Pneumatic diaphragm control valve selection is correct or not is very important. The selection of control valve, generally according to the characteristics of the medium being transferred (temperature, pressure, corrosion, viscosity), regulatory requirements, installation location and other factors, features of the reference of various types of control valve and rational selection. In the specific use, general should consider the main aspects of the following problems.
1 regulating valve structure selection
Select regulating valve structure need to consider two points:
(1) conditions to regulate the media, such as temperature, pressure and medium physical, chemical properties (such as corrosion, viscosity) to select. For example, strong corrosion medium diaphragm, high temperature medium can be used with the structure of the pterygoid radiating fin.
(2) the structure of regulating valve, also need to determine the flow characteristics of regulating valve (spool shape). The general is to flow characteristics according to the characteristics of regulating system to select the valve, and then consider the situation of the process piping to select the ideal flow characteristics corresponding, makes the valve installed in the pipeline system specific, working flow characteristic distortion can meet the requirements of regulation system. More use of current is equal percentage flow characteristics.
2 the gas valve open and close type selection
Pneumatic actuator Pneumatic open and closed two types. A control pressure signal valve closed, no control signal pressure to open for air to close. On the other hand, for the air to open.
Open air, air to close the main choice from the view of safety requirements for production process. Consider the principle is: the pressure signal is interrupted, should ensure the safety of equipment and operators. If the valve is in small harm to the open position, should choose air close, so that the air supply system failure, interruption of gas supply, the valve can automatically open, to ensure the safety of. On the contrary, valve in the closed against small, should choose to open gas valve. For example, the fuel gas or fuel oil heating furnace should use air to open valve, namely when the interrupt signal should be cut into the furnace, so as to avoid accidents caused by the temperature too high. As the regulating valve into the equipment flammable gas should be selected with air to open, to prevent an explosion; if the medium is easily crystallized material, selected gas closed, to prevent clogging.
3 valve caliber choice
Regulating valve caliber choice appropriate or not will directly affect the control effect. Choosing the size too small, can make the medium flows through the valve is not up to the maximum flow required. In the case of large disturbance, system for medium flow (numerical control variables) deficiency and out of control, so that the control effect becomes worse, if in an attempt to open the bypass valve to make up for the deficiency of medium flow, it will produce distortion makes the valve flow characteristics; choosing the size too large, not only will a waste of equipment investment, but also makes the valve often work in a small opening, also can become poor regulation performance, easy to adjust the system becomes unstable.
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