Selection of valves for coal chemical industry

II. Process requirements for valves: Direct coal liquefaction involves conditions of high temperature, high pressure, and exposure to hydrogen in the hydrogenation units, alongside corrosive and abrasive environments typical of coal chemical processes. Therefore, the source of raw materials is extremely important. To date, no effective method has been found to control the selection of raw materials. Generally, specific and clear requirements are set for the composition of valve materials, especially regarding the content of harmful elements such as S, P, O, N, and total carbon equivalent. Although these requirements are often higher than those applicable to ordinary materials, they alone are not sufficient to ensure the quality of the final product, as there are many other trace elements that affect the mechanical properties of the raw materials. Strictly speaking, our requirement is merely to control those trace elements that commonly impair the mechanical properties of materials; it is neither possible nor necessary to list all elements that could potentially have a detrimental effect on the mechanical properties of metal materials. Therefore, foundries should strictly control the source of raw materials. The raw materials processed not only need to be melted but also further refined, with particular emphasis on enhancing control at the furnace level; only in this way can the quality of castings be ensured. While ensuring the quality of raw materials, there are also some special requirements for such devices: (1) For the cast valves used in modern coal chemical plants, the precision casting process cannot be employed. Since coal liquefaction is a hydrocracking process, and due to the special penetration ability of hydrogen molecules into metal materials, castings produced by precision casting are relatively porous and have poor uniformity; therefore, precision casting is not suitable for producing valve castings in environments involving hydrogen or high temperatures and pressures ； (2) Austenitic stainless steels must undergo solution treatment (the solution treatment temperature is 1050 ± 10°C); for 321 and 347 grades, stabilization treatment is also required (the stabilization temperature is 900 ± 10°C) ； (3) Heat treatment furnaces should not use coal-fired heaters; instead, electric heaters or natural gas heaters should be used. The casting arrangements within the furnace should facilitate air circulation inside it. Since coal-fired heaters increase the temperature differences across various parts of the furnace, coal-fired heaters cannot be used ； (4) The casting test bars should be selected as coupled test pieces. “The \"separate\" test bars cannot truly represent the properties of the casting itself, either during the casting process or during heat treatment; they exhibit significant discrepancies from the actual mechanical properties of the casting. Therefore, it is not possible to use such \"separate\" test bars for testing ； (5) All valves must undergo radiographic inspection of the castings, with the inspection covering the valve body, sealing elements, and valve cover. Cast steel is prone to defects during the solidification process; special attention should be paid to the critical parts of the cast steel, areas with stress concentration, and sections with weak load-bearing capacity. For carbon steel and alloy steel cast valves, magnetic particle or liquid penetrant inspection should be performed on each individual valve. Inspection scope: the outer surfaces of the valve body, valve cover, and sealing elements, as well as the accessible inner surfaces and valve stem. For stainless steel cast valves, liquid penetrant inspection should be performed on each one individually. Inspection scope: the outer surfaces of the valve body, valve cover, and seals, as well as the accessible inner surfaces and valve stem ； (6) The total area of all repair welds on each pressure-bearing casting shall not exceed 10% of the casting’s surface area ； The number of major weld repairs per pressure-bearing casting shall not exceed 1 for DN50–DN100 ； DN150 to DN250: no more than 2 ； DN300 to DN350: no more than 3. The repair welding for the aforementioned casting defects should be carried out before the final heat treatment ； When a defect is detected during radiographic inspection and it can be repaired by welding, 1 welding repair is permitted. After patch welding, radiographic inspection should be conducted again; once the inspection is successful, the casting must undergo heat treatment once more. Repair welding must be carried out in accordance with welding procedures and process qualification certificates; the physical and chemical properties as well as corrosion resistance of the filler metal should be similar to those of the base metal. After the final heat treatment of defects in all stressed components, repair by welding is not permitted. It is necessary to possess environmental embrittlement resistance that allows for long-term use in harsh conditions. For valves operating in high-temperature and high-pressure hydrogen environments, a certain amount of hydrogen is absorbed into the inner walls of the valve during operation. During the shutdown process, if the cooling rate is too fast, the adsorbed hydrogen does not have enough time to diffuse out, resulting in supersaturated hydrogen remaining within the vessel walls. This can lead to subcritical crack propagation at temperatures below 150°C, posing a threat to the safe operation of the valve. Valve manufacturers need to pay attention to controlling the δ-ferrite content in TP347 during valve welding; the maximum value in the as-welded state should be around 10% (to prevent hot cracks during welding, the lower limit can be set at no less than 3%). This is to avoid excessive phase transformations during the final heat treatment after welding, which could lead to brittleness.

III. Structural requirements for valves: The design of valve structures should prevent coking of the coal slurry from causing the valves to fail, and it should also facilitate cleaning. Oil-coal slurry has one characteristic: if its flow is poor or it remains stationary, that is, if the flow conditions of the medium are not satisfactory, deposits will form and polymerization reactions may occur, leading to coking and the jamming of valves. Currently, all the isolation valves used in pipelines for directly liquefying coal slurry are ball valves. When it is necessary to shut down the pipeline by closing these ball valves, the oil-coal slurry inside the valve ball cannot be discharged and remains in the valve cavity, which may lead to coking and jamming. When the ball valve is closed, the coal slurry causes deposition and coking, preventing the valve ball from being moved again and leading to the damage and peeling off of the valve’s wear-resistant layer. Therefore, in fact, choosing a ball valve under these operating conditions is not the most suitable option. IV. Wear resistance requirements for valves: Ball valves used in slurry applications should employ a metal hard-seal design, with the valve seat and the ball made of the same material to ensure that they have the same coefficient of expansion; this prevents the ball from getting stuck under high-temperature conditions. Since many operating conditions for valves involve high temperatures and pressures, experience shows that some valves function properly when tested at normal temperatures, but encounter difficulties in opening and closing under high-temperature conditions. The reason for this is asynchronous thermal expansion between the valve core and the valve body. Therefore, the manufacturer should conduct high-temperature opening and closing tests before shipping the product. However, the high-temperature opening and closing test is by no means about placing the entire valve in a heat source to raise its temperature throughout; the test results obtained in this way do not correspond to the actual conditions. Because, in actual use, the valve heats up due to the high temperature of the medium; in this case, the valve core heats up first while the outer surface of the valve body warms up more slowly. If the entire valve is placed in a heat source, then the valve body heats up first and the valve core later, which is the exact opposite of what happens under actual operating conditions, and thus it fails to serve the purpose of testing. The high-temperature opening and closing test should establish a temperature gradient consistent with the actual operating conditions. The expansion rate of the coating should be similar to that of the base material. Otherwise, cracking is likely to occur during alternating high-temperature and normal-temperature conditions or at high temperatures, which in turn makes the coating more prone to peeling off. For supersonic spraying (HVOF) or similar methods, the surface hardness of the coating is 64–68 HRC, with a bond strength of not less than 10 MPa ； For metallurgical bonding or similar methods, the surface hardness of the coating is 62 to 68 HRC, with a bond strength of not less than 70 MPa. The effective thickness of the coating (excluding the transition layer) is 0.2 to 0.5 mm. The valve seat should have a scraper design. When the sphere rotates, it enables a brushing action to prevent particle deposition between the valve ball and the valve seat. During design, it should be taken into account that a scraper can be used to remove particles trapped between the sphere and the valve seat. However, this scraper design can cause another problem under certain operating conditions: the presence of the scraper results in the formation of sharp corners, and such sharp corners inevitably lead to stress concentration, which further hinders the bonding between the coating and the substrate. Under abrasive conditions, this can easily result in the peeling off of the coating, thereby damaging the valve seat.

Apart from some general-purpose valves that can be used in certain coal chemical processing applications, which valves are specifically designed for use in coal chemistry? These include metal hard-sealed wear-resistant ball valves, such as lockhopper valves, slag locking valves, ash locking valves, vent valves, and balance valves. Black water control valve, gray water control valve, high pressure difference control valve. Disk valves include: lockhopper valves, slag locking valves, ash locking valves, vent valves, and balance valves. Balance valve, slag valve for high temperatures. Coal powder control valve, coal powder three-way directional valve. Coal powder switch valve, water-coal slurry cut-off valve, oil-coal slurry cut-off valve, ash-water eccentric valve. Oxygen valve, a ball valve specifically for oxygen. Pneumatic hard-sealed ball valve. Orbital ball valve. Sliding valve, pressure relief control valve. Pulse cleaning valve (reverse valve), a cleaning valve specifically designed for ash removal filters. Pneumatic eccentric spherical control valve.