Design of main units and parts of steam turbines

General concepts of the structure of steam turbines

Main technical requirements for steam turbines and their characteristics

A steam turbine is a rotary blade engine in which the pressure energy of the steam coming from the boiler is first converted into the kinetic energy of the steam flowing out of the nozzles at high speed, and then, on the rotor blades, into the mechanical energy of shaft rotation. Nozzles are guide vanes designed to convert the internal energy of steam into the kinetic energy of the ordered motion of molecules.

Superheated steam obtained in the steam generator at a temperature of 600 C and a pressure of 30 MPa is transferred to the nozzles through steam pipes.

If the steam had a certain initial velocity and initial pressure before entering the nozzle, then after leaving the nozzle, as a result of the expansion of the steam, its velocity increases to a value and the pressure decreases to a value. The velocity of steam entering the working blade is called the absolute velocity. The steam temperature also drops significantly.

After leaving the nozzle, the steam is fed to the turbine blades. If the turbine is active, then there is no expansion of steam between its blades, therefore, the steam pressure does not change. The absolute speed of steam movement decreases from to due to the rotation of the turbine at a speed of V. V is the peripheral or transfer speed.

Structurally, the turbine is made in the form of several stages, each of which consists of one crown of nozzle blades and one crown of blades.

Jet turbines are those turbines in which the expansion of steam occurs not only in the nozzles before the steam enters the blades, but also on the blades of the working wheel itself. This is achieved by the fact that the channel formed by the blades is made narrowing.

Further expansion of steam to pressure occurs in the channels between the blades. The absolute speed of steam in the nozzle increases to a value, and at the beginnings between the blades decreases due to the rotation of the blades to a value.

Currently, turbines are made multi-stage, and the same turbine can have both active and reactive stages.

Steam turbine device

The turbine consists of three cylinders (HP, MPC and LPC), the lower halves of the housings of which are designated 39, 24 and 18, respectively. Each of the cylinders consists of a stator, the main element of which is a stationary housing, and a rotating rotor. The half coupling of the rotor of the electric generator (not shown) is connected to the half coupling 12, and the rotor of the exciter is connected to it. The chain of assembled individual rotors of cylinders, generator and exciter is called shaft line. Its length with a large number of cylinders (and the largest number in modern turbines is 5) can reach 80 m.

The shaft line rotates in liners and plain support bearings on a thin oil film. As a rule, each of the rotors is placed on two support bearings. The steam expanding in the turbine makes each of the rotors rotate, the powers arising on them are added up and reach the maximum value on the half-coupling.

Each of the rotors is placed in cylinder body. At high pressures (and in modern turbines it can reach 30 MPa » 300 atm), the cylinder body (usually HPC) is made double-walled. This reduces the pressure difference on each of the housings, allows its walls to be made thinner, facilitates tightening of flange connections and allows the turbine to quickly change its power if necessary.

All housings must have horizontal connectors 13, which are necessary for installing rotors inside the cylinders during assembly, as well as for easy access inside the cylinders during inspections and repairs. The steam inside the turbine has a high temperature, and the rotor rotates in liners on an oil film, the temperature of which, both for fire safety reasons and the need to have certain lubricating properties, should not exceed 100 °C (and the temperature of the supplied and discharged oil should be even lower). Therefore, the bearing shells are removed from the cylinder housings and placed in special structures - supports Thus, the rotating ends of each of the rotors of the corresponding cylinder must be removed from the non-rotating stator, and in such a way as to exclude any (even the slightest) contact of the rotor with the stator, on the one hand, and to prevent significant steam leakage from the cylinder into the gap between the rotor and the stator, on the other, since this reduces the power and efficiency of the turbine. Therefore, each of the cylinders is equipped with end seals of a special design.

The turbine is installed in the main building of the thermal power plant on the upper foundation plate. Rectangular windows are made in the plate according to the number of cylinders, in which the lower parts of the cylinder bodies are placed, and the pipelines feeding the regenerative heaters, fresh and reheated steam lines, and the transition pipe to the condenser are also output.

After manufacturing, the turbine undergoes control assembly and testing at the manufacturer's plant. After that, it is disassembled into more or less large blocks, brought to a good marketable condition, preserved, packed in wooden boxes and sent for installation at the thermal power plant.

When the turbine is operating, steam from the boiler through one or more steam lines (this depends on the turbine capacity) first goes to the main steam valve, then to the stop valve (one or more) and, finally, to the control valves. From the control valves, steam is fed through bypass pipes to the steam inlet chamber of the HPC inner casing. From this cavity, the steam enters the flow part of the turbine and, expanding, moves to the HPC outlet chamber. In this chamber, in the lower half of the HPC casing, there are two outlet pipes. Steam pipes are welded to them, directing steam to the boiler for intermediate superheating.

The reheated steam enters through the pipelines through the stop valve to the control valves, and from them - to the steam inlet cavity of the MPC. Then the steam expands in the flow part of the MPC and enters its outlet pipe, and from it - into two bypass pipes (sometimes they are called receiver pipes), which supply steam to the steam inlet chamber of the LPC. HPC and MPC, LPC are almost always made double-flow: having entered the chamber, the steam diverges into two identical flows and, having passed them, enters the outlet pipes of the LPC. From them, the steam is directed downwards into the condenser. In front of the front support, there is a turbine control and regulation unit. Its control mechanism allows starting, loading, unloading and stopping the turbine.

A typical working blade consists of three main elements: a profile section 1; a tail 2, used to attach the blade to the disk; a spike 6 of a rectangular, round or oval shape, made on the end of the profile section of the blade in one piece.

The blades are made of stainless steel containing 13% chromium, by stamping and subsequent milling and are assembled on the disk through two special wells, into which the locking blades with tails of a special shape are then installed.

A bandage tape is rolled separately, in which holes are punched corresponding to the shape of the spikes and the distance between them. The tape is cut into pieces with a strictly calculated number of blades to be combined. The bandage tape is put on the studs, which are then riveted. A row of adjacent blades (usually from 5 to 14), connected by a bandage tape (bandage), is called a package of working blades. The main purpose of packaging is to ensure vibration reliability of the working blades (to prevent their breakage from fatigue due to vibrations). After riveting the studs on the bandages of the working blades, the rotor is installed on a lathe and the seal ridges are finally turned.

The main element of the flow part of the turbine, determining its entire appearance, is the working blade of the last stage. The longer it is and the larger the diameter on which it is installed (in other words, the larger the area for the passage of steam of the last stage), the more economical the turbine. Therefore, the history of turbine improvement is the history of the creation of the last stages. In the early 50s, LMZ developed a 960 mm long working blade for the last stage with an average diameter of 2.4 m, and on its basis turbines with a capacity of 300, 500 and 800 MW were created. In the late 70s, a new 1200 mm long working blade was created for a stage with an average diameter of 3 m. This made it possible to create a new steam turbine for thermal power plants with a capacity of 1200 MW and for nuclear power plants with a capacity of 1000 MW.

The oil for lubricating the shaft journals is supplied by pumps from an oil tank installed at the lower mark of the condensation room. The size of the oil tank depends on the turbine power: the higher the power, the more cylinders and, consequently, rotors and their supports that require lubrication. In addition, with increasing power, the diameter of the journals increases, and these two circumstances require a large oil consumption and, accordingly, a large-capacity oil tank, reaching 50-60 m3. For lubricating the bearings, either special (turbine) mineral oil or synthetic non-flammable oils are used. The latter are much more expensive, but more fireproof.

From the pumps, the oil passes through the pipelines through oil coolers and goes to the tanks located in the bearing caps, and from there to the holes and to the sample that distributes the oil over the entire width of the shaft journal. Due to hydrodynamic forces, the oil is "driven" under the shaft journal, and thus the shaft "floats" on the oil film, without touching the babbitt filler. The oil, having passed under the shaft journal, exits through the end gaps of the liner and flows to the bottom of the bearing housing, from where it is directed by gravity back to the oil tank.

Types of steam turbines and areas of their use

To understand the place and role of steam turbines, let's consider their general classification. Of the wide variety of steam turbines used, first of all, we can distinguish transport and stationary turbines.

Transport steam turbines are most often used to drive the propellers of large ships.

Stationary steam turbines are turbines that maintain their location unchanged during operation. This book only considers stationary steam turbines.

In turn, stationary steam turbines can be classified according to a number of features.

1. By purpose, turbines are divided into power, industrial and auxiliary ones.

Power turbines are used to drive an electric generator included in the power system and to supply heat to large consumers, such as residential areas, cities, etc. They are installed at large state district power plants, nuclear power plants and thermal power plants. Power turbines are characterized, first of all, by high power, and their operating mode is a constant rotation frequency, determined by the constancy of the network frequency.

Industrial turbines are also used to produce heat and electricity, but their main purpose is to service an industrial enterprise, for example, a metallurgical, textile, chemical, sugar refinery, etc. Often, the generators of such turbines operate on a low-power individual electrical network, and are sometimes used to drive units with variable rotation frequency, such as blast furnace blowers. The capacity of industrial turbines is significantly less than that of power turbines.

Auxiliary turbines are used to ensure the technological process of electricity production - usually to drive feed pumps and boiler blowers.

Feed pumps of power units with a capacity of up to 200 MW are driven by electric motors, and those with a higher capacity are driven by steam turbines fed by steam from the main turbine extraction. For example, power units with a capacity of 800 and 1200 MW are equipped with two and three feed turbo pumps with a capacity of 17 MW each, respectively, while power units with a capacity of 250 (for CHP) and 300 MW have one feed turbo pump with a capacity of 12 MW; At 1000 MW power units for NPPs, two 12 MW feed pumps are used.

The boilers of 800 and 1200 MW power units are equipped with two and three blowers, respectively, which are also driven by steam turbines with a capacity of 6 MW each. The main manufacturer of auxiliary steam turbines in Russia is KTZ.

2. By the type of energy received from the steam turbine, they are divided into condensing and cogeneration.

In condensing turbines (type K), steam from the last stage is discharged into the condenser, they do not have adjustable steam extraction, although, as a rule, they have many unregulated steam extractions for regenerative heating of feedwater, and sometimes for external heat consumers. The main purpose of condensing turbines is to ensure the production of electricity, so they are the main units of powerful thermal power plants and nuclear power plants. The capacity of the largest condensing turbo units reaches 1000-1500 MW.

Cogeneration turbines have one or more adjustable steam extractions, in which a given pressure is maintained. They are designed to generate heat and electricity, and the largest of them has a capacity of 250 MW. A cogeneration turbine can be made with or without steam condensation. In the first case, it can have heating steam extractions (T-type turbines) for heating network water for heating buildings, enterprises, etc., or industrial steam extraction (P-type turbines) for the technological needs of industrial enterprises, or both (PT and PR-type turbines). In the second case, the turbine is called a back-pressure turbine (P-type turbines). In it, steam from the last stage is sent not to the condenser, but usually to the industrial consumer. Thus, the main purpose of a back-pressure turbine is to produce steam of a given pressure (within 0.3-3 MPa). A backpressure turbine may also have a controlled heating or industrial steam extraction, and then it is classified as a TR or PR type.

Heating turbines with heating steam extraction (type T) are designed so that at maximum heating load the stages located after the extraction zone do not generate power. In recent years, a number of turbines have been designed so that even at maximum load the last stages generate power. Such turbines are of the TK type.

3. Based on the initial steam parameters used, steam turbines can be divided into turbines of subcritical and supercritical initial pressure, superheated and saturated steam, without intermediate superheating and with intermediate superheating of steam.

As is already known, the critical pressure for steam is approximately 22 MPa, therefore all turbines, the initial steam pressure before which is less than this value, are classified as steam turbines of subcritical initial pressure. In Russia, the standard subcritical pressure for steam turbines is chosen equal to 130 atm (12.8 MPa), in addition, there is a certain percentage of turbines for an initial pressure of 90 atm (8.8 MPa). Subcritical parameters are used for all steam turbines for NPPs and CHPs (except for the 250 MW cogeneration turbine), as well as turbines with a capacity of less than 300 MW for CHPs. The subcritical initial pressure of foreign steam turbines is usually 16-17 MPa, and the maximum unit capacity reaches 600-700 MW.

All powerful condensing power units (300, 500, 800, 1200 MW), as well as a 250 MW cogeneration power unit, are designed for supercritical steam parameters (SCP) — 240 atm (23.5 MPa) and 540 °C. The transition from subcritical steam parameters to SCP allows saving 3-4% of fuel.

All turbines of thermal power plants and combined heat and power plants operate with superheated steam, and nuclear power plants - with saturated steam (with a low degree of humidity).

All powerful condensing turbines for subcritical and supercritical steam parameters are manufactured with intermediate superheat. Of the cogeneration turbines, only the LMZ turbine for subcritical parameters with a capacity of 180 MW and the TMZ turbine on SCP with a capacity of 250 MW have intermediate superheat. Obsolete condensing turbines with a capacity of 100 MW and less and numerous cogeneration steam turbines up to a capacity of 185 MW are built without intermediate superheat.

4. Based on the turbine usage zone in the electrical load schedule, steam turbines can be divided into base and semi-peak. Base turbines operate continuously at or near nominal load. They are designed so that both the turbine and the turbine unit have the highest possible efficiency. Nuclear and cogeneration turbines should certainly be classified as this type of turbine. Semi-peakturbines are designed to operate with periodic stops at the end of the week (from Friday night until Monday morning) and daily (at night). Semi-peak turbines (and turbo units), given their small number of operating hours per year, are made simpler and, accordingly, cheaper (for reduced steam parameters, with a smaller number of cylinders).
5. By design features, steam turbines can be classified by the number of cylinders, rotation speed, and number of shaft lines.

By the number of cylinders, turbines are distinguished as single- and multi-cylinder. The number of cylinders is determined by the volumetric steam flow at the end of the expansion process. The lower the steam density, i.e. the lower its final pressure, and the greater the turbine power, i.e. the greater the mass flow, the greater the volumetric flow and, accordingly, the required area for steam to pass through the working blades of the last stage. However, if the working blades are made longer and their rotation radius is larger, then the centrifugal forces tearing off the profile part of the blade can increase so much that the blade will break off. Therefore, with an increase in power, they first switch to a two-flow LPC, and then increase their number. Condensing turbines can be made with one cylinder up to 50-60 MW, two cylinders - up to 100-150 MW, three cylinders - up to 300 MW, four cylinders - up to 500 MW, five cylinders - up to 1300 MW.

By rotation frequency, turbines are divided into high-speed and low-speed. High-speed turbines have a rotation frequency of 3000 rpm = 50 rpm. They drive an electric generator, the rotor of which has two magnetic poles, and therefore the frequency of the current it generates is 50 Hz. Most steam turbines for thermal power plants, combined heat and power plants and partially for nuclear power plants in our country and almost all over the world are built for this frequency. In North America and in parts of Japan, high-speed turbines are built for a rotation frequency of 3600 rpm = 60 rpm, since the accepted grid frequency there is 60 Hz.

Earlier it was said that since the low initial parameters make the steam efficiency in NPP turbines low, and reducing capital costs requires increasing the capacity, i.e. the mass of steam passed through, the volumetric flow rate at the turbine outlet is so significant that it is advisable to switch to a lower rotation frequency. Since the number of magnetic poles in the electric generator must be whole and even, the transition to using a four-pole electric generator and obtaining the same grid frequency as with a two-pole electric generator requires a halving of the frequency. Thus, low-speed turbines in our country have a rotation speed of 1500 rpm = 25 rpm.

For NPPs built for warm climate conditions, i.e. for high cooling water temperature and, accordingly, high pressure in the condenser), it is possible to build high-speed nuclear turbines. Steam is supplied to the HPC of the turbine from the reactor compartment through four steam pipelines. After passing the HPC, the steam is supplied to the vertical SPP, and after them, with the help of a receiver, it is distributed to three identical two-flow LPCs. Each LPC has its own condenser, which is also clearly visible on the model.

By the number of shaft lines, turbines are divided into single-shaft (having one shaft line — rotors of individual cylinders and a generator connected by couplings) and twin-shaft (having two shaft lines, each with its own generator and connected only by steam flow).

Main technical requirements for steam turbines and their characteristics

In order to see how perfect a machine a steam turbine is, it is enough to consider the technical requirements imposed on it. They are formulated in state standards (GOST). Here we will focus only on the most important of them.

First of all, a number of requirements are imposed on the turbine, which can be covered by one term — reliability. Reliability of a technical object is its ability to perform specified functions in a specified volume under certain operating conditions. In relation to a steam turbine, reliability is uninterrupted power generation with the provided fuel consumption and the established system of operation, maintenance and repairs, as well as the prevention of situations dangerous to people and the environment.

It is important to emphasize that the concept of reliability includes the concept of efficiency. A turbine that operates smoothly but with low efficiency due to wear or with limited power due to internal failures cannot be considered reliable. Reliability is a complex property characterized by such subproperties as failure-free operation, durability, maintainability, storability, controllability, survivability, and safety. Without going into strict definitions of these subproperties, we will note the main ones.

Failure-free operation is the property of a turbine to continuously maintain a working condition for a certain period of time. The mean time between failures for thermal power plant turbines with a capacity of 500 MW or more must be at least 6,250 hours, and at least 7,000 hours for smaller power plants, and at least 6,000 hours for NPP turbines. If we take into account that there are 8,760 hours in a calendar year and that the turbine is out of operation for some time (for example, on the instructions of the power system dispatcher), this means that failures due to the turbine's fault should occur on average no more than once a year.

The full established service life of a thermal power plant turbine must be at least 40 years, and at least 30 years for NPP turbines. In this case, two important circumstances are stipulated. First, this service life does not apply to quickly wearing parts, such as rotor blades, seals, and fasteners. For such parts, the average service life before major repairs (inter-repair period) is important. In accordance with GOST, it must be at least 6 years (in addition, a planned system of routine and scheduled preventive maintenance is implemented at thermal power plants and nuclear power plants).

For thermal power plant turbines, or more precisely for their parts operating at temperatures above 450 °C, in addition to such a durability indicator as service life, another indicator is introduced - resource - the total operating time of the turbine from the start of operation until reaching the limit state. At the design stage, the limit state is defined as the designated resource. By definition, this is a resource upon reaching which the operation of the turbine must be stopped regardless of its technical condition. In fact, upon reaching the designated resource, the turbine can retain significant additional operability (residual resource) and, given its high cost, the service life of the turbine is extended. Given the illogicality of the term "designated resource" in relation to the turbine, the term "design resource" began to be used. Thus, the estimated (assigned) resource is the operating time of the turbine, which is guaranteed by the manufacturer; when it is reached, the question of its further operation should be considered.

For many years, the estimated resource was 100 thousand hours, now it is usually 200 thousand hours. The most important requirement for a turbine is high efficiency. The efficiency of a turbine is estimated by the efficiency of its cylinders.

The efficiency of a cylinder is characterized by the share of the working capacity of steam that can be converted into mechanical energy. The highest efficiency is shown by the MPC: in good turbines it is 90-94%. The efficiency of the HPC and LPC is significantly lower and averages 84-86%. This decrease is due to the significantly more complex nature of the steam flow in grates of very small (several tens of millimeters in the first stages of the HPC) and very large (1 m or more) in the last stages of the LPC grate height. It is difficult to calculate this flow and select blade profiles for it even with modern computing tools. In addition, a significant part of the flow path of the LPC operates with wet steam, moisture droplets have a velocity significantly lower than steam, and have a braking effect on the rotating working blades.

In addition to the technical requirements listed above, GOST contains numerous other requirements, in particular, to the turbine protection system in emergency situations, to maneuverability (the range of long-term operation - usually 30-100% of the rated power; the duration of start-up and shutdown, the number of possible starts, etc.), to the turbine regulation and control system, to maintainability and safety (fire safety, vibration level, noise, etc.), methods for monitoring the parameters of working media (steam, oil, condensate), transportation and storage.