Inventory of the valves used in coal-to-oil production

Thread Content

This post was last edited by yinkuilin6868 on 2015-11-23 08:36. The production equipment for coal-to-oil conversion mainly consists of systems such as reactors, towers, tanks, and pipelines; valves are important control devices within this setup. Valves such as general-purpose gate valves, globe valves, check valves, ball valves, and butterfly valves need to withstand conditions such as high temperatures (480°C), high pressures (30 MPa), exposure to hydrogen, oxygen, flammability, explosiveness, corrosion, and wear. For key components, metal-sealed, wear-resistant ball valves designed for high temperatures and pressures are commonly used, which is also in line with the development trend of high-tech products in the valve industry. Coal-to-oil metal-sealed wear-resistant ball valves come in various structural forms, such as metal-sealed two-piece, split-type, eccentric hemispherical, and track ball valves, depending on the installation location and process conditions. 1. Metal-sealed two-piece ball valve The metal-sealed two-piece ball valve is of a fixed-ball hard-seal design. It can withstand high temperatures, features double-sided sealing, has good structural rigidity, and provides reliable sealing performance; it is commonly used in pipelines of hydrogenation units that operate under high temperatures and in environments where flammability and explosiveness are a concern. 2. Metal-sealed split ball valve The metal-sealed split ball valve is a hard-sealing ball valve in which the valve body is divided down the centerline of the valve stem; the ball and the upper and lower sections of the valve stem form a single shaft structure with high coaxiality. The valve features high temperature resistance, double-sided sealing, good structural rigidity, and reliable sealing performance. Due to the limitations imposed by the sealing structure at the joint where the packing is located in systems with two valve bodies, it is commonly used in pipelines operating at medium pressure levels, as well as in the slag discharge sections of hoppers and reactors with low torque; it functions as a slag locking valve, ash locking valve, or hopper locking valve. 3. Metal-sealed eccentric hemispherical valve: The ball in this type of valve is in the form of a cast hemisphere with a radially eccentric structure; it may have V-shaped notches, and the sealing area is an annular hemispherical surface. The valve stem and the upper part of the ball are connected by spur or rectangular toothed splines, which offers the advantages of uniform stress distribution and good alignment. The lower part of the sphere is centered and positioned relative to the shaft sleeve of the bottom cover. Since this valve can withstand high temperatures, features good single-sided sealing performance, operates smoothly, and prevents wear between the sealing surfaces when the sealing elements are separated, its service life is extended. It is commonly used in pipelines for ash, fibers, or slurry media that require regulation and shutdown. 4. Metal-sealed track ball valve: The ball in this type of valve has a cast hollow hemispherical structure, with the sealing surface being an annular spherical platform. The 2 cross pins at the upper part of the ball engage with the 7°–10° wedge surfaces at the lower part of the valve stem, while the lower part of the ball and the bottom hole of the valve body are connected by a ball joint. The valve stem is equipped with both trapezoidal threads that drive it up and down, and helical grooves that enable the valve stem to rotate 90° simultaneously under the action of the support fixing pin sleeve. During the rotation to open or close the valve, the swinging of the ball causes the sealing surface to separate from or come into contact with the valve seat, thereby enabling the valve to open or close. This prevents idle friction between the spherical sealing surfaces and reduces wear on those surfaces. Due to the severe stress conditions on the wedge surface at the lower part of the valve stem and the cross pin of the ball at the extreme positions during opening and closing operations, such valves are mainly used in pipelines carrying ash, slag, and slurry at medium to low pressure levels and with medium to small diameters, for the purpose of opening and closing. 5. Split-ball valve The split-ball valve (spherical gate valve) also belongs to the category of metal-sealed valves. It is widely used in the coking and cracking systems of the petrochemical industry. However, due to its complex structure, large number of components, large size, and heavy weight, its application scope is limited to some extent. The common diameter of metal-sealed wear-resistant ball valves for coal-to-oil conversion is 50 ~ 400 mm, with a maximum of up to 600 mm. The pressure rating range is from 2.0 to 26.0 MPa, with a maximum of up to 42 MPa. Depending on the different operating conditions of the valve, the valve body material is selected as carbon steel (WCB), heat-resistant steel (WC9), austenitic stainless steel (CF8M), or a double-layer stainless steel with an inner lining (F51). The sphere is made of austenitic stainless steel (316), with a surface coating of high-hardness cemented carbide material. The valve stem and ball can have a combined rod structure. The valve stems are typically made of heat-treated precipitation-hardening stainless steel (17-4PH), and after aging treatment, their hardness ranges from 35 to 40 HRC. The main sealing surface at the front end of the valve seat is coated with a high-hardness cemented carbide material. The auxiliary sealing area at the rear of the valve seat provides elastic sealing properties, as well as resistance to heat and slag, and it also serves to scrape and clean the surface. The opening and closing torque of such valves is several times greater than that of soft-sealed ball valves of the same specification; the selection of the torque output by the driving mechanism should be determined based on the level of manufacturing technology and the quality of processing (surface roughness and concentricity). The spherical sealing pair of the valve is the core component of metal-sealed wear-resistant ball valves; these technical challenges must be addressed through aspects such as structural design, wear-resistant materials, spraying processes, and the grinding and fitting of the spherical surfaces. On spherical surfaces, nickel-based tungsten carbide (Ni-WC) is typically applied by supersonic spraying on 304 or 316 substrates, in order to improve the bonding strength of the sealing surface (75 MPa) and achieve a low porosity level (3%–5%). The surface hardness reaches > 832HV (> 65HRC). For the spraying powder, imported products from German Stack, American Praxair, or Japanese Fukumi can be used. The main sealing surface of the valve seat is made by thermal spraying of nickel-based alloys (Ni55, Ni60) on a 304 or 316 substrate, resulting in a surface hardness of 55–60 HRC. The roundness of the sphere should be between 0.01 and 0.02 mm, and its surface roughness Ra value should be below 0.4 μm. The sealing surface of the sphere should be roughened first, then finely ground using a bowl-shaped diamond grinding wheel or a grinding disc fitted with diamond grinding strips. Finally, the valve seat sealing surface and the spherical surface are ground together in a single pairing process, in order to ensure the proper fit of the valve sealing pair and achieve zero leakage in the metal seal. 　　Due to the high material costs, high technical requirements, complex manufacturing processes, and significant quality risks associated with metal-sealed wear-resistant ball valves, there are stringent demands regarding the choice of materials for the valve body and seals, as well as regarding the roundness of the ball, the smoothness of its surface, the fit between the sealing surfaces, and the concentricity of the ball shaft. As a result, the price of such valves is very high, comparable to that of imported similar valves, and it is 6 to 10 times higher than that of soft-sealed ball valves of the same specification. For the same metal hard-sealed ball valve, imported valves cost 3 to 5 times more than domestically produced ones. The sealing surfaces of imported wear-resistant valves utilize a variety of advanced spraying techniques; some have even reached the stage of tungsten carbide coating, as seen in the wear-resistant valves produced by companies such as Moges or Everlasting in the United States. This enables these valves to achieve better sealing performance and a longer service life.