Direct liquefaction is also called hydroliquefaction. During the hydrogenation process, valves, pipelines and related equipments are in high temperature and high pressure hydrogen, and hydrogen damage is a big problem. The corrosion of high temperature and high pressure hydrogen sulfide in the presence of hydrogen is also serious. Because of this, austenitic stainless steel is usually used as valve material to resist high temperature hydrogen sulfide corrosion. In this way, hydrogen embrittlement of stainless steel, sulfide stress corrosion cracking of austenitic stainless steel and hydrogen induced stripping of surfacing layer may occur. In addition, there is the problem of tempering brittle failure of Cr-Mo steel. Moreover, the possible damage caused by corrosive media such as ammonia and hydrogen sulfide in logistics must be carefully considered. Due to the presence of coal-oil slurry in coal direct liquefaction reaction, the wear of coal slurry on valve, pipeline and other equipment materials must be considered. Therefore, it is required that the materials used to manufacture valves should have comprehensive performance that meets the requirements of use. Specifically, there should be:
(1) The compactness, purity and homogeneity, which are used to describe the material's intrinsic properties, are superior, especially for thick (or large cross-section) steel.
(2) Requirements of chemical composition, room temperature and high temperature mechanical properties that meet the requirements of design specifications should be met.
(3) It is necessary to have environmental embrittlement resistance which can be used in harsh environment for a long time.
In the bidding documents for valves, there are clear requirements for the density of valves. For forged valves, the requirements of forging ratio and grain size of forgings are usually adopted to achieve the control of density. However, for casting valves, only in the relevant technical documents, it is mentioned that the density of the casting should be uniform, and the shrinkage and porosity of the casting should be eliminated. Quantitative indicators are seldom seen. In fact, it is precisely the foundry valves. Often because of the different quality control of foundry process, the quality of valve castings varies greatly. The main influencing factors are: the choice of molding materials, the setting of gating risers, the choice of location and quantity of chilled iron, the difference of solidification sequence, and the difference of cooling time. It will lead to great difference in density and homogeneity. Follow-up heat treatment process is also one of the key steps of valve quality assurance. The mechanical properties of final valve castings will be affected by temperature control of heat treatment furnace, code placement of castings in heat treatment furnace, holding time, cooling mode and speed.
II. Valve Technology Requirements
Coal direct liquefaction has the characteristics of high temperature, high pressure and hydrogen facing in hydrogenation unit, and the coexistence of corrosion and wear conditions in coal chemical industry. Therefore, the source of raw materials is very critical. At present, there is no effective method to control the selection of raw materials. Generally speaking, it is harmful to the composition of valve materials, especially to the components of valve materials. Element content, such as S, P, O, N and total carbon equivalent, puts forward corresponding clear index requirements. Although this requirement is often higher than the basic common requirements of materials, for the final product quality, these component indicators are not enough, because 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 which are harmful to the mechanical properties of metal materials, and it is not possible to list all the elements that may be harmful to the mechanical properties of metal materials. Therefore, foundries should strictly control the source of raw materials. For the processed raw materials, not only should they be smelted, but also further refined, especially to strengthen the control in front of the furnace. Only in this way can the quality of castings be guaranteed.
On the premise of guaranteeing the quality of raw materials, there are some special requirements for this kind of device.
(1) Precision casting technology can not be used for foundry valves of modern coal chemical plant. Because coal liquefaction is a hydrocracking process, due to the special penetration of hydrogen molecule to metal materials, and the casting produced by precision casting is relatively loose and poor uniformity, therefore, it is not suitable to use precision casting process to obtain valve castings under the conditions of hydrogen and high temperature and high pressure.
(2) Austenitic stainless steel should be treated by solid solution heat treatment (solid solution heat treatment temperature is 1050 10 C). For 321 and 347 materials, stable heat treatment (stable temperature is 900 10 C) should also be carried out.
(3) Coal-fired heating furnace should not be used in heat treatment furnace. Electric heating furnace or natural gas heating furnace should be used. Castings placed in the furnace should be conducive to air circulation in the furnace. Because coal heating furnace will increase the temperature difference of each part of the furnace body, therefore, coal-fired heating furnace cannot be used.
(4) The bar of castings should be selected as joint specimens. The split test bar can not really represent the characteristics of the castings either in the casting process or in the heat treatment process, and there is a big error with the actual mechanical properties of the castings themselves. Therefore, the split test bar can not be used.
(5) All valves must be checked by X-ray. The scope of inspection includes valve body, sealing element and valve cover. Casting steel is prone to defects in solidification process. Particular attention should be paid to key parts of steel casting, stress concentration area and weak pressure bearing capacity. For carbon steel and alloy steel casting valves, magnetic powder or liquid penetration inspection should be carried out one by one. Scope of Inspection: Outer surface of body, bonnet and sealing element and touchable inner surface and stem. For stainless steel casting valves, liquid penetration inspection should be carried out one by one. Scope of inspection: Outer surface of valve body, cover and seal, inner surface and stem that can be touched;
(6) The total repair welding area of each pressure-bearing casting shall not exceed 10% of the surface area of the casting; the number of major repair welding for each pressure-bearing casting shall not exceed 1, DN150-DN250 shall not exceed 2 and DN300-DN350 shall not exceed 3.
The repair welding of the casting defects mentioned above should be carried out before the final heat treatment, and once repair welding is allowed if defects are found in the radiographic inspection and can be repaired. After repair welding, the castings should be re-photographed and inspected. After passing the inspection, the castings must be re-heat treated. Repair welding shall have welding procedures and certificates of technological appraisal. The physical and chemical properties and corrosion resistance of the filler metal shall be close to that of the parent metal. After final heat treatment, the defects of all compressed components are not allowed to be repaired by welding.
It should have environmental embrittlement resistance which can be used in harsh environment for a long time.
For valves operated in high temperature and high pressure hydrogen environment, under the operating state, a certain amount of hydrogen will be absorbed in the inner wall of the valve. In the process of shutdown, if the cooling rate is too fast, the absorbed hydrogen can not diffuse out, and the supersaturated hydrogen remains in the wall, it may cause subcritical crack propagation when the temperature is below 150 C, which threatens the safe use of the valve. Valve manufacturer should pay attention to controlling the content of delta ferrite in TP347 when welding valves. The maximum value of delta ferrite in TP347 is 10%. In order to prevent thermal cracking in welding, the lower limit can be controlled not less than 3%. It can avoid more phase transformation and brittleness in the final heat treatment process after welding when the content is too high.
III. Valve Structural Requirements
The design of the valve structure should avoid the coking of coal slurry and make the valve invalid and easy to clean. Coal-oil slurry has one characteristic: if the flow is not smooth or static, that is, the medium flow is not good, it will deposit and may occur polymerization reaction, resulting in coking and lock the valve. At present, all the cut-off valves used in direct liquefied coal slurry pipeline are ball valves. When the operation needs to cut off the pipeline to close the ball valve, the oil-coal slurry inside the ball can not be discharged and deposited in the ball chamber, it is possible to coke and lock up. When the ball valve is closed, coal slurry can not hit the ball again because of deposit and coking, and the wear-resistant layer of the ball valve is destroyed and peeled off. Therefore, in fact, the choice of ball valve is not the most appropriate choice under this condition.
IV. Valve Wear Resistance Requirements
The ball valve used in slurry condition should be metal hard seal, and the material of the valve seat and ball is the same, so as to ensure that both have the same expansion coefficient and that the ball will not stuck at high temperature. Because many working conditions of valves are under high temperature and pressure, according to experience, some valves have no problems in testing at room temperature, but it is difficult to open and close at high temperature. The reason is the asynchronous thermal expansion between the valve core and the valve body. Therefore, the manufacturer should do high temperature start-up and shut-down test before leaving the factory. However, the high temperature opening and closing test is by no means to put the whole valve into the heat source to raise the whole temperature of the valve, so the test results obtained are inconsistent with the actual situation. Because, in the real use process, the valve is heated because of the high temperature of the medium. At this time, the valve core is heated first and the outer surface of the valve slowly warms up. If the whole valve is put into the heat source, the valve body is heated first and the valve core is heated later, which is just opposite to the actual working condition, and the purpose of testing can not be achieved. The temperature gradient should be established in high temperature start-up and shut-down test, which is consistent with the actual working conditions.
The expansion rate of the coating should be similar to that of the basic material. Otherwise, it is easy to crack during the alternating process of high temperature and normal temperature or at high temperature, which makes the coating peel off more easily. For supersonic spraying (HVOF) or similar methods, the surface hardness of the coating is 64-68 HRC and the bonding strength is not less than 10 MPa; for metallurgical fusion or similar methods, the surface hardness of the coating is 62-68 HRC and the bonding strength is not less than 70 MPa. The effective thickness of the coating (excluding the transition layer) is 0.2-0.5 mm. The seat shall be designed with a scraper. When the ball rotates, it can provide a brush hanging action to prevent the particle deposition between the ball and the seat. In the design, it should be noted that the particles between the sphere and the valve seat can be brushed off by using the scraper. However, this kind of scraper design will bring another problem under some working conditions: because of the scraper design, an acute angle is formed at the scraper, which will inevitably lead to stress concentration, which is more disadvantageous to coating. The combination between the base material and the coating is more likely to cause the peeling of the coating and the damage of the valve seat under the abrasion condition.