Four technical requirements for valve products in modern coal chemical industry

1 valve material requirements

Direct liquefaction is also called hydroliquefaction. In the hydrogenation process, its valves, pipelines and related equipment are in high temperature and high pressure hydrogen, hydrogen damage is a big problem. The corrosion is also very serious when hydrogen sulfide and hydrogen coexist at high temperature and high pressure. Because of this, in order to resist the corrosion of high temperature hydrogen sulfide, austenitic stainless steel is usually used as valve material. 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 temper embrittlement of Cr Mo steel. Moreover, the possible damage caused by ammonia, hydrogen sulfide and other corrosive media in the logistics must be carefully considered. Due to the existence of coal oil slurry in direct coal liquefaction reaction, the abrasion of coal slurry on valves, pipelines and other equipment materials must be considered. Therefore, it is required that the materials used in the manufacture of valves should have comprehensive performance in line with the use requirements. Specifically, it should have:

(1) The compactness, purity and homogeneity, which are used to describe the intrinsic properties of materials, should be superior, which is particularly important for thick (or large section) steel.

(2) It should meet the requirements of chemical composition, room temperature and high temperature mechanical properties.

(3) It should be able to use in harsh environment for a long time.

In the bidding documents of valves, there are clear requirements for the density of valves. For forged valves, the requirements for controlling the density are generally achieved through the forging ratio and grain size of forgings. 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 cavity and porosity of the casting should be eliminated. Few quantitative indexes are found. In fact, it is precisely the casting valve. Because of the different quality control of the casting process, the quality of the valve castings varies greatly. The main influencing factors are: different choice of molding materials, different setting of pouring and riser, different choice of position and quantity of chill, different solidification sequence, and different cooling time, which will lead to different densification and homogeneity It's big. The subsequent heat treatment process is also one of the key steps of valve quality assurance. The temperature control of heat treatment furnace, stacking of castings in heat treatment furnace, holding time, cooling mode and speed and other factors will affect the mechanical properties of final valve castings.



2 valve process requirements

Direct coal liquefaction has the characteristics of high temperature, high pressure and hydrogen in the hydrogenation unit, as well as 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, the composition of valve materials, especially the content of harmful elements, such as s, P, O, N, has not been found And total carbon equivalent, etc., put forward the corresponding clear index requirements, although this requirement is often higher than the basic general requirements of materials, but for the final product quality, it is not enough to rely on these composition indexes alone, because the trace elements that affect the mechanical properties of raw materials are far more than these. Strictly speaking, our requirement is only to control the common trace elements harmful to the mechanical properties of materials, and it is not possible to list all the possible harmful elements to the mechanical properties of metal materials. Therefore, the foundry should strictly control the source of raw materials, for the processed raw materials should not only be melted, but also be further refined, especially to strengthen the control before furnace, only in this way can it be possible to ensure the quality of castings.

On the premise of ensuring the quality of raw materials, there are some special requirements for such devices

(1) Precision casting process can not be used for casting valves in modern coal chemical plant. Because coal liquefaction is a hydrocracking process, due to the special penetrability of hydrogen molecules to metal materials, the castings produced by precision casting are relatively loose, and the uniformity is poor. Therefore, it is not suitable to use the 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 with solution heat treatment (solution heat treatment temperature is 1050 ± 10 ℃), and 321 and 347 materials should be treated with stabilization heat treatment (stabilization temperature is 900 ± 10 ℃);

(3) The heat treatment furnace should not use coal-fired heating furnace, but should use electric heating furnace or natural gas heating furnace. Coal fired heating furnace can't be used because it will increase the temperature difference between different parts of the furnace;

(4) The selection of casting test bar should be conjoined test piece. No matter in the casting process or heat treatment process, the "split" test bar can not really represent the characteristics of the casting itself, which has a large error with the actual mechanical properties of the casting itself. Therefore, the "split" test bar can not be used for inspection;

(5) All valves must be inspected by X-ray. The scope of inspection includes valve body, sealing element and valve cover. Defects are easy to occur in the solidification process of steel castings. Special attention should be paid to the key parts of steel castings, stress concentration areas and parts with weak bearing capacity. For cast carbon steel and alloy steel valves, magnetic particle or liquid penetrant inspection shall be carried out one by one. Inspection scope: outer surface of valve body, valve cover and sealing element, accessible inner surface and valve stem. For cast stainless steel valves, liquid penetrant inspection shall be carried out one by one. Inspection scope: outer surface of valve body, valve cover and seal, accessible inner surface and valve stem;

(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 for dn50-dn 100, 2 for DN 150-dn 250 and 3 for DN 300-dn 350.

The repair welding of the above casting defects shall be carried out before the final heat treatment; when defects are found in the radiographic inspection and can be repaired by repair welding, one repair welding is allowed. After repair welding, the casting should be re photographed for inspection, and the heat treatment must be carried out again after passing the inspection. Repair welding shall have welding procedure and process qualification certificate, and the physical and chemical properties and corrosion resistance of filler metal shall be close to that of parent metal. After the final heat treatment, the defects of all pressure components are not allowed to be repaired by welding.

For the valve operating in high temperature and high pressure hydrogen environment, a certain amount of hydrogen will be absorbed in the inner wall of the valve under the operating state. In the process of shutdown, if the cooling speed is too fast, the adsorbed hydrogen will not diffuse out, resulting in supersaturated hydrogen remaining in the vessel wall, which may cause subcritical crack propagation when the temperature is lower than 150 ℃, and threaten the safe use of the valve. The valve manufacturer should pay attention to the control of δ ferrite content in TP347 during valve welding, and the maximum value should be 10% in the welding state (in order to prevent hot cracks in welding, the lower limit can be controlled not less than 3%), so as to avoid more phase transformation and brittleness in the final heat treatment process after welding when the content is too much.



3 valve structure requirements

The design of valve structure should avoid coal slurry coking to make the valve invalid and facilitate cleaning. Coal oil slurry has one characteristic: if the flow is not smooth or static, that is, the medium flow condition is not good, it will deposit and polymerization may occur, resulting in coking and locking the valve. At present, all the cut-off valves used in the direct liquefaction of 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 valve ball can not be discharged and deposited in the ball cavity, which may cause coking and locking. When the ball valve is closed, the ball valve can not be hit again due to the deposition and coking of coal slurry, and the wear-resistant layer of the ball valve is damaged and peeled off. Therefore, in fact, the selection of ball valve in this condition is not the most appropriate choice.



4 valve wear requirements

The ball valve used under slurry condition shall adopt the form of metal hard seal, and the material of the valve seat and ball shall be the same, so as to ensure that they have the same expansion coefficient, and the ball will not be "stuck" under high temperature conditions. Because many of the working conditions of valves are under high temperature and high pressure, according to the experience, some valves have no problems in the test at room temperature, but it is difficult to open and close under high temperature. The reason is that there is asynchronous thermal expansion between the valve core and the valve body. Therefore, the manufacturer should do high temperature opening and closing test before leaving the factory. But the high temperature opening and closing test is by no means putting the whole valve into the heat source to raise the whole temperature of the valve, so the test results are not consistent with the actual situation. Because, in the real use process, the valve is heated due to the high temperature of the medium. At this time, the valve core is heated first and then the outer surface of the valve is slowly heated. If the whole valve is put into the heat source, the valve body will be heated first and the valve core will be heated later, which is just opposite to the actual working condition and can not achieve the purpose of testing. The temperature gradient consistent with the practical working condition should be established in the high temperature opening and closing test.

The expansion rate of coating and basic material should be similar. Otherwise, in the alternating process of high temperature and room temperature or at high temperature, it is easy to crack, which makes the coating peel off more easily. For 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 70MPa. The effective thickness of the coating (excluding the transition layer) is 0.2-0.5mm. The valve seat shall be of scraper type design. When the ball rotates, it can provide a brush hanging action to prevent the particle deposition between the ball and the valve seat. During the design, it should be noted that the particles between the ball and the valve seat can be removed by using the scraper. However, this scraper design will bring another problem under some working conditions Because of the attached scraper design, an acute angle is formed at the scraper, and this acute angle is bound to produce stress concentration phenomenon, which is more unfavorable to the combination between the coating and the substrate. Under the abrasion condition, it is easier to cause the coating peeling off, resulting in the damage of the valve seat.