With the continuous improvement of the degree of industrial automation, regulating valve as the final executing mechanism of the automatic adjustment system has been more and more widely used. The application of the regulating valve is directly related to the quality and safety of production. Therefore, I combined long-term design of automatic control systems, instrument modeling and installation, commissioning and maintenance experience, talk about the application of diaphragm control valve.
1 Pneumatic diaphragm control valve selection 1.1 According to the use of requirements Selection of pneumatic diaphragm control valve from the valve body and the valve body (including the valve seat) consists of two parts, according to different use requirements have different structural forms, Pneumatic diaphragm control valves used by the company mainly include direct single-seat valves, two-seat control valves and high-pressure angle type control valves. The single-seat valve has a small leakage, and the unbalanced force caused by the thrust of the fluid on the single-seat valve is very large. Therefore, the direct-seat single-seat valve is suitable for applications requiring small leakage, small pipe diameter, and low pressure difference between the front and rear of the valve. Pass-through double seat valve body has two upper and lower valve cores, which are generally offset due to the opposite thrust of the fluid acting on the upper and lower valve cores; therefore, the double seat valve has a very small imbalance force, allowing a large pressure difference between the front and rear of the valve. However, due to the complex flow path in the valve body, the erosion damage to the valve body is severe when it is used for high pressure difference, and it is not suitable for high viscosity, medium containing suspended particles or fibers. In addition, due to the limitation of processing conditions, the two valve seats are difficult to close both the upper and lower valve cores at the same time. Therefore, the amount of leakage is large when the valve is closed. Especially when used under high or low temperature conditions, due to the different coefficients of thermal expansion of the materials, it is more likely to cause serious problems. The leak. The angle-type high-pressure valve body is of a right-angle type, with a simple flow path, low resistance, and small erosion by high-speed fluids. It is particularly suitable for high-pressure differentials, high-viscosity and fluids containing suspended particulate matter, and can also be used to treat vapor and liquids. Miscible, flash cavitation occasions. This valve body can avoid coking, sticking and clogging, easy to clean and self-cleaning.
1.2 According to the safety selection Pneumatic diaphragm control valve has two forms of air opening valve and air closing valve. According to the safety and application requirements of different production processes, when the signal pressure is interrupted, the regulating valve is in the open or closed position, which depends on the degree of hazard caused by the process. If the hazard is small when the valve is in the closed position, the valve is opened by air. When the signal pressure is interrupted, the regulating valve is in the closed position. If not, the pneumatic closed valve is selected.
1.3 Flow characteristics Selecting a pneumatic diaphragm control valve during the design of the automatic control system should focus on the flow characteristics. Typical ideal characteristics are straight line flow characteristics, equal percentage flow characteristics (logarithmic flow characteristics), fast opening flow characteristics, and parabolic flow characteristics. Straight line flow characteristics In the case of the same change in the relative opening, the relative change in the flow hourly flow rate is large; when the flow rate is large, the relative change in flow rate is small. Therefore, the linear flow control valve in the case of a small opening (small load) regulation performance is not good, not easy to control, often oscillating, so the linear flow characteristics of the valve should not be used for small openings, it is not suitable for load changes Larger regulation system, suitable for the adjustment system with relatively stable load and little change. Percentage flow characteristics of the regulating valve in a small load regulation effect is weak, large load regulation role, it is close to the closure of the regulation of weak, smooth and smooth work, and close to fully open when the adjustment effect, work sensitive and effective, to a certain extent , Can improve the regulation quality, so it is suitable for occasions with large changes in load, both at full load production and half load production are better to adjust the role.
1.4 The choice of control valve diameter should be based on the known fluid to calculate the required flow coefficient CV, and then select the appropriate valve diameter according to the product technical parameter table. When calculating CV, pay attention to the difference between liquids, gases, water vapor, and other vapors.
