Flow characteristics of control valve
refers to the relationship between valve opening size and relative flow of medium under the conditions that a valve maintains constant differential pressure. There are three flow characteristics of control valve: parabolic characteristic, linear characteristics and equal percentage characteristic.
(1) Parabolic characteristic
Parabolic characteristic is approximately midway between linear and equal percentage characteristic. It provides fine throttling at flow capacity and approximately linearly characteristics at high flow capacity.
(2) Linear characteristic (linearity)
Flow capacity increases linearly with valve travel. When the flow is big, relative value of flow will in small changes, and when the flow is small, the change will be big.
(3) Equal percentage characteristic (logarithmic)
Flow capacity increases exponentially with valve trim travel. And the advantage of equal percent characteristic is that when the flow is small, the change of flow will also be small, and vice versa. That is to say it has same regulation accuracy even in different opening size.
From the analysis above, we can see equal percentage characteristic has best and stable regulation performance. And parabolic characteristic is better than linear characteristic in regulation performance. In fact, we can choose any of these flow characteristics according to the applications requirements.