3. Requirements for valve structure
The design of the valve structure should avoid the failure of the valve caused by the coking of the coal slurry and facilitate cleaning. Coal oil slurry has a characteristic. If the flow is not smooth or stationary, it will deposit and possibly polymerize, resulting in coking and block of the valve. At present, the shut-off valves used in the direct liquefaction coal slurry pipeline are all ball valves. When the pipeline needs to be stopped 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 be coked and blocked. When the ball valve is closed, the coal slurry will not be able to hit the valve ball again due to the deposition and coking, and the wear-resistant layer of the ball valve will be damaged and peeled off. Therefore, the selection of ball valves is not the most suitable choice under this working condition.
3. Requirements for the wear of the valve
Metal hard sealing should be adopted for the ball valve used in slurry conditions, and the material adopted for the valve seat and the ball should be the same to ensure that the two have the same expansion coefficient; the blocking of the ball will not occur under high temperatures. Since many valves are under high temperature and high pressure, problems don't happen when tested at normal temperatures according to the use experience. However, it is difficult to open and close the valve under high temperatures. The reason is the asynchronous thermal expansion between the valve core and the valve body. Therefore, the manufacturer should do a high-temperature opening and closing test before the valve leaves the factory. However, the high-temperature opening and closing test is by no means putting the whole valve into the heat source to increase the entire temperature of the valve. Test results obtained in this way are inconsistent with the actual situation, because the valve is heated up due to the high temperature of the medium in the actual use process. At this time, the valve core is heated first and the outer surface of the valve is then heated slowly. If the entire valve is put into the heat source, the valve body will be heated first and the valve core will heat up later, which is just the opposite of the actual working condition, and cannot achieve the purpose of testing. A temperature gradient consistent with the actual working conditions should be made for the high-temperature opening and closing test.
The expansion rate of the coating and the base material should be similar. Otherwise, in the alternating process of high temperature and normal temperature or high temperature, cracks will likely occur, so that the coating is more likely to peel off. For supersonic spraying (HVOF) or similar methods, the coating surface hardness is 64 to 68HRC, and the bonding strength is not less than 10MPa. For metallurgical fusion or similar methods, the coating surface hardness is 62 to 68HRC, and the bonding strength is not less than 70MPa. The effective thickness of the coating is 0.2 to 0.5mm, not including the transition layer. The valve seat shall be of scraper design to have a scrapping action as the ball rotates, preventing particle deposition between the ball and seat. When designing, it should be noted that the scraper can be used to scrape the particles between the ball and the valve seat. However, this scraper design will bring another problem in some working conditions. An acute angle is formed due to the scraper design, which will make stress concentration happen and is more unfavorable for the bonding between the coating and the base metal. Under abrasive conditions, it is more likely to cause the coating to peel off, resulting in damage to the valve seat.
