Control valve sizing is relatively easy. The available information plays important role and that is why control valve sizing is done differently for different design stage i.e. FEED (or conceptual design) or details design or troubleshooting. Most of the engineers size the valve for only maximum case and somehow overlook minimum case requirement.
Control valve, like any other process unit operations gives best performance in certain range and if it has to operate beyond that, it loose control and leads to process upsets.
Control valve sizing can be represented by simple term Cv. Cv of control valve can be written as
cv ∝f√（ρ/∆p）
Where
简历是阀流量系数
F is mass flowrate
ρ是在入口条件下流体的比重
ΔP是跨控制阀的压力下降
Above equation is valid for single phase and correction needs to be applied for two or multiple phases. The proportional constant is typical vendor correction factors and varies between 0.85 and 1.0. Some vendors apply various factors as viscosity correction, density correction, etc.
根据舞台的不同，需要使用工程判断来估计简历要求
- 饲料的早期阶段（概念设计）：在这种情况下，正在进行上游和下游设备规范准备。完整的详细信息不可用。因此，可以假定跨控制阀的压降为1-3 bar。对于水平控制阀或降压阀，压力下降应为上游和下游单元操作之间的差异。
- Later stage of FEED and Detail design, usually all the upstream as well as downstream equipment details are known and upstream and downstream piping details are also available.
When sizing the control valve few things needs to be known such as flow variations, fluid variations, temperature variations, pressure drop variations.
Typical schematic of any control valve system is as follows
We should know following
Psou= Source pressure and elevations of liquid level
Inlet pipe (including lengths, material-roughness), fittings details (elbows, valves, tees, etc), elevations details (from source till inlet of control valves), etc.
Outlet pipe (including lengths, material-roughness), fittings details (elbows, valves, tees, etc), elevations details (from control valves to the end equipment), etc.
pend =结束或目的地压力
有了这些细节，我们可以估计
- 压力at the inlet of control valve, Pin i.e. difference between Psou and pressure drop in inlet pipe (including elevation). The pressure drop is termed as ΔPin
- 在控制阀出口处的压力，p嘴，即在出口管中添加吊站和压降（包括海拔）。压降称为ΔPOUT。
Above detail can be written as,
Psou-Pend = ΔPin+ΔPcv+ΔPout
Above equation is valid for system without any pressure increase (like with pump or compressor). The same can be written as
ΔPcv = Psou+ Pincr-Pend - ΔPin - ΔPout
在哪里，PINCR是泵或压缩机的压力增加。请使用性能曲线估算当特性或性能曲线可用时压力增加。
请注意，管道压力下降是流量的函数。泵或压缩机中的排放压力（放电头）也是流量的功能。如果流量增加，则管道中的压降更高。从本质上讲，这意味着，对于相同的源和最终压力，控制阀通常会随着流量增加而获得较小的压降。
For proper of sizing of control valves all possible flowrate scenario and its pressure drop across (and effectively Cv) to be reviewed and proper characteristics can be selected. Ratio of maximum Cv i.e. Cvmax and minimum Cvmin is determines rangeability of control valve.
Depending upon the requirement the valve characteristic can be selected. Usually control works when the operating Cv (Cvmax and Cvmin) is between 15% and 85% of selected Cv. The range of useful operations changes from operator to operator as well as vendor to vendor. Cv of 15% to 85% of rated Cv is approximate healthy range of control valve operations.
当尺寸控制阀尺寸时，供应商或设计工程师使用CVMAX为85％并选择Design CV，将其称为CVRED。如果CVMIN和CVRAID的比率超过15％，则选定的对照阀将适用于整个对照阀范围。这表示控制阀的线性特征。以下图中表示其他类型的特征 -
The characteristic of control valve are function of trim. Trim type can be selecting depending on the range of CV required. The characteristics of control valve are as follows
- 线性 - 这种类型的控制阀通常是地球阀，这是线性行为，流量容量随阀门的打开而线性增加。
- Equal percentage - This type of control valves are typically globe valve and flow capacity increases exponentially with valve opening. Equal increments of valve opening produce equal percentage changes in the Control valve. Other natures like parabolic (Low flow capacity) and hyperbolic (High flow capacity) are achieved by modifying trim to suit to the requirements.
- Quick opening – This type of control valves opens quickly provide larger opening and provides large changes in flow for very small increase in opening. These types of valve are used for on-off service, such as sequential operation in either batch or semi-continuous processes.
在服务是两相的情况下，需要进行特殊护理，高于临界条件，流体可以在压降后使用闪光，甚至下游也非常接近液体的蒸气压。这些情况会导致控制阀内的空气。
