engleski

Determination of Useful Life of Steam and Gas Turbines Parts

Today the damages due to low cycle thermal fatigue(LCTF) are a major cause of fracture (of breakdowns) of steam and gas turbine parts. LCTF is often the result of overtop of allowable range of plastic deformation induced by cyclic thermal stresses that occur mostly at transient loading (e.g. during start-up and shut-down). Most parts, subject to thermally induced stresses simultaneously operate in the creep range (C), which additionally exhausts the allowable plastic deformation. In that case it comes current conditions in design, manufacturing and exploitation of the steam and gas turbines. Therefore already in their design phase special attention is paid to the calculation and analysis of useful life in LCTF&C conditions. In such a manner it becomes a part of the design procedure of modern turbomachinery, with the aim of realising sufficiently long, safe, useful life (about 200-300 thousand hours) at high manoeuvrability of operation (103-104 or more changes in operation conditions or complete shut-downs) and high efficiency. Determination of the useful life becomes even more complex due to numerous factors: increase of mass and dimensions of parts of steam and gas turbines, increase in the working parameters (power, temperature, pressure), demand for manoeuvrability of operations due to large inequality of loading diagrams of electric power systems, increased demands on the low- cycle stresses due to variations in the mode of operation, construction and manufacturing technology of components, the level of starting technological micro-defects, activity of working fluid, etc. Among classical methods there is no worked-out in detail, safe enough, generally accepted method for useful life estimation of steam and gas turbines components. Results from most studies show that the majority of available methods can predict useful life to within a factor 2 to 3. In determining the useful life in LCTF&C conditions the basic difficulties are related to determination of stresses state and the form of real loading cycles. At present, the reliability of calculation of stresses state is considerably increased by means of numerical methods. The approach for determination of useful life in LCTF&C conditions of steam and gas turbines parts, which will be presented in this paper is classical method in which calculation of stresses state by numerical modelling is used. Because an error of 10% in the stresses level can falsify the useful life by up to 100%, for estimated regimes it is necessary to calculate the state of stresses with highest accuracy. Therefore special attention at calculation of the stresses state is dedicated to setting of thermal and mechanical boundary conditions. The method consists from determination of the useful life of part until crack initation and remaining useful life of part after crack initiation. The calculation by this approach will be presented on "Jugoturbina" steam turbine which was about 30 years in exploitation in "Petrokemija d.d. - Kutina" with aim at determination remaining useful life. The metod is also applicable on the other high thermal loaded equipments of power stations, e.g. steam boilers, steam pipelines, etc.

steam and gas turbines; useful life; low cycle thermal fatigue; creep

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