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.
• 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
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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.
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
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