1. Requirements for valve materials
Direct liquefaction is also called hydroliquefaction. In the hydrogenation process, the valves, pipelines and related equipment are used in high-temperature and high-pressure hydrogen, and damage caused by hydrogen is a big problem. Corrosion is also serious when high-temperature and high-pressure hydrogen sulfide and hydrogen coexist. Because of this, austenitic stainless steel is usually selected for the valve to resist the corrosion of high-temperature hydrogen sulfide. There may be hydrogen embrittlement for stainless steel, sulfide stress corrosion cracking for austenitic stainless steel, and hydrogen-induced peeling for the surfacing layer. There is also the problem of temper brittle failure of Cr-Mo steel. Moreover, the possible damage caused by corrosive media such as ammonia and hydrogen sulfide in the transportation must also be carefully considered. The wear of the coal slurry on materials of equipment such as valves and pipelines must be considered due to the existence of oil-coal slurry in the direct coal liquefaction reaction. Therefore, it is required that the materials used to manufacture valves have comprehensive properties which meet the requirements for use. Specifically, the requirements for the material are as follows:
(1) The compactness, purity and homogeneity of the material should be superior, which is particularly important for thick steel or steel with large sections.
(2) The chemical composition, room temperature and high-temperature mechanical properties must meet the requirements of the design specification.
(3) The material should have anti environmental embrittlement performance that can be used for a long time in harsh environments. In the valve bidding documents, there are clear requirements for the density of the valve. For forged valves, the requirements for density control are generally achieved through those for the forging ratio and grain size of the forgings. However, for cast valves, the uniform density of the casting body only was mentioned in the relevant technical documents, and the shrinkage cavity and shrinkage porosity of the casting should be eliminated. Quantitative indicators are rarely seen. In fact, casting valves have different quality due to the different quality control of the casting process. The main influencing factors are a different selection of molding materials, positions and quantity of cold iron, different settings of pouring risers, different solidification sequences and cooling time. These will lead to great differences in its density and homogeneity performance. The subsequent heat treatment process is also one of the most critical steps in valve quality assurance. Factors such as the temperature control of the heat treatment furnace, the stacking of castings in the heat treatment furnace, the holding time, the cooling method and speed will affect the mechanical properties of the final valve casting.
2. Requirements for the valve process
Direct coal liquefaction not only has the characteristics of high temperatures, high pressure in the hydrogenation unit, but also has the characteristics of the coexistence of corrosion and wear conditions in the coal chemical industry. Therefore, the source of raw materials is very important. At present, there is no effective method to control the selection of raw materials. General speaking, put forward corresponding clear index requirements for the composition of valve materials, especially the content of harmful elements, such as S, P, O, N and total carbon equivalent. Although the requirements are often higher than the basic general requirements of the material, these component indicators alone are not enough for the quality of the final product. The trace elements affecting the mechanical properties of raw materials are far more than these. Strictly speaking, our requirement is only to control the common trace elements that are harmful to the mechanical properties of materials, and it does not and is impossible to list all possible elements which may be harmful to the mechanical properties of metals. Therefore, the foundry should strictly control the source of raw materials; the processed raw materials should not only be smelted, but also further refined, especially paying attention to the control before the furnace. Only in this way can it be possible to ensure the quality of castings. Under the premise of ensuring the quality of raw materials, there are some special requirements for this type of device.
(1) The precision casting process cannot be used for the casting valves of modern coal chemical plant, because coal liquefaction is a hydrocracking process. The castings produced by precision casting are relatively loose and have poor uniformity due to the special penetration of hydrogen molecules to metals. Therefore, it is not suitable to use the precision casting process to obtain valve castings used for hydrogen conditions.
(2) Austenitic stainless steel should be subjected to solution heat treatment. The temperature of solution heat treatment is 1050±10°C. For 321 and 347, stabilization heat treatment should be performed. The stabilization temperature is 900±10°C.
(3) The coal-fired heating furnace should not be adopted for the heat treatment furnace. An electric heating furnace or a natural gas heating furnace should be used. Make sure the airflow circulation in the furnace when stacking castings in the heating furnace. Since the coal heating furnace will increase the temperature difference between the various parts of the furnace body, coal-fired heating furnaces cannot be used.
(4) Conjoined casting test bars shall be selected. The split test bar cannot represent the characteristics of the casting itself, both the casting process and the heat treatment process; there is a great error with the actual mechanical properties of the casting. Therefore, the split test bar cannot be used for inspection.
(5) All valves must undergo radiographic inspection. The scope of inspection includes valve bodies, sealing components, and valve bonnets. Steel castings are prone to defects in the solidification process, and special attention should be especially paid to key parts of steel castings, stress concentration areas and weak pressure bearing parts. For carbon steel and alloy steel casted valves, magnetic particle or liquid penetrant inspection shall be carried out one by one. External and internal surfaces of valve bodies, bonnets and sealing elements and stems should be inspected. For stainless steel casted valves, liquid penetration inspection shall be carried out one by one, including external and internal surfaces of valve bodies, bonnets and sealing elements and stems.
(6) The sum of all repair welding areas of each pressure-bearing casting shall not exceed 10% of the surface area of the casting; major repair welding of each pressure-bearing casting shall not be performed more than one time for valves with DN50 to DN100, two times DN150 to DN250 and three times DN300 to DN350.
Repair welding of the above casting defects should be carried out before the final heat treatment; when defects are found during the radiographic inspection, and they can be repaired by repair welding, repair welding for one time is allowed. After the repair welding, the radiographic inspection should be conducted. After the inspection is qualified, the casting must be heat treated. Welding regulations and process appraisal certificate should be provided for repair welding. The physical, chemical properties and corrosion resistance of filler metal should be close to those of the parent metal. Welding repair are not permitted after the final heat treatment of all pressure bearing components with defects.
Having environmental embrittlement resistance for long-term use in harsh environments
For valves operating in a high-temperature and high-pressure hydrogen environment, a certain amount of hydrogen will be absorbed in the inner wall of the valve under operating conditions. In the shutdown process, if the cooling rate is too fast, the adsorbed hydrogen will not diffuse in time, resulting in supersaturated hydrogen remaining in the wall of the vessel, which may cause subcritical crack propagation at a temperature lower than 150°C, and pose a threat to the safety of the valve. Valve manufacturers need to pay attention to controlling the content of 6 ferrite in TP347 during valve welding. The maximum value in the welded state is 10%. In order to prevent hot cracks during welding, the lower limit can be controlled by not less than 3% to avoid many phase changes due to the excessive ferrite in the final heat treatment process after welding, resulting in brittleness.
