Pneumatic control valve flow characteristics

in due to the influence of many factors, such as changes in river closure area at the same time, also occurred the change of pressure difference before and after pneumatic valve, and the change of pressure difference will cause the change of the flow. Keeps the same pressure difference before and after pneumatic valve, pneumatic control valve flow characteristics called the ideal flow characteristic; Control structure characteristics of the pneumatic valve is refers to the pneumatic valve core displacement and fluid circulation, the relationship between the sectional area, it is purely decided by the pneumatic valve core size and geometry shape, and related control pneumatic valve geometry, but also considered on the basis of the differential pressure under the influence of flow coefficient, therefore,
according to the characteristics of the process object, choosing the right pneumatic valve flow characteristic of pneumatic control valve flow characteristics is medium flows through the control of pneumatic valve relative flow and relative displacement ( Control of the pneumatic valve relative opening degree) The relationship between general change control pneumatic valve pneumatic valve core and circulation section of a pneumatic valve seat, can control the flow. But in fact, due to the influence of many factors, such as changes in river closure area at the same time, also pneumatic valve pressure difference before and after the change, and the change of pressure difference will cause the change of the flow. Keeps the same pressure difference before and after pneumatic valve, pneumatic control valve flow characteristics called the ideal flow characteristic; Control structure characteristics of the pneumatic valve is refers to the pneumatic valve core displacement and fluid circulation, the relationship between the sectional area, it is purely decided by the pneumatic valve core size and geometry shape, and related control pneumatic valve geometry, but also take into account the flow coefficient under the condition of invariable pressure difference, therefore, ideal flow characteristic and structure characteristics of pneumatic control valve is different. Rational flow characteristic is mainly composed of linear, such as percentage, parabolic and quick opening four. Application in actual production process, the control pressure difference before and after pneumatic valve is always changing, the flow characteristic of then referred to as work flow characteristic, because the control of pneumatic valves and process equipment used in series or parallel, often flow changes because of the change of resistance loss, in the practical work because of the change of pressure difference before and after pneumatic valve make ideal flow characteristic distortion into working characteristic. Ideal flow characteristic, pneumatic control valve is commonly used in the production line, such as percentage, quick opening three, parabolic flow characteristic between straight line and equal percentage, generally can be replaced with equal percentage, and quick opening feature is mainly used for the two type adjustment and program control. Control characteristics of the pneumatic valve choice, therefore, refers to how to choose the lines and equal percentage flow characteristic. Now control the choice of pneumatic valve flow characteristics by means of the criterion of experience more, can consider from the following several aspects:
below, from the quality of the regulating system analysis is the automatic control system of a heat exchanger, it is by the adjustment object, transmitter, adjustment of instrumentation and control of pneumatic valve. K transmitter amplification coefficient, adjust the amplification coefficient, K K amplification factor of the actuator, control of pneumatic valve amplification coefficient K, K adjustment magnification coefficient of objects.
it is clear that the system's total magnification coefficient K is: K = K * * * * K K K K K, K, K, K, K respectively for the transmitter, adjusting instruments, actuators, pneumatic control valve, adjust the amplification coefficient of object, in the case of load changes, to make the control system can still maintain a predetermined quality indicator; Wants the total magnification coefficient in the entire operating range of the adjustment system remains the same. Usually, the transducer, regulator ( Had been setting) And amplification factor of the actuator is a constant, but the regulating object of amplification coefficient is always as the operating conditions vary, so the characteristics of the object often is nonlinear. Therefore, choose appropriate control characteristics of pneumatic valve, pneumatic valve amplification coefficient changes to compensation adjustment the changes to the amplification coefficient, and make the system total magnification coefficient remains constant or approximate constant, thus improve the quality of the regulating system. Therefore, the choice of the pneumatic control valve flow characteristics should be consistent with: K = K * constants for amplification coefficient along with the increase of load the phenomenon of smaller, if choose magnification coefficient varies with the load increasing and bigger percentage of characteristic control pneumatic valve, it can make the two cancel each other out, the result of the synthesis, bringing the total magnification coefficient remains the same, the approximate to linear. When the regulating object of amplification coefficient is linear, should adopt linear flow characteristic, bringing the total magnification coefficient remains constant.
consider, from the process piping control pneumatic valve are always used together with the piping, equipment, etc, because of the different system piping, piping resistance control pneumatic valve pressure drop caused by the existence of the change, therefore, work flow characteristics of pneumatic valve and pneumatic valve ideal flow characteristic also varies. Must be based on the system characteristics to choose hope to get the job characteristics, then consider piping is to select the corresponding ideal characteristics. Pneumatic control valve, the pressure difference on all process equipment and piping system, called the system total pressure difference. Pneumatic valve pneumatic control valve fully open, and the differential Δ Pmin and differential pressure of the system total Δ P is known as the S value, that is: S = Δ Pmin/Δ P
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