ABSTRACT

Experience has shown that thermal stress cracking can occur in High and Medium Pressure Rotor blade attachments (tangencial T-Slot and axial trees steeples type). For this reason, in recent years diferent manufacturers have recommended regular inspections to keep track of the problem. This paper presents the on-site validation in real conditions of the S.I.R.E.N.A. system ," ROBOTIZED INSPECTION SYSTEM OF BLADES ATTACHMENTS USING ARRAY TECHNIQUE ", performed at the high-pressure turbine rotor of the Anllares Thermal Plant, in May 1996.

RÉSUMÉ
L´expérience a demontré que des fissures apparaissent au niveau des pieds de sapin des rotores de haute et de basse préssions dûes á la fatigue thermique. Pour cette raison, le fabricant recommande un contrôle, afin d`avoir un suivi des défauts. Cette exposé, présente l´inspection par ultrason comme alternative au contrôle des pieds de sapin des rotores de haute et de basse préssions. Cette inspection est faite á l´aide d´un systéme appelé S.I.R.E.N.A, "SYSTEME D´INSPECTION DES PIEDS DE SAPIN ET DE LA TECHNIQUE ARRAY".

INTRODUCTION

This paper deals with the development of an Ultrasonic Inspection System based on Array technology for application to the inspection of high and intermediate pressure rotor blade grooves from the central bore. Development of this inspection system, carried out completely by Tecnatom, included collaboration by the Anllares Thermal Power Plant, with a pilot test of the blade grooves of a HP/IP rotor in May 1996 for validation of the technique. Additionally, in February 1997 an inspection was performed on the HP/IP rotor of the Teruel Thermal Power Plant. In both cases the circumferential blade grooves of high pressure crowns 1 and 2, and the axial blade grooves of intermediate pressure crowns 10 and 11 were successfully inspected.

Problems involved in Inspection Thermal stress-induced cracking may occur in the blade grooves of high and intermediate pressure rotors, for which reason the manufacturer recommends tracking of the component by means of periodic inspections. To date these inspections have been performed using visual inspection and magnetic particle techniques. Use of these techniques requires that the rotor blades be extracted, destroyed and replaced, this significantly increasing the problematic of said inspections. Development of the SIRENA, " System for the Robot-based Inspection of Blade Grooves using Array Technology", provides an alternative to the Inspection Procedures performed to date. This development makes it possible to guarantee the inspection of 100% of the blade grooves of all the crowns in the case of rotors with a central bore.

DEVELOPMENT

Each of the following aspects was addressed in order to successfully culminate development of the inspection system:
• Geometry of the component and defectology.
• Validation block.
• UT technique and characteristics of the Array system.
• Mechanical system.
• Results.

Geometry of the component and defectology This system is used to ultrasonically inspect the blade grooves of high pressure crowns ("circumferential T-shaped blade grooves"), as well as those from intermediate pressure crowns ("axial tree-stepple blade grooves"), on rotors fitted with a central bore. As regards the geometry of this type of components, it is important to underline the following:

• High degree of complexity.
• Large material thicknesses.
• Large differences in the thickness of the different crowns.
• Limitations as regards accessibility for the ultrasonic beam applied, from the central bore.

Validation block


Abb 4: Nachweis Wanddicke aus n-ter mit 2.5 MHz an einer 8 mm

As a starting point for development of the inspection system, and in view of the difficulties posed by the geometry of the component to be inspected, it was considered necessary to design and manufacture a block generically representing the two types of blade grooves.

This block (Fig.1), is of a material ultrasonically equivalent to that used in manufacturing the rotors, and accurately represents the geometry of the blade grooves of crowns 1, 2 and 10 of a 350 MW unit high and intermediate pressure rotor, with a central bore measuring 150 mm in diameter. A series of notches measuring 0.2 mm in width and 2 and 4 mm in depth were induced in the block by electroerosion in the zones previously identified as being critical to crack growth in this type of blade grooves.

Fig. 2 shows, as an example, a detail of the defects implemented in the validation block for the circumferential blade grooves.

Abb 4: Nachweis Wanddicke aus n-ter mit 2.5 MHz an einer 8 mm

UT technique and characteristics of the Array system
The ultrasonic technique used was the pulse-echo method, with immersion in a light, low halide content oil to prevent oxidation of the material, which is carbon steel.

In order to be able to cover the different thicknesses foreseen, due to the need to inspect the two types of blade grooves, it was considered advisable to use variable focus Array technology probes.
With a view to detecting the indications considered critical, three different angles were defined, these being necessary to suitably access the different areas of interest: 0° and 15° longitudinal wave and 45° transverse wave. The first probe (0° ), is used to detect defects in both types of blade grooves, while the other two are used only for "circumferential T-shaped" blade grooves.

These probes have a large external diameter (approximately 100 mm.) because of the long sound paths to be covered inside the material to be inspected and the need to minimize the width of the beam in the focal zone.

In addition, and in view of the possibility for distortion of the ultrasonic beam due to its impinging on a cylindrical surface (material inlet surface), the Array probes used have a Fermat curvature which corrects these beam deficiencies. This Fermat surface is possible thanks to the use of composite materials which allow the active zone to be shaped depending on design needs.
It has also been necessary to have available powerful simulation programs, on the one hand to be able to specify the manufacturer the probes to be constructed, and on the other to be able to optimize the focal laws applied.
The three probes used were manufactured by IMASONIC, in accordance with a detailed specification drawn up by TECNATOM.
Data acquisition was performed using the SUMIAD III system. The data acquired were processed and evaluated using MASERA.

Fig 3: Probe-holding module

Mechanical system
For performance of the inspection an oil-tight system was designed, in order to maintain the oil level inside the rotor and guarantee good sound transmission throughout the inspection. This immersion inspection technique poses certain problems, which were solved by submerging only the active part of the probes and by rotating the rotor on turning gear, controlled by means of an electrohydraulic system whose encoded movement allows access to be gained through 360°.

This rotation, along with the axial displacement imparted to the probe-holding module by means of a bedplate located externally, enables the system to inspect the entire volume of interest. Control of this movement was achieved using the TECNATOM universal controller SIROCO.

Fig. 3 shows a prototype of the probe-holding module used. The module used for the real inspections has a housing for the three probes involved, such that all the acquisitions are performed without the need for downtime resulting from multiple oil emptying and filling processes to the necessary level.

Fig 4: General record optained with 15° (circumferential blade grooves)

Results
The results obtained from the validation block, in which all the defects postulated for both types of blade grooves were detected, have allowed the inspection technique to be validated. Figure 4 shows a general plot of the results obtained with 15° for the circumferential blade grooves.
The manufacturer has accepted the Ultrasonic Procedure for blade grooves inspection as a replacement for the previous procedure, which implied the extraction, destruction and replacement of the blades. The inspections performed to date (Anllares TPP, 1996 and Teruel TPP, 1997) have underlined the performance of the system and the advantages of its use.

CONCLUSIONS

An Inspection Procedure based on new ultrasonic NDE techniques has been developed and brought into service. This procedure makes it possible to avoid the costly maintenance tasks that were necessary for operation of these components.

Antonio Tanarro
Head of Ultrasonic Research Group. R&D Division.
Tecnatom, s.a.
Avda. Montes de Oca, 1.
28709 San Sebastián de los Reyes. Madrid. Spain.
Telephone: (341) 651 67 00
Telex: 45.831 TCOM
Telefax: (341) 654 15 31
E-mail: tanarro@tecnatom.es
Homepage: http://www.tecnatom.es