Pneumatic diaphragm control valve selection method

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. Among various regulating valves, pneumatic diaphragm regulating valves are the most widely used because they are simple in structure, convenient to use and maintain, and have intrinsic safety characteristics. Pneumatic diaphragm control valve normal use, precise control, and the selection of a great relationship, the following combination of my design and production experience, talk about pneumatic diaphragm control valve selection considerations.

1, according to the use of requirements selection.

Pneumatic diaphragm control valve is composed of two parts, valve body and valve body (including valve seat). There are different structural forms according to different application requirements. Pneumatic diaphragm control valves 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, so the amount of leakage when closing is large, especially when used under high or low temperature conditions, due to the different thermal expansion coefficients of the materials, it is more likely to cause serious The leak. The angle-type high-pressure valve body is a right-angle type, with a simple flow path, low resistance, and small erosion due to 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.

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.

3, flow characteristics.

Selecting the pneumatic diaphragm control valve in the design process 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 the flow rate is small. Therefore, the linear flow control valve in the case of a small opening (small load) regulation performance is not good, difficult 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 the 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 role, 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.

4. Adjust the valve diameter selection.

Calculate the required flow coefficient CV according to the known fluid, and then select the appropriate valve diameter according to the product technical data sheet. When calculating CV, pay attention to the difference between liquids, gases, water vapor, and other vapors.