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Research Papers: Gas Turbines: Turbomachinery

Integrated Weld Quality Concept—A Holistic Design Approach for Steam Turbine Rotor Weld Joints

[+] Author and Article Information
C. Borgmann

Siemens AG,
Division Power and Gas,
Rheinstr. 100,
45478 Muelheim, Germany
e-mail: borgmann@siemens.com

P. Dumstorff, T.-U. Kern, H. Almstedt, K. Niepold

Siemens AG,
Division Power and Gas,
Rheinstr. 100,
45478 Muelheim, Germany

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 14, 2015; final manuscript received August 13, 2015; published online October 13, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(4), 042601 (Oct 13, 2015) (7 pages) Paper No: GTP-15-1285; doi: 10.1115/1.4031441 History: Received July 14, 2015; Revised August 13, 2015

The today's energy market requires highly efficient power plants under flexible operating conditions. Especially, the fluctuating availability of renewables demands higher cycling of fossil fired power plants. The need for highly efficient steam turbines is driven by CO2 reduction programs and depletion of fossil resources. Increased efficiency requires higher steam temperatures up to 630 °C in today's units or even more for future steam power plants. The gap between material properties in the hot and cold running parts of a steam turbine rotor is widened by increased live steam temperatures and the increased demand for flexibility. These technical challenges are accompanied by economic aspects, i.e., the market requirements have to be met at reasonable costs. The welding of steam turbine rotors is one measure to balance required material properties and economical solutions. The rotor is a core component of the steam turbine and its long-term integrity is a key factor for reliable and safe operation of the power plant. An important aspect of weld quality is the determination of permissible size of weld imperfections assessed by fracture mechanics methods. The integrity of rotor weld joints is assured by ultrasonic inspection after the final post weld heat treatment with respect to fracture mechanics allowable flaw sizes. This procedure usually does not take credit from the quality measures applied during monitoring of the welding process. This paper provides an overview of a holistic design approach for steam turbine rotor weld joints comprising the welding process and its improved online monitoring, nondestructive evaluation, material technology, and its fracture mechanics assessment. The corresponding quality measures and their interaction with fracture mechanics design of the weld joint are described. The application of this concept allows to exploit the potentials of weld joints and to assure a safe turbine operation over life time.

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References

Figures

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Fig. 1

Portfolio of welded steam turbine rotors at Siemens for low, intermediate, high pressure application

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Fig. 2

Mock-up weld for 3.5Ni–3.5Ni LP rotor with diameter of 1500 mm including artificial reflectors for nondestructive testing

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Fig. 3

Calculation procedure for the fracture mechanic assessment of the rotor weld

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Fig. 4

Cross section of the TIG NG technique for mock-up weld 10Cr-2Cr

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Fig. 5

(a) Principle of the TIG NG. 1: Oscillating TIG electrode. 2: Cold wire. 3: Oscillating motor. 4: NG work piece. 5: Weld metal. 6: Shielding gas. (b) Visual online control of the welding arc via camera monitoring.

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Fig. 6

Potential welding defects in TIG weld joints (size is not to scale)

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Fig. 7

Ultrasonic scanning directions for inspection of rotor weld joints. R, radial. T, tangential. A, axial. d, depth of weld. a, axial distance. Da, outer diameter. Di, inner diameter.

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Fig. 8

Ultrasonic inspection results on mock-up weld with artificial reflectors

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Fig. 9

Integrated weld quality concept (IWQC)

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Fig. 10

Reduction of potential welding defects by IWQC. Flaw size density distribution is shifted to smaller values for both mean (size) and variance (scatter).

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