Abstract
The problem of reducing the axial dimensions of steam turbine diaphragms is associated with the problem of steam turbine modernization performed by increasing the number of reactive blading stages and using existing foundations. Evaluation of the suitability of diaphragm design versions with established steam flow characteristics was carried out with constraints on short- and long-term strength conditions, as well as the accumulation of axial deflections due to creep. For computational research, there was introduced a methodology using the finite element method and Yu. M. Rabotnov’s theory of strain aging. The calculation of creep was reduced to solving an elastic-plastic problem with a deformation curve, which was represented by an isochronous creep curve for the time chosen. A software was used providing for the automated construction of the original computer diaphragm model with the help of guide-vane profile drawings and axial cross-sections of the diaphragm rim and body, as well as several geometric parameters. The calculated model of a welded diaphragm reproduces the main essential features of the structure, the material properties of its elements, as well as steam load. The exploratory studies of diaphragms with reduced axial dimensions were performed on the example of the second- and third-stage diaphragms of the high-pressure cylinder of the K-325-23.5 steam turbine. The original second- and third-stage diaphragm designs were considered to be basic, in relation to which, according to strength and rigidity parameters, the alternative ones were compared. Calculated data for the basic diaphragm design versions for 100 thousand operating hours were obtained. According to the calculations, maximum deflections are achieved at diaphragm edges, and the stresses, that are maximum at the points where the guide vanes are attached to the diaphragm rim and body, undergo a significant redistribution due to creep. Two approaches to the reduction of the axial dimensions of the second-stage diaphragm design of the steam turbine high pressure cylinder were involved. In the first approach, the reduction of the dimensions was achieved by proportionally reducing the guide-vane profile with a corresponding increase in the number of the guide vanes. In the second approach, the profile remained unchanged, but the axial dimensions of the diaphragm rim and body were reduced. The parameters of strength both in the elastic state at the beginning of operation and in the conditions of creep, as well as the accumulation of axial deflections were investigated. Based on the comparisons with the basic design, it was established that the second approach is more effective. Additional recommendations for the use of more heat-resistant steels for outlet guide vanes and the conditions of diaphragm attachment in the turbine casing are given.
Highlights
One of the aspects of deep turbine modernization is to increase the number of stages, in particular in the high-pressure cylinder (HPC), and the use of reactive blading, which can significantly increase efficiency
There arises the problem to assess the possibility of structural changes in the most loaded diaphragms of a steam turbine HPC according to the criteria of strength in high temperature conditions, which is considered on the example of the K-325-23.5 steam turbine
It is in the elastic region that stress deformations in the diaphragms acquire maximum values, and the assessment of short-term strength is performed according to the calculations for the elastic state
Summary
One of the aspects of deep turbine modernization is to increase the number of stages, in particular in the high-pressure cylinder (HPC), and the use of reactive blading, which can significantly increase efficiency. Steam turbine modernization is performed using the existing foundation, which necessitates the reduction of the axial dimensions of the stages. The simplest way is to reduce the axial dimensions due to the greater compactness of steam turbine diaphragms. It is necessary to ensure the basic indicators of long-term rigidity and strength of the diaphragms. There arises the problem to assess the possibility of structural changes in the most loaded diaphragms of a steam turbine HPC according to the criteria of strength in high temperature conditions, which is considered on the example of the K-325-23.5 steam turbine.
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