When specifying control valves, multiple tasks are involved. Firstly, the style of the valve must be selected. This is usually based on a comprehensive consideration of process parameters (such as pressure, temperature, corrosiveness and abrasiveness of the medium), factory preferences, and economics. Next, it is necessary to select the inherent flow characteristics of the valve (linear or equal percentage). There are empirical rules for selecting inherent characteristics. For example, some authoritative figures suggest that systems with short pipelines and minimal dynamic pressure loss, as well as other systems with low pressure consumption such as isolation valves, elbows, heat exchangers, etc., have minimal linear characteristics. For systems with long pipelines and other pressure consuming components, it is recommended to use equal percentage valves.
During normal operation, the specification person must also consider the standard of how much control range the valve should use. For example, it is generally recommended that the valve be opened between approximately 60% and 80% at the maximum required flow rate to provide sufficient but not excessive safety factor, and if possible, not less than 20% at the minimum required opening. Avoid unpredictable flow characteristics when the opening is below 10%. Once it is determined which valve style to use, one can use one of the many available valve size calculation programs to perform size calculations.
The two tools that can help the presenter select valves are charts of installed flow characteristics 1 and installed gains 2 for all valves being considered for application. Figure 1 shows the installation flow characteristics and installation gains of two possible valve selections (partial ball valves, with equal percentages of inherent flow) for the same application, including long pipeline operation and centrifugal pumps. A 6-inch valve will control the required flow range of 80 to 550 gpm within a 10% stroke range, while only controlling 30% of its full stroke capacity within a 40% opening range, making the flow highly sensitive to changes in valve position. At the low end, there is not much safety factor, and exceeding the maximum flow of 550 gpm wastes a lot of traffic, which is both expensive and unnecessary.

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