The implementation of the quality control and production mode of the stability has the close relation, so for the implementation of the choice appears to be especially important for safety explosion-proof demand is higher chemical enterprise, most actuators used the pneumatic-valve. The choice of pneumatic-valve general from the following several aspects to consider: 1) according to the process conditions, choose appropriate regulator structure and types. 2) according to the process object, choose appropriate flow characteristics. 3) according to the process parameters, calculation flow coefficient, choose the calibre of the valve. 4) according to the technical requirement, material selection and auxiliary devices. In view of the above four points below to elaborate. A, the structure of the regulator and type of pneumatic-valve is by pneumatic actuators and the two parts, pneumatic actuators are receiving input source signal, produce the corresponding thrust, make the push rod happen displacement, promote the valve movement; And the valve is to point to the pipeline connection of valve components, it accept actuator push rod thrust, change the valve stem displacement, which changes the valve opening, final control of fluid flow changes. Pneumatic-valve according to the schedule can be divided into straight tour and quarter-turn two, according to the structure types points through single seat valve, valve, high-pressure valves direct two-seater, Angle valves, sleeve valve, the diaphragm valve, butterfly valve, eccentric rotate the valve, etc. Direct comparison of the common, single seat valve leakage small, but the before and after the pressure difference can not too big, but just the opposite, valve. High pressure valve suitable for high pressure and high pressure difference measurement of the medium, but in high-pressure difference cases, fluid on the material flush and serious corrosion, in general to consider the valve core and seat material, in order to improve its service life. In high-pressure difference, high viscosity, including suspended solids and granular material fluid control can choose Angle valves. The diaphragm valve more suitable to the strong acid, strong alkali, strong corrosive medium control. Butterfly valves used in big flow, low voltage poor gas medium. Sleeve valve type the balance valve core structure, it has the characteristics of low noise, a wide range of application is one of the valve. Pneumatic-valve open and shut gas have two types. Sure regulator switch mode principle is: when the signal pressure disruptions, should guarantee the process equipment and production safety. If the valve in the signal in the open position after the break, don't interrupt the safest fluid, chooses gas closing valves; If the valve in the signal pressure after the break in the closed position, not through the most safe fluid, chooses gas open valve. Such as the fuel gas or heating furnace fuel oil regulator road rappel/abseil stations should be open valve using gas, as the signal after the break, valve shut down automatically, fuel was cut off, lest too high temperature and accident; On the way of boiler feed lines regulator, choose the gas supply closing valves, as the signal after the break, valve automatically open, still to the boiler to send water, can avoid dry up the boiler. Second, the regulator flow characteristics regulator flow characteristics is to point to the relative flow through valve medium valve core Q to relative trip f (valve's relative opening degree) the relations between: Q = f (L) when regulator ends pressure differential enables delta P remain unchanged, the flow characteristics called the intrinsic flow feature. The intrinsic flow feature of a straight line, the main percentage (logarithm), parabolic and quick opening four types. In the production of valves in the intrinsic flow feature of a straight line, percentage and quick opening of 3. Parabolic characteristics between straight lines and between, generally with the characteristics such as a substitute. Quick opening characteristics is mainly used for two type control. Be in usually, on both ends of the pressure difference may not always be the same, at this time of the intrinsic flow feature of distortion occurs, the actual working conditions in the characteristics of the work flow feature called. At this time to determine the flow characteristics in introducing a called the resistance of the coefficient S than. S = enables delta P / ∑ enables delta P ∑ enables delta P for the system total pressure difference, is valve, all the process equipment and piping system of differential pressure on the sum. From the following three aspects of work to the flow characteristics are analyzed: 1) from the control quality analysis for a simple control system, it is by the control object, transmitter, regulators and regulator several basic link of composition, system of total amplification coefficient K = K1K2K3K4K5; K1 ~ K5 respectively transmitter, regulators, says executive mechanism, valve, the controlled objects amplification coefficient. In load changes, the control system will make can keep scheduled control index, hope the total amplification coefficient of the control system in the whole operation scope remain unchanged. Generally speaking, in a certain system, K1 ~ K3 coefficient is fixed, only the object of amplification coefficient with the load of the change of the X5 camera change, for the choice of appropriate flow characteristics to abundant I, claims the changes of characteristic of object, guarantee the product of a constant K4K5 is ensure that the system's total amplification coefficient K is a steady value. 2) from process piping analysis regulator is always with pipeline, equipment together of use, the existence of the pipeline resistance will make the job characteristics and natural characteristics of different. So, should according to the characteristics of the object, and choose the right job characteristics, again according to select the corresponding pipe valve the intrinsic flow feature. Consider process piping, can consult table 3-1 to choose the inherent characteristics of the valve. Fluorine-butterfly butterfly valve pipe condition S = 1 ~ 0.6 S = 0.6 ~ 0.3 S < 0.3 valve work characteristics such as straight line percentage percentage is unfavorable the inherent characteristics of the linear control valve percentage percentage percentage is unfavorable control when S = 1 ~ 0.6, the selected the intrinsic flow feature and work flow characteristics consistent. When S < 0.3, characteristic curve distortion serious shoulds not be control. 3) from the load variation analysis in small open degree linear characteristics regulator volume flow relative variation is big, too sensitive, easy to cause oscillation, valve core seat damaged, therefore, in S value, and the load variation little of the occasion. The percentage of amplification coefficient increases with the valve stroke, flow relative variation is constant. Three, the regulator to determine the diameter of the caliber regulator is calculated in the flow coefficient of the CV based on. The definition of the flow coefficient is to point to in the valve is fully open conditions, on both ends of the pressure difference enables delta P for 100 kPa, fluid density ρ for 1 g/cm3, through the valve fluid volume flow for Q (m3 / h), its throttling formula is: C is a ratio, it and the relationship between the flow coefficient is m times, namely CV = mC. When the flow characteristics for linear m = 1.63 when, when flow feature type percentage when m = 1.97. When measuring for the liquid medium, the computation method of measurement for gas medium when should consider when the temperature
|
