How to choose pneumatic diaphragm control valve?
In the chemical production process, whether the control of a process is stable, whether the overshoot, attenuation ratio, and disturbance are within the specified range, in addition to reasonable process design and advanced equipment, an important point is whether the regulating valve can accurately act according to the main control consciousness, to accurately change the material or energy. If the flow characteristics of the regulating valve are poor, the leakage is large, and the action is unreliable, the quality of the automatic control process will deteriorate, or even lose the regulating function, thereby increasing the labor intensity and causing significant economic losses to the production. Therefore, the selection of regulating valves is very important. Pneumatic diaphragm regulating valves are widely used in chemical production process control because of their advantages such as good regulating performance, simple structure, reliable action, convenient maintenance, fire and explosion prevention, and low price. So how to choose a suitable pneumatic diaphragm regulating valve? This should be done from the following aspects.
Selection of regulating valve type
In the chemical production process, the characteristics of the regulated media vary greatly, some are high pressure, some are high viscosity, some are corrosive, and the flow state of the fluid is also different, some have very small flow, and some have very large flow. Therefore, it is necessary to choose the right type of regulating valve to meet different requirements.
Selection of the structural form of the regulating valve
When selecting the structural form of the regulating valve, is mainly based on the characteristics of the controlled process medium on-site, the process production conditions, and control requirements, combined with the flow characteristics and structural characteristics of the regulating valve itself. For example, when it is used for regulating the medium with large diameter, large flow, low pressure difference or thick slurry and suspended particles, a pneumatic diaphragm regulating butterfly valve can be selected; when a right-angle connection is required, and the medium is a high viscosity, containing suspended matter and granular medium, a pneumatic diaphragm angle regulating valve with a simple flow path, small resistance and easy flushing can be selected; when regulating the desalted water medium, since the desalted water medium contains low concentrations of acid or alkali, they are more corrosive to the rubber-lined butterfly valve and diaphragm valve, so a special ball valve for water treatment can be selected to extend the service life; when the valve is required to work at a small opening, a double-seat valve should not be selected, because the double-seat valve has two valve cores, and the lower valve core is in a closed state, with poor stability and easy to cause valve oscillation.
In addition, when selecting a regulating valve, the valve core type of the regulating valve should also be considered. The valve core is the most critical part of the regulating valve, which is divided into two categories: straight-stroke valve core and angular-stroke valve core. The valve core of the straight-stroke regulating valve is vertically throttled, while the medium flows in and out horizontally. The flow channel in the valve cavity must turn and turn, making the flow path of the valve like an inverted “S” shape, so there are many dead zones, which provide space for the precipitation of the medium and easily cause blockage. The valve core of the angular-stroke regulating valve is horizontally throttled, which is consistent with the inlet and outlet direction of the medium. Therefore, it is easy to take away the unclean medium, and the flow path is simple, and there is less space for the medium to settle, so its anti-blocking performance is good.
Again, the form of the valve cover on the regulating valve and the packing used should also be considered. When the medium temperature is -20~200℃, a common valve cover should be selected; when the temperature is higher than 200℃, a heat dissipation valve cover should be selected; when the temperature is lower than -20℃, a long-necked valve cover should be selected; in occasions with important media such as highly toxic, volatile, and permeable, a bellows-sealed valve cover should be selected. The packing in the packing chamber of the upper valve cover is made of polytetrafluoroethylene or graphite. The former has a small friction coefficient, can reduce the backlash, and has good sealing performance; the latter has a long service life, but poor sealing performance.
Selection of the action mode of the regulating valve
The selection of the air-open and air-close forms of the regulating valve is mainly based on the safety requirements of process production. The principle is: that when the instrument gas supply system fails and the gas supply is interrupted or the control signal is interrupted, the position of the regulating valve in the open or closed position is determined by the degree of harm it causes to production. If the danger is small when the valve is in the open position, the air-close valve should be selected.
