Compressor unit training materials

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Section 1 Overview of Compressors 1. Definition and Classification of Compressors In industrial production, gas with a certain pressure is often required for various purposes, and the compressor is a machine that transports and increases the pressure of gas. We know that the pressure of a gas depends on the number and intensity of gas molecules hitting a unit area per unit time. Increasing the temperature of the gas within the volume increases the movement speed of the gas molecules, and increasing the degree of impact can increase the gas pressure. But when the temperature decreases, the gas pressure decreases. It is generally required that the compressed gas should have a temperature that is not too high. Therefore, the main way to increase gas pressure is by increasing the number of gas molecules per unit area (that is, shortening the distance between molecules), which is accomplished through a compressor. The most widely used compressors today are generally divided into two categories:: One type is a positive displacement compressor, which increases the pressure by reducing the volume of gas (such as piston type, sliding vane type, Roots type, screw type) ； The other type is a turbine compressor, which uses the work done by the rotating blades on the air flow. Through the continuous acceleration and deceleration of the air flow, they are squeezed against each other due to inertia and shorten the distance between molecules to increase the pressure. Turbocompressors are generally divided into centrifugal and axial flow: 1. Centrifugal compressor: The movement of compressed gas in a centrifugal compressor is in a radial direction perpendicular to the compressor axis. The increase in gas pressure in a centrifugal compressor is that when the gas flows through the impeller, the gas is subject to centrifugal force due to the rotation of the impeller, causing its speed to increase. When the gas flows through the channels with expanded cross-sectional areas such as the diffuser, the flow speed gradually decreases, so that the speed energy is converted into pressure energy, and the pressure of the gas is increased. 2. Axial flow compressor: The movement of gas in an axial flow compressor is in a direction parallel to the compressor axis. In the axial flow compressor, the rotation of the rotor also generates a high speed for the gas. When the gas flows through the stator blades spaced apart from the moving blades, the speed of the gas gradually slows down, and the speed energy is converted into pressure energy. In terms of use, generally positive displacement compressors should be used in situations with high pressure ratio, medium and small flow rates. ； Turbine compressors are used in situations with low to medium pressure ratios and large flow rates. The flow rate of the axial flow type is larger than that of the centrifugal type, and the pressure is lower than that of the centrifugal type. 2. Definition and Classification of Steam Turbine Steam turbine, also called turbine, is a rotating prime mover that uses steam to do work. The steam from the boiler or other steam sources enters the steam turbine through the speed control valve, and flows through a series of annularly arranged nozzles (or stationary blade cascades) and moving blade cascades at high speed, and expands to do work, pushing the steam turbine rotor to rotate (converting the kinetic energy of the steam into mechanical work). The steam turbine in turn drives the motor, compressor, pump and other load machines to rotate. Steam turbines are divided according to the thermal process: 1. Condensing steam turbine. After the steam does work in the steam turbine, all the steam is discharged into the condenser for condensation. The internal pressure of the condenser is lower than the atmospheric pressure. 2. Extraction condensing steam turbine steam When the steam turbine expands to a certain level, part of the steam is extracted from the steam turbine and supplied to other steam users. ； The remaining steam is discharged into the condenser after doing work in the subsequent stages. The air compressor/supercharger and generator-driven turbine in the Shuangjia workshop are of the extraction and condensing type. 3. After the steam of the back-pressure steam turbine enters the steam turbine to expand and perform work, it is discharged from the cylinder at a pressure greater than 1 atmosphere, and its exhaust gas is supplied to other low-pressure users. 4. In multi-pressure (injection) steam turbines, if there is a certain pressure of steam that cannot be used up during the process, the excess steam is injected into an intermediate stage in the steam turbine through pipes and expands with the original steam in the turbine to perform work, thereby recovering energy. Steam turbines can also be divided into low-pressure (below 2.0 Mpa), medium-pressure (2.0~5.0Mpa), high-pressure (5.0~10.0Mpa), ultra-high-pressure (12.0~14.0Mpa) and supercritical (above 22.5Mpa) steam turbines according to steam pressure. In addition, it can also be divided into: Impulse type, reaction type, impulse type and reaction type combined steam turbine, etc. Section 2 Basic Principles and Structure of Centrifugal Compressors and Steam Turbines 1. Working Principle and Basic Structure of Centrifugal Compressors 1. Structure The first thing you see from the appearance is the casing, which is also called the cylinder. It is usually cast from cast iron or cast steel. A compressor often has two or more cylinders, which are called low-pressure cylinders, medium-pressure cylinders and high-pressure cylinders according to the pressure. The compressor body structure can be divided into two parts: (1) The rotating part (rotor), which consists of main shaft, impeller, balance plate, thrust plate and half coupling for coupling, is also called the rotor. (2) The stator part is composed of cylinders, partitions, radial bearings, thrust bearings, shaft end seals and other parts, and is often called the stator. In compressor theory, the compressor is often divided into several stages along the gas flow path. The so-called stage is the basic unit composed of an impeller and matching fixed components. As shown in Figure 1, in the middle stage of the compressor, it includes several components: impeller, diffuser, bend and return flow. The stage at the inlet of each section of the compressor is called the first stage. In addition to the above components, it should also include an air inlet chamber. ； The stage at the exhaust port of the compressor is called the final stage. It does not have bends and returners, but is replaced by an exhaust chamber. Some compressors don't even have a diffuser, and the gas comes out of the impeller directly into the exhaust chamber. In a centrifugal compressor, after the gas flows through one stage, the pressure increase is limited. If you want to compress to a higher pressure, you need to complete it through several stages, and several stages can be installed in one cylinder. A cylinder can hold up to about 10 stages, and more stages require the use of multiple cylinders. The temperature of the gas will rise after being compressed. When the required pressure is relatively high, the gas is often compressed to a certain pressure and then drawn out of the cylinder, cooled in the cooler, and then entered into the lower stage to continue compression. The compressor can be divided into several sections according to the number of cooling times. A segment can be one level or several levels. A cylinder can be divided into one segment or multiple segments. In a multi-stage centrifugal compressor, due to the different forces acting on the impeller on both sides of each stage, the rotor is subjected to a resultant force directed toward the low-pressure end. This resultant force is called an axial force. The balance plate is a part that uses the pressure difference on both sides of itself to balance the axial force. It is located on the high-pressure side of the compressor and is used to balance most of the axial force, and the remaining axial force acts on the thrust bearing. Some compressor impellers are arranged back-to-back to balance axial force. The coupling is also called the back pulley. It is the connection between the steam turbine (or drive motor) and the compressor as well as the high and low pressure cylinders of the compressor. Flexible couplings are usually used now. It allows for larger parallel misalignments, angular misalignments and combined misalignments. The stator includes a casing and fixed components inside the casing. The casing has two types: horizontally split and vertically split. Horizontal splitting facilitates disassembly and assembly machine manufacturing, but the sealing surface is large and the strength is poor. ； For situations where the pressure is high, a vertically split form is adopted. The shell is actually two cylinders, and the inner cylinder is still split horizontally. The rotor and fixed components are installed in the inner cylinder, and then installed in the outer cylinder. The outer cylinder is a complete cylinder with end caps at one or both ends. After opening, the inner cylinder can be pulled out. There are various partitions in the casing. Between the casing and the partitions, and between the partitions, fixed components such as the suction chamber, diffuser, bend and return flow are formed. 2. The role of each main component in the flow part. The main channel components that gas flows through in the compressor are the air inlet chamber, impeller, diffuser, bend, return flow device and volute. These components are called flow components. The functions of these components are described below. (1) Air intake chamber: This is a passage that evenly sucks the gas from the air intake chamber or intercooler into the impeller for pressurization. Therefore, an air intake chamber is provided at each inlet section of the compressor. (2) Impeller: The impeller is also called the working wheel. It is the heart part of the compressor. Under the action of the impeller blades, the gas rotates with the impeller at high speed. Due to the centrifugal force of the rotation and the expansion flow in the impeller, the gas pressure is increased and the speed is also increased. Therefore, the impeller is the key component to increase the energy of the gas. (3) Diffuser: After the gas is thrown out from the impeller, it has a high flow speed. A component with a gradually enlarging flow channel section is installed after the impeller outlet, which is called a diffuser. Its purpose is to further convert the flow velocity of the gas into pressure. (4) Curve: In order to introduce the gas after the diffuser to the inlet of the next stage impeller, the direction of the gas flow must be changed from centrifugal flow to centripetal flow, so a curve is set behind the diffuser to connect it. (5) Refluxer: Its function is to evenly distribute the gas coming from the curve to the inlet of the next stage impeller. (6) Volute: The main purpose of the volute is to collect the gas behind the diffuser or impeller, lead it out of the compressor, and flow to the gas delivery pipe or gas cooler. In addition, during the process of collecting gas, generally due to the gradual increase in the outer diameter of the volute, the flow cross-section also gradually expands, so it also plays a certain role in deceleration and boosting. 3. Working principle The working principle of centrifugal compression is similar to that of a centrifugal pump for transporting liquids. When the driver (such as steam turbine, electric motor, etc.) drives the compressor rotor to rotate, the gas in the impeller flow channel rotates with the impeller under the action of the impeller. Under the action of centrifugal force, the gas is thrown to the diffuser outside the impeller. As a result, a thin zone is formed in the impeller, and the inlet gas enters the impeller to fill this zone. As the impeller continues to rotate, the gas is continuously thrown out, and the inlet gas continues to enter the impeller. The gas flowing out of the impeller in the radial direction not only increases in pressure, but also increases in speed. This part of the speed is converted into pressure in the diffuser of the subsequent component, and is then introduced into the lower stage through the bend. The flow guide then uniformly introduces the gas coming from the bend into the lower impeller in a certain direction for further compression. 4. Power consumption and efficiency of cardiac compressor (1) Overview: Compressor gas needs to consume energy. Large centrifugal compressors are driven by prime movers (such as steam turbines, combustion engines, etc.). The power transmitted by the prime mover shaft end includes the mechanical losses of transmission parts such as compressor bearings, gearboxes, and couplings, as well as the internal power of the compressor. Internal power refers to the power consumed by the compressor rotor on the gas. The compressor rotor transfers energy to the gas through the impeller. In addition to the work the impeller does on the gas, the wheel resistance loss caused by the friction between the impeller disk, the outer surface of the wheel cover and the rim and the surrounding gas, and the leakage loss caused by the leakage of high-pressure gas from the impeller outlet back to the low-pressure end of the impeller also consume work. For the entire compressor, the work done by the impeller on the gas is converted into the following three parts:: a Increase the static pressure energy (compression work) of the gas to increase the gas from the inlet pressure to the outlet pressure. b Increase the kinetic energy of the gas. Under normal circumstances, the increase in kinetic energy is not significant and can often be ignored. c Overcome the flow loss of air flow in the stage. This part of the flow loss refers to the flow loss of the airflow in the impeller and in the fixed components of the stage (such as suction chamber, diffuser, bend, return flow device, volute, etc.). In short, the medium power consumption of the compressor stage consists of five parts, namely, the increase in static pressure energy, changes in kinetic energy, flow loss, wheel resistance loss and air leakage loss. Only the increase in static pressure energy is useful for increasing the pressure of the gas. (2) The increase in static pressure energy during the compression process of gas is related to the compression process of gas. Thermodynamics divides the compression process of gas into: Isothermal compression process, adiabatic compression process, polytropic compression process. The actual compression process of gas in the compressor is a variable compression range, but the heat exchange with the outside world can be ignored. Now analyze the static pressure energy increase (compression work) during each compression process. Assume that the compressor inlet and outlet parameters are P1, V1, T1 and P2, V2, T2 respectively. The energy required to compress gas is expressed in Kg.m/Kg, which represents the energy required to compress 1kg of gas. A Isothermal compression T=Const (constant) The isothermal compression work is His=RT1Ln(P2/P1) (Kg.m/Kg) B Adiabatic compressed gas has no heat exchange with the outside world during the compression process and there is no gas flow loss and friction loss. Gas temperature after adiabatic compression: T2/T1=(P2/P1)K-1/K The adiabatic compression work is: Had=K/(K-1)RT1((P2/P1)K-1/K-1) (Kg.m/Kg) C multi-variable compression process: There are flow losses and friction losses in the process, and there may or may not be heat exchange with the outside world. The calculation formula of the gas temperature in the multilateral variable process is:: T2/T1=(P2/P1)M-1/M The variable compression power is: Hpol=(M-1)RT1((P2/P1)M-1/M-1) (Kg.m/Kg) In the above formula, R is the gas constant. The lighter the compressed gas component, the greater the R. The formulas for polytropic processes and theoretical adiabatic processes have the same form, except that the adiabatic index K is replaced by the polytropic index m. The polytropic index is different from the adiabatic index. It not only changes with the type of gas, but also is related to the structure of the equipment. For centrifugal compressors, the polytropic index m is greater than the adiabatic index K. The more reasonable the machine design and control are, the closer m will be to the K value. (3) Analysis and discussion of compressor process A. Three typical compression processes. If the gas temperature and pressure ratio are the same, the isothermal compression process requires the smallest compression work and the exhaust temperature is the lowest, which is equal to the intake air temperature. This is an ideal situation that can only be approached but not achieved. The multi-variable compression process requires the largest energy head, so multi-stage compressors are often made into multiple stages, and inter-stage coolers are added to make the compression process closer to the isothermal compression process. For compressors with intercoolers, isothermal efficiency is often used to measure the perfection of the machine. B For the same gas with the same mass flow rate, if the initial temperature is the same and the compression ratio is the same, the power consumption is also the same. For example, compressing a gas from 10 atmospheres to 100 atmospheres requires the same power consumption as compressing a gas from 1 atmosphere to 10 atmospheres. The compression work required for C gas is related to the properties of the gas. For light gases, because the gas R is large, the compression work required to compress heavy gases at the same pressure is greater (as can be seen from the compression expression). However, since the same compressor compresses different gases with the same volume flow rate, the blade work provided is the same, that is, the H blades have nothing to do with the properties of the gas. Therefore, under the same pressure ratio requirement, compressing light gases requires more stages than heavy gases. D The variable process is a process with losses, and the variable index m reflects the amount of work required by the variable compression process. The loss causes the gas to gain additional heat, and an intercooler is used in order to approach the isothermal compression process. The representation of each different compression process on the PV diagram and TS diagram is shown in Figures 2-3 below. The compression work of each process is the area included by the ordinate of each compression process line and the two isobars P=P1 and P=P2. Figure 2 Various compression processes on the TS diagram Figure 3 Various compression processes on the P-V diagram 4) Compressor efficiency The efficiency of the compressor or stage is mainly used to illustrate the degree of utilization of the mechanical energy transferred to the gas. Since there are three compression processes, the compressor efficiency is correspondingly divided into variable efficiency etapol, adiabatic efficiency etaad and isothermal efficiency etais. Polyvariable efficiency refers to the ratio of the polyvariable compression work required to increase the pressure from P1 to P2 and the actual power consumed. The current variable efficiency of centrifugal compressors is about 0.7 to 0.84. Adiabatic efficiency refers to the ratio of adiabatic compression work to the actual work required when the pressure increases from P1 to P2. Isothermal efficiency refers to the ratio of isothermal compression work to the actual work consumed when the pressure increases from P1 to P2. It is impossible for an actual compressor to achieve an adiabatic compression process without losses, but it can serve as a standard for comparison. Since the variable compression work > adiabatic compression work > isothermal compression work, there is etapol > etaad > etais, so it is necessary to pay attention to which expression method is used for the machine efficiency. 2. Basic Principle and Structure of Steam Turbine A steam turbine is a rotary prime mover that uses steam to do work. The steam from the boiler or heating network enters the steam turbine through the trip throttle valve or emergency shut-off valve or speed regulating valve. It flows through a series of annularly arranged nozzles (or stationary blade grids) and moving blade grids at high speed and expands to do work to push the steam turbine rotor to rotate, converting the kinetic energy of the steam into mechanical work. This is the simple working principle of a steam turbine. Steam turbines can be divided into: Impulse type, reaction type, impulse type and reaction type combined steam turbine. First, let’s introduce the working principles of these types of steam turbines. 1 Working principle of steam turbine (1) Impulse steam turbine The simplest structure of the impulse steam turbine is shown in Figure 4. The impeller is equipped with a circle of moving blades and the nozzle, forming a simple machine that performs work. We call the power-producing unit of a steam turbine composed of a nozzle and its matching moving blades. A steam turbine consisting of one stage is called a single-stage steam turbine. The nozzle is also called a static blade. It is a channel with a special and ever-changing cross-sectional shape. After the steam enters the nozzle, it expands and consumes the pressure energy of the steam, that is, the heat energy of the steam. The pressure and temperature of the steam decrease, but the flow rate of the steam increases, and a high-speed airflow is obtained. The function of the nozzle is to convert the thermal energy of steam into kinetic energy. Moving blades are also called working blades. A full circle of blades filled on the outer circumference of the impeller is often called a moving cascade. When the high-speed airflow from the nozzle flows to the moving blades, the speed and direction are certain. Then the airflow changes its original speed and direction due to the obstruction (force) of the moving blades. At this time, the airflow must give a reaction force to the moving blades, promote the movement of the blades, and convert part of the kinetic energy into the mechanical work of the impeller rotation. It can be seen from the above that there are two energy conversions during the continuous operation of the steam turbine, namely: thermal energy → steam kinetic energy → rotor mechanical energy. In order to better understand the working principle of the steam turbine, let's analyze the moving blade type of the impulse steam turbine. If we use an upright flat plate and let the high-speed air flow impact its surface, the flat plate will move due to the impact of the air flow. However, due to the large disturbance and eddy current loss generated near the surface of the flat plate, as shown in Figure 5-a, a large amount of useful energy in the steam cannot be well utilized and even causes waste. Therefore, after a lot of practical improvements, the moving blades of steam turbines are now made into curved shapes. If you want to generate the maximum force, you must make the steam injection direction consistent with the movement direction of the moving blades, and then turn 180° away from the moving blades, as shown in Figure 5-b. At this time, the impact force on the moving blades is shown in Figure 6. Figure 5 Analysis diagram of impulse turbine blades Figure 6 The centrifugal force of steam particles acting on the blades. The airflow flows to the curved surface at the speed of C1. It is equivalent to the moving blades of the steam turbine and can move in the direction parallel to the airflow. After the airflow enters the steam channel formed by the inner arc of the curved flow channel, it gradually changes its flow direction along the inner arc. Finally, when it leaves the steam channel, the speed of the C2 direction is exactly opposite to the C1 direction. When the airflow flows through the curved surface, it actually moves in a circular motion. Therefore, each steam particle that makes up the airflow is subject to a centripetal force exerted on it by the blades. At the same time, the blades receive an equal and opposite reaction force from the steamflow. If the centrifugal force of airflow particles is represented by a vector. The centrifugal force P1 at 1 point can be decomposed into the axial component P1Z and the component force P1U in the direction of motion. The centrifugal force P2 at 2 points can also be decomposed into P2Z and P2U. The axial separations P1Z and P2Z exactly cancel each other out because the two forces are equal in magnitude and opposite in direction, and they act together on the same branch line of a blade. Similarly, the axial separation at other points cancels each other out, so the sum of the components of the centrifugal force of the airflow in the axial direction is zero, that is, P1Z+P2Z+…..=0. The sum of the components in the direction of movement of the curved surface is equal to P, that is, P1U+P2U+…..=0. Under the action of this P force, the curved surface (blade) moves to the right, generating rotational motion through the impeller and shaft. If it drives compressors, pumps, fans and other machinery, it can output mechanical power. This is how impulse turbines work. In fact, due to limitations in mechanical structure and other aspects, the airflow flowing out of the nozzle cannot be exactly the same as the moving direction of the moving blades, but forms an included angle. The moving blade is not a semi-circular arc, but consists of several curves, usually circular arcs and parabolic arcs. As shown in Figure 7. (2) Speed ​​stage and pressure stage The concept of stage has been introduced before. From a structural point of view, a stage of a steam turbine is a device that combines a nozzle (several or nozzles arranged around the entire circumference) and a row of moving blades. From the perspective of the action principle, it is the basic unit that can create high-speed airflow, convert speed energy into mechanical energy, and generate thrust to perform external work. Levels can be divided into pressure levels and speed levels, which are briefly introduced as follows. A pressure stage In the case where the available steam energy is very large, only one stage cannot fully utilize this energy. At this time, we connect the stages composed of nozzles and moving blades in series on the same shaft, and utilize the energy of the steam in several stages. From a structural point of view, it is a row of nozzles and a row of moving blades, followed by a row of nozzles and a row of moving blades, and so on. The steam pressure is the highest at the inlet of the first row of nozzles, and then decreases step by step. This is the common structure of a multi-stage steam turbine, and each stage is called a pressure stage. B Speed ​​stage In addition to the pressure stage, some steam turbines are also equipped with speed stages. Speed ​​level is also called complex speed level or Curtis level. The speed stage is a little more complex in structure than the pressure stage. Figure 8 is a schematic diagram of a single-stage impulse steam turbine with double-row speed stages. It utilizes steam energy more fully than a single-stage impulse steam turbine. The rotor consists of shaft 1, impeller 2, and double-row moving blades 3 and 6. ； The stator part consists of nozzle 4, guide vane 7, cylinder 5, exhaust pipe 8, etc. Figure 8 Schematic diagram of a single-stage impulse turbine with dual-row speed stages. If the speed of the impulse stage leaving the moving blades is still very high when the impulse stage is working, it means that the kinetic energy of the steam has not been fully utilized to do work. In order to utilize this part of the energy, a second moving blade cascade is installed on the rim of the same impeller so that the steam flows through the two rows of rotating blade cascades. Part of the steam energy in the first row of moving blade cascade channels is converted into mechanical energy, while the remaining steam energy is continued to be converted into mechanical energy by the second row of moving blade cascades. In order to make the steam flow flow into the second moving blade cascade in a certain direction, a row of fixed blades is installed between the first and second rows of moving blade cascades to play a guiding role, called guide blades, which are installed on the cylinder. The working principle of the speed stage is the same as that of the impulse pressure stage. The difference is that the speed of the steam is utilized in the first and second rows of moving blades respectively. In addition to the dual-row speed stage, there are also three-row speed stages, but the dual-row speed stage is commonly used. It is often used to make low-power steam turbines to drive fans and other various pumps. It can also be used as the first stage of multi-stage steam turbines. (3) Reaction steam turbine Reaction steam turbine uses reaction force and impact force to convert the speed energy of steam into mechanical energy. The working principle of reaction steam turbine is also based on the law of inertia and the law of action and reaction. Figure 9 is a schematic structural diagram of a reaction steam turbine. The moving blades are installed on the drum, and the shaft, balance piston and drum form the rotor. The stator blade is installed on the cylinder and forms a stator with the intake and exhaust pipes. The reaction stage is still composed of a row of stationary blades and a row of moving blades. How it works is: The airflow in the stationary cascade is similar to that when passing through the nozzle. The pressure decreases, the volume expands, and the speed increases. ； And its moving blade cascade is also made into a steam channel with a gradually shrinking cross-section. The air flow is further decompressed in the moving blade cascade and the expansion is accelerated. According to the law of inertia, if a moving object is not affected by external forces, it will continue to move at its original speed and direction. Since the airflow acquires acceleration in the moving blade cascade, there must be an external force acting on the airflow. This force is obtained by reducing the pressure and temperature of the airflow in the moving blade cascade, that is, converting the thermal energy of the airflow into kinetic energy. In the moving blade cascade, the airflow is further decompressed, accelerated, and leaves at high speed. At this time, the air flow must give an equal and opposite force to the moving blade cascade, causing the rotation of the moving blade cascade to drive the rotation of the shaft and perform external work. This is how a reaction steam turbine works. The practical application of the reaction principle in steam turbines is shown in Figure 10. This is a schematic cross-section of a stage in a reaction steam turbine. The steam reaches a higher speed after expanding in the stationary blade cascade. After the steam leaves the stationary blade cascade, it enters the moving blade cascade airway and changes direction along the inner arc of the airway wall. Therefore, the moving blades are subject to the impact force generated by the impulse principle, which is recorded as P impact. ； And because the airflow expands and depressurizes from P1 to P2 in the moving blade cascade airway, the moving blades are acted upon by the reaction force P caused by the reaction principle. The resultant force of P impulse and P reaction is Pu. In addition, the pressure difference between the front and rear of the moving blade also causes an axial force P-axis. The resultant force of Pu and P axis is P total, which is the force acting on the moving blade. The component force along the direction of movement of the moving blades causes the moving blades to move to the right and do mechanical work. Therefore, the force acting on the moving blades of the reaction steam turbine stage includes both impulse and reaction forces. Figure 9 Schematic diagram of single-stage reaction steam turbine Figure 10 Stages of reaction steam turbine Practical reaction steam turbines all adopt multi-stage type, and their working principle is basically the same as that of the single-stage reaction steam turbine analyzed previously. For the convenience of analysis, the aforementioned impulse stage actually refers to the situation where no expansion occurs in the moving blade cascade. Some people call it a pure impulse stage. The steam turbine commonly used in modern times is actually an impulse steam turbine with reaction degree. In this kind of steam turbine, the steam flow also expands in the moving blade cascade, but the expansion degree is smaller than that in the nozzle. The so-called reaction degree is the proportion of the degree of steam expansion in the moving blade cascade to the total expansion degree in the stage, or the ratio of the ideal enthalpy drop in the moving blade cascade. ρ is often used to represent the reaction degree. Pure impulsive stage ρ=0 ； Reactionary level ρ=0.5 ； Impulsive level 0 with reactionary degree