Comprehensive Guide to Steam Turbines

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1) What is the residual velocity loss of a turbine? Answer: When steam leaves the moving blades, it has a certain residual velocity, that is, it possesses certain kinetic energy; this unutilized kinetic energy is referred to as residual velocity loss. 2) What is a impulse turbine? Answer: The expansion of steam mainly occurs in the nozzles or stationary vanes; a turbine in which the moving vanes operate on the principle of impulse is called an impulse turbine. 3) What is a turbine stage? Answer: The most basic unit of a steam turbine, consisting of a row of nozzles or stationary vanes and a row of rotating vanes. 4) What is a reaction turbine? Answer: In this type of turbine, the expansion of steam occurs not only in the stationary blades but also in the rotating blades. The enthalpy drop in the stationary blades and rotating blades is roughly equal, with the rotating blades primarily operating based on the reaction principle. 5) Explain the importance of manufacturing multi-stage steam turbines from a materials science perspective? Answer: As steam parameters increase, the enthalpy drop in the steam also increases. If a single-stage steam turbine is still used, in order to ensure operation at an optimal speed ratio, the peripheral speed must be increased accordingly. This results in a larger diameter of rotating components and greater centrifugal forces acting on them. However, the allowable stress of materials is limited; therefore, the power output of a single-stage steam turbine is constrained by material limitations. To obtain a prime mover with high power output, multi-stage steam turbines must be employed. 6) What are the mechanical losses of a steam turbine? Answer: When the main shaft of a steam turbine rotates, it must overcome the frictional resistance of the bearings. Additionally, it drives the main oil pump, governor, etc. All these processes consume a portion of the useful work, resulting in losses, which are collectively referred to as mechanical losses. 7) What is the variable-load operation of a steam turbine? Answer: When the operating conditions of a turbine, such as load, parameters of the fresh steam, and vacuum, do not match the design conditions, it is referred to as variable-condition operation of the turbine. 8) What is the internal power of a steam turbine? Answer: When only the various internal losses of the turbine are taken into account, including the throttling loss at the steam inlet, the losses within each stage, and the pressure loss in the exhaust pipe, the power generated by the turbine is referred to as the turbine’s internal power. 9) Can a back-pressure steam turbine meet the requirements of both electrical and thermal loads simultaneously? Answer: No. The backpressure machine operates based on the magnitude of the heat load, and its power generation capacity varies according to that heat load. 10) What is cylinder heat dissipation loss? Answer: Although insulation is applied to the cylinder, the surface temperature of the insulation remains higher than the ambient temperature. Therefore, heat is still dissipated to the surrounding cooler air, resulting in cylinder heat loss. 11) What is the purpose of cylinder flange heating and bolt heating devices? Answer: High-pressure steam turbines are generally equipped with cylinder flange heating and bolt heating systems, which serve to reduce the temperature difference between the cylinder, flanges, and bolts during startup and operation under varying conditions, thereby shortening the startup time. 12) What are the advantages and disadvantages of the cat’s-paw type supports for the lower cylinder? Answer: This type of support is simple and easy to install and maintain. However, its load-bearing surface and the horizontal midplane of the cylinder are not in the same horizontal plane. When the cylinder is heated, the temperature of the cat claw increases rapidly, causing it to expand and raising the centerline of the cylinder. Meanwhile, the centerline of the rotor supported by the bearings remains unchanged, which results in a reduction in the radial clearance between the lower cylinder and the rotor components. This can even lead to friction between the moving and stationary parts. 13) What are the advantages and disadvantages of upper cylinder paw supports? Answer: Its load-bearing surface and the horizontal midplane of the cylinder lie in the same horizontal plane, which allows for better alignment of the cylinder and the rotor’s centers; however, installation and maintenance are relatively complex. 14) What is the function of the sliding sleeve system? Answer: To ensure that the cylinder can expand freely when heated and maintain alignment between the cylinder and the rotor center. 15) What are the internal losses of a steam turbine? Answer: Throttling loss of the steam distribution mechanism ； Pressure loss in the exhaust pipe ； Intra-stage losses of the turbine. 16) What are the external losses of a turbine? Answer: Steam leakage loss at the end shaft seal ； Cylinder heat dissipation loss ； Mechanical losses. 17) Where is the area in the cylinder that experiences the highest pressure and temperature? Answer: The steam inlet chamber. 18) When is the temperature difference between the flange and the bolt greatest? Answer: During the cold start of the unit. 19) For the cold tightening of cylinder bolts, it is necessary to repeat the tightening process at least once in sequence in order to ensure uniform tension across all bolts. Why? Answer: The tightening force of the bolts in the middle often decreases because the adjacent bolts share the force exerted on the flange gap during tightening, thereby reducing the tightening force of the bolts in the middle. 20) What are the causes of bolt seizure? Answer: When bolts operate at high temperatures for extended periods, a high-temperature oxide film forms on their surfaces. When the nuts are tightened or loosened, improper techniques can cause this oxide film to be damaged, resulting in burrs on the thread surface ； Alternatively, poor thread machining quality, poor surface finish, scratches, gaps that do not meet standards, and non-uniform bolt material can all easily lead to thread seizure. 21) After scraping the bottom of the bearing housing, how can the longitudinal level of the cylinder and the jacking value of the journal be basically restored to their pre-repair values? Answer: Shims can be placed under the cylinder pawls and at the bearing shell supports in order to restore the longitudinal levelness of the cylinder and the camber value of the shaft journals to their values before repair. 22) Under what conditions are the conditions met to lift the cylinder head? Answer: Once the cylinder has cooled to below the specified temperature, the insulation is removed. Then, all the flange connection bolts and set screws for various components such as the cylinder joint surfaces, steam guide pipes, front and rear shaft seal pipes, and flanged steam supply pipes are removed, as well as the thermal measurement elements. Only after it is confirmed that there are no connections between the upper cover, the lower cylinder, and other pipelines can the upper cover of the cylinder be lifted. For units with an inner and outer cylinder, the inner cylinder should have no connections. 23) After lifting the cylinder head by 100–150 mm, what inspections should be carried out before continuing with the lifting? Answer: A comprehensive inspection should be conducted once again. Special attention must be paid to inspecting the interior of the cylinder, ensuring that no internal components such as connecting parts or baffles become detached. Only after confirming that everything is in order can the slow lifting process proceed. 24) When lifting the cylinder head, the rotor tends to rise. What is the reason for this? Answer: This indicates that the flanges of certain shaft seal sleeves or partition sleeve inside the large cover are severely stuck in the corresponding grooves of the upper cylinder. Therefore, when the shaft seal sleeve or partition sleeve is lifted, lifting operations should be stopped and measures should be taken to address the situation. 25) What preparations should be made inside the cylinder before reinstalling it after turbine maintenance? Answer: All components inside the lower cylinder and bearing housing should be lifted out and thoroughly cleaned. The steam extraction holes, drain holes, etc. should be temporarily sealed and removed, and the seals on the nozzles should be torn off. Thoroughly inspect and clean the cylinder condenser and the interior of the bearings. Use compressed air to check whether the holes of each pressure gauge are unobstructed. 26) What are the advantages and disadvantages of rigid couplings? Answer: The advantages are a simple structure, small size, no noise, and no need for lubrication. Two rotors can be supported by three bearings, which reduces the number of bearings and shortens the length of the rotors. The disadvantages are that they transmit vibrations and axial displacement during operation; as a result, high precision is required when aligning the rotors, and they are sensitive to any deviation in their alignment. 27) Why is it necessary to number the bolts on the cylinder during removal? Answer: For large bolts of M25 size or larger, nuts, and washers with special thicknesses used on the cylinder, they must be matched according to their numbers. Clear numbering should be done before disassembly. Place it back in its proper position during reinstallation. It can prevent problems such as thread jamming, thread interlocking, and contact between the top of the inner hole of the cover nut and the bolt. 28) How should the threads be treated when formally assembling the cylinder bolts? Answer: When applying grease to bolt threads, the grease must be washed off and the threads dried. Then, they should be polished with black lead powder or molybdenum disulfide. Care must be taken to prevent any accumulation of black lead powder or molybdenum disulfide in the threads, which could cause them to seize up. 29) How to perform the trial threading of cylinder bolts? Answer: First, measure the play gap of the threads. Carefully check whether the threads on the bolts and nuts are intact. Then, apply molybdenum disulfide powder. Gently turn the nut all the way in by hand. If any resistance is encountered, it must be unscrewed to determine the cause; under no circumstances should a hammer be used to strike it or force it in. For the large cylinder bolts, trial threading at the bottom of the cylinder must also be carried out according to the aforementioned procedure. 30) How to carry out the inspection and cleaning of cylinder bolt bottoms and bolts? Answer: First, clean the rust off the bolts using a wire brush and kerosene, then use a fine file and a edged flap sandpaper to repair any damage or burrs on the threads, washers, and the bottom surfaces of the nuts. The repaired bolts must be inspected with nuts; they should be able to be tightened to the end by hand. When the nut seat or the bottom seat of the cylinder bolt is damaged, tap repair should be carried out. 31) What should be done when threads are seized? Answer: When the nut and bolt are seized, never force them together with excessive torque. If the temperature is high, wait until the bolt has cooled to room temperature. Then, pour a little lubricating oil into the threads, turn the nut back and forth using an appropriate torque, while simultaneously tapping it with a hammer. It is also possible to gently heat the nut; after gradually smoothing out the burrs inside the threads, the nut can then be removed. When it is not possible to remove the nut, a skilled gas welder can use an oxy-acetylene torch to cut off the nut and protect the screw. 32) What should be noted when rotating the nut of a heat-tightening bolt? Answer: It is not allowed to use excessive torque to force it open, nor should one use a hammer to strike and twist it forcefully. Otherwise, the threads may undergo plastic deformation at high temperatures or develop burrs upon pulling out, resulting in galling and damage. 33) How to determine the dead point of thermal expansion of a cylinder? Answer: In the sliding pin system, the intersection point of the center line of the cross pin installed between the low-pressure cylinder and the metal base plate, and the center line of the longitudinal pin installed between the turbine’s bearing housing, exhaust chamber, and foundation base plate, is the dead center for cylinder expansion. 34) How to reinstall the inspected and qualified slip pins? Answer: It needs to be cleaned thoroughly. Apply dry graphite powder or molybdenum disulfide powder, then reinstall it in its original position according to the marks. Be careful not to turn the head upside down or flip it over. 35) When lifting the cylinder head, who should be assigned to monitor those areas? Why? Answer: Special personnel should be assigned to monitor the four corners of the large cover, as well as the guide rods and the large bolts on both sides. To check whether the four corners of the lid rise evenly ； Is there any resistance when moving the guide rod? Do the bolts come into contact with the screw holes in the cover? ； Have the internal components of the cylinder been affected by issues such as sticking with the upper cover? ； When lifting the large cover away from the guide rod, it is necessary to hold the four corners of the large cover firmly to prevent it from swinging or rotating, which could cause damage to the blades. After measuring one position, turn 90° to take a measurement at the second position. The purpose of the second measurement is first to verify the accuracy of the first measurement, and second to check whether the impeller has undergone deformation or misalignment. 36) When servicing the rotor coupling, what should be done if burrs form on the bolts and holes during disassembly? Answer: To ensure the stability of the rotor center and correct assembly, the coupling bolts must be reamed and custom-fitted. To ensure that the rotor’s mass balance is not disrupted, the bolts should also be fixed in numbered positions. Since bolts often develop pull-out burrs that cause jamming, when such burrs are present, they should be removed with an emery stone to keep the surface smooth. If the damage is severe, the hole should be enlarged and new bolts should be used. 37) How to check and handle rotor balance weights? Answer: Are there any signs of loosening or erosion? If it is loose, it should be tightened. If the scouring amount is too high, the counterweight needs to be replaced. 38) When repairing impeller keyway cracks with bushings, what is the sequence for assembly? Answer: First, measure the interference between the shaft and the sleeve, then install the sleeve. After the sleeve has cooled down, measure the interference between it and the impeller, and then install the impeller. After the impeller has cooled down, check its deviation. Then install the radial keys and assemble the impellers of adjacent stages. 39) Where are the key areas to check for cracks in the impeller rim? Answer: Generally, rim cracks occur mostly at the blade root groove and along the circumferential direction. These areas are subjected to the centrifugal force of the blades as well as the alternating stresses generated by blade vibration, and stress concentration is severe at these cross-sections. 40) The twisting method and mechanical compression method for straight shafts – which types of shafts and what types of bending conditions are they suitable for? Answer: A shaft with a small diameter and minimal bending. Since both the twisting method and the mechanical compression method involve applying force in a cold state, residual stresses remain after the shaft is straightened, and its stability is poor. 41) What precautions should be taken when cleaning the rotor blades using sandblasting? Answer: The sand particles must be screened. The compressed air pressure is between 0.4 and 0.6 MPa. Pay attention to personal safety protection during sandblasting. Make sure to keep a proper distance between the spray gun and the blades to prevent damaging them. In addition, the gap between the set of impellers and the shaft seal sleeve must also be sealed with tape. Wrap the shaft journals, thrust discs, emergency stop devices, and other shaft-end components with cloth. Prevent sand particles from getting trapped in gaps, which could cause imbalance in the rotor’s mass and other problems. 42) How can the direction of vibration be determined after a turbine rotor blade fails due to fatigue from vibration? Answer: Fatigue cracks occur in the blades due to resonance fatigue; generally, the direction of crack distribution is perpendicular to the vibration direction of the blade. If the blade fractures due to tangential vibration, the fatigue cracks are axially distributed. If fracture occurs due to axial vibration, the fatigue cracks are tangentially distributed. 43) How to re-rivet the leaf rivet after it is severely worn? Answer: One method is to remove some of the leaf shoulder and deepen the groove on the double ring. Another approach is to update the thinner reloop when its strength permits. Both methods involve rewinding the willow to raise the rivet head 2.5 mm above the loop. 44) What are the two most commonly used cross-sectional shapes for labyrinth steam seals? What are their respective advantages and disadvantages? Answer: They are the fir-tree shape and the comb-tooth shape. The advantage of the fir-tree-shaped steam seal is its compact structure, while its disadvantages are its complex shape and high cost; it has been replaced by the comb-toothed steam seal. The comb-toothed steam seal is further divided into two types: the one with comb teeth on the steam seal ring and the one with J-shaped teeth fitted on the shaft. The advantage of the steam seal ring with comb teeth is its simple structure and ease of replacing spare parts, but the main shaft is prone to thermal bending when friction occurs between the moving and stationary parts ； The advantages and disadvantages of the J-shaped toothed steam seal are exactly the opposite of those of the ring-type steam seal. 45) Why are shaft seal sleeves generally not installed in the high-temperature areas of turbines? What is an alternative method? Answer: Shaft seal sleeves are mounted on the rotor shaft using keys and the heat-shrink method. For the high-pressure rotor of the high-pressure cylinder, shaft seal sleeves are generally not used; instead, steam seal protrusions are directly machined on the main shaft. This is because of the high operating temperature; the shaft seal sleeve is directly exposed to high-temperature steam. Materials with low mass expand rapidly when heated, while larger shafts heat up more slowly. This difference makes it easy for loosening to occur between them, leading to vibration of the rotor. Furthermore, the high-pressure rotor is generally of a solid-forged design, and the steam seal protrusions are created by machining them on the main shaft, which makes the structure easy to implement and relatively simple. 46) What are the effects of an excessive radial gap in the turbine seal on the turbine unit? Why cannot the radial gap of the seal be made too small? Answer: When the radial gap of the seal is large, it not only increases steam leakage and reduces the efficiency of the stages, but it also leads to increased steam leakage at the diaphragm seal, which in turn increases the pressure difference before and behind the impeller. This results in an increased axial thrust on the rotor, posing a risk to the safety of the unit. On the contrary, if the radial clearance of the steam seal is too small, it may cause friction between the steam seal teeth and the main shaft, leading to localized overheating of the shaft and subsequent bending. This is because during startup and operation of the unit, the cylinder and rotor inevitably experience thermal deformation, and the relative position (center) between the cylinder and the rotor cannot remain exactly the same as it was during maintenance. 47) What methods can be used to remove hard deposits on turbine blades? Answer: (1) Manually use tools such as scrapers, sandpaper, and wire brushes for cleaning ； (2) Sandblasting for cleaning: Care should be taken to ensure that the sand particles are not too large, the air pressure should not be too high, and the distance between the nozzle of the sandblaster and the blades should not be too close, in order to prevent damage such as pitted surfaces on the blades ； (3) Clean by heating with a sodium hydroxide solution. After cleaning by heating with a sodium hydroxide solution, it is necessary to remove any remaining sodium hydroxide to prevent caustic embrittlement of the steel. In short, no matter which cleaning method is used, the blades or impellers must not be damaged. 48) Why are radial keys often used in the final impeller? Answer: Since the final-stage impeller transmits large torque and experiences high centrifugal stress in its inner hole, using axial keys would result in severe stress concentration in the key slots; therefore, radial keys are used instead to transmit torque. 49) Why are two dial indicators used to measure the runout of the thrust disc and the coupling? Answer: To eliminate the impact of axial movement of the rotor on the accuracy of the measurement data during the measurement process. 50) How is the ellipticity of a journal measured? Answer: Using an outside diameter micrometer, by measuring the difference between the maximum and minimum diameters in the same cross-section. It is the ellipticity of the journal at that cross-section. 51) What are the common causes of impeller damage? Answer: Mechanical damage, water hammer damage, corrosion and rust damage, erosion damage, vibration-induced fracture, etc. 52) Why is it not allowed to use a torch to heat the inner hole of the bolts when tightening the cylinder joint under heat? Answer: The flame temperature of a torch is too high and concentrated, which can lead to uneven heating; this may cause the inner wall of the bolt hole to overheat, even reaching a state of local melting. This not only alters the metallic properties of the bolt but also generates significant thermal stress, resulting in cracks or even breakage, thereby affecting the bolt’s lifespan and operational safety. 53) What principles should be followed to determine the sequence of tightening and loosening the bolts at the cylinder joint surfaces? Answer: The gap at the cylinder flange joint surfaces is mainly caused by the vertical curvature resulting from the weight of the lower cylinder. When tightening or loosening these bolts, it is necessary to start from the point where the vertical curvature is greatest, that is, where the gap is largest, and then tighten or loosen the bolts symmetrically from there towards the front and back. By tightening the bolts in this sequence, the vertical arc gap can be pushed toward the ends and eliminated; by loosening the bolts in this manner, it is prevented from becoming difficult to remove or even damaging the bolts at the point where the vertical arc gap is greatest. 54) What are the causes of fracture in high-temperature cylinder bolts? Answer: The causes of fracture in high-temperature cylinder bolts include: (1) Improper techniques or operations when heating or cooling the bolts, which leads to localized overheating of the bolt hole walls and the formation of cracks ； (2) The bolt material has defects, such as elemental segregation ； (3) Improper heat treatment of the material results in inadequate strength, or the steel exhibits significant thermal brittleness ； (4) Stress concentration occurs due to the unreasonable structure of the bolt itself or improper processing ； (5) The additional stress on the bolt is too high ； (6) Excessive bolt preload, etc. 55) What are the common spraying methods used to repair the gaps at the cylinder joint surfaces using the spraying technique? Answer: The common spraying methods for repairing gaps at cylinder joint surfaces via spraying include: (1) Oxygen-acetylene flame spraying, in which a steel wire is melted using an oxygen-acetylene flame, and the molten wire is atomized and sprayed onto the surface of the workpiece that has been pre-treated to make it rough. (2) Plasma spraying involves using a plasma gun to ionize the working gas, turning it into plasma; the powdered coating material is then melted by this plasma and sprayed onto the surface of the workpiece. 56) Under what circumstances is it sufficient to simply remove cracks in the cylinder? Answer: When the depth of the crack in the cylinder is less than one-third of the thickness of the cylinder wall, and after verification it is confirmed that the cylinder’s strength allows it, it is enough to remove the crack completely and then smooth out the resulting notch with a gradual transition; no further treatment is required. 57) What factors determine the magnitude of the deformation at the inner and outer openings of the cylinder? Answer: It is related to the thickness of the cylinder wall and flange metal, as well as the structural dimensions ； It is related to the start/stop operating conditions as well as the operation of heating the flanges and bolts ； It is also related to the insulation of the cylinder, as well as the manufacturing process. 58) After the camber measurements of the rotor journals in the turbine are completed, how can one analyze and determine changes in the rotor position by examining the changes in the line connecting the centers of the rotors? Answer: First, it is necessary to check whether the horizontal points (where the camber is zero) on the continuous curve formed by the center lines of the rotors meet the requirements specified by the manufacturer. If the position at which the camber is zero shifts significantly, it indicates that the positions of the rotors change greatly. 59) Due to the settlement of the turbine foundation, the vertical alignment between the moving and stationary parts of the turbine changes. How is this generally handled? Answer: Civil engineering measures should be taken to prevent the foundation from continuing to sink. If the centering relationship between the moving and stationary parts can be restored by adjusting the shaft seal sleeve and partition, and the requirements for safe operation are met, it is generally not necessary to adjust the position of the bearing housing and cylinder. 60) Briefly describe the lead wire method for measuring bearing shell tension? Answer: During measurement, after assembling and tightening the upper and lower half-shaft bearings, place a section of wire in front of and behind the joint between the top shim and the bearing seats on both sides of the shaft bearings. Then fasten the bearing cover and tighten the bolts slightly in a uniform manner. After that, loosen the bolts to lift off the bearing cover, and measure the thickness of the compressed wires. The tightening force is equal to the average thickness of the wires on both sides minus the average thickness of the wire at the top. If the result is negative, it indicates that there is a gap between the shaft bearings and the bearing cover. 61) What should be done if the oil baffle ring clearance is too small or too large? Answer: If the gap is too small, it can be filed down with a file until the gap reaches the proper size; thereafter, a triangular scraper or an M-shaped scraper can be used to sharpen it. When the gap is too large, special tools can be used to twist and deform the copper teeth in order to reduce the gap of the oil retaining ring; if this still does not meet the requirements, new copper teeth should be installed. 62) What are the consequences of an excessive thrust clearance? Answer: If the thrust clearance is too large, it will increase the variation in the axial clearance of the flow-through area when there is a change in the direction of the rotor’s thrust. Moreover, it will exert excessive force on the non-working bearing pads, leading to instability in the rotor’s axial position. In some units, due to the structural characteristics of their speed control systems, oscillations in the governor occur, resulting in unstable load levels. 63) What are the main factors that constitute the axial thrust of a multi-stage steam turbine? Answer: The axial thrust of a multi-stage steam turbine is primarily determined by the following factors: (1) Due to the reaction degree during operation, there is a pressure drop between the front and back of the moving blades, which generates a certain amount of axial thrust. (2) The axial thrust generated on the wheel surface by the pressure difference before and after the rotor. Especially when the vapor leakage from the diaphragm seal is high, it increases the pressure difference before and after the impeller. (3) On the rotor shaft, the pressure difference before and after the steam seal shoulder creates an axial force. (4) The axial thrust generated by the pressure difference before and after the hub when the rotor has a stepped structure. 64) What methods are commonly used to balance the axial thrust of a steam turbine? Answer: To balance the axial thrust of the turbine, the following methods are commonly used: (1) Providing balance holes on the impeller. (2) Adopt a balanced piston structure or increase the diameter of the front shaft seal. 65) What are the operational causes that lead to damage of turbine blades? Answer: The operational factors that cause damage to turbine blades include: (1) operating outside the specified frequency range. (2) Operation beyond capacity. (3) The main steam temperature is too low or too high. (4) Steam quality is unacceptable. (5) Vacuum level is too high or too low. (6) Water hammer occurs. (7) Improper operation during startup/shutdown or load variation. (8) Severe vibration of the unit caused by various factors. (9) Leakage of steam due to improper closure of the main steam valve causes blade corrosion. (10) Low load or excessively high vacuum causes erosion due to flow blockage at the lower part of the last-stage blades. (11) Water erosion on the back arc surface on the steam-inlet side of the blade tip. 66) From which aspects should we analyze the causes of damage and breakage of steam turbine blades? Answer: Examine the visible signs of blade damage, analyze the cross-section of the damaged blades, review operational and maintenance technical documents, determine the vibration frequency characteristics of the blades, conduct material testing and strength verification on the blades, and perform a comparative analysis with similar types of units.