Risk-Based Planning of Diagnostic Testing of Turbines Operating with Increased Flexibility

Abstract

<jats:p>An increase in the share of renewable sources in the energy mix makes coal-fired power plants operate in new conditions that require more dynamic operation and adequate flexibility. The frequency of the power unit start-ups increases and so does the frequency of changes in loads. This intensifies some life consumption processes, such as low-cycle fatigue and crack propagation in the turbine components. Further operation of power unit elements that have already been in service for a long time has to be supplemented with new diagnostic and repair procedures that take into account the intensification of life consumption processes. This article gives predictions about the propagation rate of potential cracks in the turbine rotor for different scenarios of the power unit’s long-term operation. A method is presented of rational selection of the diagnostic testing time based on risk analysis. The method is used to estimate the optimal interval after which diagnostic testing of a 200 MW turbine rotor should be carried out. Changes in the rotor steel crack toughness are evaluated based on the results of testing of microspecimens cut out of the rotor. Turbines with more frequent start-ups and shorter start-up times necessitate performance of diagnostic testing of the rotor central bore after about 12 years of turbine operation.</jats:p>

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

“…For fatigue-creep life consumption, two variants of the initial wear of the material were considered: Z 0 = 0 and Z 0 = 0.5. Similar analyses were presented in [5,14] for other crack initial dimensions and for three operation scenarios in order to present the potential development of destruction processes for turbine rotors. In this paper, the estimation of the crack dimension and the damage degree over time is used to determine safe values of the stresses during start-ups.…”

“…When an analyzed plant is intended to be operated as the power-regulatory unit, some start-ups are carried out "on demand", which means that the heating process may be much faster than in the manufacturer's recommendations. In such a situation, the level of circumferential stresses (σ Z ) affecting the crack propagation in area I may increase to 300 MPa [5,14]. The effective stresses in the thermal grooves (σ red ) then reach the highest level in the entire rotor and may increase to 400 MPa [5], and even more due to the significant contribution of thermal stresses in this area.…”

“…Table 2 presents a summary of the previously determined values. In order to determine the optimal time of carrying out preventive actions, the NPV index [5,14] was used, described by the formula:…”

“…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.…”

On-Line Control of Stresses in the Power Unit Pressure Elements Taking Account of Variable Heat Transfer Conditions

“…This means faster start-ups and more frequent increases and decreases in the power output [1][2][3]. Due to that, large temperature gradients arise in elements of the power unit and of the turbine, and this creates a higher risk of a serious failure [4]. At such an operating regime it is impossible to ensure safety based on start-up characteristics only-it becomes necessary to constantly monitor stress levels in critical areas of machinery and equipment elements.…”