The relationship between control valve capacity and valve stem travel is known as
Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs. Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim.
The most common characteristics are shown in the figure above. The percent of flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on constant pressure drop across the valve and are called inherent flow characteristics.
The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics.
When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel.
When the actual flow in a system is plotted against valve opening, the curve is called the Installed Flow Characteristic.
In most applications, when the valve opens, and the resistance due to fluids flow decreases the pressure drop across the valve. This moves the inherent characteristic: