The regulating valve can be divided into three kinds of pneumatic, electric and hydraulic, namely the pneumatic control valve with compressed air as power source, electric control valve with electric power source, with liquid medium (such as oil) pressure as the power of electro-hydraulic control valve, in addition, according to its function and characteristics, there are solenoid valve, electronic, intelligent, fieldbus-type control valve. The valve has many types of products, the structure is also varied, and is constantly updated and changed.
Control valve body A lot of types, commonly used to have straight-through single seat, through two-seater, angular, diaphragm, small flow, three-way, eccentric rotation, butterfly, sleeve type, spherical, etc. 10 kinds. Before selecting the valve, we should carefully analyze the media, process conditions and parameters of the control process, collect enough data, understand the requirements of the system to the regulating valve, and determine the type of valve to be used according to the data collected. In the specific selection, the following considerations can be considered:
(1) The shape structure of the spool is mainly considered according to the flow characteristics and unbalance force chosen.
(2) Abrasion resistance when the fluid medium is a high concentration of wear particles of the suspension, spool, seat junctions every time the shutdown will be seriously rubbed. So the valve flow path to smooth, valve internal material to be hard.
(3) Corrosion resistance due to the corrosive medium, in the case of the adjustment function, as far as possible to choose a simple structure valve.
(4) Medium temperature, pressure when the medium temperature, pressure high and changes, should choose the valve core and seat material by temperature, pressure change small valve.
(5) Prevent Flash and cavitation flash and cavitation only in liquid medium. In the actual production process, flash and cavitation affect not only the calculation of flow coefficient, but also the formation of vibration and noise, so that the service life of the valve is shortened, so in the selection of valves should prevent the valve from flashing and cavitation.
After the output force of the actuator is determined, the corresponding executing agency is selected according to the requirements of the process use environment. For on-site explosion-proof requirements, the use of pneumatic actuator, and junction box for explosion-proof type, can not choose the electric actuator. If there is no explosion-proof requirements, the pneumatic, electric actuator can be selected, but from the energy-saving considerations, should try to choose electric actuator. For hydraulic actuator, its use is not as pneumatic, electric actuator wide, but with high adjustment accuracy, speed and smooth movement characteristics, therefore, in some cases, in order to achieve better adjustment effect, must choose hydraulic actuator, such as power plant transparent machine speed regulation, Temperature control of catalytic reactor in refinery.
Regardless of the type of actuator, its output force is used to overcome the load of the effective force (mainly refers to the imbalance force and unbalanced moment plus friction, sealing force, gravity and other related forces). Therefore, in order to make the regulating valve work properly, the executing agency should be able to produce sufficient output force to overcome various resistance, ensure the high seal and valve opening.
For the dual-acting pneumatic, hydraulic, electric actuator, there is generally no reset spring. The magnitude of the force has nothing to do with its direction, so the key to selecting the actuator is to find out the maximum output force and the rotational torque of the motor. For the single acting pneumatic actuator, the output force is related to the opening of the valve, the force on the regulating valve will also affect the movement characteristics, so it is required to establish the force balance in the entire control valve open range.
The flow characteristic of the regulating valve refers to the relationship between the relative flow of the medium flowing through the valve and the displacement (relative opening of the valve), the ideal flow characteristics are linear, equal percentage (logarithmic), parabola and fast opening 4 kinds, characteristic curve and spool shape as shown in Figure 1 and Figure 2. The common ideal flow characteristics are straight line, equal percentage (logarithm), fast open three kinds. Parabolic flow characteristics between the line and equal percentages, generally available as a percentage of characteristics to replace, and the fast-opening characteristics are mainly used in two-bit regulation and program control, so the selection of control valve characteristics is actually linear and equal percentage flow characteristics of the choice.
The selection of flow characteristics of the regulating valve can be calculated theoretically, but the methods and equations are very complicated. At present, the use of empirical guidelines, specific from the following aspects: ① from the regulation of the adjustment of quality analysis and selection of ② from the process of piping, ③ from the load change analysis.
The selection and determination of the valve caliber is mainly based on the flow capacity of the valve, i.e. CV. In a variety of engineering instrumentation design and selection, the adjustment valve to the CV calculation, and provide control valve design instructions. From the adjustment valve CV calculation to the valve diameter determination, the general need to follow the following steps:
(1) Calculate the flow of determination. The existing production capacity, equipment load and the status of the media, determine the calculation of flow Qmax and qmin.
(2) to determine the pressure difference before and after the valve. According to the selected valve flow characteristics and system characteristics of the S (resistance coefficient), and then determine the calculation of pressure difference.
(3) Calculate CV. The Cmax and cmin are obtained by selecting the appropriate calculation formulas and graphs according to the adjusted media.
(4) Choose CV. According to Cmax, select the >cmax and its closest level C in the selected product standard series.
(5) Check the opening of the control valve. Generally require the maximum calculation of the flow of open ≯90%, the minimum calculation of the flow of open ≮10%.
(6) checking the actual adjustable ratio of the control valve. The general requirements of the actual adjustable ratio ≮10.
(7) Seat diameter and nominal diameter of the determination. After verification is appropriate, determine according to C.
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