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1. What are the state parameters of a working fluid? What is the meaning of enthalpy? (1) The state parameters are: temperature (T), specific volume (υ), pressure (p), thermodynamic energy (u), enthalpy (h), and entropy (s). (2) Basic state parameters: temperature (T), specific volume (υ), pressure (p). (3) Enthalpy: For m kilograms of a working substance, the sum of its internal energy and pressure potential energy is called enthalpy; its unit is kJ, denoted by H. For 1 kg of a working substance with pressure p and specific volume υ, the sum of its internal energy u and pressure potential energy is called specific enthalpy. h=u+pυ2, what is the speed of sound? What is the Mach number? (I learned about this back in college during my fluid mechanics course; I’ve long forgotten it, so I’m reviewing it now.) (1) Speed of sound: It is the speed at which pressure waves caused by weak disturbances propagate through a continuous medium; it is denoted by α. It is also called the speed of sound. It can be seen that the speed of sound is a constant. It is related to the properties and state of the gas; therefore, the speed of sound is also a state parameter. As it flows, therefore, the speed of sound is not a constant value; it changes depending on the properties of the gas or the flow velocity, and mechanical conditions are required first. It can be derived that, that is. In a geometric configuration where the flow velocity changes, the state of the gas at each cross-section of the flow channel is constantly changing; therefore, the speed of sound at each cross-section also changes continuously. That is why the speed of sound is referred to as the “local speed of sound”. (2) Mach number: The ratio of the gas flow velocity c to the local speed of sound a is called the Mach number. Denoted by Ma. Therefore, Ma = c/a. When Ma＜1, the flow velocity is less than the local speed of sound, which is called subsonic ； When Ma > 1, the flow velocity is greater than the local speed of sound, which is known as supersonic ； When Ma = 1, the flow velocity is equal to the local speed of sound. 3. The relationship between air flow velocity and pressure (only the conclusion is given, without any derivation): As the velocity of the air flow increases during its movement, the pressure necessarily decreases ； As pressure increases, the flow rate must decrease. Since the enthalpy decreases when pressure drops, meaning thermal energy is reduced, and the total energy of the airflow remains constant during its flow, kinetic energy increases; thus, the speed of the airflow must increase. 4. The relationship between changes in flow velocity and changes in the cross-sectional area through which air flows (Learning this lays the groundwork for later learning about nozzles): When the air flow velocity changes, variations in certain parameters cause a corresponding change in the cross-sectional area through which air flows. The geometric conditions favorable for changes in flow velocity can be derived from the basic equations of isentropic flow: it can be seen from these formulas that, when the flow velocity changes, the way in which the cross-sectional area of the airflow changes depends not only on whether the velocity is increasing or decreasing, but also on whether the flow is subsonic or supersonic. 5. How do the airflow parameters change within the nozzle? How does the flow cross-sectional area change in a nozzle under different sound speed conditions? What are the requirements for nozzle design in airflow with different sound speeds? (This question explains the original intention behind the design of nozzle structure; it’s worth learning.*) (1) Changes in the parameters of the vapor flow within the nozzle: pressure decreases while flow velocity increases. (2) Changes in the flow cross-sectional area of airflow under different sound speed conditions in the nozzle: dp < 0, dc > 0. From the formula in Question 4, it can be deduced that: a. When the airflow is supersonic, the Mach number Ma > 1; thus, dA > 0, meaning the flow cross-sectional area gradually increases ； b. When the airflow is subsonic, i.e., Mach number Ma < 1, it can be concluded that dA < 0; thus, the cross-sectional area through which the airflow passes gradually decreases ； c. When the airflow is at sonic speed, the Mach number Ma = 1; it can be deduced that dA = 0. At this point, the cross-sectional area through which the airflow passes becomes minimal. (3) Requirements for nozzle design in different sound speed flow conditions: For subsonic flow, a converging nozzle is required ； For supersonic airflow, a gradually expanding nozzle must be used ； To enable the airflow to increase continuously from subsonic to supersonic speeds, a scaled nozzle, also known as a Laval nozzle, is required. Only when the cross-sectional shape of the nozzle matches that of the airflow can the airflow expand sufficiently within the nozzle, achieving an ideal acceleration effect. The shapes of various nozzles are shown in the figure: 6. What shape of nozzles are used in steam turbines? The nozzles in turbines are generally designed as beveled nozzles, as shown in the figure below. Why are the nozzles in turbines made to be beveled nozzles? One reason is to give the nozzle airflow a proper direction as it enters the moving blades, thereby driving them to perform work. Another reason is that when the outlet pressure of the nozzle is less than the critical pressure, the steam at section AB in the throat can only achieve sonic flow; the inclined section is gradually expanding, so the steam continues to expand in this inclined section, resulting in supersonic flow at the outlet section of the inclined part. 7. Reaction degree of the turbine stage? (1) Definition: The term reactance refers to the degree of expansion of steam within the turbine blade cascade. It is denoted by the symbol ρ. (2) Algorithm: The degree of reaction is defined as the ratio of the ideal enthalpy drop hb of the steam in the moving blade row (also known as the isentropic enthalpy drop) to the total stagnation isentropic enthalpy drop of the entire stage. As shown in the following formula: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps77.tmp.jpg (3) Meaning of the formula: When the degree of reaction is equal to 0, it means that hb = 0; this indicates that the steam does not expand in the moving blade row, but only expands in the nozzles. Such stages are purer impulse stages. The so-called reaction stage refers to a stage with a reaction degree of approximately 0.5, in which the steam expands to roughly the same extent in the nozzle and the rotor blades. The efficiency of reaction stages is higher than that of pure impulse stages; therefore, ordinary impulse stages generally have a slight degree of reaction. 8. Relations between impulse stage, reaction stage, and speed stage: (1) Impulse stage: In a pure impulse stage, steam expands only in the nozzle cascade, resulting in a decrease in pressure; it does not expand in the rotor cascade, so the pressure remains constant. The rotor blades are symmetric, and the inlet angle of these blades is equal to their outlet angle. As a result, the velocity of the airflow through the rotor cascade (i.e., the relative velocity) remains theoretically constant. Due to the curvature of the blade shapes, the direction of the airflow changes, which generates a force on the rotor blades that causes the impeller to rotate and perform work. Pure impulse stages have high performance but low flow efficiency, and are no longer used in modern turbines. (2) Reaction stage: In the reaction stage, the steam not only expands and accelerates in the nozzle, but also continues to expand and accelerate as it flows through the passages of the moving blade row. That is, in the moving blade row, not only does the direction of the steam flow change, but its relative velocity also increases. Therefore, the moving blades are subjected not only to the impulse force of the high-speed airflow at the nozzle exit but also to the reaction force as the steam leaves the moving blade cascade; thus, the reaction stage does work due to both the impulse force and the reaction force. Since the steam expands more rapidly in the moving blade cascade, and it is the combined force of impulse and reaction that causes the impeller to rotate and perform work, the efficiency of the reaction stage is higher than that of the impulse stage, but its capacity to perform work is lower. (3) Speed stage: The characteristic of a speed stage is that it has two or three rows of moving vanes on one impeller, with a row of stationary guide vanes mounted on the cylinder between these rows of moving vanes. 9. Why do modern high-power and high-parameter steam turbines mostly use a two-row velocity stage in the first stage? The first stage of a multi-stage steam turbine often employs a double-row speed stage, which allows the pressure and temperature of the steam to drop significantly after this stage. This not only reduces the total number of stages in the turbine, resulting in a more compact design, but also lowers the requirements for metal materials in the cylinders and blades located downstream of the speed stage, thereby reducing the cost of the turbine. 10. What forces does the turbine cylinder have to withstand during operation? (1) The pressure difference inside and outside the cylinder exerts a certain force on the cylinder wall ； (2) The forces exerted by the diaphragm and nozzle on the cylinder; these are caused by the pressure difference before and after the diaphragm, as well as the reaction force when the steam flows through the nozzle ； (3) The weight of the cylinder itself and the various components installed on it ； (4) In units where the bearing housing is cast as one piece with the cylinder, or where the bearing housing is bolted to the cylinder, the cylinder also has to bear the weight of the rotor as well as the unbalanced forces generated during the rotor’s rotation ； (5) Forces exerted by the inlet and exhaust pipes on the cylinder ； (6) During operation of the turbine, thermal stresses caused by temperature differences exist in various parts of the cylinder. Especially during rapid startup, shutdown, and changes in operating conditions, large temperature fluctuations occur, resulting in significant thermal deformation due to thermal stresses in the cylinders and flanges. 1. Why is steam introduced into the multilayer cylinder interlayer? There are two reasons: (1) when a turbine uses multi-layer cylinders, steam at moderate pressure is usually introduced into the gap between the inner and outer cylinders. When the unit is operating normally, due to the high temperature of the inner cylinder, heat is continuously radiated to the outer cylinder, posing a risk of overheating it; at this point, the steam flow in the interlayer acts to cool the outer cylinder. (2) During the cold start of the unit, in order to heat the inner and outer cylinders as quickly as possible, thereby reducing the differential expansion between them and the thermal stresses and shortening the startup time, the steam flow in the interlayer serves to heat the cylinders. 2. What are the functions of the nozzle and the diaphragm? (1) Nozzle: The nozzle is one of the main components that make up a steam turbine. Its function is to convert the thermal energy of steam into the kinetic energy of a high-speed steam stream, causing this stream to be ejected from the nozzle in a specific direction and enter the rotor blades, thereby driving the rotor to rotate and perform work. The first-stage nozzle is directly mounted on a dedicated nozzle chamber at the high-pressure end of the cylinder. (Hehe, here are a few pictures of the nozzles for everyone to take a look at.) Picture of the nozzle’s shape (half at the top and half at the bottom); photos of the nozzle’s arc section: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps78.tmp.jpg file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps79.tmp.jpg. (2) Separators: These are used to hold the nozzles in place and to separate the different stages of the impellers. Each stage of an impulse turbine consists of a diaphragm and a rotor blade wheel. Reaction turbines do not use a diaphragm-type structure; the nozzles at each stage (also known as stator vanes) are installed directly on the cylinder. Let’s also take a few photos to have a look: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps7A.tmp.jpg file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps7B.tmp.jpg 3. What are the characteristics of the nozzles in the first stage of a steam turbine? The first-stage nozzles are directly mounted on a dedicated nozzle chamber at the high-pressure end of the cylinder, and are divided into varying numbers of arc segments. Due to the high pressure of the working steam in the first-stage nozzle, its volumetric flow rate is low. To give the blades of the first-stage nozzle a certain height and thereby reduce flow losses, this nozzle is often designed with partial steam inlet, meaning that nozzles are only arranged along a portion of the arc. Each nozzle segment is directly controlled by its respective control steam valve, which is used to adjust the amount of steam supplied to the turbine in order to meet the demands of the load (currently, there are basically 4 control valves; the nozzles are divided into upper and lower halves, with 2 steam guide tubes in the upper half and 2 in the lower half, each corresponding to its own nozzle segment). Therefore, the first-stage nozzle is also known as the regulating-stage nozzle. 4. Materials related to expansion differential* (Thank you to “I Dedicate My Youth to Electricity” – I learned this from him, and I’ve added my own simple analysis; please let me know if there are any mistakes.) (1) Definition of expansion differential: The relative expansion of the turbine rotor and the cylinder is known as the expansion differential. *It is conventionally specified that when the rotor expansion is greater than the cylinder expansion, the differential expansion is considered a positive differential; whereas when the cylinder expansion is greater than the rotor expansion, the differential expansion is considered negative. Based on cylinder classification, it can be further divided into high difference, medium difference, low I difference, and low II difference. The expansion difference value is a very important operating parameter; if it exceeds the limit, the thermal protection system will activate and cause the main unit to trip. (2) The main factors causing the expansion difference to increase in the positive direction are briefly described as follows: a. The warm-up time during startup is too short; the speed or load is increased too rapidly. (The meaning is roughly that the warming up is insufficient, resulting in uneven expansion of the rotor and the cylinder block.) b. The heating temperature of the heating device located in the cylinder jacket or flange is too low, or the flow rate is too low, which leads to a weak heating effect. (This is also caused by thermal stress.) c. The sliding performance of the slider system or bearing plate is poor, making it prone to sticking. (Resulting in improper expansion of the cylinder) d. The steam temperature at the shaft seal is too high, or the amount of steam supplied to the shaft seal is excessive, causing excessive elongation of the journal. (e) Excessive rotor expansion; during unit startup, parameters such as steam inlet pressure, temperature, and flow rate are too high. (This is equivalent to the rotor being too hot, with the cylinder block’s expansion failing to keep up with that of the rotor.) f. Wear of the thrust bearing leads to increased axial displacement. (The rotor displacement is too large.) g. The insulation effect of the cylinder’s thermal insulation layer is poor, or the insulation layer has come loose. During cold seasons, the temperature inside the turbine hall is excessively low, or there are drafts of cold air. (The cylinder body is cooled; its expansion fails to keep up with that of the rotor.) h. Cold steam (or cold water) flows into the interlayer of the double-layer cylinder. (The cylinder block is cooled, so its expansion cannot keep up with that of the rotor.) i. An inaccurate zero point of the expansion difference indicator or worn contacts cause numerical deviations. j. Multi-rotor units: the mutual influence resulting from changes in the expansion difference between adjacent rotors. k. Effect of vacuum changes. (So now I don’t know why?) ) l. Impact of speed changes. (This is the same as above.) m, the impact of changes in steam extraction at various levels: if the primary steam extraction is stopped, the effect on the pressure difference is quite significant. n. The bearing oil temperature is too high. o. Due to the Poisson effect during the coasting phase after the unit is shut down. (What is the Bosan effect?) ? I don’t understand.) (3) The main reasons for an increase in the differential pressure to a negative value: a) A rapid drop in load or sudden load shedding. b. A sudden drop in main steam temperature, or the inlet steam temperature during startup being lower than the metal temperature. c. Water impact. d、Excessive heating of the cylinder clamp and flange heating device. e. The shaft seal steam temperature is too low. f. Change in axial displacement. g. The bearing oil temperature is too low. h. The starting speed rises suddenly; due to the reduction in the axial size of the rotor under the effect of centrifugal force, this change is particularly noticeable at low differential pressures. i. High-temperature steam flows into the cylinder interlayer; it may originate from the steam heating device, or from leaks in the steam inlet sleeve or the shaft seal. During startup, a heating device is generally used to control the degree of expansion of the cylinder, while the rotor’s degree of expansion is primarily controlled by the inlet temperature and flow rate of steam in the turbine, as well as the temperature and flow rate of steam used for shaft sealing. During startup, the expansion difference generally moves in the positive direction. When a turbine is shut down, as the load and speed decrease, the rotor cools down faster than the cylinders; as a result, the expansion difference generally moves in a negative direction. This is especially severe during shutdowns under sliding-parameter conditions. It is necessary to use steam heating devices to supply cooling steam to the cylinder jackets and flanges in order to prevent the expansion difference protection from activating. After the turbine rotor stops rotating, a negative expansion difference may worsen; therefore, shaft seal steam at a certain temperature should be maintained to avoid adverse consequences. 1. Related to the turbine slide system (The turbine slide system has always been a weak point in the maintenance or operational management of turbines; it doesn’t even receive enough attention, and it is often overlooked in studies related to turbines. Let’s address this today as a sort of make-up lesson.) First, here is a system diagram: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps7C.tmp.jpg. (1) Function of the turbine slide system: To ensure that the cylinder can expand smoothly when heated, thereby keeping the center of the cylinder aligned with that of the rotor and preventing accidents. A sliding pin is a component mounted on the cylinder block that allows the cylinder to expand and contract smoothly. (2) Classification of sliding pins: Based on their structural design, installation location, and functions, sliding pin systems typically consist of vertical pins, longitudinal pins, horizontal pins, cat’s-claw horizontal pins, diagonal pins, and corner pins. I will introduce them one by one below, and try to include images to make it easier to understand. As shown in the figure below: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps7D.tmp.jpg a. Vertical pin: The vertical pin is used to guide the cylinder to move in a vertical direction ； b. Vertical pin: The vertical pin guides the bearing housing and cylinder to slide axially ； c. Cross pin: The cross pin guides the cylinder to slide horizontally and works in conjunction with the vertical pin to determine the fixed point of expansion, known as the dead center. In condensing steam turbines, the dead centers are mostly located at or near the center of the low-pressure exhaust port; this way, when the turbine expands due to heat, it has less impact on the large and bulky condenser. (3) Upper and lower cat-claw supports for high-pressure and medium-pressure cylinders (first, 2 photos): a. Lower cat-claw support method: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps7E.tmp.jpg b. Upper cat-claw mid-plane support method (the previous company used a horizontal mid-plane support method for the upper cat claws; no photos were taken, so let’s use a structural schematic instead): file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps7F.tmp.jpg (4) Advantages and disadvantages of the upper and lower cat-claw support structures: a. Upper cylinder cat-claw support: As can be seen from the diagram, in this support method, the force-bearing surface lies in the same plane as the horizontal mid-plane of the turbine cylinder, which helps to maintain alignment between the cylinder and the turbine rotor. However, during maintenance, shims must be placed under the lower cat-claw support; otherwise, the lower cylinder will sink. b. Cylinder-bottom cat’s-paw support: Its advantage is easy maintenance, as there is no need to install maintenance shims. The drawback is that the force-bearing surface of the cat claw and the mid-plane of the cylinder are not in the same plane; when the cylinder heats up and the temperature of the cat claw rises causing it to expand, this will raise the center line of the cylinder. Since the center of the rotor supported by the bearing housing remains unchanged, this results in a reduced gap between the lower cylinder and the rotor components, and in severe cases, it can lead to dynamic and static friction. 2. How does the turbine lubricating oil function in the bearings? During operation, a oil film is formed between the shaft journal and the bearing shells, creating liquid friction that enables the steam turbine to operate safely and stably. The lubricating oil enters at the horizontal joint surface of the lower bearing shell; the oil that is drawn in as the shaft rotates first passes through the gap between the shaft and the upper bearing shell, and then through the gap between the shaft and the lower bearing shell, thereby forming an oil film. To reduce friction losses and enable the oil to circulate in order to cool the shaft journals, oil grooves are engraved on the brass surfaces of the bearing shells, extending toward both ends near the joint surface to ensure even distribution of oil along the entire length of the bearing shells. After being lubricated in the bearing shells, the oil flows out through the gaps at both ends of the bearing shells. It gathers in the bearing housing and then flows back to the oil tank through the oil pipes. 3. Why is axial thrust generated during the operation of a steam turbine? Due to the effect of steam, when it emerges from the nozzle, it forms an angle with the direction of motion of the rotor blades (the circumferential velocity direction). Therefore, in addition to the force in the circumferential direction, the steam exerts an axial force on the blades as well. Furthermore, there is a pressure difference before and after each stage of impellers and vanes, and where there is a pressure difference, there is an axial thrust. Therefore, the high and medium pressure cylinders as well as the low pressure cylinder of high-power steam turbines are arranged in a reverse symmetrical pattern, and the impellers are equipped with an odd number of balance holes – all of these measures are intended to reduce the axial thrust on the turbine rotor. Of course, thrust bearings are an important means of balancing the thrust on a steam turbine rotor. Regarding the principles and maintenance of thrust bearings (these things aren’t easy to deal with; I’ve repaired thrust bearings for 600MW units at Dongfang Electric several times and have some insights), I’ll strive to learn more about it* and will discuss it in detail in future posts. (Check out the old photos for a more intuitive view.) The image below shows the layout of the high and low-pressure rotors as well as the direction of steam flow: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps80.tmp.jpg. The next image shows the layout of the low-pressure rotor and the direction of steam flow: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps81.tmp.jpg. The final image shows the balance holes on the impeller: file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps82.tmp.jpg. 1. Methods for dealing with common hydrogen leakage points in generators and important precautions (summarized by myself; it was posted in previous threads, and I’ve analyzed it again today, adding the ninth point, which relates to temperature*.)(1) Sand holes and cracks in hydrogen pipelines: Isolate the affected section and repair it by welding or replace it with a new pipe segment (seal both ends with plugs and purge the section with sand holes or cracks using inert gas or carbon dioxide). (2) Hydrogen system valve stem and flange hydrogen leakage: Tighten it; if tightening does not resolve the issue, isolate the component and replace the gasket or packing (tighten it slowly using copper tools). (3) Internal leakage in the drain valves and vent valves of the hydrogen system: isolate for replacement or seal online. (4) Hydrogen leakage at the outlet flange of the generator stator temperature: Try tightening it or shut down the machine to replace the gasket. (5) Leakage in the generator seal pads: 1) If the leakage is caused by too low a seal oil pressure, increase the seal oil pressure appropriately, but not to excessively high levels, to prevent oil from entering the generator. 2) If leakage is caused by excessive wear of the sealing pads, the machine must be stopped and hydrogen removed. After the replacement is complete, the sealing pads should be disassembled for inspection; they are then ground and adjusted so that the sealing gap is within the acceptable range before being reinstalled. If the wear is too severe, new pads must be used. Additionally, it is necessary to check the sealing condition between the seal pad and the generator’s contact sealing surface, as leaks can also occur here. (Here’s an online photo of the seal ring leaking for everyone to see; can you see it? The seal oil is being blown out.) file:///C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ksohtml\wps83.tmp.jpg (6) Excessive wear on the journal of the main shaft at the seal ring location causes leakage: if the wear is not severe, adjusting the sealing clearance of the seal ring will suffice; if the wear is severe, laser repair of the main shaft should be carried out whenever possible. (7) If the generator’s casing has sand holes or cracks, sealing the leaks under pressure can be attempted if conditions permit. If this method does not work and the leakage is severe enough to pose a risk to the safe operation of the unit, the machine should be shut down for welding repairs, or the manufacturer should be consulted for a solution. (8) Generator hydrogen cooler leakage: This is also a common point of leakage, but it is the most difficult to locate. Generally, each set of hydrogen coolers is equipped with an exhaust valve at the top, which is used to isolate the cooling water for that set of hydrogen coolers. Isolating the hydrogen coolers requires reducing the load on the unit; this must be done in consultation with the power generation department, as failing to do so could lead to excessive temperatures in the generator coils and cause the unit to shut down. By opening the exhaust valve and using a hydrogen detection instrument pointed at the drainage outlet, it is possible to determine whether there is a hydrogen leak in that set of hydrogen coolers. After identifying the leaking hydrogen cooler, a work order is prepared to isolate that cooler; the inlet chamber of the hydrogen cooler as well as its end seal cover are removed. A hydrogen detector is used to locate the heat exchange tubes in the leaking cooler and they are marked; then copper plugs are used to seal both ends of those leaking heat exchange tubes tightly. (The total number of blocked tubes should not exceed 10% of the total number of heat exchange tubes; if too many tubes leak, the system must be shut down to replace them with new tubes.)