths according the geometrical requirements. With the JNA-Scanner-

System Westinghouse has developed a specific scanner system for automated UT inspections of

nozzles in the main coolant pipe in pressurized water reactors. The system permits to scan the primary

pipe from the outer surface with UT transducers.

The JNA scanner consists on carriageway mounted parallel to the axis of the main coolant

pipe and a system carriage on which a bent boom is mounted perpendicular, see figure 5. The

carriageway is mounted on the main coolant pipe with 2 straps placed at each extremity. A system to

tight the straps by mechanical buckle hold the manipulator in place on the primary pipe.

MANIPULATOR

Figure 5 - JNA scanner system

The JNA system allows 2 types of translational displacements:

• A displacement in regards with X axis (parallel to the primary pipe axis): this movement is

performed through a toothed belt powered by a drive unit mounted on the end of the rail. The unit for

the boom drive is mounted on the system carriage.

• A displacement in accordance with Y axis (circumferentially to the primary pipe): this

movement is performed trough gears powered by a drive unit mounted on the unit for the boom.

In addition to the two translational displacements a rotating unit is used which consists of:

• A frame that permits to interface with universal joint suspension. A Plexiglas shoe shaped

according the primary pipe OD is screwed on the lower part in order to be in contact with inspected

component.

• A rotating device in integrated in the frame. In consists in a toothed wheel that ensures the

rotation and permits to integrate the probe

• A drive unit is fixed on the frame to generate the rotating movement.

A universal joint suspension system permits to press the device on the component surface through a

string system. Forks ensure the holding of the frame. The probe holder system is capable to carry

either phased array probe heads or standard probe heads with e.g. pulse-echo transducers.

Finally the whole inspection system and the procedure have been proven by practical trials. Full scale

specimen where available for a 14 inch and partially for a 6 inch nozzle. Both nozzles were with

respect to material, welding process and geometry an exact copy of the on-site configuration.

Upfront manual testing using a varied range of probe types with different refraction angles,

frequencies, crystal sizes and wave modes were initially performed to find the optimal inspection

PERFORMANCE DEMONSTRATION

technique to detect and size the expected flaw. The probe types that gave the best results were then

used together with the JNA scanner to perform automated examination in the laboratory. The full

inspection system was used as is later on during in-service-inspection.

Probe skew angles were varied as well to find optimum probe orientation. The optimum probe

orientation was found in line with the results of the theoretical modelling. In general the inspection

system was able to detect these flaws introduced into the specimen – notches and real cracks – which

were in line the inspection target.

For automated inspections of primary pipe nozzle to shell welds a complex mathematical description

to position the ultrasonic probe with respect to postulated failure orientation is needed. Mathematical

modelling has been used to optimize the inspection approach and to reduce the number of practical

trials. A specific scanner had to be used to follow the required scan paths on a complex geometry.

Based on practical trials on full scale specimen the complete inspection system has been

proven and the applicability of the modelling technique has been validated. In the meantime the

described technique has been successfully applied in field applications in the Koeberg units 2 in 2007

and unit 1 in 2008.

REFERENCES

1) ASME Boiler and Pressure Vessel Code: Section XI, Rules for Inservice Inspection of

Nuclear Power Plants

2) ENIQ Report EUR 17299: European Methodology for Qualification

SUMMARY