The inherent flow characteristic of the control valve is the relationship between the flow and travel of the valve to a constant pressure drop across the valve.
The inherent flow characteristics of the valve are determined by the valve orifice and plug geometry. There are three types of inherent flow characteristics, namely linear and an equal percentage.
Linear flow Characteristics:
The linear flow characteristics show that the flow rate is directly proportional to the path of the valve. Control valves with linear flow characteristics are commonly specified for liquid level control. The sensitivity of a control valve with a linear flow characteristic is always constant.
Equal percentage characteristic:
These valves have a valve plug shaped so that each increase in valve lift increases the flow rate in a certain percentage of the previous flow. The relationship between the elevation of the valve and the size of the hole (and, therefore, the flow) is not linear but logarithmic.