Basic knowledge of steam turbine

1. Basic knowledge of steam turbines

Basic concept knowledge of steam turbine

1, critical pressure of water

When the pressure of water reaches 225.65Kg/cm2 and the temperature reaches 374. 15℃, the density of water and steam is the same, and the boundary between water and steam cannot be separated. Water becomes steam without vaporization, and water is no longer evaporated by boiling vaporization. This pressure is called the critical pressure of water.

3. Limit vacuum of steam turbine

1After the steam enters the turbine, the useful heat for doing work is reduced to the difference between the heat of new steam and the heat of exhaust steam. Assuming that the pressure and temperature of fresh steam remain unchanged, when the vacuum of condenser is ultra-high, the heat content of exhaust steam is small, so the heat of doing work with1steam increases. Therefore, when the vacuum is high, the steam consumption of the steam turbine can be reduced.

However, the improvement of vacuum is not infinite. The improvement of vacuum is limited by the expansion capacity of the last stage blade, and the vacuum equivalent to this capacity is called extreme vacuum. When the vacuum increases beyond this limit, the turbine load will not continue to increase and the economic benefit will be improved.

4. Differential expansion of steam turbine

The relative expansion difference between cylinder and rotor is called expansion difference. Positive expansion difference means that the cylinder expands slowly and the rotor expands quickly, while negative expansion difference means that the cylinder does not contract and the rotor contracts, or the cylinder expands quickly and the rotor expands slowly. This phenomenon occurs in steam turbines with flange heating devices.

5, the critical speed of steam turbine

Because there is eccentricity between the center of gravity of the shaft and the center of gravity of the rotor, centrifugal force is generated when the shaft rotates, which is the main reason for the vibration and shaft bending of the steam turbine. When the rotor rotates, the center of gravity rotates with the center line of the shaft. Every time an important official rotates, it will produce a vibration, which is the forced vibration of the shaft caused by centrifugal force. The number of times the shaft is forced to vibrate per second is called the frequency of forced vibration.

When the forced vibration frequency of the rotor coincides with the free vibration frequency of the rotor, that is, when the number of changes in the direction of centrifugal force causes the forced vibration frequency of the rotor to be the same as or proportional to the free vibration frequency, * * vibration occurs, and the vibration of the rotor is particularly large at this time, which is called the critical speed of the rotor.

6. supercooling

The difference between steam turbine exhaust temperature and condensate temperature is called supercooling. If the degree of supercooling is large, the economy of steam turbine will be reduced. Due to the low temperature of condensed water, more heat will be taken away by cooling water, and the heat loss is great.

7. Cooling rate

The cooling water required for condensation of exhaust steam per ton is called cooling ratio = cooling water quantity/exhaust steam quantity.

Generally, the cooling ratio of condenser is 50~60, and there are more.

8, the relationship between exhaust temperature and vacuum

The boiling temperature of water is related to the pressure of gases (such as air and water vapor) on the surface of water.

Why is there water vapor in the exhaust chamber of steam turbine when the temperature is only tens of degrees? This is because the pressure in the exhaust chamber is lower than atmospheric pressure, so the saturation temperature of water vapor is very low. Although it is only a few tens of degrees, the water is still in a steam state.

We say condensation.

2. Basic knowledge of steam turbine

& lt； I >； Basic concept knowledge of steam turbine

1, critical pressure of water

3. Limit vacuum of steam turbine

4. Differential expansion of steam turbine

5, the critical speed of steam turbine

6. supercooling

7. Cooling rate

The cooling water required for condensation of exhaust steam per ton is called cooling ratio = cooling water quantity/exhaust steam quantity.

Generally, the cooling ratio of condenser is 50~60, and there are more.

8, the relationship between exhaust temperature and vacuum

The boiling temperature of water is related to the pressure of gases (such as air and water vapor) on the surface of water.

We say condensation.

3. The basic knowledge of steam turbine

A rotating power machine that converts steam energy into mechanical work.

Also known as a steam turbine. Mainly used as the prime mover of power generation, it can also directly drive various pumps, fans, compressors and ship propellers.

You can also use the exhaust steam or intermediate extraction steam of the steam turbine to meet the heating needs in production and life. Steam turbine is a rotating power machine that converts steam energy into mechanical work, and it is one of the main equipment of steam power plant.

Steam turbine is a kind of turbine machinery, also known as steam turbine. In the first century, Herod of Alexandria described the steam spinning ball, also known as Fengshen wheel, as the prototype of the earliest reactive steam turbine. In 1629, Blanca, Italy, proposed a runner that rotates by steam impacting blades.

At the end of 19, Laval in Sweden and Parsons in Britain built practical steam turbines respectively. Laval built the first 5 HP (3.67 kW) single-stage impulse turbine in 1882, which solved the problems related to nozzle design and strength design.

Single-stage impulse turbine has little power and is rarely used now. At the beginning of the 20th century, France Lato and Switzerland Zolai made multi-stage impulse steam turbines respectively.

Multi-stage structure has opened the way for improving steam turbine power, and has been widely used, and the unit power is also increasing. Parsons obtained the British patent in 1884, and manufactured the first 10 horsepower multistage reactive steam turbine, which was in a leading position in power and efficiency at that time.

At the beginning of the 20th century, Curtis made a multi-speed steam turbine, and each speed stage generally had two rows of moving blades. After the first row of moving blades, guide blades are installed on the cylinder to guide the steam flow to the second row of moving blades. At present, speed-class steam turbines are only used for small steam turbines, mainly driving pumps and blowers. , often used as the first stage of small and medium-sized multistage steam turbines.

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. 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 with 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.

According to the structure, there are single-stage steam turbines and multi-stage steam 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, there is an impulse turbine, and its steam mainly expands in nozzles (or stationary 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 the thermal characteristics, there are condensing steam turbine, heating steam turbine, back pressure turbine, extraction steam turbine and saturated steam turbine. 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.

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 fresh steam pressure can be divided into various pressure grades, and the fresh steam pressure is usually 24.5~26 MPa.

4. Basic knowledge of steam turbine

Basic concept knowledge of steam turbine 1, critical pressure of water When the pressure of water reaches 225.65Kg/cm2 and the temperature reaches 374. 15℃, the density of water and steam is the same, and the boundary between water and steam is inseparable. Water becomes steam without vaporization, and water is no longer evaporated by boiling vaporization. This pressure is called the critical pressure of water.

2. When the velocity of steam is equivalent to the speed of sound, the state of steam is called critical state, and the section that produces critical state is called critical section. All the parameters on this section are called critical parameters, that is, critical speed CC, critical pressure PC, critical pressure ratio εC, critical specific volume υC, critical flow rate GC, etc. 3. After the limit vacuum steam of steam turbine enters the steam turbine, the useful heat for doing work is reduced to the difference between the heat of new steam and the heat of exhaust steam. Assuming that the pressure and temperature of fresh steam are constant, when the vacuum of condenser is ultra-high, the heat content of exhaust steam is small, so the heat of work done by1steam increases.

Therefore, when the vacuum is high, the steam consumption of the steam turbine can be reduced. However, the improvement of vacuum is not infinite. The improvement of vacuum is limited by the expansion capacity of the last stage blade, and the vacuum equivalent to this capacity is called extreme vacuum. When the vacuum increases beyond this limit, the turbine load will not continue to increase and the economic benefit will be improved.

4. The relative expansion difference between differential expansion cylinder and rotor of steam turbine is called differential expansion. Positive expansion difference means that the cylinder expands slowly and the rotor expands quickly, while negative expansion difference means that the cylinder does not contract and the rotor contracts, or the cylinder expands quickly and the rotor expands slowly. This phenomenon occurs in steam turbines with flange heating devices.

5. Critical speed of steam turbine Due to the eccentricity between the center of gravity of the shaft and the center of gravity of the rotor, centrifugal force is generated when the shaft rotates, which is the main reason for the vibration and shaft bending of the steam turbine. When the rotor rotates, the center of gravity rotates with the center line of the shaft. Every time an important official rotates, it will produce a vibration, which is the forced vibration of the shaft caused by centrifugal force. The number of times the shaft is forced to vibrate per second is called the frequency of forced vibration.

If the degree of supercooling is large, the economy of steam turbine will be reduced. Due to the low temperature of condensed water, more heat will be taken away by cooling water, and the heat loss is great. 7. The cooling rate is the cooling water required for the condensation of exhaust steam per ton, which is called cooling rate = cooling water/exhaust steam.

Generally, the cooling ratio of condenser is 50~60, and there are more. 8. Relationship between exhaust temperature and vacuum The boiling temperature of water has a certain relationship with the pressure of gas (such as air and water vapor) on the water surface.

The greater the air pressure on the surface of water, the higher the boiling point of water. After the temperature of water reaches the boiling point, if heating is continued, water will become steam at the same temperature. In the process of water changing into steam, the temperature of water does not rise. Water at this temperature is called saturated water, and steam at the same temperature is called saturated steam. Why is there water vapor in the exhaust chamber of steam turbine when the temperature is only tens of degrees? This is because the pressure in the exhaust chamber is lower than atmospheric pressure, so the saturation temperature of water vapor is very low. Although it is only a few tens of degrees, the water is still in a steam state.

We call it condensation.

5. Steam turbine operators should know this knowledge.

Working principle Steam turbine is a kind of external combustion rotating machinery which can convert steam heat energy into mechanical work. After entering the steam turbine, the steam from the boiler passes through a series of annular nozzles and blades in turn, converting the thermal energy of the steam into mechanical energy for rotating the rotor of the steam turbine.

Steam is converted into energy in different ways in a steam turbine, which constitutes a steam turbine with different working principles. Edit the supporting facilities of this paragraph. The steam turbine usually works under the conditions of high temperature, high pressure and high speed. It is a relatively precise heavy machinery, which generally needs to be matched with boilers (or other steam generators), generators (or other driven machinery), condensers, heaters, pumps and other supporting equipment. Work together.

The structural components of this section are composed of two parts: the rotating part and the static 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. 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.

Rotor rotor is made of alloy steel forgings. The governor end of the high-pressure rotor is connected with the long shaft through a rigid coupling, and the upper shaft of this section is equipped with 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. Coupling The coupling is used to connect the rotors 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: this kind of 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.

The right coupling of the semi-flexible coupling is forged into a whole with the main shaft, 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. There are usually two kinds of elastic couplings commonly used in medium-capacity units, namely 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.

The stationary blade partition is used to fix the stationary blade and divide the cylinder into several steam chambers. The moving blades are installed on the rotor impeller or drum and receive the nozzle blades.