Development of life assessment procedures for power plant headers operated under flexible loading scenarios

Abstract

Publication InformationFarragher, TP, Scully, S, O Dowd, NP, Leen, SB (2013) 'Development of life assessment procedures for power plant headers operated under flexible loading scenarios'. International Journal Of Fatigue, 49 :50-61. PublisherElsevier ScienceDirect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Abstract A finite element methodology for thermomechanical fatigue analysis of a subcritical power plant outlet header under realistic loading conditions is presented. The methodology consists of (i) a transient heat transfer model, (ii) a sequential anisothermal cyclic viscoplastic model and (iii) a multiaxial, criticalplane implementation of the Ostergren fatigue indicator parameter. The methodology permits identification of the local thermomechanical stress-strain response at critical locations and prediction of fatigue life and cracking orientation for complex transient, anisothermal, cyclic elastic-plastic-creep material behaviour. Measured plant data, in the form of steam and pipe temperature transients and steam pressure data, are employed to identify heat transfer constants and validate the predicted thermal response, with particular attention given to plant start-up and attemperation effects. The predictions indicate out-of-phase temperature-strain response at the header inside surface and in-phase response on the outside surface. Cooling transients are predicted to control damage and crack initiation at the inner bore, whereas heating transients are predicted to have a more damaging effect at weld locations. A representative test cycle is presented, which is shown to capture the salient thermo-mechanical cyclic damage of the realistic cycle. The predicted results correlate well with industrial experience in terms of crack (initiation) orientation, location and life. IntroductionThere is a need to understand the performance of candidate power plant materials for higher temperatures and pressures. 9-12% Cr steels are an example of such candidate materials, owing to their excellent mechanical properties, and steels such as P91 and P92 are already being utilised in power generation plant [1]. There is an increasing trend towards shifting of energy supply from conventional fossil fuel power plant to sustainable energies, e.g. wind power. To allow for the variable nature of renewable energy supply, there is a requirement to operate existing plant, designed for 'base-load' operation, with relatively infrequent shutdown and start-up, in 'load-following' mode, characterised by increased frequency of shutdown and start-up. This issue has increased the importance of design and assessment of materials and components for high temperature, pressurised plant with respect to thermomechanical fatigue (TMF) and creep-fatigue failure.Current header design does not consider cyclic thermal stresses and load cycling that can lead to creep-f...

A dislocation-based model for high temperature cyclic viscoplasticity of 9–12Cr steels

“…Thus, the initial lath width is predicted to have no effect on the plastic strain range at 600 °C. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 13 0.05 α 2 3.0 Q CR [68] 400 kJ/mol D Fe,0 [69] 1.6 10 -4 m 2 /s Q δ [42] 303 kJ/mol d e [14] 6b r M23C6 [41] 45.0 nm δ 0 [15] 0.7 μm r NbC [41] 25.0 nm f M23C6 [53] 0.019 r VN [41] 20.0 nm f NbC [53] 0.001 ρ i,0 [21] 1.0 10 11 mm -2 f VN [53] 0.004 t w [21] 100 b 1.E-12…”

“…This change is driven by (i) a requirement to reduce emissions from fossil fuel plant, (ii) the control of energy cost by continuous mixing of energy sources and (iii) the intermittent nature of renewable energy. The complex operational cycles [1,2] which current and future plant must endure, therefore, leads to thermo-mechanical fatigue (TMF) and creep-fatigue (CF) failure. These failures commonly occur within the heat affected zone (HAZ) of welded connections (Type IV cracking) [3], sometimes as early as 5-10% of the plant design life.…”

Multiaxial cyclic viscoplasticity model for high temperature fatigue of P91 steel

“…4, illustrates the ability of the model to operate across a broad range of strain-rates. This result enables reliable extrapolation from accelerated laboratory test conditions to the strainrates typically observed in modern plant [8,9,22]. Fig.…”

“…P91 steel is a 9Cr martensitic steel typically used for plant header and piping systems due to its high creep strength [3]. A number of material models capable of simulating the constitutive behaviour of candidate materials already exist, including the unified Chaboche model [4][5][6][7] and the twolayer viscoplasticity model [8,9]. However, a hyperbolic sine material model has been chosen as it is more representative of the key mechanisms of deformation [10], enabling strain-rate independence of the material parameters and offers greater accuracy through the full stress regime [1,2,11].…”

Stress analysis in the beater relief of a beater wheel

“…Stosunkowo rzadko opisywane autorskie rozwiązania dotyczą problemów, w których istotne zjawiska mają charakter lokalny. Wśród nich dużą grupę stanowią publikacje dotyczące zagadnień z pogranicza zmęczenia, mechaniki pękania oraz niezawodności w różnych dziedzinach techniki [7][8][9]12]. Sporo publikacji poświęcono analizie zjawisk w różnych połączeniach: śrubowych [10,14] oraz spawanych [11,13].…”