The purpose of the control ball valves is to regulate the flow rate of fluids through pipes in the system. This is accomplished by placing a valve in the path of flow of liquid. It acts to offer resistance to the flow of a liquid, where the resistance and flow can be controlled by a signal from a process control. This signal is derived from the sensors in the plant. It is to be noted that when a control valve is supplied by the manufacturer it is attached to an actuator- it uses the controller signal to activate the final control element.

The different types of control valves are classified by a relationship between the valve stem position and the flow rate through the valve. Three basic types of valves based on the relationship between stem position and flow rate are Quick Opening, Linear and Equal Percentage. The relationship between the flow rate through the valve and the stem position at a constant pressure drop is called as the Inherent valve characteristics. The relationship between the flow through the valve and the stem position for a valve installed in the process line at varying pressure drop is known as Effective valve characteristics.

In the Quick Opening Flow characteristics, the maximum flow is produced as soon as the valve begins to open. Hence they are used for ON/OFF applications. Because of the extreme nature of the quick open characteristics the inherent and the effective valve characteristics are similar. The linear flow characteristics are usually specified in those process systems where the majority of the pressure drop is taken through the valve. The piping effects have a tendency to push the linear characteristics towards the quick open characteristics. With an Equal Percentage characteristic, the change in flow per unit of valve position is directly proportional to the flow occurring just before the change is made.