What knowledge do you need to know about steam turbine operation?

principle of operation

Steam turbine is a kind of external combustion rotating machinery which can convert steam thermal energy into mechanical work. After the steam from the boiler enters the steam turbine, it passes through a series of annular nozzles and blades in turn, converting steam heat energy into mechanical energy for the rotation of the steam turbine rotor. Steam is converted into energy in different ways in a steam turbine, which constitutes a steam turbine with different working principles.

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supporting facilities

Steam turbines usually work at high temperature, high pressure and high speed. It is a relatively precise heavy machinery, which generally needs to work in coordination with boilers (or other steam generators), generators (or other driven machinery), condensers, heaters and pumps.

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structural parts

It consists of a rotating part and a stationary part. The rotor includes a main shaft, an impeller, moving blades and a coupling. Stator includes steam inlet, steam turbine cylinder, diaphragm, stator cascade, gland seal and bearing, etc.

cylinder

The cylinder is the shell of the steam turbine, and its function is to separate the flow passage of the steam turbine from the atmosphere to form a closed steam chamber, thus ensuring that the steam completes the energy conversion process inside the steam turbine. Nozzle chamber, diaphragm, diaphragm sleeve and other components are installed in the cylinder. The outside of the cylinder is connected with steam inlet, exhaust and extraction pipes.

Generally, the high-pressure section of the cylinder adopts alloy steel or carbon steel casting structure, and the low-pressure section can adopt casting structure or welded structure formed by welding simple castings, section steel and steel plates according to the requirements of capacity and structure.

High-pressure cylinder has two forms: single-layer cylinder and double-layer cylinder. Single-layer cylinders are mostly used for steam turbines with medium and low parameters. Double cylinders are suitable for steam turbines with relatively high parameters. Divided into high-pressure inner cylinder and high-pressure outer cylinder. The high-pressure inner cylinder is separated by a horizontal split surface to form an upper cylinder and a lower cylinder, and the inner cylinder is supported on the horizontal split surface of the outer cylinder. The high-pressure outer cylinder is supported on the front bearing box by four cat claws. The cat claws are cast together from the lower cylinder and located at the upper part of the lower cylinder to keep the support point on the horizontal center line.

The intermediate pressure cylinder consists of an inner intermediate pressure cylinder and an outer intermediate pressure cylinder. The middle pressure inner cylinder is separated on the horizontal dividing plane to form an upper cylinder and a lower cylinder. The inner cylinder is supported on the horizontal split surface of the outer cylinder, and the outer boss machined on the outer cylinder and the annular groove on the inner cylinder cooperate with each other to maintain the axial position of the inner cylinder. The intermediate pressure outer cylinder is separated by a horizontal split surface to form an upper cylinder and a lower cylinder. The intermediate pressure outer cylinder is also supported by two pairs of cat claws on the middle bearing box and the front bearing box of the 1 low pressure cylinder.

The low-pressure cylinders are of reverse split type, and each low-pressure cylinder consists of an outer cylinder and two inner cylinders, all of which are welded by plates. The upper half and the lower half of the cylinder are vertically divided into three parts, but during installation, the vertical joint surface of the upper half of the cylinder has been connected into a whole by bolts, so the upper half of the cylinder can be hoisted as a part. The low-pressure outer cylinder is supported by the apron, which is integrated with the lower half of the cylinder and extends to both ends along the lower half of the cylinder. The low-pressure inner cylinder is supported on the outer cylinder. Each skirt seat plate is installed on the foundation seat plate, which is fixed on the foundation by grouting. The position of the low-pressure cylinder is fixed by the sliding pin between the skirt plate and the foundation plate.

rotator

The rotor as a whole is made of alloy steel forgings. At the governor end of the high-pressure rotor, a rigid coupling is connected with a long shaft, and this section is installed on the shaft.

There is a main oil pump and an overspeed trip mechanism.

After all rotors are processed and all blades are assembled, full-speed rotation test and accurate dynamic balance are carried out.

Sleeve rotor: Impeller, shaft sleeve, coupling and other parts are processed separately, and then hot-sleeved on the stepped shaft. Interference fit is adopted between each component and the spindle to prevent the impeller from loosening due to centrifugal force and temperature difference, and the torque is transmitted by the key. Sleeve structure is often used for the rotors of medium and low pressure steam turbines and the low pressure rotors of high pressure steam turbines. At high temperature, the impeller and spindle are easy to loosen. Therefore, it is not suitable for high-pressure rotors of high-temperature steam turbines.

Integral forging rotor: the impeller, shaft sleeve, coupling and other parts are integrally forged and cut with the spindle, and there are no hot sleeve parts, which solves the problem that the connection between the impeller and the shaft is easy to loosen at high temperature. This kind of rotor is often used in high and medium pressure rotors of large steam turbines. Compact structure, strong adaptability to start-up and variable working conditions, suitable for running at high temperature, and good rotor rigidity, but large forgings, high processing technology requirements, long processing cycle, and the quality of large forgings is difficult to guarantee.

Welded rotor: the low-pressure rotor of steam turbine has a large mass and bears a large centrifugal force. When the sleeve rotor is used, the inner hole of the impeller will undergo large elastic deformation during operation, so it is necessary to design a large assembly interference, but this will cause large assembly stress. If the integral forging rotor is used, the quality is difficult to guarantee, so the welded rotor combined with sectional forging and welding is adopted. It is mainly formed by splicing and welding a plurality of impellers and an end shaft. The welded rotor has light weight, small forgings, compact structure and high bearing capacity. Compared with the integral forged rotor with the same size and central hole, the welded rotor has high strength, good rigidity and light weight, but it requires high welding performance. The application of this kind of rotor is limited by welding technology, inspection method and material type.

Combined rotor: it is composed of integral forging structure and sleeve structure, and has the advantages of both rotors.

connect

The coupling is used to connect the rotor of the steam turbine and the generator and transmit the torque of the steam turbine to the generator. There are three kinds of couplings commonly used in modern steam turbines: rigid couplings, semi-flexible couplings and flexible couplings.

Rigid coupling:

The coupling has simple structure and small volume; Work without lubrication and noise; But the transmission of vibration and axial displacement requires a high degree of neutrality.

Semi-flexible coupling

The right coupling and the main shaft are forged into a whole, and the left coupling is sleeved on the opposite shaft end with a hot sleeve and a double key. Two pairs of wheels are connected by a corrugated semi-flexible sleeve, and the sleeve is fastened by two bolts. The corrugated sleeve is rigid in the torsion direction and rigid in the bending direction. This kind of coupling is mainly used between steam turbine and generator to compensate the height difference caused by vacuum pumping, temperature difference and hydrogen charging between bearings, which can reduce the mutual interference of vibration and has low centering requirements. It is usually used for devices with medium capacity.

Flexible coupling usually has two forms, gear type and snake spring type.

This coupling can weaken or eliminate the transmission of vibration. The requirement for neutrality is not high, but it needs lubrication during operation, which is complicated in production and high in cost.

Stator blade

The diaphragm is used to fix the stator blades and divide the cylinder into several steam chambers.

rotor blade

The moving blades are installed on the rotor impeller or drum wheel, receiving the high-speed airflow sprayed by the nozzle cascade, converting the kinetic energy of steam into mechanical energy, and making the rotor rotate.

The turbine turns.

Generally speaking, a leaf consists of three parts: leaf shape, leaf root and leaf tip.

The blade profile is the working part of the blade, and a steam channel is formed between the blade profiles of adjacent blades. When steam flows, kinetic energy is converted into mechanical energy. According to the change law of blade profile section, blades can be divided into straight blades with equal cross section, straight blades with variable cross section, twisted blades and curved and twisted blades.

Straight blade with equal cross section: the profile and area of the cross section are the same along the height direction of the blade, which is convenient to process and low in manufacturing cost, which is beneficial to realize the universality of the blade profile at some stages. But its aerodynamic performance is poor, and it is mainly used for short blades.

Bending and torsion blade: The continuous torsion of the centroid line of the cross section can well reduce the profile loss of long blades, which has good fluctuation characteristics and strength, but the manufacturing process is complicated and is mainly used for long blades.

Blade root is a connecting part that fixes the blade on the impeller or drum. Ensure firm connection under any working conditions, and strive for simple manufacture and convenient assembly.

T-shaped blade root: it is convenient to process and assemble, and is mostly used for medium and long blades.

Fungal leaf roots: high strength, widely used in large machines.

Forked blade root: simple processing, convenient assembly, high strength and good adaptability.

Fir-tree-type leaf root: the leaf root has large bearing capacity, good strength adaptability and convenient disassembly, but it is complex to process and requires high precision. Mainly used for blades with heavy load.

Short blades and medium-long blades of steam turbines are usually connected together at the tip of the blade with a shroud to form a blade group. Long blades are only connected in the middle of the blade body through lacing, or are free blades.

The role of the shroud: increase the stiffness of the blade, change the natural vibration frequency of the blade and avoid vibration, thus improving the vibration safety of the blade; Reduce bending stress caused by steam flow; The blade can form a closed channel, and a shroud seal can be installed to reduce the air leakage loss at the top of the blade.

Lacing: The function of lacing is to increase the stiffness of the blade and improve its vibration characteristics. However, lacing increases the loss of steam flow, and at the same time, lacing will weaken the strength of the blade. Therefore, under the condition of meeting the vibration requirements of blades, lacing should be avoided as far as possible, and some long blades are designed as free blades.

vapour lock

The gap between rotor and static body will lead to steam leakage, which will not only reduce the efficiency of the unit, but also affect the safe operation of the unit. In order to prevent steam leakage and air leakage, there must be a sealing device, usually called a steam seal.

Steam turbine

According to different installation positions, the gland can be divided into runner gland, diaphragm gland and shaft end gland.

bearing

Bearing is an important part of steam turbine, which is divided into radial support bearing and thrust bearing, and is used to bear all the weight of rotor.

Turbine force and determine the correct position of the rotor in the cylinder.

1. Multi-wedge bearing (three oil wedges, four oil wedges): light load, large power consumption, high-speed small machine.

2. Circular bearing: able to bear heavy load and high tile temperature.

3. Elliptical bearing: can bear heavy load.

4. Tilting tile bearings: 2, 4, 5 and 6 watt bearings with good stability, wide bearing range and high fuel consumption.

5. Thrust bearing: 1) Fixed tile type: small bearing capacity, used for small units.

2) Tilting tile type:

① Close contact type: line contact on the back of ceramic tile.

② kingsbury type: point contact on the back of ceramic tile.

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kind

There are many types of steam turbines and different classification methods.

By structure

There are single-stage turbines and multi-stage turbines; Single-cylinder steam turbines installed in one cylinder at all levels, and multi-cylinder steam turbines packaged in several cylinders at all levels; Single-shaft steam turbines installed on one shaft at all levels, and double-shaft steam turbines installed on two parallel shafts at all levels.

According to the working principle

Impulse turbine whose steam mainly expands in nozzles (or blades) at all levels; Reactive turbine with steam expanding in stator and rotor blades; And the kinetic energy of the steam expanded in the nozzle is utilized on several rows of moving blades.

According to thermal characteristics

There are condensing, heating, back pressure, extraction and saturation turbines. The steam discharged from condensing turbine flows into the condenser, and the exhaust pressure is lower than the atmospheric pressure, so it has good thermal performance and is the most commonly used turbine. The heating turbine not only provides power to drive generators or other machinery, but also provides heat for production or life, with high heat utilization rate; Back-pressure turbine The exhaust pressure of which is greater than the atmospheric pressure; Extraction steam turbine is a steam turbine that can extract steam from the intermediate stage for heating; Saturated steam turbine is a steam turbine with saturated steam as new steam.

According to the purpose

It can be divided into power plant turbines, industrial turbines and marine turbines.

By number of cylinders

It can be divided into single-cylinder turbine, double-cylinder turbine and multi-cylinder turbine.

other

In addition, it can also be classified according to initial steam pressure (low pressure, medium pressure, high pressure, ultra-high pressure, subcritical and supercritical) and arrangement mode (uniaxial and biaxial).

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Marine steam turbine

Steam turbine is an important power plant on modern ships, including steam turbine and gas turbine. The steam turbine uses the steam discharged from the boiler to rush to the impeller with blades through the nozzle, and the impeller rotates to drive the propeller to propel the ship. Steam turbine has high power and efficiency, and is suitable as the main engine of large ships.

The gas turbine first heats the air through the compressor and then enters the combustion chamber. Fuel burns in the combustion chamber to produce high-temperature gas, which then enters the turbine and impacts the blades on the turbine, making the turbine rotate at high speed and driving the propeller to work. Gas turbines do not need boilers, so they are light in weight, small in size and high in power, and can be used as the main engine of large ships.

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superiority

Compared with the reciprocating steam engine, the steam flow in the steam turbine is continuous and high-speed, and the flow rate per unit area is large, so it can generate more power. High-power steam turbines can use higher steam pressure and temperature, so the thermal efficiency is higher. /kloc-since the 20th century, the development of steam turbines has been to increase the single unit power and improve the thermal economy of the unit on the basis of continuously improving safety, reliability and durability and ensuring convenient operation.

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development prospect

The appearance of steam turbine promoted the development of electric power industry. By the beginning of the 20th century, the power of a single steam turbine in power station had reached 10 MW. With the increasing application of electric power, the peak load of power stations in new york and other big cities in the 1920s approached 65,438+000 MW. If the power of a single machine is only 10 MW, it is necessary to install nearly 100 units. Therefore, in the 1920s, the unit power was increased to 60 MW, and in the early 1930s, 1.65 MW and 208 MW steam turbines appeared.

Since then, the economic recession and the outbreak during the Second World War have made the increase of single turbine power stagnate. In 1950s, with the post-war economic development, the demand for electricity increased by leaps and bounds, and the single unit power began to increase continuously, and large steam turbines of 325-600 MW appeared one after another. In 1960s, a 1000 MW steam turbine was built. A 1300 MW steam turbine was built in 1970s. At present, the single machine power widely used in many countries is 300 ~ 600 MW.

Steam turbines are widely used in various sectors of social economy. There are many types of steam turbines and different classification methods. The steam of steam turbine expands from the inlet to the outlet, and the volume of steam per unit mass increases by hundreds or even thousands of times, so the height of blades at all levels must be lengthened step by step. The exhaust area of high-power condensing steam turbine is very large, and the last stage blades must be made very long.

The thermal economy of steam turbine device is expressed by the heat consumption rate or thermal efficiency of steam turbine. The heat consumption rate of steam turbine is the steam heat consumed per unit output mechanical work, and the thermal efficiency is the ratio of output mechanical work to steam heat consumed. For the whole power station, the boiler efficiency and power consumption of the power station should also be considered. Therefore, the heat consumption rate of the power station is higher than that of a single steam turbine, and the thermal efficiency of the power station is lower than that of a single steam turbine.

A power station with a total power of 1000 MW turbine generator consumes about 2.3 million tons of standard coal every year. If the absolute value of thermal efficiency can be increased by 1%, 60,000 tons of standard coal can be saved every year. Therefore, the thermal efficiency of steam turbine devices has been paid attention to. In order to improve the thermal efficiency of steam turbine, in addition to continuously improving the efficiency of steam turbine itself, including improving the blade profile design at all levels (reducing flow loss) and reducing the loss of valves and exhaust pipes, measures can also be taken from the perspective of thermodynamics.

According to the principle of thermodynamics, the higher the new steam parameters, the higher the thermal efficiency of thermal cycle. The pressure and temperature of new steam used in early steam turbines are low, and the thermal efficiency is lower than 20%. With the increase of single machine power, the fresh steam pressure has increased to 3 ~ 4 MPa and the temperature is 400 ~ 450℃ in the early 1930s. With the continuous improvement of high-temperature materials, the steam temperature is gradually increased to 535℃, the pressure is also increased to 6 ~ 12.5 MPa, and some of them have reached 16 MPa, and the thermal efficiency is above 30%. In the early 1950s, a steam turbine with a fresh steam temperature of 600℃ was used. Then there was a new steam turbine, and the steam temperature reached 650℃.

According to the different output power of modern large steam turbines, the new steam pressure can be divided into various pressure grades. Usually, supercritical parameters such as fresh steam pressure of 24.5 ~ 26 MPa, fresh steam temperature and reheat temperature of 535 ~ 578℃ or subcritical parameters such as fresh steam pressure of 16.5 MPa, fresh steam temperature and reheat temperature of 535℃ are adopted. The thermal efficiency of these turbines is about 40%.

In addition, the lower the exhaust pressure of steam turbine, the higher the thermal efficiency of steam cycle. However, the exhaust pressure mainly depends on the vacuum degree of the condenser, which in turn depends on the temperature of the cooling water and the equipment for vacuumizing (usually called vacuum pump). If the exhaust pressure is too low, it is necessary to increase the flow rate of cooling water, increase the heat exchange area between cooling water and condenser cooling medium, reduce the temperature of cooling water used and vacuum the equipment, so that the last stage blades are longer. At the same time, however, low vacuum will accelerate the steam flow of steam turbine cylinder (low pressure cylinder) and aggravate the differential expansion of steam turbine cylinder (low pressure cylinder). The common exhaust pressure of condensing turbine is 5~ 10 kPa (a standard atmospheric pressure is 10 1325 Pascal). In order to reduce the weight and size of marine steam turbines, the exhaust pressure of 0.006 ~ 0.0 1 MPa is often used.

In addition, the measures to improve the thermal efficiency of steam turbine include regenerative cycle, reheat cycle and heating steam turbine. Improving the thermal efficiency of steam turbine is of great significance to energy saving.

Developing large-scale steam turbines is an important direction for the development of steam turbines in the future, in which developing longer last stage blades is the key to further developing large-scale steam turbines. The research on improving thermal efficiency is another direction of steam turbine development, and it is an important trend to adopt higher steam parameters and secondary reheating, develop peak shaving units and popularize the application of heating steam turbines.

The number of steam turbines in modern nuclear power plants is increasing rapidly, so it is of great significance to study steam turbines with good performance suitable for different reactor types.

1983 The installed capacity of steam turbines using geothermal energy in the world has reached 3 190 MW, but the utilization of deep geothermal resources with higher temperature such as lava remains to be explored. Steam turbine power station using solar energy is already under construction, and ocean temperature difference power generation is also under study. All these new energy turbines have yet to be tested and studied.

In addition, in the process of steam turbine design, manufacture and operation, it is also an important content of steam turbine research in the future to adopt new theories and technologies to improve the performance of steam turbines. For example: three-dimensional flow theory in gas dynamics, wet steam two-phase flow theory; Finite element method of strength and fracture mechanics analysis: fast Fourier transform, modal analysis and laser technology in vibration: computer technology in design, manufacturing technology, test and measurement and operation monitoring; Ultrasonic detection and loss calculation in life monitoring. In addition, the application of new working fluids such as freon, as well as new structures, new processes and new materials will be developed.

At present, the bottleneck of development is mainly in materials. The material problem is solved, and the power of a single chip can be greater.

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Common problems of steam turbine

In the process of steam turbine operation, steam turbine leakage and cylinder deformation are the most common equipment problems, and the tightness of cylinder joint surface directly affects the safe and economic operation of the unit. It is an important job to repair and scrape the cylinder joint surface to make it tight. In the process of dealing with steam leakage at the joint surface, the reasons should be carefully analyzed, and various methods should be comprehensively used according to the deformation degree and gap size to meet the requirements of tight joint surface.

Causes of air leakage in steam turbine cylinder

1. The cylinder block is cast, so it must be aged after leaving the factory, that is, it must be stored for a period of time, so as to completely eliminate the internal stress generated in the casting process of the cylinder block. If the aging time is short, the treated cylinder will be deformed in the future operation, which is why some cylinders will still leak steam in the future operation after the first steam leakage treatment. Because the cylinder is still deforming.

2. The stress on the cylinder during operation is very complicated. Besides bearing static loads such as the pressure difference between gas inside and outside the cylinder and the weight of components installed in it, it also bears the reaction force of steam flowing out of the stator and the force of various connecting pipes on the cylinder under cold and hot working conditions. Under the interaction of these forces, the cylinder is plastically deformed and leaked.

3. The cylinder load increases or decreases too fast, especially when the engine starts, stops and the working conditions change rapidly, the temperature of the cylinder changes greatly, the heating mode of the cylinder is incorrect, and the insulation layer is opened too early when the engine stops and overhauls, resulting in large thermal stress and deformation of the cylinder and flange.

4. The cylinder produces stress during machining or after repair welding, but it is not tempered to eliminate it, resulting in large residual stress in the cylinder and permanent deformation in operation.

5. In the process of installation or maintenance, due to the reasons of maintenance technology and maintenance technology, the expansion gap of inner cylinder, cylinder diaphragm, diaphragm sleeve and steam sleeve is inappropriate, or the expansion gap of lifting lug pressure plate is inappropriate, which produces huge expansion force to deform the cylinder after operation.

6. The used cylinder sealant has poor quality, excessive impurities or wrong model; If there are hard impurity particles in the cylinder sealant, it will make it difficult to closely combine the sealing surfaces. Bokos high-temperature sealing paste is the latest sealing material for steam turbine cylinders, which can be used for high, medium and low pressure cylinders, avoiding cylinder leakage caused by improper selection.

7. Insufficient tightness of cylinder bolts or unqualified bolt materials. The tightness of cylinder joint surface is mainly realized by the tightness of bolts. Thermal stress and high temperature generated when the unit starts or stops or increases or decreases the load will cause stress relaxation of bolts. If the stress is insufficient, the pre-tightening force of the bolt will gradually decrease. If the bolt material of the cylinder is not good, under the action of thermal stress and cylinder expansion force for a long time, the bolt will be stretched, plastically deformed or broken, resulting in insufficient sealing and cylinder leakage.

8. The tightening sequence of cylinder bolts is incorrect. Generally, cylinder bolts are tightened from the middle to both sides at the same time, that is, from the place with the largest vertical radian or the place with the largest stress and deformation, which will transfer the gap at the place with the largest deformation to the free ends before and after the cylinder, and finally the gap will gradually disappear. If it is tightened from both sides to the middle, the gap will be concentrated in the middle, and the joint surface of the cylinder will form an arch gap, resulting in steam leakage. [ 1]

Turbine oil leakage

In the continuous production of modern industry, due to the influence of medium corrosion, erosion, temperature, pressure, vibration and other factors, the leakage of equipment, pipelines, valves and containers is inevitable. Leakage plugging technology with pressure is a special technical means to repair the leakage part by using temperature and pressure without affecting normal production, and realize re-plugging. Because this technology is implemented under the condition that the process medium, pressure and flow rate are not reduced and the medium leaks, it is essentially different from the traditional shutdown and plugging, and its economic value is more significant.

It is an ideal method to use Meijiahua technology products to implement on-site plugging, especially in equipment maintenance and plugging without stopping under pressure in flammable and explosive situations. In particular, it is safe, convenient, time-saving and reliable for the on-site treatment of low-temperature and low-pressure equipment and pipelines such as "dripping, leaking, leaking and infiltrating" in power and chemical industries. It can not only stop to stop plugging, but also deal with the leaking parts on line without affecting production, so as to achieve the purpose of re-plugging, with remarkable economic benefits.