Steam Turbine Last-Stage Blade Root Non-Destructive Testing Inspections Practical Experience

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1 Steam Turbine Last-Stage Blade Root Non-Destructive Testing Inspections Practical Experience Paul McKay RWE npower PAGE 1

2 Overview > Last Stage Blade Design > Root Defects > Inspection Methods > Practical Experience > Validation > Recommendations RWE npower PAGE 2

3 Steam Turbine Last Stage (L-0) Blades Highly Twisted Aerofoil Axial-entry Fir Tree Root Design Curved Root Geometry to Accommodate Aerofoil Twist RWE npower PAGE 3

Pressure side (concave) the stresses are greatest towards the ends of the root On the

4 Steam Turbine Last Stage (L-0) Blades - Failure Locations Areas of highest stress concentration occur in the upper serrations on all designs inspected to date On the Pressure side (concave) the stresses are greatest towards the ends of the root On the Suction side (convex) the stresses are greatest in the central region of the root RWE npower PAGE 4

Cracking can occur as a result of several damage

corrosion Crack initiation location and propagation

5 Steam Turbine Last Stage (L-0) Blades - Failure Locations Observed cracking is consistent with highly stressed areas Cracking can occur as a result of several damage mechanisms including fretting, cyclic fatigue or stress corrosion Crack initiation location and propagation orientation can vary with blade design and damage mechanism RWE npower PAGE 5

6 Steam Turbine Last Stage (L-0) Blades - Inspection Options In-situ Phased Array (PAUT) Ex-situ Fluorescent MT Ex-situ Fluorescent PT Ex-situ Eddy Current (ET) RWE npower PAGE 6

7 Inspection Strategy Options - Process Damage to blade roots suspected Are you aware of the damage mechanism? No NDT Scenario 1 Yes NDT Scenario 2 In-situ inspection: PAUT or Ex-situ inspection: MT, PT, ET Yes Are growth rates and critical crack sizes known? No Initial in-situ inspection: Visual, PAUT, MT (end face), swabbing, etc. Yes Yes Are blades damaged? Are blades damaged? No Maybe Manage Risk Through Component Life Cycle Using Inspection Programme Permanent Solution e.g. Blade Modification Or Replacement Reduce Operational Risk Until Suitable Future Opportunity For Action/Investigation RWE npower PAGE 7

8 Inspection Strategy Options > Known defective blades should always be removed, but the risks may still remain unless remedial action can be taken: > Scenario 1 Damage mechanism is well understood and regular inspection is capable of detecting sub-critical defects Continue operating with periodic inspections (in-situ or ex-situ as appropriate) > Scenario 2 Damage mechanism is not fully understood (or is eventdriven rather than hours/starts-driven) Assess current situation and, depending on outcome, option to continue operating with conservative inspection intervals and/or changes in operation. Inspect after any potentially damaging operational event > This is all well and good in principle but RWE npower PAGE 8

9 What confidence can I have in the NDT inspections? > Both scenarios are dependent on having quantified NDT results > Despite having over ten years of experience in inspecting curved axial entry last stage blade roots, no opportunities to validate NDT techniques on real defects have existed at RWE until 2013 > Phased array techniques have been validated using machined slots (typically 5mm x 1mm) with non-preferential orientation (relative to the ultrasonic beam) to add conservatism > Assumed that fluorescent MT or PT would be capable of detecting >1-2mm long defects if applied carefully, provided surface condition of root is good RWE npower PAGE 9

10 Opportunities for validation > Three separate instances of last-stage blade root cracking have been detected at RWE plant in the past two years > All three instances occurred at separate sites on different blade types from multiple OEMs > All three instances were discovered during planned in-situ inspections utilising PAUT > Subsequent de-blading of rotors provided further evidence to support the intended detection threshold of PAUT RWE npower PAGE 10

11 Station A > Cracks detected in 2 blade roots (5 th inspection of affected turbine) 2 cracks in total > Both blades cracked in upper serration near centre of blade on suction (convex) side > Metallurgical assessment revealed cracks were very fine trans-granular (length 15-25mm, depth <3mm) > Both cracks readily detectable by the initial in-situ PAUT > Neither crack initially detected by subsequent fluorescent MT nor PT > Both cracks readily detectable by subsequent ET RWE npower PAGE 11

12 Station B > Cracks detected in 11 blade roots (1 st inspection) - 15 defects in total, > All blades cracked in upper serration, near centre of blade on suction (convex) side or near outlet end of pressure (concave) side > Metallurgical assessment revealed cracks were facetted near the opening then smoother as the crack deepens, consistent with high-cycle fatigue (HCF) (length mm, depth <5.5mm) > 12 largest cracks readily detectable by initial in-situ PAUT > 14 cracks subsequently detected on site by combination of fluorescent MT or PT > All cracks subsequently detected by ET after de-blading RWE npower PAGE 12

13 Station C > Cracks detected in 17 blade roots (2 nd inspection) 21 cracks in total > All blades cracked in upper serration, near centre of blade on suction (convex) side or near outlet end of pressure (concave) side > Metallurgical assessment revealed cracks generally had a wide oxidefilled opening then became very fine as the crack deepened > length <1-92mm, depth <4.5mm. Multiple initiation sites resulting in long cracks with a shallow profile or long intermittent defects > 7 cracks readily detectable by in-situ PAUT > 16 cracks subsequently detectable by MT and 16 by PT > All cracks detected by ET after de-blading RWE npower PAGE 13

14 Phased Array UT Performance > Overall Detection 21 out of 38 defects > Validation based on detection of 5mm long x 1mm deep slots > 16 of 17 missed defects either below this threshold or of intermittent nature with individual elements below threshold > Remaining missed defect 6mm, depth unknown to date > Smallest defect detected (of those currently sectioned for metallurgical assessment): 4.7 x 1.1mm on pressure side RWE npower PAGE 14

15 Phased Array UT Performance Continuous crack detectable by PAUT Intermittent crack NOT detectable by PAUT RWE npower PAGE 15

16 MT Performance > Overall Detection 29 out of 38 defects > Initial on-site MPI using technique compliant with BS EN ISO detected 25 of 38 defects. Second inspection party detected fewer still. > Very small and very fine (tight) cracks not detected unlikely to produce sufficient flux leakage > By improving surface condition (polishing) and modifying technique, MT performance increased, but still did not detect all defects RWE npower PAGE 16

17 MT Performance comparison of techniques Original MT Technique No Defect Detected Optimised MT Technique Intermittent Defect Detected RWE npower PAGE 17

18 PT Performance > Overall Detection 25 out of 38 defects > Initial on-site PT using technique compliant with BS EN detected 25 of 38 defects. Note: not all 25 detected were the same as were detected by MT > Very small cracks not detected > Very fine (tight) cracks only detectable after 20 hour soak/24 hour development > By improving surface condition (polishing) and modifying technique (long soak and development times), PT performance increased, but still did not detect all defects RWE npower PAGE 18

19 Eddy Current Testing Performance > All known defects in all blades tested are detectable with eddy current (either using arrays, where available, or single coil probes) > Very small cracks, including intermittent defects, can be detected, however techniques are not fully validated > Signal-to-noise ratio for small/intermittent defects is poor, so further work needs to be done to minimise false calls > Appears to give greatest sensitivity to defects and relatively short inspection time, but this comes at cost of high capital expenditure for array probes conformed to root geometry RWE npower PAGE 19

20 Approximate Defect Detection Thresholds* 10 Defect Depth 1 PAUT, ET, MT, PT (mm) ET, MT, PT ET only 1 10 Defect Length (mm) * Based on data using RWE techniques on the three case studies in this presentation RWE npower PAGE 20

21 Summary of Full Results Station Defects Present Total Defects Detected PAUT MT PT ET A B * 7* 15 C [20] Total [29] * - Results for optimised techniques in laboratory conditions not available - Revised total after extensive surface preparation beyond requirements of BS EN ISO RWE npower PAGE 21

22 Conclusions > Experience gained with these three inspections has helped to quantify the ability of various NDT techniques > No NDT technique gives a complete guarantee of a defect free blade > MT/PT results are dependant on technique application and specific competency/training of inspection personnel > Based on these examples, a properly designed eddy current testing inspection should give the greatest confidence in finding small defects, but requires blades to be removed from the rotor and may lead to false calls > Where in-situ inspection is desirable, periodic phased array inspection can give confidence that cracks above a certain size (approximately 5mm long by 1mm deep in these examples, using RWE proprietary techniques) are not present so that turbines can be run at reduced risk RWE npower PAGE 22

23 Further Information > Ultrasonic Inspection of Curved-Entry Turbine Blade Roots Presented at Power-Gen Europe, Amsterdam, 8 th -10 th June 2010: Paper/Presentation available on request > Contact: Mr Paul McKay paul.mckay@rwenpower.com Dr Chris Ward chris.ward@rwenpower.com RWE npower PAGE 23

Qualification of New Component- Specific Inspection Techniques for Turbine & Generator Service

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