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Research Paper: Gas Turbines: Structures and Dynamics

Crack Initiation in 14% Cr Low Pressure Turbine Blade Steel

[+] Author and Article Information
W. Hahn

EDF Energy,
West Burton Power Station,
Retford, Nottinghamshire DN22 9BL, UK
e-mail: wolfgang.hahn@edfenergy.com

G. Tasker

EDF Energy,
West Burton Power Station,
Retford, Nottinghamshire DN22 9BL, UK
e-mail: geoff.tasker@edfenergy.com

E. Naylor

Consultant-Fluids
Fraser Nash Consultancy Ltd.,
Cayman House,
First Avenue, Centrum 100,
Burton-upon-Trent, Staffordshire DE14 2WN, UK
e-mail: e.naylor@fnc.co.uk

M. Kidd

School of Mechanical, Civil
and Aerospace Engineering,
University of Manchester,
P.O. Box 88, Pariser Building,
Manchester M60 1QD, UK
e-mail: moray.kidd@manchester.ac.uk

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 18, 2013; final manuscript received December 27, 2013; published online February 11, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 136(6), 062504 (Feb 11, 2014) (7 pages) Paper No: GTP-13-1425; doi: 10.1115/1.4026453 History: Received November 18, 2013; Revised December 27, 2013

The impact of multiple erosion pits and crack initiation was investigated for a 500 megawatt (MW) steam turbine unit with three low pressure (LP) rotors on the steam end and generator end of the stage L0 blades. These units have been subjected to two-shifting operation and have been retrofitted with new high pressure (HP) turbine units over the life history of the turbines. Droplet erosion damage was exacerbated by operating conditions causing multiple crack initiation sites concentrated above the root platform. A method of accumulated damage was employed using pit counting and the number of cycles referenced back to turbine revolutions in line with the accumulated damage model developed from the damage function analysis and Palmgren–Miner approaches. The number of rotational cycles were calculated from the starts and running hours for pre- and post-retrofit scenarios and compared and correlated to the number of pits formed during the completed cycles. The macro crack size represented the critical crack size or a damage number of one. It was found that there was a significant shift in the number of rotations before and after the HP turbine retrofit to achieve a damage rate of one. An accumulated damage model was developed for the post HP turbine retrofit and the LP turbine last stage blades fitted from new, based on the empirical evidence from the analysis. Assessments on the erosion distribution in the zoned areas revealed evidence of cracking, manifesting 18 mm away from the highest probability distribution with a standard deviation of 2 mm. The area where cracking first initiated on multiple samples was found to coincide with the mechanical change in the section blending in with the blade trailing edge. The damage model was implemented on a ive running plant and successfully applied over a period of two years using the most conservative approach, based on the lower bound values.

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References

Engelhardt, G., Macdonald, D. D., Zhang, Y., and Dooley, B., 2005, “Deterministic Prediction of Corrosion Damage in Low Pressure Steam Turbines,” Centre for Electrochemical Science and Technology, The Pennsylvania State University, University Park, PA.
MinerM. A., 1945, “Cumulative Damage in Fatigue,” ASME J. Appl. Mech., 67, pp. A159–A164.
Lalanne, C., 2002, Fatigue Damage, Mechanical Vibration and Shock, Hermes Penton Science, London.
O'Connor, P. D. T., Newton, D., and Bromley, R., 2002, Practical Reliability Engineering, 4th ed., John Wiley and Sons Ltd, West Sussex, UK.
Naylor, E., 2013, “Exhaust Hood Survey and Analysis,” Frazer Nash Consultancy— Systems and Engineering Technology, Ref. No. FNC/42289/72495V.

Figures

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Fig. 1

Damage rate versus number of rotational cycles for different wetness fractions

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Fig. 2

Outlined area under investigation—blade 36

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Fig. 3

SEM specimen images of the area under investigation—blade 36

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Fig. 4

SEM of specimen at 100 μm for the area under investigation showing 80 μm pit size

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Fig. 5

Trailing edge damage observation from blade 42

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Fig. 6

Droplet recirculation path and areas of erosion impact from the CFD analysis. (a) Droplet size impact during off design conditions as modeled and visually observed.

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Fig. 7

First sign of erosion damage on newly fitted last stage blades for unit 2

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Fig. 8

(a) and (b) pitting rate as a function of running hours for unit 2

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Fig. 9

Pitting rate as a function of running hours and starts for unit 3

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Fig. 10

Accumulated damage for rotations to failure

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Fig. 11

Gaussian distribution deviation due to mechanical features in the blade

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Fig. 12

Gaussian distribution displayed with reference to observed erosion damage

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Fig. 13

Model for accumulated damage leading to crack initiation

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