Steam turbine maintenance planning based on forecasting of life consumption processes and risk analysis

Abstract

Flexible operation of coal-fired power plants contributes to the intensification of the life consumption processes, which is a serious problem especially in the case of units with a long in-service time. In steam turbine rotors, the crack propagation rate and material wear caused by low-cycle fatigue increase. The aim of the research is an attempt to forecast the development of these processes and to estimate the probability of critical elements damage, such as the high-pressure and intermediate-pressure rotors. In the stress state analyses, the finite element method (FEM) is used, the Monte Carlo method and the second order reliability method (SORM) is apply to calculate the probability of failure. It is proposed to use risk analysis to plan preventive maintenance of the turbine. The optimal intervals for carrying out diagnostic tests and prophylactic repairs is determined for various operating scenarios and various failure scenarios. This enables a reduction of the costs while ensuring the safety of the turbine's operation.

Risk-Based Operation and Maintenance Planning of Steam Turbine with the Long In-Service Time

“…The presented research concerns the operation and maintenance of the long in-service 200 MW steam turbine, mainly the high-pressure rotor. In this rotor, two risk areas were identified [5], which are marked in Figure 1. In area I, located in the central bore of the impulse stage, there is a risk of crack propagation, which may cause one of the most serious turbine failures.…”

“…The failure occurs when the wear limit value Z g = 1 for life-consumption processes is exceeded. The probability of such an event is then defined as [5,15]:…”

“…Both the crack propagation and life-consumption processes strongly depend on the stress level during variable load and the quasi-steady state. It is assumed that under creep conditions (after the relaxation process), the stress in area I is approximately 55 MPa [5]. In area II in turn, it is so low that the total creep life is assumed at the level of 10 6 h. The rotor start-up is considered as the variable load state of the greatest importance for the damage processes.…”

“…Maintaining energy production from long-operated 200 MW coal-fired units in Poland is a key task to ensure adequate safety of the power system operation by guaranteeing energy supplies to consumers, as well as balancing the current demand, especially in the presence of unstable renewable sources [1][2][3][4]. Due to the significant life consumption of individual elements [5], it is necessary to develop a strategy for further operation under flexible load regime, with the lowest possible financial outlays for preventive maintenance of the unit.…”

“…The presented article is a continuation of research related to the critical components of 200 MW turbines, such as the high-pressure rotor. The previously published papers [5,14] focused on presenting the methodology for forecasting the development of degradation processes such as crack propagation or low-cycle fatigue. The optimization of time interval of preventive repairs based on the risk analysis was also described.…”