Phased Array Inspections of Low Pressure Steam Turbine blade Curved Roots

PHASED ARRAY ULTRASONIC INSPECTION OF LOW PRESSURE STEAM TURBINE CURVED BLADE ROOTS

1 Public Domain Information

TWO INSPECTION APPROACHES: MANUAL & AUTOMATED

The inspection is designed to detect stress corrosion cracking and fatigue damage in fir tree serrations on low pressure turbines.

Defects are generally found in the top serration of the blade root, however they can also be found in all serrations, on both the blade root and disc steeple.

Inspection Techniques often use a combination of conventional UT and phased array technology, in order to ensure coverage is maximised.



1.DEVELOPMENT OF MANUAL INSPECTION TECHNIQUE



2.DEVELOPMENT OF AUTOMATED INSPECTION TECHNIQUE Originally presented at Power-Gen Europe 2010: The following paper has been edited to make for easier reading and is a good example

of Advanced NDT developments in Power Generation.

PHASED ARRAY ULTRASONIC INSPECTION OF LOW PRESSURE STEAM TURBINE ROTORS

CURVED AXIAL ENTRY FIR TREE ROOTS

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; the curvature of the root attachment allows more

flexibility in the aerodynamic design of the aerofoil with improved inter-blade spacing.

The tendency is 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 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.

1 Introduction

Low Pressure (LP) steam turbine 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; engineered to overcome the mechanical forces while providing the ideal shape for efficient

aerofoil dynamics. The tendency of these blade root designs to suffer failure induced by stress corrosion

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

Figure 1 below illustrates typical the design configurations, 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.

Figure 1 Curved Axial Entry Fir Tree Roots

2.1 Areas of vulnerability

Industry experience of failures of these blade configuration type shows that Stress Corrosion Cracking (SCC),

has been the most prevalent type of defect. SCC is 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 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.

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



2.2 Scanning Surfaces

Due to the complex geometries, there are limited available surfaces 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



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 surfaces, 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.



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 of 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 inspections, 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.

The flexibility of fast prototyping, allows all couplant feeds and fixing mechanisms to be incorporated into the

jig, see Figure 11. This allows PAUT transducers to be placed accurately in remote positions, with positive and

repeatable detection in critical regions with increased coverage, see Figure 12.

Figure 11 Typical Jig

Figure 12 Typical inspection jig refracting through radii



3.2 Implementation of mechanised aerofoil scans

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

In order to perform in-situ inspections where manual manipulation of the PAUT transducer is difficult, special

manual scanning frames had to be designed. The special 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 Examples of Manual Scanners



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.



4 Conclusions

Inspection coverage of the affected areas of curved blade root 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, higher levels of coverage, and offer a significant saving over traditional

inspection methods.

Acknowledgements:

Tediko – Czech Republic

Chris Charlesworth

Phased Array Inspections of Low Pressure Steam Turbine blade Curved Roots

Engineering

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