One of the most common control element in the process control industry is the control valve. A控制阀helps regulate the flow of a fluid (gas, steam, water, or chemical substance) in order to keep the regulated process variables (pressure and flowrate) as close as possible to the desirable set point. Control valves can generally be designed to handle all types of fluids at temperatures from the cryogenic range up to 500+ degrees Celsius steam temperatures.
Control valves are typically operated withinstrument air因此，其特征为气动驱动瓣膜。电力和电湿度致动器也可以与对照阀一起使用。但是，与气动执行器相比，它们通常更复杂，因此更昂贵。它们提供了在没有仪器空气或仪器气体供应源的情况下，在低环境温度下，气动供应线中的冷凝水可能会冻结，或者在气门运动中需要异常大的驾驶力（STEM）力来进行阀。气动类型控制阀通常是地球阀.
Control valves are typically designed to operate in aclosed loop，即有关过程变量的信息，将连续馈回控制器设置点，以便为过程变量提供连续的自动校正。这通常称为feedback loop.
为了能够顺序阀的最佳控制e suited for the application, the user has to provide the valve manufacturer with all the necessary information to select and design a suitable control valve.
- Type of fluid through the valve
- Process properties of the fluid (viscosity，蒸气压等），以便能够告知任何特殊设计条件，例如闪烁的可能性等。
- Normal, maximum and minimum flowrate values through the control valve need to be specified, in order to be able to calculate the design flow coefficient Kv/Cv of the valve and the valve's capacity, i.e. the flow rate through the valve under steady conditions. Theflow coefficient, Cv,expressed in m3/h, is the flow rate that flows through the valve resulting to a static pressure drop of 1 bar across the valve. Knowing fluid details at various operating points and taking into consideration the valve specific construction details (cage etc), it is possible to calculate a Cv value for each one of these operating conditions. The selected Cv value of the valve must be greater than the Cv value at the worst operating condition.Knowing maximum and minimum flow rate also enable us to calculate the valve's turndown ratio, i.e. the maximum controllable ratio of maximum / minimum flow and we are also able to calculate other operating characteristics like shut-off DP pressure and maximum actuating force, thereby allowing us to select the most suitable actuator.
- 正常工作，阀门入口和出口处的最大压力和最小压力。
- Pressure dropduring normal flow and shutoff conditions.
- Installation (indoors / outdoors)
- 最大允许noise level
- 建筑材料，端连接类型
- Information about trim characteristic, i.e. whether the valve will be oflinear characteristic(flow capacity increases linearly with valve travel),equal percentage characteristic(flow capacity increases exponentially with valve trim travel) orquick opening characteristic(large changes in flow can be achieved for very small changes in valve trim travel).
- Action desired on air failure: valve to open (FAIL OPEN), close (FAIL CLOSE), or retain last controlled position (FAIL LAST POSITION)
- Instrument air supply available, operating and design pressure values for the instrument air supply.
Figure 3 - Typical pneumatic-type control valve datasheet
