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.
Control valve is a type of final control element that is most commonly used for process control systems. Control valve serves to regulate flow of fluid so it can be determined in accordance with controller’s requirement. Valve has important role in steam boiler.
Most of control valve characteristics that are used in industrial process are as follow:
Linear Characteristic
The control valve will give increment value of flow rate close to each increment travel (proportional).
Equal percentage characteristic
The percentage of valve opening (valve travel) will give same percentage of amount of flow rate.
Quick opening characteristic
Control valve provide maximum change in flow with small valve opening and maintain linear relationship at all positions of fluid movement. Each additional valve opening provides sharp change in flow rate and when the valve approaching full opening position, flow changes near zero.
