Precautions For The Use Of Valves In Coal Chemical Industry

- Aug 30, 2019-

First, the material requirements of valve

Direct liquefaction is also called hydro liquefaction. In the hydrogenation process, the valves, pipes and related equipment 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 and hydrogen coexist is also very serious. Because of this, in order to resist high temperature hydrogen sulfide corrosion, austenitic stainless steel is usually used as the valve material. In this way, there may be damage such as hydrogen embrittlement of stainless steel, sulfide stress corrosion cracking of austenitic stainless steel, and hydrogen induced peeling of the weld overlay layer. There is also the problem of temper brittle failure of Cr-Mo steel. Moreover, the damage that may be caused by corrosive media such as ammonia and hydrogen sulfide present in the stream must also be carefully considered. Due to the presence of oil coal slurry in the direct liquefaction of coal, the wear of coal slurry to equipment such as valves and pipes must be considered. Therefore, the materials used to manufacture the valves are required to have a comprehensive performance that meets the requirements of use. Specifically, it should have:

(1) The density, purity and homogeneity of the endogenous properties of the material are superior, which is especially important for thick (or large) steel.

(2) Requirements for chemical composition, room temperature and high temperature mechanical properties that meet the design specifications.

(3) It is required to have environmentally resistant embrittlement properties that can be used for a long time in harsh environments.

In the valve bidding documents, there is a clear requirement for the density of the valve. For forged valves, the forging ratio and grain size of the forgings are generally required to achieve the control of the density. However, for casting valves, it is only mentioned in the relevant technical documents that the density of the casting body should be uniform, and the shrinkage and shrinkage of the castings should be eliminated, and few quantitative indicators are seen. In fact, it is precisely the casting valve, often because of the different quality control of the casting process, resulting in a large difference in the quality of the valve castings, the main influencing factors are: different choice of molding materials, different settings of the riser, the choice of cold iron position and quantity Different, the difference in the coagulation sequence, and the difference in cooling time, etc., will result in a large difference in density and homogeneity. The subsequent heat treatment process is also one of the most critical steps in the quality assurance of the valve. The temperature control of the heat treatment furnace, the stacking of the casting in the heat treatment furnace, the holding time and the cooling method and speed all affect the mechanical properties of the final valve casting.

Second, the process requirements of valve

The direct liquefaction of coal has the characteristics of high temperature; high pressure and hydrogen in the hydrogenation unit, and the coexistence of coal chemical corrosion wear conditions. Therefore, the source of raw materials is very important. At present, no effective method has been found to control The selection of raw materials, in general, the composition of the valve material, especially the content of harmful elements, such as S, P, O, N and total carbon equivalent, etc., put forward corresponding clear requirements, although this requirement is often higher than the material Basic general requirements, however, for the final product quality, these component indicators alone are not enough, because the trace elements that affect the mechanical properties of raw materials are much more than that. Strictly speaking, our requirement is only to control the trace elements of the common mechanical properties of the hazardous materials. It is not possible or impossible to completely list all the possible harmful elements of the mechanical properties of the metal materials. Therefore, the foundry should strictly control the source of raw materials, and should not only smelt the processed raw materials, but also further refine it, especially to strengthen the pre-furnace control, so that it is possible to ensure the quality of the castings.

Butterfly valve

Under the premise of ensuring the quality of raw materials, there are some special requirements for this type of device:

(1) For the casting valves of modern coal chemical plants, precision casting processes cannot be used. Because coal liquefaction is a hydrocracking process, due to the special penetrability of hydrogen molecules to metal materials, precision castings are relatively loose and have poor uniformity. Therefore, for both hydrogen working conditions and high temperature and high pressure working conditions. It is not suitable to obtain valve castings by precision casting process;

(2) Austenitic stainless steel is subjected to solution heat treatment (solution heat treatment temperature is 1050 ± 10 °C), and for 321 and 347 materials, stabilization heat treatment (stabilization temperature is 900 ± 10 °C);

(3) The heat treatment furnace should not adopt a coal-fired heating furnace. The electric heating furnace or the natural gas heating furnace should be used. The casting of the castings in the heating furnace should facilitate the circulation of the gas in the furnace. Since the coal heating furnace will increase the temperature difference between various parts of the furnace body, the coal-fired heating furnace cannot be used;

(4) The selection of the casting test bar shall be the joint test piece. The "separate" test bar cannot truly represent the characteristics of the casting itself, whether it is the casting process or the heat treatment process, and there is a large error with the actual mechanical properties of the casting itself. Therefore, the "separate" test bar test cannot be used;

(5) All valves must be inspected for the inspection of the castings. The scope of inspection includes the valve body, sealing components, and bonnet. The steel castings are prone to defects during solidification, especially for key parts of steel castings, stress concentration areas, and Special attention should be paid to parts with weak pressure capability. For carbon steel and alloy steel casting valves, magnetic powder or liquid penetration inspection shall be carried out one by one. Inspection range: The outer surface of the valve body, bonnet and sealing element and the accessible inner surface and valve stem. For stainless steel casting valves, liquid penetration inspection should be performed on a piece-by-piece basis. Inspection range: the outer surface of the valve body, bonnet and seal and the accessible inner surface and valve stem;

(6) The sum of all repair welding areas of each pressure-bearing casting shall not exceed 10% of the surface area of the casting; the number of major repair welding of each pressure-bearing casting shall not exceed DN50 to DN100; no more than 2 DN150 to DN250 ; no more than 3 DN300 ~ DN350.

The repair welding of the above casting defects shall be carried out before the final heat treatment; when it is found to be defective during the radiographic inspection and it is repairable, repair welding is allowed once. After the repair welding, the film shall be re-inspected. After the inspection, the casting must be reheated. The repair welding shall have the welding procedure and the process qualification certificate. The physical, chemical properties and corrosion resistance of the filler metal shall be close to the parent metal. All defects of the pressurized components are not allowed to be repaired by welding after the final heat treatment.

Anti-environmental embrittlement performance for long-term use in harsh environments

For valves operating in a high temperature and high pressure hydrogen environment, a certain amount of hydrogen is absorbed in the inner wall of the valve under operating conditions. In the process of shutdown, if the cooling rate is too fast, the adsorbed hydrogen will not be able to diffuse out, causing supersaturated hydrogen to remain in the wall, which may cause subcritical crack propagation at temperatures below 150 °C, and safe use of the valve. Bring threats. When the valve manufacturer welds the valve, it is necessary to pay attention to controlling the δ ferrite content in TP347. The maximum value of the welded state is 10% (to prevent thermal cracking during welding, the lower limit can be controlled not less than 3%) to avoid the content. When too much, the phase change occurs in the final heat treatment after welding, resulting in brittleness.

Third, the structure requirements of valve

stainless steel Check valve

The valve structure should be designed to avoid coking of the coal slurry to make the valve fail and easy to clean. There is a characteristic of coal slurry: if the circulation is not smooth or stationary, that is, the flow condition of the medium is not good, the deposition will occur and polymerization may occur, and coking will occur and the valve will be locked. At present, the shut-off valves used in the direct liquefaction coal slurry pipeline are all ball valves. When the operation needs to cut off the pipeline to close the ball valve, the oil coal slurry inside the valve ball cannot be discharged and is deposited in the ball cavity, which may cause coking. When the ball valve is closed, the coal slurry can not be hit again due to deposition coking and the wear layer of the ball valve is damaged and peeled off. Therefore, in fact, the choice of ball valve under this condition is not the most suitable choice.

Fourth, the wear requirements of valve

The ball valve used in the slurry working condition should be in the form of a metal hard seal, and the valve seat and the ball are made of the same material to ensure that the two have the same expansion coefficient, and the ball will not be stuck in the high temperature condition. Since many working conditions of the valve are under high temperature and high pressure, according to the experience, some valves have no problem at normal temperature, but it is difficult to open and close under high temperature conditions. The reason is between the valve core and the valve body. This is caused by unsynchronized thermal expansion. Therefore, the production plant should do high temperature opening and closing test before leaving the factory. However, the high temperature opening and closing test is not to put the whole valve into the heat source, so that the whole temperature of the valve rises, so the test results obtained are inconsistent with the actual situation. In the actual use process, the valve is heated due to the high temperature of the medium. At this time, the valve core is hot and the outer surface of the valve is then slowly heated. If the whole valve is put into the heat source, the valve body is hot and the valve core is hot. After the heat, the opposite of the actual working conditions, cannot get the purpose of the test. The high temperature opening and closing test shall establish a temperature gradient consistent with the actual working conditions.

The expansion ratio of the coating to the base material should be similar. Otherwise, cracks are likely to occur during high temperature and normal temperature alternating processes or high temperatures, which makes it easier to peel off the coating. For supersonic spraying (HVOF) or similar methods, the surface hardness of the coating is 64-68HRC, and the bonding strength is not less than 10MPa. For metallurgical fusion or similar methods, the surface hardness of the coating is 62-68HRC, and the bonding strength is not less than 70 MPa. . The effective thickness of the coating (excluding the transition layer) is 0.2 to 0.5 mm. The seat should be designed with a scraper. A hanging brush action is provided when the ball is rotated to prevent particle deposition between the ball and the valve seat. When designing, it should be noted that the scraper can be used to brush the particles between the ball and the seat. However, this type of scraper design poses another problem in some working conditions: because the scraper design is attached, at the scraper An acute angle is formed, and this acute angle tends to cause stress concentration, which is more unfavorable for the bonding between the coating and the substrate. In the abrasive condition, the coating is more likely to be peeled off, and the valve seat is damaged.