The flow characteristic of a control valve is defined as the relationship between the flow coefficient and valve travel as the valve travels from 0 to 100 percent open. The purpose of characterizing a control valve is to provide uniform control loop stability over the expected range of operating conditions; i.e., to match the valve gain to the system gain for optimum performance.
The three most common flow characteristics are illustrated in Figure 40 and they are described below. Variations of these characteristics may also be encountered.
Quick Opening – Throughout the first 60 percent of the rated valve travel, the quick-opening characteristic provides a very large change in C v for every incremental change in travel. Above 60 percent of the rated travel, each incremental change in valve position results in smaller and smaller changes in Cv. Another way to describe this characteristic is that it provides high valve gain at low percentages of valve travel, and it provides low valve gain at high percentages of valve travel. This characteristic is appropriate for on-off applications, for emergency relief applications, or when high, linear valve gain at low percentages of travel is desired.
Linear – When a linear flow characteristic is specified, the control valve Cv is directly proportional to valve travel. In other words, valve gain is constant over travel. This characteristic is commonly selected when the pressure drop remains constant over the entire operating range of the system.
Equal Percentage – With an equal percentage characteristic, equal increments of change in valve travel produce equal percentage changes in the existing flow coefficient. Refer to Figure 41 and note that between 0 and 50 percent valve travel, each 10 percent increase in travel produces a relatively small increase in Cv. Beyond 50 percent travel, each 10 percent increase in valve travel results in larger and larger changes in Cv. In other words, this characteristic provides low valve gain at low percentages of valve travel, and high valve gain at higher percentages of valve travel.