Basics of Steam Turbine Operation
From the standpoint of compliance with the operating characteristics of the STU during their operation, the main attention is paid to the constant and variable operating modes of the steam turbine
Constant operating mode of the steam turbine
For modern powerful turbo plants at thermal and nuclear power plants with a unit capacity from several hundred MW to 1000-1500 MW, which, as a rule, are operated in a constant maximum load mode, such indicators as efficiency, reliability, durability and maintainability come to the fore.
The efficiency of the STU is characterized by both the efficiency factor (efficiency) of the turbo unit (TU) and the specific gross heat consumption (i.e. excluding energy costs for the TU's own needs). The efficiency indicators for cogeneration turbine plants with adjustable extraction for heating and hot water supply are the specific steam consumption in the cogeneration mode, the specific heat consumption in the condensation mode, the specific heat consumption for electricity generation, etc. The specific gross heat consumption for high-power condensing turbines is at the level of 7640–7725 kJ/(kW h); for thermal power plants – 10200 kJ/(kW h) and 11500 kJ/(kW h) for nuclear power plants. The specific gross heat consumption for cogeneration turbine units at a cooling water temperature of 20°C in condensation mode is approximately 8145–9080 kJ/(kW h), and the specific steam consumption in cogeneration mode is no more than 3.6–4.3 kg/(kW h).
Reliability and durability are characterized by a number of quantitative indicators, such as mean time between failures, total assigned service life, total assigned resource of components, average service life between major repairs, technical utilization factor, availability factor, and others. The total assigned service life of a power unit manufactured before 1991 is at least 30 years, and that of equipment manufactured after 1991 is at least 40 years. The total assigned resource (park resource) of the main components operating at temperatures above 450°C is 220 thousand operating hours. For high-power turbines, the mean time between failures is set at no less than 5,500 hours and the availability factor is no less than 97%.
The variable operating mode of a steam turbine primarily involves changing the steam flow through the flow path - downwards from the nominal value. In this case, minimum losses with variable, i.e. "partial", steam flow are achieved with nozzle regulation, when the valves (valve) servicing one specific group of nozzles are fully open. Heat drops change significantly only at the regulating and last stage of the flow path. Heat drops of intermediate stages remain almost constant with a decrease in steam flow through the turbine. The operating conditions of the intermediate stages and, consequently, the efficiency all stages of high pressure (except the first stage), medium pressure and low pressure (except the last stage) remain practically unchanged.
The greater the lift of the valve servicing any one group of nozzles, the smaller the increment in flow per "unit" of its lift. Upon reaching h/d ? 0.28 (where h is the linear displacement of the valve when it opens, and d is the valve diameter), the increment in steam flow through the valve practically stops. Therefore, to ensure a smooth loading process, it is envisaged to open the valve servicing the next group of nozzles with some "overlap", i.e. somewhat earlier than the previous valve opens completely.
For the last stage of the low-pressure cylinder, a decrease in the relative volumetric flow rate of steam to a value below 0.4 GV 2 leads to the formation of vortices in the main flow both at the root of the last stage working blades and at their periphery, which is dangerous from the point of view of dynamic off-design stresses in these blades, which are already loaded to the limit.
Basics of Steam Turbine Operation
The requirements for maneuverability and reliability of modern steam turbines during their operation are related to the general operating conditions of power systems, daily and annual energy consumption schedules, the structure of generating capacities in power systems, their condition and technical capabilities. Currently, the schedules of electrical loads of power systems are characterized by great unevenness: sharp load peaks in the morning and evening hours, dips at night and on weekends, when it is necessary to ensure a rapid increase and decrease in loads. Maneuverability is understood as the ability of a power unit to change its capacity during the day to cover the load schedule of the power system. In this regard, the periods of loading and unloading of the turbo unit, as well as start-up from various thermal states (hot - after a preliminary downtime of less than 6-10 hours, not cooled - after a preliminary downtime from 10 hours to 70-90 hours, cold - after a preliminary downtime of more than 70-90 hours) are important. Also taken into account are the number of stops and starts over the entire service life, the lower limit of the adjustment range, i.e. the lower limit of the load interval when the power changes automatically without changing the composition of the auxiliary equipment, and the possibility of operating at the load of own needs after the load is reset.
The reliability of the power unit largely depends on how well the turbine itself and its auxiliary equipment are protected from the hazardous effects of non-stationary processes. Statistics on equipment damage show that the overwhelming majority of failures occur precisely at the moment of transient operating modes, when one or another set of parameters changes. In order to avoid the development of an emergency situation, an emergency shutdown of the turbine is used: with or without a vacuum breakdown.
With a vacuum breakdown, the turbine (for turbines with a rotor speed of 3000 rpm) should be stopped immediately in the following cases: when the speed increases above 3360 rpm; in case of sudden increase in vibration by 20 µm (vibration velocity of 1 mm/s) or more on any of the bearings; in case of sudden increase in oil temperature at the drain of any bearing above 70°C; in case of drop in oil pressure on the bearings below 0.15 MPa; in case of increase in babbitt temperature of any of the bearings above 100°C.
Sudden forced shutdown is also necessary in case of any impacts in the flow part of the turbine, rupture of steam pipes, any ignition on the turbine or generator.
Shutdown without vacuum breakdown is provided for in case of the following deviations from the normal operating mode: in case of deviation of fresh steam or reheat steam parameters by the value: up to ±20°C – for temperature and up to +0.5 MPa – for fresh steam pressure; in case of sudden change in fresh steam or reheat steam temperature at a rate of more than 2°C per minute; after 2 minutes of generator operation in motor mode; if atmospheric membranes in the exhaust pipe of the low-pressure cylinder are damaged; if oil leaks are detected.
Turbine protection systems for powerful steam turbines provide for shutdown upon reaching the following values: upon reaching the rotor axial shift of –1.5 mm towards the regulator or +1.0 mm towards the generator (the protection is triggered by a vacuum breakdown in the condensers); upon reaching the relative expansion of the RND-2 (low-pressure rotor) of –3.0 mm (the rotor is shorter than the casing) or +13.0 mm (the rotor is longer than the casing); upon an increase in the temperatures of the exhaust pipes of the low-pressure cylinder to 90°C and above; upon a drop in the oil level in the oil tank by 50 mm (immediate shutdown of the turbine is required).
The operation of turbines at full or partial constant load is provided in accordance with the factory operating instructions. The turbine start-up is also regulated by detailed factory instructions and does not allow deviations from the specified start-up schedules.