A. V. Koroliov, Ya. O. Komarova
Odessa National Polytechnic University,
1, Shevchenko ave., Odesa, 65044, Ukraine
The features of transferring turbines to power below nominal are discussed in the article. This topic is due to the underload of NPP units, as well as the policy of transferring NPPs to maneuvering modes. A turbine unit is the main equipment of a nuclear power plant, taking the load from a generator connected to the power grid.
Thermodynamic and mechanical view on the operation of a turbine with throttle steam distribution at reduced load is presented in the article. It was taken into account that the power maneuvering mode is realized by changing the power on the turbine shaft with a constant operating frequency.
It is shown that when the turbine power changes to 75 % of the nominal value, the steam speed decreases by 13.4%, and when the power changes by 50 %, the speed decreases by about 30%.
From the energy conservation equation and from geometric considerations on the trapezoid of the expansion process, a similarity is drawn in the h-s diagram, which showed
that in order to reduce the turbine power it is necessary to reduce the available heat drop per stage more significantly and, accordingly, significantly increase the throttle rate of the steam going to the turbine.
The analysis of the dependence of energy conservation efficiency (ЕСЕ) turbines from the degree of throttling of steam at the turbine inlet, leading to a change in the ratio of the peripheral speed of the blade and the absolute steam velocity at the exit of the nozzle grate is conducted. It is also shown that the steam velocity at the exit of the nozzle stage depends on two independent factors — a decrease in mechanical power on the shaft and a decrease in steam pressure in front of the nozzle stage.
It is also shown in the article, that in the case of decrease in the power of turbine, a more significant role is played by an increase in the vibration of the working blades when steam enters not on the edge but on the back of the blade, as well as an increase in the axial force acting on the turbine shaft.
To study this phenomenon, an experimental study of the vibration of the scapula was carried out, which showed almost linear increase in the level of vibration of the scapula with a change in the angle of attack.
It is shown that the reduced power mode leads not only to a decrease in the efficiency of the turbine, but also to an increase in the vibration of the blades due to the deviation of the angle of attack of the blades from the design.
Keywords: turbine, nuclear power plant, efficiency, maneuvering modes, blade vibration.
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