As the degree of industrial automation continues to improve, as the final actuator of automatic control system, control valve gets more and more widely used. The application of control valve is directly related to the quality and safety of production. In a variety of control valves, pneumatic membrane valve as a simple structure, use, easy maintenance, and with the nature of safety features of the type of valve, the most widely used. Pneumatic film regulating valve of the normal use, precise control, and the selection has a great relationship, the following combination of my design and production experience, talk about the selection of pneumatic film regulating valve considerations. 1, according to the requirements of the selection. Pneumatic diaphragm control valve by the valve body and the valve body (including the valve seat) of two parts, according to the different requirements of the use of different structures. Pneumatic membrane valve mainly through the single-seat valve, two-seat control valve and high-pressure angle control valve. Through the single-seat valve leakage is small, the thrust of the fluid on the single-seat valve unbalanced force formed by a large, so through the single-seat valve is suitable for small leakage requirements, small diameter and low pressure differential valve before and after the occasion. Through the two-seater valve body has two upper and lower valve spool, due to fluid acting on the spool in the opposite direction and generally offset; so the two-seat valve unbalance force is small, allowing the valve before and after a larger pressure. However, due to the complexity of the flow path in the valve body, the erosion damage to the valve body when used for high pressure differential is more serious and should not be used for medium with high viscosity, suspended particles or fibers. In addition, due to the processing conditions, the upper and lower two-seater valve spool is not easy at the same time off, so when the closure of the large amount of leakage, especially in the case of high temperature or low temperature, the material due to different thermal expansion coefficient is more likely to cause serious Leakage. Angular high-pressure valve body is right angle, the flow path is simple, the resistance is small, high-speed fluid erosion is also small, especially suitable for high pressure differential, high viscosity and suspended solids containing particles of fluid, can also be used to deal with vapor Mixed phase, easy flash steam cavitation occasions. This body prevents coking, sticking and clogging for easy cleaning and self-cleaning. 2, according to the safety selection. Pneumatic membrane valve with air valve and air valve closed in two forms. Depending on the safety of the production process and the use of requirements, when the signal pressure is interrupted when the control valve is open or closed position, the size of the hazardous process technology may be. If the valve is in the closed position harm is small, the choice of gas valve, the signal pressure is interrupted, so that the control valve is in the closed position, on the contrary, the choice of gas valve. 3, the flow characteristics. In the design process of automatic control system choose pneumatic valve should pay attention to flow characteristics. Typical ideal features are linear flow characteristics, equal percentage flow characteristics (logarithmic flow characteristics), fast open flow characteristics and parabolic flow characteristics of four. Linear flow characteristics in the relative changes in the opening of the same circumstances, when the flow rate is relatively small changes in the value of flow; flow is large, the relative change in the value of small flow. Therefore, the linear flow control valve in the small opening (small load) the case of poor regulation performance is not easy to control, often produce oscillation, so the linear flow characteristics of the valve should not be used for small openings, it is not suitable for load changes Larger regulating system, but suitable for the load is relatively stable, little change regulation system. The regulating valve with percentage flow characteristics is weak in regulating action under small load and strong in regulating load. It is weak in regulating function close to closing and stable in work, while it has strong regulating function close to full opening and is sensitive and effective in work. To a certain extent, , Can improve the quality of regulation, so it is suitable for load changes in the larger occasions, both in full load and half load production are better play a regulatory role. 4. Control valve diameter choice. Calculate the required flow coefficient CV according to the known fluid, then select the appropriate control valve bore diameter according to the product specification. Pay attention to the difference between liquids, gases, water vapor and other vapors when calculating CV.
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