Application of supersonic spraying technology in hard sealing ball valve

       1 Overview 

Metal hard-sealed ball valves have been widely used in petroleum, chemical, metallurgy and urban construction. By choosing different materials, the metal hard-sealed ball valve can be applied to various media such as water, steam, oil, nitric acid, acetic acid and urea. Through the optimized design of the structure, the metal hard-sealed ball valve can open and close without friction and low torque. In use, the main failure mode of metal hard sealing ball valve is erosion wear. Although the service life and scratch resistance of metal hard-sealed ball valves can be improved by surface heat treatment or surfacing of cemented carbide, they still cannot meet the requirements of transporting special media such as steel ash, coal powder, silicon powder and alumina powder. . If the sphere is treated with supersonic flame spraying technology, a carbide cermet coating can be formed on the surface of the sphere. The bonding strength of the coating and the matrix can reach 70MPa, the porosity is less than 2%, and it has a uniform and dense structure. The characteristics of hard phase WC, hardness>1000HV, good wear and corrosion resistance, good mechanical and mechanical properties, can significantly improve the performance of the ball valve.

2. Working principle

Figure 1HVO/AF supersonic spraying device

The supersonic spraying device (Fig. 1) uses kerosene as fuel and oxygen and compressed air as combustion accelerant. The control device delivers kerosene and oxidant to the spray gun at a certain pressure and flow rate, and is atomized and mixed into a liquid through a high-performance atomizing nozzle. After the mist is sprayed into the combustion chamber of the spray gun, the liquid mist is ignited and burned by the spark plug to form high temperature and high pressure gas, which is accelerated to supersonic speed by the Laval spray gun. The powder feeding system sends the spray powder from the low pressure area of the Laval spray gun nozzle into the supersonic jet, and after the jet is heated and accelerated, it is sprayed from the spray gun, and sprayed at a high speed to the surface of the workpiece to form a coating. Since the multi-functional HVOF spraying technology can use two combustion accelerants, oxygen and compressed air, and has the functions of HVOF and HVAF at the same time, the speed and temperature of the flame can be continuously adjusted in a wide range.

3. Spraying process

Before spraying, the workpiece must undergo surface cleaning treatment and sandblasting roughening treatment. The general spraying material is WC-12Co, and the mass percentage Wt of WC and Co is 88% and 12% respectively, and it is manufactured by sintering and crushing method. Tungsten carbide has high hardness at room temperature, especially good hot hardness, and its hardness decreases very little at 1000 °C (Figure 2).

Fig.2 Temperature-hardness curve

By comparing the erosion wear test of WC-12Co coating and substrate samples at 30∀ and 90∀, it is 4.5 times that of the substrate at 30∀, and 5.2 times that of the substrate at 90∀. Because of its strong bonding force, high hardness, and complete wear-resistant microstructure, the coating has high erosion wear resistance. The test shows that the erosion of WC-12Co coating is characterized by partial plastic erosion. The binder phase Co can absorb part of the energy of the plastic erosion particles due to its own plastic deformation, while the hard phase WC has the effect of resisting the erosion particles, and the two complement each other, so the coating has good erosion wear resistance. .

4. Application

In petroleum, chemical and coking units, due to the high temperature of the pipeline, the medium is easy to adhere or coke on the valve cavity and the surface of the seal. Ordinary hard-sealed ball valves (Figure 3) often have poor sealing due to coking on the surface of the ball or even stuck. plug. For this reason, the structure of the sealing surface of the valve seat is changed to a scraper shape with a sharp edge (Figure 4), which can scrape off the adhesion or coke layer on the surface of the sphere during the valve opening and closing process, thereby ensuring the reliable sealing of the sealing pair. .

Figure 4: Scraper type high temperature metal hard sealing ball valve

In the trial production process, the sphere is made of F304 forging, and the surface is nitrided to improve the hardness. The material of the valve seat is F304, and it is surfacing with Stellite carbide. It has been found that reliable sealing can be ensured when the medium is clean at room temperature. When the temperature of the medium rises above 400!, due to the low thermal hardness of Stellite alloy, the surface of the sphere is nitrided, and the wear resistance is poor. When scraping the coke, it is easy to damage the sealing surface of the valve seat and the surface of the sphere, so it is difficult to meet the requirements of the working conditions. Through analysis, the supersonic spraying technology was used to spray WC-12Co on the surface of the sphere to form an erosion-resistant coating with a thickness of about 0.3 mm and a hardness of 60-62 HRC. The base of the valve seat sealing surface is still processed into a scraper shape, and a coating of 0.3mm thickness is also sprayed on its surface. By changing the spraying process, the hardness of the coating is 55~58HRC, so that the hardness of the sealing surface of the valve seat and the surface of the sphere form a certain hardness difference. After the spraying is completed, the sphere is ground with a diamond grinding tool to a surface roughness of Ra 0.8 μm, and the sealing surface of the valve seat and the sphere are manually ground. The grinding material is made of natural diamond powder (granularity 1200 mesh) prepared with oil acid. After grinding the surface roughness of the sealing surface to Ra0.2μm, the assembly test is performed, and the sealing is reliable at room temperature. Using special heating equipment, the valve was heated to the design upper limit temperature of 540 ° C, kept for 8 hours, and the operation test showed that the valve was flexibly opened and closed, and the sealing test was normal. After repeated tests, the spherical surface has no obvious change. Due to the good thermal hardness and erosion and wear resistance of the supersonic sprayed WC-12Co coating, even if the temperature rises above the design upper limit of 540 °C during use, it can still maintain good erosion and abrasion resistance. In addition, there is a difference of about 5HRC between the hardness of the blade surface and the spherical surface. When the blade of the valve seat scrapes the coke on the surface of the ball, it will not damage the surface of the ball and the sealing surface of the valve seat, which can ensure the sealing performance and service life.

5Conclusion

The application of supersonic spraying technology in hard-sealed ball valves greatly improves the application range and service life of the valve. For example, other materials such as Stellite alloy powder can be used to manufacture metal coatings that meet the requirements of different working conditions. According to user feedback information, the service life of hard-sealed ball valves sprayed with supersonic spraying technology has been increased by about 20 times in water, steam, oil and other media, and under severe erosion conditions such as coal powder and silicon powder pipelines The average lifespan can also be increased by more than 5 times.