Pneumatic diaphragm control valve consists of two parts: valve body and valve body (covering valve seat). It is divided into straight single-seat valve, two-seat regulating valve and pressure angle regulating valve according to different requirements. Pneumatic diaphragm control valve has two kinds of rituals, air opening valve and air closing valve. The selection depends on the hazardous volume caused by the process production when the signal pressure is interrupted when the throttle valve is opened or closed. If the hazard is small when the valve is closed, use the gas to open the valve, otherwise select the gas to close the valve.

1 Pneumatic diaphragm control valve selection

1.1 According to the selection of causative requirements

Pneumatic diaphragm control valve consists of two parts: valve body and valve body (covering valve seat). It is divided into straight single-seat valve, two-seat regulating valve and pressure angle regulating valve according to different requirements. 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 huge. Therefore, it must be used in applications where the small leakage of the pipe diameter and the pressure difference between the front and rear of the valve are small. Pass-through double-seat valve body has upper and lower two valve cores, because the effect of fluid used for the opposite direction of the upper and lower valve cores is roughly offset, so the unbalanced force of the double-seat valve is small, allowing the valve to have a larger pressure difference before and after. However, due to the complex internal flow of the through-seat double-seat valve body, the erosion and damage of the valve body is severe under the high-pressure difference condition, and it is not suitable for high-viscosity, medium containing suspended particles or fibers.

In addition, because of the processing conditions, the two seat valve upper and lower valve cores are not easy to close at the same time, so when the leakage is large, especially in high temperature or low temperature applications, because of the thermal expansion coefficient of the material is not the same, easier Trigger a serious leak. The angle-type high-pressure valve body is a right-angle type, simple flow, small resistance, erosion by high-speed fluid is also small, surprisingly used for high pressure differential, high viscosity and fluid containing suspended particles, can also be used for gas liquid Miscible, easy flash cavitation occasions, can avoid coking, adhesion and congestion, easy to clean and self-purification.

1.2 According to the safety performance selection

Pneumatic diaphragm control valve has two kinds of rituals, air opening valve and air closing valve. The selection depends on the hazardous volume caused by the process production when the signal pressure is interrupted when the throttle valve is opened or closed. If the hazard is small when the valve is closed, use the gas to open the valve, otherwise select the gas to close the valve.

1.3 Selection based on flow characteristics

In the design process of the automatic control system, the pneumatic diaphragm control valve should be selected to consider the flow characteristics. Typical ideal characteristics include straight line flow characteristics, equal percentage flow characteristics (logarithmic flow characteristics), fast opening flow characteristics, and parabolic flow characteristics. When the linear flow characteristics are the same with respect to the relative opening degree, the relative variation of the flow rate in the flow hours is large, and the relative variation in the flow rate is small when the flow rate is large. Therefore, the linear flow control valve has poor adjustment performance under a small opening (small load), is not easy to tamper with, and often induces vibration. Therefore, it is not suitable for a small opening, and is not suitable for an adjustment system with a large load variation. It only has to be used for relatively smooth and variable adjustment systems. Percentage flow characteristics of the regulating valve in the small load to adjust the small section of the role of weak, large load to adjust the small section of the role of strong, it is close to the closure of the small section of the song effect is weak, smooth office work, and close to the full time when the small section of the role of strong Because the office is sensitive, it can be used in occasions where the load variation is large, and it can be used to better adjust the effect both in full-load production and half-load production.

1.4 adjustment valve aperture selection

The required flow coefficient CV should be calculated based on the known fluid, and then the appropriate valve diameter should be selected according to the product technical parameter table. When calculating CV, pay attention to differences in liquids, gases, water vapor, and other vapors.

2 Pneumatic diaphragm control valve installation

The following points should be added when installing the regulating valve.

(1) The regulating valve shall be installed vertically on the leveling pipe. If it is required to install the leveling statically, it shall be added normally.

(2) When the nominal diameter of the selected valve is not the same as the diameter of the process pipe, a reducer shall be installed to perform the affiliation.

(3) The installation site should have good conditions, the temperature should be -25 ~ 55 °C.

(4) Do your best to avoid installation in a place where there is a vibration source, or else take the necessary anti-vibration reinforcement measures.

(5) When mounting, be sure to point the direction of the arrow on the valve body or flange to the direction of the media.

(6)Before installation, weld slag and other debris should be carefully removed from the pipeline. After installation, the valve plug should be at the maximum opening, and the pipeline and regulating valve should be cleaned again to prevent the jamming and damage of debris. Throttle parts.

(7) Pneumatic regulators should be installed for easy repair.

(8) The straight pipe section before the valve should be as long as possible.

(9) Apply 3 to 5 times the pipe diameter of the straight pipe section to the outlet pipe.

(10) In order to ensure that the failure of the regulating valve and regulating system will not affect the production and safety incidents, it is usually necessary to install the bypass and side valve. The side valve must not be installed directly above the regulating valve, so that the corrosive medium in the side valve leaks to the regulating valve. Valves must be installed before and after the valve. For high temperature, high pressure, thawing, and viscous media, the shower valve must also be installed.

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