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I. Introduction to Turbines A turbine (also known as a steam turbine) is a rotary power machine that converts the energy of steam into mechanical work. It is primarily used as a prime mover for power generation, and can also directly drive various pumps, fans, compressors, and ship propellers. Additionally, the exhaust steam or intermediate steam extracted from the turbine can be utilized to meet heating needs in industrial and domestic applications. During the operation of a steam turbine, leakage and cylinder deformation are the most common equipment problems. The tightness of the cylinder joints directly affects the safe and efficient operation of the unit. Repairing and grinding these joint surfaces to ensure their tightness is an important part of cylinder maintenance. When dealing with leaks at these joint surfaces, it is necessary to carefully analyze the causes; depending on the degree of deformation and the size of the gaps, various methods can be employed together to achieve the required level of tightness. II. Causes of air leakage in turbine cylinders 1. The cylinders are cast, and after leaving the factory they undergo aging treatment, which involves storing them for a certain period of time to allow any internal stresses generated during the casting process to be completely eliminated. If the aging time is short, the processed cylinders will still deform during subsequent operation; this is why some cylinders continue to leak air even after the first leakage repair. Because the cylinder is still continuously deforming. 2. The forces acting on the cylinder during operation are quite complex. In addition to the static loads such as the pressure difference between the gases inside and outside the cylinder and the weight of the various components housed within it, the cylinder must also withstand the reaction forces exerted on the stationary parts when steam flows out of the stationary blades. Moreover, there are forces from the connecting pipes under different temperature conditions that act on the cylinder. As a result of these interacting forces, the cylinder undergoes plastic deformation, leading to leaks. 3. Rapid changes in the load on the cylinder, especially during quick starts, stops, and changes in operating conditions, lead to large temperature fluctuations. Incorrect methods of warming up the cylinder, as well as opening the insulation layer too early during shutdown for maintenance, all result in significant thermal stress and thermal deformation within the cylinder and on its flanges. 4. Stress was generated in the cylinder during mechanical processing or after repair welding, but the cylinder was not tempered to eliminate this stress; as a result, significant residual stress remained in the cylinder, causing permanent deformation during operation. 5. During installation or maintenance, due to maintenance procedures and techniques, the expansion gaps of the inner cylinder, cylinder partition, partition sleeve, and steam seal sleeve may become inappropriate, or the expansion gaps of the lugs and pressure plates may also be unsuitable; as a result, large expansion forces are generated during operation, causing the cylinder to deform. 6. The cylinder sealant used is of poor quality, contains too many impurities, or is of the wrong type ； If there are hard impurity particles in the cylinder sealant, it will make it difficult for the sealing surfaces to fit together tightly. Bokosi high-temperature sealant is the latest sealing material for turbine cylinders; it can be used in high, medium, and low-pressure cylinders, thereby preventing cylinder leaks caused by improper selection of models. 7. The tightening force of the cylinder bolts is insufficient, or the material of the bolts is of poor quality. The tightness of the cylinder joint is primarily achieved through the tension of the bolts. The thermal stress and high temperatures generated during the start-up, shutdown, or load adjustment of the turbine unit can cause stress relaxation in the bolts; if the stress is insufficient, the preload on the bolts will gradually decrease. If the material of the cylinder bolts is of poor quality, these bolts will elongate over time due to thermal stress and the expansion force of the cylinder, resulting in plastic deformation or fracture. This leads to insufficient tightening force, and consequently leakage occurs in the cylinder. 8. The sequence of tightening the cylinder bolts is incorrect. For ordinary cylinder bolts, tightening is carried out simultaneously from the center toward both sides, that is, from the point where the vertical deflection is greatest or where the stress-induced deformation is most significant. This causes the gap at the area with the greatest deformation to shift toward the free ends at the front and back of the cylinder, until the gap eventually disappears. If it is tightened from both sides toward the center, the gaps will concentrate in the middle, resulting in an arch-shaped gap at the cylinder joint and causing steam leakage. III. Methods for dealing with leaks in turbine cylinders. The causes of deformation and leakage at the cylinder joints vary, and the locations where these issues occur as well as the degree of deformation and leakage differ as well. First, a long straight edge and feeler gauge should be used to check the degree of deformation at the cylinder joints. During maintenance, appropriate measures should be taken based on the cause of the leakage and the extent of the deformation. The specific methods are as follows: 1. For joint surfaces where the cylinder is significantly deformed or where there is severe steam leakage, the method of grinding and scraping the joint surface is used. If the deformation of the upper cylinder’s joint surface is within 0.05 mm, using that surface as a reference, red lead is applied to the lower cylinder’s joint surface or blue paper is pressed there; the lower cylinder surface is then ground and scraped based on these marks. If the joint surface of the upper cylinder is highly deformed, apply red lead to it and use a large straight edge to create marks in order to level the upper cylinder. 2. Use appropriate cylinder sealing materials. Since there are currently no unified **standards or industry standards for turbine cylinder sealants, the raw materials and formulations used vary, resulting in uneven product quality ； When selecting a sealant for turbine cylinders, it is necessary to choose it from reputable manufacturers with guaranteed product quality, in order to ensure the tightness of the cylinders after maintenance work. Products such as BOCOS high-temperature sealant from Germany are single-component, paste-like sealants intended for industrial use; they represent high-quality sealing compounds suitable for applications where high temperatures and pressures exist on smooth, flat sealing surfaces (butt joints). 3. Method of local patch welding: Since the cylinder joint surfaces are scratched or corroded by steam, an appropriate welding rod is used to fill in those scratches; a flat tool is then used to level them out, ensuring that the weld seam and the joint surface are in the same plane. When the joint surface of the cylinder is severely deformed or there is significant air leakage, weld one or two seal strips 10–20 mm wide on the joint surface of the lower cylinder to eliminate the gap; then measure using a straight edge or by fitting the upper cylinder onto it, and scrape with red lead until the gap is eliminated. The process for this operation is also simple: preheat the cylinder to 150°C before welding, and then perform segmented or skip welding at room temperature. Austenitic welding electrodes such as A407 and A412 should be used, and after welding, the area should be covered with asbestos cloth to retain heat and facilitate slow cooling. After cooling to room temperature, grind and scrape it. 4. Coating or spraying of the cylinder mating surface: When there is significant air leakage at the cylinder mating surface with a gap of around 0.50 mm, the coating process can be used to reduce the amount of work required for grinding and polishing. Use the cylinder as the anode and the coating tool as the cathode; apply the electrolyte solution repeatedly to the joint surface of the cylinder. The thickness of the coating should depend on the size of the gap at the cylinder’s joint, while the type of coating should be determined by the material of the cylinder and the polishing process used. Spraying is a method in which metal powder is heated to a molten state or to a plastic state using a specialized high-temperature flame gun, and then sprayed onto the surface of the treated cylinder to form a coating with the desired properties. Its features include simple equipment and easy operation; the coating is durable, and the cylinder temperature after spraying remains at only 70°C–80°C, preventing deformation of the cylinder. Moreover, a heat-resistant, wear-resistant, and corrosion-resistant coating can be achieved. It should be noted that before coating and spraying, the cylinder surface must be polished, degreased, and textured; after coating and spraying, the coating must be scraped to ensure a tight bond between the surfaces. 5. Method of using padding at the joint surface: If the leakage due to local gaps in the joint surface is not severe, a copper mesh with a mesh size of 80–100 can be used. This mesh is heat-treated to reduce its hardness, then cut into appropriate shapes and placed over the areas where steam is leaking from the joint surface, followed by the application of cylinder sealant. If the gap at the joint surface is large and leakage is severe, grooves 50 mm wide and 5 mm deep can be made in the upper and lower joint surfaces; a toothed gasket made of IGr18Ni9Ti can be inserted there. The thickness of this toothed gasket is generally 0.05–0.08 mm greater than the depth of the grooves, and stainless steel gaskets of the same shape can be used for adjustment. 6. Methods for controlling bolt stress: If the deformation of the cylinder mating surfaces is small and uniform, new bolts can be installed in the areas with gaps, or the pre-tensioning force of the bolts can be increased appropriately. Tighten from the center outwards simultaneously, that is, tighten the bolts starting from the point where the vertical arc is at its maximum or where the deformation due to stress is greatest. Theoretically, the preload of control bolts can be calculated using the formula d/L ≤ A. However, since the data required for this calculation and the methods for making such measurements are still under investigation, this approach has not yet been widely adopted; instead, the preload is usually determined empirically within the allowable maximum stress limit for the bolts. IV. Application Instructions for Mijiahua-BIRKOSIT High-Temperature Sealant 1. Technical Overview BIRKOSIT High-Temperature Sealant is primarily used for high-temperature flat sealing in applications such as turbines, high-temperature flanges, and valves that require such sealing. It has been recognized and certified by manufacturers abroad including Siemens, Alstom, and GE. BIRKOSIT Dichtungskitt is a one-component, paste-like sealant designed for industrial use; it is a high-quality sealing compound suitable for applications where there are high temperature and pressure requirements on smooth, flat sealing surfaces (butt joints). Spread it on a dry surface using a trowel or rubber spatula. Since sealed products do not cure, their consistency changes only slightly, allowing them to retain their flexibility and elasticity; therefore, there is no time constraint regarding their use – they can be put into operation immediately after installation, without any need to wait. To repair high-temperature sealing surfaces with scratch damage, BOCOS High-Temperature F series repair products should be used. 2. Technical data: (1) Temperature resistance: Can withstand hot steam and gases up to 900°C, as well as hot and cold water, light fuels, lubricants, crude oil, and natural gas. (2) Pressure resistance: The excellent adhesion at the sealing surfaces and joints ensures a pressure resistance of up to 250 bar (255 kg/cm2); flanges without sealing rings can withstand pressures of up to 450 bar (459 kg/cm2), while screw connections can handle pressures of up to 550 bar (561 kg/cm2). (3) Plastic deformation: Plasticity is unlimited; therefore, even under the most demanding conditions, the sealing membrane will not rupture. (4) Application areas: Various industrial enterprises such as steam turbines and gas turbines, power plants (nuclear power plants), gas plants and water treatment plants, oil refineries, smelting plants, shipyards, paint and rubber manufacturing industries, and chemical plants. It is used for sealing the cylinder mating surfaces of steam turbines, gas turbine wheels, and flue gas turbine wheels in power plants (including nuclear power plants), as well as for high-temperature sealing of industrial turbine machinery and equipment flanges, and for sealing high-temperature heat pipe threads. It is particularly suitable for sealing metal joints: steam turbines and gas turbines, compressors, pumps, enclosures, flange joints, etc. (5) Service life: For high-pressure turbines, we guarantee that the products can withstand high-temperature steam, gases, heat, and cold water at temperatures up to 900°C and pressures up to 250 bar, as well as light fuels, lubricants, mineral oils, and natural gas. When used in accordance with the application guidelines and operating properly, we guarantee a service life of 10 years for the gland seals (joint connections) of steam turbines and gas turbines. V. Application Case Description: This section provides an overview of the field testing conducted by Alstom Power (Switzerland) using Megagard-Bocce’s high-temperature sealants on the joints between steam turbines and gas turbines. 1. The Megagard-Bocce sealants were used to conduct sealing tests on the joints of the following turbines: (1) High-pressure turbines. High-pressure turbines (HPT) have two sealing surfaces: an inner sealing surface and an outer sealing surface. The inner sealing surface is the load-bearing surface (inner seal). (2) In medium-pressure turbines and low-pressure turbines, the joints on the medium-pressure turbine (IPT) and the low-pressure turbine (LPT) form a continuous seal with a central hole. The inner sealing surface is the load-bearing surface, extending all the way to the central hole. . 2. Selective damage to the sealing surface of the joint (1) For damage areas of ≥0.5–1.0 millimeters, Meijiahua-Bokosis should be mixed with graphite materials to create a thick, elastic, and highly resistant sealing compound that can level damage areas up to 1.0 millimeters in size. (2) Damage spots of ≥1.0–2.0 millimeters should be treated using a ceramic-metal sealing mixture; once cured, they can be machined (through grinding, sharpening, scraping, etc.). 3. Instructions for use: During the inspection to be carried out, corrosion can be observed on the horizontal joints of the intermediate pressure turbine casing, with the corrosion occurring at the inlet area. The customer’s representative suggested welding repairs, while our steam turbine department recommended using Megalox-Bocce high-temperature sealant. At first, the customer was concerned that the sealant might be washed away, but after approximately 25,000 hours of operation, during the next inspection, the durability and reliability of Meijiahua-Bokoses were proven. Through the photos, the corroded areas can be seen very clearly. 4. Application Areas 1. Applications in steam turbines and gas turbines 2. Applications in compressors 3. Applications in expansion equipment 4. Applications in generator equipment 5. Applications for various high-temperature and high-pressure flanges and sealing rings 6. Applications in other types of equipment