The valve’s flow characteristic is the relationship of the change in the valve’s opening to the change in flow through the valve. The most frequently used characteristics are quick opening, linear, and equal percentage, as shown in Figure.
The quick-opening valve is predominantly used for on/off control applications. A relatively small movement of the valve stem causes the maximum possible flow rate through the valve.
For example, a quick-opening valve may allow 85 percent of the maximum flow rate with only 25 percent stem travel.
The linear valve has a flow rate that varies linearly with the position of the stem.
The equal percentage valve is manufactured so that a given percentage change in the stem position produces the same percentage change in flow. Generally, this type of valve does not shut off the flow completely in its limit of travel.