Влияние на долговечность работы высокотемпературных роторов длительной статической прочности и малоцикловой усталости

Abstract

Рассмотрены вопросы влияния на работоспособность и ресурс ротора паровой турбины длительной статической прочности и малоцикловой усталости. Рассмотрены вопросы, связанные с определением остаточного ресурса службы высокотемпературных роторов турбоагрегата. Проанализированные факторы, определяющие длительность надежной эксплуатации турбоустановки: исчерпание длительной пластичности из-за ползучести материала; накопление повреждений в критических зонах роторов из-за малоцикловой усталости. Рассмотрены зоны ротора среднего давления с наиболее высокой вероятностью появления трещин.

A significant part of heat and mechanical equipment at Ukrainian thermal power plants has reached the end of normative and extended operating life. The power plants were commissioned 30-40 years ago and now appear to be obsolete morally and physically. The damage of the elements has increased in the zone of high temperatures and this reduces the technical and economic indicators of turbo-installations and requires additional costs for its repairing. Meanwhile a significant part of the nodes and parts that operate in the zone of moderate temperature remains suitable for further operation. It is necessary to identify the effect of long-term durability and low cycle fatigue on the operation of steam turbine rotors. Design operating life is established taking into account the creep of the material under prolonged static loading and low cycle (thermal) fatigue. The limited period of permissible operation at the stage of fracture development is evaluated during operation, if necessary. The probability of cracks in the stress concentration zones is the highest. Such zones in the turbine rotors appear to be the rims of the discs with slots for shank blades, axial channel and thermal grooves in the seals area, radius transitions from the disk to the rotor body. The thermal stresses for thermal grooves are critical in terms of the probability of cracking, but this zone does not limit the service life of turbines that operate in the base mode. The assessment of low cycle fatigue is carried out in critical zones of the nodes, which are exposed to thermal stresses due to thermal cycling under variable-load operation. It is typical that fractures due to low cycle fatigue appear after the turbine units achieve a working time of 2·105 hours and even less, in some cases. Accumulated irreversible residual deformation in metal due to the creep and thus a metal plastic property decrease is the main feature of the problem of high-temperature rotors operating life. The main factors that limit the failure-free operation of the structure during long-term loading are limiting deformations or destructive stresses depending on the operating conditions. The design creep deformation usually lays within 0,3-1 %. The lower service life total deformation assigned, the bigger is the stress margin with respect to long-term strength. Normative reserve with respect to long-term strength is ensured if the accepted maximum permissible deformation of creep is no more than 1%. Metal creep and low cyclic fatigue are the main factors that determine the operation time of high-temperature rotors.