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. Therefore, I have long been engaged in the design of automatic control system, instrument modeling and installation, commissioning and maintenance experience, talk about the membrane valve applications. 1 Pneumatic membrane control valve selection 1.1 According to the requirements of the selection Pneumatic membrane control valve by the valve body and valve body (including the valve seat) of two parts, according to different requirements have different structural forms, the current Henan Yingqing Chemical Co., Ltd. Pneumatic membrane valve used by the company 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, double valve seat up and down the two spools 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. 1.2 According to the safety selection of pneumatic diaphragm 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. 1.3 Flow Characteristics In the design of automatic control system, the choice of pneumatic diaphragm control valve should focus on the 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. 1.4 The choice of regulating valve diameter should be based on the known fluid to calculate the required flow coefficient CV, and then select the appropriate technical parameters of the product valve diameter. Pay attention to the difference between liquids, gases, water vapor and other vapors when calculating CV.
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