When we in the choice of pneumatic control valve should pay attention to what the problem? Then the following Shanghai OLS valve to tell.
First the structure and type of control valve, pneumatic control valve is a pneumatic actuator and valve is composed of two parts, the pneumatic actuator gas source for receiving input signals, to produce the thrust, so putting shifted, push the valve action; and the valve is a valve body assembly part connected with the pipeline, it accept the implementing agencies of the putter thrust, change the valve stem displacement, thereby changing the valve opening, the ultimate control of fluid flow changes. Electric three regulating valve according to stroke can be divided into two straight stroke and angle of travel, according to the type of structure through single-seat valve, two-seat valve, high pressure valve, through the angle valve, sleeve valve, diaphragm valve, butterfly valve, eccentric rotary valve. Valve through which the more common, single-seat valve leakage is small, but the valve before and after the pressure is not too big, and two-seat valve just the opposite. High pressure valve is suitable for medium and high differential pressure measurement of high pressure, but in the pressure differential, fluid erosion of material and cavitation serious, generally to consider the valve core and valve seat materials, to improve its service life. Can choose the angle valve to control pressure differential, high viscosity, suspended solids and particulate matter in the fluid. Control valve is more suitable for acid, alkali and other strong corrosive media. Butterfly valve is suitable for gas flow, low dropout medium. Sleeve valve spool with balanced structure, with low noise characteristics, is one of the most widely used valve. Pneumatic control valve is open air and gas off two types. Determine the mode switch valve principle is: when the pressure signal is interrupted, should ensure the safety of equipment and production technology. If the valve is in the open position in the signal interruption, the fluid does not break the most secure, then use air to close valve; if a single cut-off valve valve in the closed position in the signal after interruption of fluid pressure, not through the most secure, then use air to open valve. If the control valve heating furnace fuel gas or fuel oil pipeline, should be used to open gas valve, when the signal is interrupted, the valve automatically shut down, cut fuel furnace temperature is too high, in order to avoid accidents; boiler feed water regulating valve on the pipeline, should use air to close valve, when the signal is interrupted, the valve automatically opens, still within the boiler water, can avoid the boiler dry burning.
The second is the flow characteristics of regulating valve, control valve flow characteristics is the relative flow Q medium flows through the valve and valve core relative travel has ( relative to the valve opening ) the functional relationship between: Q=f ( L ) when the control valve at both ends of the pressure difference△P remained unchanged when the valve flow characteristics, known as the inherent flow characteristics. Inherent flow characteristics are linear, equal percentage ( logarithmic ), 4 types of parabolic and so quick. Inherent in the production of valve flow characteristics are linear, equal percentage and quick opening of 3. Parabolic characteristic between the lines and other between a hundred, a hundred other characteristics generally used instead. Quick open feature is mainly used for the two bit control.
Finally is the valve diameter and material and auxiliary equipment, determine the valve diameter is based on Calculation of valve flow coefficient CV. The definition of flow coefficient is defined in the valve fully open conditions, valves at both ends of the pressure difference△ P is 100 kPa, the fluid density is 1g / cm3, the volume of fluid flow through the valve is Q ( m3 / h ), the throttle formula is: C is a scaling factor, its relationship with the flow coefficient is m times, namely CV =mC. When the linear flow characteristics when m=1.63, when the flow characteristics such as the percentage of type m=1.97. Type ( 4--1 ) is when the method is liquid medium for measuring, when the measured gas should be considered when the temperature and pressure effect on medium volume, calculated the C values of the two situations: when the valve before and after the pressure difference△ P less than 0.5 times of the valve before the pressure P1, namely the△P<0.5P1 in addition, when the medium for superheated steam, calculating the value of C to consider the degree of superheat steam. After determining the value of complete CV, to calculate the opening of the valve, the valve open degree greater than 90% requires the maximum flow, minimum flow opening degree is not less than 10%. In the normal condition, the valve opening should be between 15%~85%. Finally, according to the CV value to determine the adjustment valve diameter. The valve body valve cast iron material, its pressure rating, temperature range, corrosion resistance and so should not be less than the requirements for process piping, but the fluid medium is flammable and explosive special consideration should be made.
Therefore, with the development of industrial automation, the higher of the pneumatic valve are required, these requirements include: new structures, new materials, high performance, communication with the computer and the excellent dynamic performance. It is a difficult and important task to study and learn new adjusting valve.
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