Session 2A (1) – Phased Array Applications Chairman – … Phased array ultrasonic testing (PAUT) Axial coverage of the blade roots is achieved by careful manipulation of the ultrasonic

ConferenceProgramme

ConferenceTopics

Listof Exhibitors

UsefulLinks

FutureEvents

AboutUs

NDT 2010 Conference Topics

14.00 Phased array ultrasonic inspection of low pressure steam turbine rotors, Curved Axial Entry Fir tree roots

Author - Mr. Charlesworth

The last stage blades of low pressure steam turbine rotors are among the most highly stressed components in modern power generating plants. The ongoing drive for increased efficiency has seen the proliferation in the number of designs incorporating larger last stage blades with curved axial entry fir tree roots (CAEFTR); the curvature of the root attachment allows more flexibility in the aerodynamic design of the aerofoil with improved inter-blade spacing. The tendency of CAEFTRs to suffer failure induced by stress corrosion cracking, high-cycle fatigue cracking, or low-cycle high strain fatigue, is well documented, and can be shown to be most likely to occur in the first two serrations of the blade root. Finite element analysis and actual failures confirm the regions under highest risk and have driven developments in ultrasonic phased array techniques to achieve detection of defects in these regions whilst in-situ; in turn avoiding the huge costs associated in decommissioning and dismantling rotors to perform alternative NDE surface inspections. Due to the complexity of the root geometry there are many difficulties in applying ultrasonic techniques due to limited scanning surfaces, inter-blade spacing, and disorientation of the active ultrasound trajectory and the region under test. In this presentation the author will show that the application of novel phased array techniques and unique inspection design has led to increased sensitivity to smaller defects and comprehensive coverage of CAEFTR rotor designs in-situ. It will also be shown how the application of these techniques has negated the need to upgrade both equipment and resources to the use of 2d arrays, thereby reducing inspection costs significantly whilst achieving higher repeatability and sensitivity. Novel inspection design has led to the ability to reduce the number of scans required, enabled single scan encoded data recording over the whole blade root, increased sensitivity, improved detectability, and increased coverage, whilst reducing inspection costs and time.

Session 2A (1) – Phased Array Applications Chairman – Prof B Drinkwater

RWE Power International

1

Phased array ultrasonic inspection of low pressure steam turbine

rotors - Curved Axial Entry Fir tree roots

Chris Charlesworth


Electron, Windmill Hill Business Park

Whitehill Way, Swindon

Wiltshire, SN5 6PB

United Kingdom

Tel: +44 (0)1793 892900

Fax: +44 (0)1793 892421 [email protected]

Abstract

The last stage blades of low pressure steam turbine rotors are among the most highly

stressed components in modern power generating plants. The ongoing drive for

increased efficiency has seen the proliferation in the number of designs incorporating

larger last stage blades with curved axial entry fir tree roots (CAEFTR); the curvature of

the root attachment allows more flexibility in the aerodynamic design of the aerofoil

with improved inter-blade spacing.

The tendency of CAEFTRs to suffer failure induced by stress corrosion cracking,

high-cycle fatigue cracking, or low-cycle high strain fatigue, is well documented, and

can be shown to be most likely to occur in the first two serrations of the blade root.

Finite element analysis and actual failures confirm the regions under highest risk and

have driven developments in ultrasonic phased array techniques to achieve detection of

defects in these regions whilst in-situ; in turn avoiding the huge costs associated in

decommissioning and dismantling rotors to perform alternative NDE surface

inspections. Due to the complexity of the root geometry there are many difficulties in

applying ultrasonic techniques due to limited scanning surfaces, inter-blade spacing, and

disorientation of the active ultrasound trajectory and the region under test.

In this presentation the author will show that the application of novel phased array

techniques and unique inspection design has led to increased sensitivity to smaller

defects and comprehensive coverage of CAEFTR rotor designs in-situ. It will also be

shown how the application of these techniques has negated the need to upgrade both

equipment and resources to the use of 2d arrays, thereby reducing inspection costs

significantly whilst achieving higher repeatability and sensitivity. Novel inspection

design has led to the ability to reduce the number of scans required, enabled single scan

encoded data recording over the whole blade root, increased sensitivity, improved

detectability, and increased coverage, whilst reducing inspection costs and time.


2

1 Introduction

LP rotors are constructed from the largest aerofoil blades, the last stage of which can

consist of up to 120 individual blades measuring around one metre in length and

weighing in excess of 15kg. When rotating under full load at 3000rpm the LP rotor’s

last stage blades (LSB) are subjected to several tonnes of centrifugal and torsional

forces. A common method of blade attachment to the rotor shaft utilises curved axial

entry fir tree roots (CAEFTR); engineered to overcome the mechanical forces while

providing the ideal shape for efficient aerofoil dynamics(1)(2)

. The tendency of

CAEFTRs to suffer failure induced by stress corrosion cracking, high-cycle fatigue

cracking, and low-cycle high strain fatigue is well documented(3)

. If these blades were to

fail and become detached from the rotor shaft during operation they would cause

catastrophic failure of the rotor, leading to a potential explosion as well as possible risk

to life and collateral damage.

A number of non-destructive evaluation (NDE) techniques are deployed for early

detection of CAEFTR cracking. As direct visual access to root cracking is not possible,

the most common technique requires decommissioning of the rotor and blade removal

in order to perform magnetic particle inspection (MPI). MPI is the most sensitive and

comprehensive method of inspection, but is prohibitively expensive due to the cost of

decommissioning and the loss in generation during extended outage periods. The most

established alternative to MPI utilises ultrasonic testing (UT), using combinations of

single element pulse echo and phased array techniques to allow for early detection of

root cracking(4)(5)

. The difficulties of comprehensively inspecting the blade roots of

steam turbines stems from geometric factors which lead to efficient rotor design, but

have a dramatic effect on the inspection capability in the affected regions. Limited

accessible land from which to introduce ultrasound, constantly varying relative

geometry, and lack of physical access all add to the difficulties in achieving consistent

and sensitive coverage.

In this paper the author will outline some of the techniques developed, which

overcome the difficulties of inspecting such complex components, illustrating the

solutions innovated to improve coverage on CAEFTR designs. It will be shown that, a

combination of advanced methodologies, simulation software, and novel inspection

design, has resulted in high levels of coverage across many rotor configurations.

2 Curved Axial Entry Fir Tree Roots (CAEFTR) Figure 1 below illustrates the design configuration of typical CAEFTR, consisting

of fir tree root blocks to the blades which are inserted axially into mating fir tree steeple

grooves on the disk head. Depending on the manufacturer and design of rotor, keyways

are then utilised to lock the blades and prevent axial movement in service.


3

Figure 1 Curved Axial Entry Fir Tree Roots

2.1 Areas of vulnerability

RWE power international’s experience of failures of CAEFTRs spans over 7 years

in development and validation for third party customers and deployment of inspection

techniques both within and outside the company. Stress corrosion cracking has been the

most prevalent type of defect, induced where environmental conditions or poor water

chemistry have been primary concerns. There are however instances of fatigue cracking

which, due to their tendency to propagate rapidly, are more serious and present the

potential for blade detachment from the turbine. Finite element analysis (FEA) suggests

that the top serration is the most highly stressed area of the blade root, see Figure 2 and

Figure 3, representing the region where cracking is most likely to lead to total blade

detachment from the turbine; in some cases however, FEA suggests that the second

serration down is the most highly stressed.

Operational experience has shown that defects have the tendency to initiate in the

first serration down from the aerofoil-root platform, in the centre of the convex side, and

the extreme ends of the concave side. Typical examples of defects found in several

different rotor designs are illustrated in Figure 4 showing concave side cracking, and

Figure 5 showing convex side cracking. The main target areas for inspection of

CAEFTR has therefore been focussed on coverage of the top two serrations, with

particular focus on the extremities of the concave side roots and the centre portion of the

convex side roots.

Blades in Rotor

Blades in Rotor

Rotor Disk Steeples with blades removed

Loose Blades


4

Figure 2 Blade root Concave: Von Mises equivalent tension [MPa]

Figure 3 Blade root Convex: Von Mises equivalent tension [MPa]

Figure 4 Crack propagating in the top serration of the concave side

Figure 5 Crack propagating in the top serration from the centre of the convex side 2


5

2.2 Available scanning lands

Due to the complex geometry associated with CAEFTR designs, there are limited

available lands from which to perform ultrasonic testing, varying from one blade design

to another; some having generous platforms from which to scan whilst others have

almost none. The aerofoil however is common to all blade designs and offers the main

scanning surface from which the majority of coverage is attained; see Figure 6 and

Figure 7. Due to the shape of the aerofoil in relation to the root, limitations to the

coverage achieved exist, requiring further scanning from any available platforms

(Figure 8), or end faces of the root block, (

Figure 9). In addition to pulse echo phased array techniques, novel tandem phased

array techniques have been developed to further increase sensitivity in difficult but

critical regions, see Figure 10.

Figure 6 Root inspection from convex aerofoil

Figure 7 Root inspection from concave aerofoil

Figure 8 Root inspections from platforms


6

Figure 9 Root shear wave inspection from end face

Figure 10 Tandem arrangement from end face and platform

2.3 Phased array ultrasonic testing (PAUT)

Axial coverage of the blade roots is achieved by careful manipulation of the

ultrasonic transducer along the aerofoils, platforms, and end face lands, as seen above.

In order that the critical inspection areas are interrogated in the most efficient and

comprehensive way, phased array ultrasonic testing (PAUT) is utilised. PAUT is

constrained by the same physical limitations as conventional pulse echo ultrasonic

testing (UT) but offers superior control over ultrasound transduction; having the ability

to steer the ultrasonic beam over many angles of trajectory, many times per second,

enables sectorial images to be built up, offering the inspector a clear view within the

volume of the material. PAUT also offers the ability to focus the ultrasonic beam at

distances shorter than the natural focal depth of the transducer by introducing time

delays into the firing sequence of the individual elements. This produces narrower beam

profiles and higher sound intensities at the critical areas and thus improving sensitivity

and resolution. In order to take full advantage of the capabilities of PAUT it is necessary

to very accurately position the transducer on the available inspection surfaces in a

positive and repeatable manner. The biggest challenge has been to achieve this whilst

the turbine is in-situ with access to only the outlet side of the root block.

Concave root serrations

Transmit

Reflection from Defect

Receiver trajectory


7

3 Scanning Aids and bespoke Inspection solutions A number of solutions have been developed, some unique, to aid in the aim to

increase coverage and sensitivity to defects, and to reduce development time and costs

associated with CAEFTR rotor inspections.

3.1 Inspection jigs and formers

Difficult in-situ access and limited inspection surfaces lead to the development of

bespoke Rexolite wedges, designed to provide the appropriate refraction of the

ultrasonic waves through very limited flat lands on turbine blades, and having the

secondary but crucial function of fitting the surface geometry and acting as positioning

jigs. Rexolite wedges were designed utilising advanced modelling and simulation

software, manufactured using CNC technology, and successfully deployed on a number

of steam turbine applications. Further development of this concept, enabling in-situ

inspection of CAEFTR, has been achieved utilising fast prototyping methodology; the

cost and time required to manufacture solid Rexolite wedges is prohibitive, and a faster

more effective solution was developed. Multi-probe jigs, designed to accurately position

PAUT transducers, were simulated, modelled, and manufactured using

stereolithography (STL). Rexolite wedges were then retrofitted into the STL jigs,

producing a composite former able to clamp the PAUT transducer in position, and

provide the appropriate wedge angle for accurate refraction into very small surfaces.

Due to the flexibility of fast prototyping, all couplant feeds and fixing mechanisms are

incorporated in the jig, see Figure 11. This methodology has allowed PAUT transducers

to be placed accurately in remote positions, with positive and repeatable detection in

critical regions; the utilisation of previously unused non-flat inspection lands has been

enabled, such as the radii between the aerofoil and platform regions, thereby increasing

coverage, see Figure 12.

Figure 11 Typical CAEFTR inspection jig

Rexolite wedges


8

Figure 12 Typical CAEFTR inspection jig refracting through radii

3.2 Implementation of mechanised aerofoil scans

As described in section 2.2, much of the coverage for inspection of CAEFTRs is

achieved from the aerofoil, interrogating the root serrations at the opposite side. There

are a number of difficulties in performing aerofoil scanning as manual manipulation of

the PAUT transducer is limited almost entirely by the inter-blade spacing. Skewing of

the ultrasonic beam or transducer is also necessary to compensate for the geometric

mismatch between the aerofoil and the blade root (Figure 13); thereby optimising the

geometric reflections and maximising sensitivity to defects in the serrations. Skewing of

the beam has the effect of normalising the ultrasound trajectory to the root serrations

and, although straightforward with manual manipulation of the transducer, is more

challenging where access is limited. One solution would be the introduction of 2D

Phased array transducers with which the ultrasonic beam can be steered and focussed in

3 dimensions. There are however technical and commercial reasons why this is not seen

by the author as an effective solution: - transducer cost and limited application, vast

increase in capital equipment, reduced portability, and complexity of phased array law

generation.

Figure 13 Aerofoil – blade root geometric mismatch

In order to perform in-situ inspections where manual manipulation of the PAUT

transducer is difficult, manual scanning frames, commissioned by RWE PI, were

Beam

trajectories

normal to

aerofoil

Ideal refracted

beam trajectories normal to root

Aerofoil-Platform

radius


9

designed and built by Zetec, Canada. The bespoke frames allow the transducer to be

mechanically driven around the aerofoils, via an encoded manual drive module, and

accommodate the ability to manipulate the skew of the ultrasonic beam, see Figure 14.

Full automation utilising motor drives was discounted due to the desire to limit

equipment required and thereby sustain portability. The inclusion of the encoded

manual drive module allows the application of encoded line scans, thereby producing

permanent records of inspection data and allowing offline data processing to increase

detection capabilities.

Figure 14 Zetec Mechanised manual scanner

In order to reduce the inspection development and validation times associated with

new rotor designs, a flexible scanning frame was conceived which would accommodate

a range of rotor designs, whilst retaining all the functionality of the bespoke frame. The

scanner, was developed and built in partnership with Phoenix Inspection Ltd; consisting

of a flexible steel track which is able to conform to the aerofoil when clamped onto the

blade, and allowing fitment to a range of rotors. The PAUT transducer is driven around

the encoded track, enabling the correct scan trajectory around the aerofoil, see Figure

15. This concept offers huge advantages over the bespoke (fixed) scanning frames

which incur significant cost and long lead times.

Figure 15 Flexible scanning frame


10

3.3 Fixed continuous wedge

Further development of the aerofoil scan technique was made by the author to

improve coverage, sensitivity, and coupling of the PAUT transducer. It has been shown

how coverage of the blade root is achieved by scanning around the aerofoil, requiring

skewing to account for the geometric mismatch between them. When encoded line

scans are recorded they must be carried out at fixed skews, requiring several to achieve

sensitive coverage in all target areas. In addition, the wedge designed to refract the

ultrasonic beam into the blade material must be profiled to couple closely with the

aerofoil surface. The profile of the wedge is only correct at a given position around the

scan surface and therefore, either several wedges with different profiles are required

over several separate scans, or the wedge profile is made as a compromise, fitting

reasonably well but not exactly at all positions. The compromise taken in wedge design

has a detrimental effect of coupling and therefore repeatability and sensitivity of the

inspection.

To mitigate these issues the author conceived of a single continuous Rexolite

wedge, one surface of which matches the exact profile of the blade root platform and

aerofoil, and the other face on which the PAUT transducer is scanned, see Figure 17.

Careful modelling of the scanning surface provides the appropriate angles in three

dimensions to refract the ultrasonic beam into the component at the optimised

trajectory. The invention means that, whatever the position of the PAUT transducer

along the scan path, the ultrasonic beam will be refracted into the material at a trajectory

which is normal to the root serrations under interrogation, with no need for skew. As

there is no requirement to mechanically skew the ultrasonic transducer, single line scans

can be taken with consistent sensitivity across the whole scan. Careful design of the

scanning surface of the wedge allows the ultrasonic beam to be further refracted in the

lateral direction, thereby achieving increased coverage toward the outer limits of the

root serrations. The wedge provides perfect coupling to the inspection surface, idealised

sound refraction, ability to single line scan, and improved coverage.

Figure 16 Fixed continuous Rexolite Wedge


11

Figure 17 Effect of the fixed wedge on beam trajectory

4 Conclusions Inspection coverage of the affected areas of CAEFTR designs has been significantly

improved by the innovations in mechanised scanner design and innovative wedge

design. Advanced simulation software tools, coupled with modelling capabilities, and

fast prototyping technology have facilitated the production of bespoke jigs; able to place

PAUT transducers in remote positions and interrogate the most critical root regions,

thereby significantly improving coverage.

The ability to inspect rotors with minimal inter-blade spacing has been achieved

using bespoke scanning frames with innovations such as skewing of the PAUT

transducer. Further development has lead to the invention of flexible scanning frames

which allow new rotor designs to be inspected in-situ without the extensive costs and

lead times associated with fixed frames.

It has been shown how the continuous wedge negates the requirement to skew the

PAUT transducer when scanning around the aerofoil of the blade. Moreover, coupling

to the scanning surface is idealised, single line scans are facilitated, and sensitivity to

small defects in previously un-reachable areas improved. The need to upgrade to 2d

arrays has been mitigated, so existing technology and portability are maintained.

In-situ inspection of steam turbines is critical in reducing the cost of maintenance

and length of service down-time, but and increasing the ability for early detection of

defects. The solutions outlined in this paper offer advanced detection capabilities for

CAEFTR, higher levels of coverage, and offer the station managers a significant saving

over traditional inspection methods.

Ultrasound trajectory

on traditional wedge

Ultrasound trajectory

on continuous wedge


12

References

1. GE Energy, Schenectady, NY, USA, Final report to the US Department of

Energy, High Efficiency Steam Turbines with Extra Long Buckets, December

2005

2. Amir Mujezinovic, GE, Schenectady, NY, USA, Bigger blades cut costs,

Modern Power Systems, February 2003

3. Philippe DUMAS, IMASONIC, Besancon, France, Michael CLOSSEN- VON

LANKEN SCHULZ, Dr. Michael OPHEYS, Hans RAUSCHENBACH,

Siemens Power Generation, Mülheim/Ruhr, Germany, Nondestructive

Inspection of Blade Roots and Blade Attachment Grooves - Ideal Application of

Phased Array Inspection Technique

4. Michael Clossen, Dr Michael Opheys, Hans Rauschenbach, Michael siegel;

Siemens Power Generation, “Lifetime Extension Through Advanced Non-

Destructive Examination Methods”, Power-Gen Europe 30th

May to June 1st

2006, Cologne, Germany.

5. Dr Michael F. Opheys, Hans Rauchenbach, Michael Siegel, Graham Goode;

Siemens AG Power Generation, Detlev Heinrich; Celgelec AT Gmbh & Co,

“Blade Root / Blade Attachment Inspection by Advanced UT and Phased Array

Technique”, 6th

International Charles Parsons Turbine Conference, 16-18

September 2003, Trinity College, Dublin.

6. Hans Rauschenbach, Michael Siegel; Siemens Germany, and Laurent Capponi;

EDF, France, “Modern Non-Destructive Methods for examining Turbine Blades

and Blade Roots in Steam Turbine Service”, International Journal for Electricity

and Heat Generation, VGB Powertech 9/2008.

Documents

Session 2A (1) – Phased Array Applications Chairman – … Phased array ultrasonic testing (PAUT) Axial coverage of the blade roots is achieved by careful manipulation of the ultrasonic