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This post was last edited by Desert Fish on 2022-4-29 at 12:57. I. Basics of Thermodynamics: 1. What is gauge pressure? What is absolute pressure? The value obtained by using a pressure gauge to measure pressure is the value above atmospheric pressure; this is known as gauge pressure. It refers to the pressure value measured relative to atmospheric pressure. The value that takes atmospheric pressure into account is the true value of pressure; in engineering, this pressure is referred to as absolute pressure. ?The relationship between gauge pressure and absolute pressure is as follows: P_gauge = P_absolute – B or P_absolute = P_gauge + B. Here, P_gauge refers to the gauge pressure of the fluid, P_absolute refers to the absolute pressure of the fluid, and B represents the atmospheric pressure at that location and time (approximately equal to 1 engineering atmosphere). 2. What is vacuum? What is vacuum degree? When the pressure inside a sealed container is lower than atmospheric pressure, the part with pressure lower than atmospheric pressure is referred to as vacuum. Vacuum expressed as a percentage is called vacuum degree. That is: divide the measured vacuum value by the local atmospheric pressure value and then convert it into a percentage. Expressed in a formula: Vacuum level = --h_vacuum-- × 100% / h_atmosphere. 3. What is economic vacuum? What is ultimate vacuum? Economic vacuum refers to the vacuum level at which the increase in load on the steam turbine generator resulting from an increase in vacuum is maximized, relative to the additional electrical power consumed by the circulation water pump. If the vacuum is increased further, the expansion capacity of the last-stage nozzles in the turbine reaches its limit, and the power output of the turbine no longer increases; at this point, the vacuum is referred to as the ultimate vacuum. 4. What is the relationship between the specific volume of a gas and pressure and temperature? The specific volume of a gas is closely related to pressure and temperature; when the temperature remains constant, an increase in pressure results in a decrease in the gas’s specific volume ； If the pressure remains constant and only the temperature is increased, the volume of the gas expands and its specific volume increases. The relationship between them is: Pv = RT, where P -------? pressure ； v-------? specific volume ； T-------? Absolute temperature ； R-------? Gas constant. 5. What is vaporization? What is condensation? The process by which a substance changes from a liquid state to a gaseous state is called vaporization. There are two ways of vaporization: evaporation ； Boiling. The phenomenon in which a substance changes from a gaseous state to a liquid state is called condensation. Under certain pressure, the boiling point of a liquid is also the condensation temperature of its vapor. Condensation and vaporization are two opposite thermodynamic processes. 6. What is superheated steam? What is the superheat of steam? At the same pressure, when saturated steam is reheated, its temperature begins to rise above the saturation temperature; at this point, the steam is called superheated steam. The difference between the temperature of superheated steam and the temperature of saturated steam is called the superheat of the steam. The greater the superheat, the more thermal energy is stored in the steam, and the greater its ability to do work. 7. What is enthalpy? Enthalpy is an important state parameter of a gas. The physical meaning of enthalpy is: the total energy possessed by a gas at a certain state, which equals the sum of its internal energy and pressure potential energy. 8. What is entropy? Entropy is a derived parameter in thermodynamics. A slight change in entropy serves as an indicator of whether heat transfer is occurring or not. The introduction of entropy can conveniently reflect the conversion of heat in thermodynamic processes as well as the thermal efficiency of cycles. 9. What is the latent heat of vaporization of a liquid? The heat required to heat 1 kilogram of saturated water to 1 kilogram of dry saturated steam at constant pressure is called the latent heat of vaporization of that liquid. 10. What is the latent heat of condensation? The heat released when 1 kilogram of steam completely condenses into water at the same temperature, under constant pressure, is called the latent heat of condensation. 11. What is the relationship between the heat of vaporization and the heat of condensation? At a certain pressure and temperature, the heat of vaporization of a liquid is equal to the heat of condensation at the same pressure and temperature. In other words, when the temperature and pressure are constant, the amount of heat released when 1 kilogram of saturated steam condenses is equal to the amount of heat absorbed when 1 kilogram of saturated water vaporizes. 12. What is the cyclic thermal efficiency? What does it indicate? The cyclic thermal efficiency is defined as the ratio of the work done by the working fluid per complete thermodynamic cycle to the heat absorbed by the working fluid from the heat source during each such cycle. The cyclic thermal efficiency indicates the degree to which thermal energy is converted into work within a cycle; the higher the efficiency, the greater the proportion of heat absorbed by the working fluid from the heat source that is converted into useful work ； Conversely, the lower the efficiency, the less heat is converted into useful work. 13. What processes make up the Carnot cycle? How is it represented on a T--S diagram? The Carnot cycle consists of two reversible isothermal processes and two reversible adiabatic processes. It is shown on the T---- S diagram. The 1---2 process is a reversible isothermal endothermic process ； The 2---3 process is a reversible adiabatic expansion process ； 3---4 is a reversible isothermal exothermic process ； 4---1 represents a reversible adiabatic compression process. 14. What devices are used to implement the Rankine cycle? What role does each thermal device play in this cycle? The Rankine cycle is the basic thermal cycle in thermal power plants, and it is implemented through four main thermal devices: a steam boiler, a turbine, a condenser, and a feed water pump. The functions of various thermal equipment are as follows: ??(1) Function of the boiler: The boiler consists of a economizer, a furnace water wall, and a superheater; it heats the feedwater at a constant pressure to produce superheated steam, that is, main steam, which is then sent to the turbine through main steam pipes ； ?(2) Function of the turbine: Steam enters the turbine where it undergoes adiabatic expansion to perform work, converting thermal energy into mechanical energy; the exhaust steam after performing this work is discharged into the condenser ； ?(3) Function of the condenser: To cool the exhaust steam from the turbine, causing it to condense into saturated water at a constant pressure, which is equal to the pressure of the turbine’s exhaust steam ； ? ?(4) The feedwater pump compresses the condensate adiabatically to increase its pressure, after which it is sent back to the boiler; the water supplied to the boiler is known as feedwater. 15. How is the Rankine cycle represented on a T-S diagram? As shown in the figure: the process 4---5---6---1 represents the heating, vaporization, and superheating of the working fluid (water) at constant pressure in the boiler ； ? The 1---2 process is a process in which superheated steam does work through isentropic expansion in a turbine ； ? The 2 --- 3 process is a process in which the steam that has done work (waste steam) is discharged into the condenser, where it condenses at constant pressure while releasing heat ； The 3---4 process is a isentropic compression process of condensate water in the feed water pump. 16. What is the steam consumption rate? What is the formula for calculating it? The amount of steam consumed by a steam turbine generator set to produce 1 kilowatt-hour of electrical energy is referred to as the steam consumption rate, which is denoted by the letter d. Calculation formula: ?d = D/Nf ?? ?? The symbol d represents the steam consumption rate, in kilograms per kilowatt-hour ； D-----Steam consumption per hour of the turbine, ? kg/h ； Nf---Power generation, in kilowatts. 17. What is the heat consumption rate? How is it calculated for condensing steam turbines? The amount of heat required to generate 1 kilowatt-hour of electrical energy by a turbine generator set is referred to as the heat consumption rate. It is denoted by the letter q. Calculation formula: q = d (io – t). Here, q represents the heat consumption rate, in kJ/kW·h ； ? ? ?d----fuel consumption rate, ? kilograms/kilowatt-hour ； ? ? ?io-----Initial enthalpy of steam, ?? kJ/kg ； ? ? ?t------Feedwater enthalpy, in kJ/kg. 18. What is heat conduction? Heat conduction refers to the phenomenon of heat transfer between objects that are in direct contact with each other, or within different parts of the same object. Common heat conduction phenomena in thermal power plants include heat transfer between the inner and outer surfaces of tube walls, cylinder walls, and drum walls. 19. What is convective heat transfer? Heat exchange between a flowing fluid and a solid wall surface, or heat exchange between flowing fluids, is referred to as convective heat transfer. Common examples of convective heat transfer in thermal power plants include: flue gas and the economizer ； Working fluid to water wall ； Steam turbine exhaust gas to condenser copper tubes ； Circulating water on copper pipes ； Air for heaters, etc. 20. What is thermal radiation? Rays with wavelengths ranging from 0.4 to 40 micrometers can be absorbed by objects and then converted into heat energy; such rays are called thermal rays. The process by which heat rays transfer thermal energy is called thermal radiation. Thermal radiation is a form of heat transfer that does not require direct physical contact between substances. An example is the heat transfer between the flames in the furnace and the water-cooled wall screens in thermal oxidizers or wall-type reheaters in power plants. 21. What is thermal stress? When heavy metal components are heated and cooled unidirectionally, the temperature in different parts of them is uneven, which results in uneven thermal expansion as well. As a whole, the component is continuous, with forces of mutual restraint and constraint existing between its various parts; this prevents the heated portion from expanding and causes it to be compressed ； The cold parts are stretched, thereby generating stress inside the component. The stress resulting from uneven heating is called thermal stress. 22. What is metal fatigue? Under the action of long-term alternating stresses, even though the stress values are far below the ultimate strength limit, metal materials can still be damaged. This phenomenon is known as metal fatigue. During the startup and shutdown of a turbine, if there are large variations in steam temperature, the temperature difference with the metal increases, causing significant thermal stress to be exerted on both the rotor surface and the cylinder walls. Although the impact duration is short, its force is very large; it is even more dangerous when the material is brittle. It is necessary to not only examine the material’s yield limit but also consider the thermal fatigue damage that results. The rotating blades of steam turbines experience resonance due to the repeated action of shock steam forces; if resonance occurs, it can lead to fatigue fracture in severe cases. The thermal fatigue damage suffered by the rotor is caused by multiple alternating thermal stresses. Given that the frequency of thermal stress cycles is very low—for instance, one start-stop cycle or one load increase-decrease cycle constitutes one cycle—the entire rotor experiences tensile thermal stress during startup and compressive thermal stress during shutdown. The direction of the thermal stress in the entire rotor is opposite to that in the inner bore, and their magnitudes add up. It can reach 8–10 times during sudden temperature changes, which makes it easy to develop thermal fatigue cracks and leads to rotor damage when operating conditions change suddenly. 23. What is metal creep? What are the stages of the creep process? The process by which metal materials gradually undergo plastic deformation under long-term high temperatures and static stress is known as metal creep. The typical process is shown in the figure: When a material is subjected to a certain load under specific pressure and temperature conditions, it undergoes elastic deformation, as indicated by the OA segment; this deformation is not caused by creep. Stage I, AB, is an unstable stage during which plastic deformation progresses rapidly, but for only a short time. Stage II BC is a stable stage, during which plastic deformation develops slowly and the creep rate remains constant. In Phase III, the CD non-acceleration phase, the creep rate increases rapidly again, and the material breaks and is damaged upon reaching point D. We will absolutely not allow the material to operate in its state at Stage III. ?II. Equipment Principles: 1. What is the characteristic curve of a centrifugal pump? A curve that shows the relationship between the main performance parameters is called a characteristic curve or performance curve. These characteristic curves include the flow rate--head curve at a certain speed (Q---H), the flow rate--power curve (Q---N), and the flow rate--efficiency curve (Q---η). On the pump’s characteristic curve, the data for head H, power N, and pump efficiency η at each flow rate can be found. 2. What is the proportional law for centrifugal pumps? For the same pump, when its speed changes, the relationships between flow rate, head, and power and this speed are as follows: Q/Q’ = n/n’, H/H’ = (n/n’)², and N/N’ = (n/n’)³. It can be seen from these formulas that, as the pump’s speed changes, the flow rate is directly proportional to that speed ； Head is proportional to the square of the rotational speed ； Power is proportional to the cube of the rotational speed. This relationship is known as the proportional law of centrifugal pumps. 3. What is cavitation? What phenomena occur when a pump is subject to cavitation? The inlet of a pump is the area where the liquid pressure is lowest; therefore, it is possible for the pressure of the liquid at the inlet to drop below the saturation pressure corresponding to its temperature. In such cases, vaporization occurs, and bubbles are formed and escape. In areas of high pressure within the liquid, the pressure surrounding the bubbles is greater than the vaporization pressure; as a result, the bubbles burst and condense. If this occurs near a metal surface, the liquid particles continuously strike the metal surface, causing it to become honeycombed or sponge-like in structure. Furthermore, the oxygen in the air causes chemical corrosion on the metal surface through heat release during condensation. This phenomenon is cavitation. Cavitation in the pump is the cause of noise, leading to a significant decrease in the pump’s flow rate, head, and efficiency, as well as fluctuations in the ammeter pointer. 4. Why is fire-resistant oil used as the medium in some turbine speed control systems? As the unit power and steam parameters continue to increase, the lifting force of the control valve in the control system grows larger. Raising the oil pressure of the oil-driven motor is one way to address this increase in the lifting force of the control valve. However, an increase in oil pressure can lead to oil leaks, and conventional turbine oils have a low ignition point, which can easily cause fires. Anti-fire fuel has a high auto-ignition point; even if it comes into contact with the surface of hot steam pipes, it will not catch fire. Moreover, flames cannot be sustained or spread in anti-fire fuel. Thereby **reducing the threat of fires to the power plant. ? The main feature of flame-retardant fuel is its flame-retardant properties, but it also has drawbacks such as certain toxicity, high cost, and poor viscosity-temperature characteristics (that is, temperature has a significant impact on viscosity). Therefore, the control system and the lubrication system are generally separated into two independent systems. The control system uses high-pressure fire-resistant oil, while the lubrication system uses ordinary turbine oil. ? 5. What determines the capacity of the main fuel tank? What is the circulation rate of turbine oil? ? The oil storage capacity of the turbine’s main oil tank is determined by the size of the oil system, and it should be sufficient to meet the oil requirements of the lubrication and control systems. The larger the engine, the more oil is required for the control and lubrication systems. The larger the fuel tank capacity, too. The circulation ratio of the turbine oil is equal to the ratio of the output volume of the main oil pump per hour to the total oil volume in the tank, and it should generally be less than 12. If the circulation rate is too high, the turbine oil stays in the tank for a short time, leaving no time for air and moisture to be separated, which causes the quality of the oil to deteriorate rapidly and shortens its service life. ? 6. What determines the lubricating oil pressure of a steam turbine? The lubricating oil pressure in a turbine is calculated during design based on factors such as the rotor’s weight, speed, the design of the bearing shells, and the viscosity of the lubricating oil, in order to ensure that a proper oil film is formed between the shaft journals and the bearing shells, as well as to provide sufficient oil for cooling purposes. Therefore, the lubricating oil pressure in turbines is generally set at 0.12–0.15 MPa. Excessively high lubricating oil pressure can cause oil leakage from the oil seals and bearing vibration. Too low oil pressure results in a poor formation of the oil film, and may even cause loss of oil supply that damages the bearing shells. 7. Why is an oil fume exhaust fan installed in the turbine oil tank? ? The function of the exhaust fan installed in the fuel tank is to remove gases and water vapor from the tank. This prevents water vapor from condensing in the fuel tank on one hand ； On the other hand, it keeps the pressure in the tank below atmospheric pressure, allowing the bearing return oil to flow smoothly into the tank. Conversely, if the fuel tank is airtight, a large amount of gas and water vapor will accumulate inside the tank, creating positive pressure that can affect the return of oil to the bearings; it also makes it easy for water to accumulate in the fuel tank. The fume exhaust fan also helps to remove harmful gases, preventing the oil quality from deteriorating. ? 8. What are the hazards of degraded turbine oil quality? The quality of turbine oil is closely related to the proper operation of the turbine. The deterioration of the oil quality alters the properties of the lubricant as well as the strength of the oil film, resulting in inadequate lubrication of various lubricated components; this in turn causes the bearing metals to melt and get damaged ； It can also cause the components of the control system to corrode, rust, and stick, leading to serious consequences such as the failure of the control system and protective devices to function properly. Therefore, it is necessary to pay attention to the supervision of the quality of turbine oil. 9. What is the viscosity of turbine oil? What is the viscosity index? ?? ?? ?? Viscosity is the standard for determining whether turbine oil is thick or thin. High viscosity means the oil is thick and does not flow easily ； Low viscosity means the oil is thin and flows easily. Viscosity is measured in Eng units, with the viscosity of commonly used turbine oils ranging from 2.9 to 4.3 Eng units. Viscosity has a significant impact on the lubrication performance of bearings; too high viscosity can cause the bearings to overheat, while too low viscosity can lead to the breakdown of the oil film. When the oil quality deteriorates, its viscosity increases. ? 10. Why must ventilation holes and vents be installed on the turbine bearing cover? Generally, a negative pressure exists inside bearings, usually due to the suction effect of the oil flowing out of them. Due to the negative pressure formed inside the bearing, steam is drawn in along with water droplets, which then condense. To prevent negative pressure from forming inside the bearing, ventilation holes or vents are provided on the bearing cover to connect with the atmosphere. On the other hand, providing an air vent on the bearing cover can also serve to remove the vapor generated by the turbine oil in the bearings due to heating, thereby preventing the pressure inside the bearing housing from exceeding atmospheric pressure. During operation, care should be taken to keep the ventilation holes unobstructed to prevent blockages. ? 11. What are the static and dynamic characteristics of a control system? ? The operating characteristics of a control system are of two types: dynamic characteristics and static characteristics. Under stable operating conditions, the relationship between the turbine’s power and speed is the static characteristic of the control system. The characteristics of the transition process from one stable operating condition to another are known as the dynamic characteristics of the control system; they refer to the patterns of change over time in parameters such as the unit’s power, speed, and the opening degree of the control valves during this transition. 12. What is the static characteristic curve of a control system? What are the requirements for the static characteristic curve? ? The static characteristic curve of the control system is the curve that shows the relationship between load and speed under stable conditions. The static characteristic curve of the control system should be a smoothly descending curve, without any horizontal sections in the middle, with steeper slopes at both ends. If there is a horizontal section in the middle, it will cause spontaneous fluctuations or instability in the load during operation. The left end of the curve is steeper, which mainly helps the turbine to stabilize at a certain speed for the paralleling and de-paralleling of the generator. It also reduces the impact of external load fluctuations on the unit under low loads after grid connection. The steeper slope on the right side is intended to maintain a stable and economical load for the unit; when the grid frequency drops, it allows the turbine to handle a lower load, thereby preventing overload of the turbine. ? 13. What is the speed variation rate of a control system? What are the requirements for the rate of speed change? ? From the static characteristic curve of the control system, it can be seen that when the unit operates alone, from no-load to rated load, the speed of the turbine decreases from n2 to n1. The ratio of this speed change to the rated speed n0 is called the speed variation rate, denoted by δ. That is, δ = (n2 – n1) / n0 × 100%. A control system with a smaller δ has the advantage of being able to respond flexibly to changes in load, and is suitable for units used to handle frequency regulation tasks ； Regulation systems with a larger δ have strong load stability and are suitable for units that provide basic load ； If δ is too large, the unit is prone to overspeeding when load is shed ； A control system with too small a δ value may experience oscillations; therefore, it is generally set between 4% and 6%. The greater the steepness of the speed variation rate and the static characteristic curve, the larger the speed variation rate; conversely, it is smaller. 14. What is the delay rate of a control system? ? During operation, the control system must overcome the frictional forces within the various moving components. Additionally, due to factors such as gaps between components and their degree of overlap, the static characteristics differ during acceleration and deceleration, resulting in two curves that are almost parallel to each other. In other words, the rotational speed must be varied by a certain amount; only after overcoming the resistance and clearance does the adjustment of the valve’s movement in the opposite direction begin. The ratio of the maximum possible variation in rotational speed Δn under the same load to the rated rotational speed n0 is called the droop rate. It is usually denoted by the letter ε, that is, ε = Δn/n0 × 100%. ? 15. What is the impact of an excessively high delay rate in the control system on the operation of the turbine? ? The effects of an excessively high delay rate in the control system on the operation of the turbine are as follows: (1) When the turbine is running at no load, an overly high delay rate in the control system causes instability in the turbine’s speed, thereby making it difficult to synchronize it with other systems. ? (2) After the turbine is connected to the grid, excessive lag rate can cause fluctuations in the load. (3) When the unit load is suddenly reduced to zero, the excessive delay rate prevents the control valves from closing immediately, resulting in a sudden increase in speed and triggering the emergency shutdown device (ETS protection). If the emergency safety device fails to function, it can lead to serious accidents such as vehicle speeding out of control. 16. Why are dynamic and static characteristic tests conducted on control systems? ? The purpose of testing the static characteristics of a control system is to determine its static characteristic curve, speed variation rate, and lag rate, in order to fully assess whether the control system operates correctly, reliably, and flexibly ； Analyze the causes of defects in the control system in order to properly eliminate them. ? The purpose of testing the dynamic characteristics of the control system is to determine the speed rise curve during load rejection, so as to accurately evaluate the quality of the transition process and improve the dynamic control quality of the control system. ? 17. What are the dynamic characteristic tests of a control system? The dynamic characteristics of a control system refer to the properties of the transition process from one stable operating condition to another; in other words, they describe how parameters such as the power, speed, and opening degree of the control valves of the turbine unit change over time during this transition. When a turbine is operating at full load and the load is suddenly removed, this represents the greatest change in operating conditions; at such times, the turbine’s power, speed, and the opening degree of the control valves experience the most significant changes. As long as the dynamic performance indicators of the control system meet the requirements when this operating condition changes, other changes in operating conditions will also meet the requirements; therefore, the dynamic characteristic test uses the scenario of the turbine losing all its load as the test condition. The full-load rejection test is, in other words, a dynamic characteristic test. 18. Why are the main steam valves usually opened by hydraulic pressure and closed by spring force? ? This is because in any accident scenario, including when the oil supply is cut off, the automatic main steam valve should still be able to close quickly. Therefore, the main steam valve is generally designed to close under spring force. ? 19. What is the function of the turbine axial displacement protection device? ? The axial clearance between the turbine rotor and stator is very small. When the axial thrust on the rotor becomes excessive, causing the bearing materials to melt, the rotor will experience unauthorized axial movement, which leads to friction between the rotating and stationary parts and results in severe damage to the equipment. Therefore, turbines are equipped with axial displacement protection devices. Its function is to emit an alarm signal when the axial displacement reaches a certain value ； When the axial displacement reaches a dangerous level, the protection device activates, shutting off the steam supply to the turbine and stopping it. ? 20. What is the function of a low-vacuum protection device? A decrease in vacuum during turbine operation not only affects the turbine’s output and reduces its thermal efficiency, but an excessive drop in vacuum can also compromise the turbine’s safety due to high exhaust temperatures and increased axial thrust. Therefore, high-power steam turbines are all equipped with low vacuum protection devices. When the vacuum level drops to a certain value, an alarm signal is emitted; when it reaches the specified limit, the machine will shut down automatically. To protect the turbine from damage. Turbine, typical knowledge, characteristic curves, speed control valves, characteristic control system mmexport1576201326076.jpg (4.24 KB, downloads: 0) https://attach01.hcbbs.com/forum/201912/13/1576201333864-91312.jpg mmexport1576201477666.jpg (5.47 KB, downloads: 0) https://attach01.hcbbs.com/forum/201912/13/1576201484823-4944.jpg mmexport1576201538135.jpg (5.8 KB, downloads: 0) https://attach01.hcbbs.com/forum/201912/13/1576201542954-30345.jpg