When selecting a control valve, first of all, collect the physical parameters of the complete process fluid and the operating conditions of the control valve. The composition, temperature, density, viscosity, normal flow rate, maximum flow rate, minimum flow rate, maximum flow rate and minimum flow rate of the main fluid Import and export pressure, the maximum pressure and so on. In the technical aspects of the main control and determine the structure of the valve itself, flow characteristics, rated flow coefficient Kv value, caliber size, the process allows the calculation of pressure and actuator selection, materials and installation aspects. The following principles should be followed when choosing a regulating valve. A control valve structure type: Should be able to meet the media temperature, pressure, mobility, flow direction, the adjustment range and tightness requirements. Two regulating valve flow characteristics: should be able to meet the system characteristics of a reasonable compensation. The flow characteristics of the control valve refer to the relationship between the relative flow of the medium through the valve and the relative displacement of the valve stem. The mathematical expression is as follows: Q / Qmax = f (l / L), where Q / Qmax is the relative flow, Valve at a certain opening when the flow Q and Qmax ratio of full-open flow; l / L is the relative displacement of the valve in a certain degree of opening valve spool displacement l and full-open displacement L ratio. The general rule of thumb is that the flow characteristics of the regulating valve should be opposite to the characteristics of the regulating object and to the characteristics of the regulator so that the overall characteristics of the regulating system approach a linearity. The selection of flow characteristics is usually carried out under the requirements of the process system, but the following actual situation is also to be taken into account. 1, the linear flow characteristics of the scope of application: ①differential pressure changes small, almost constant; ② process parameters of the main changes in the linear; ③ most of the system pressure loss distribution in the control valve (to change the opening, the valve differential pressure changes Relatively small); ④ small external interference, a small change in the given value, the adjustable range of small requirements. 2, the percentage of the characteristics of the scope of application: ①actual adjustable range; ② opening changes, valve pressure changes relatively large; ③ piping system pressure loss; ④ process system load fluctuations; ⑤ regulating valve often small Opened under the opening. 3, in addition to the above two commonly used flow characteristics, there are other flow characteristics of the parabolic characteristics and quick-opening characteristics of the valve. Three regulating valve caliber: should be able to meet the process on the flow requirements. According to the known fluid conditions, calculate the necessary Kv value, select the appropriate control valve diameter.
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