The relationship between the control valve opening (also known as ‘valve travel’) and the flow through valve is known as the flow characteristic of that valve.
An inherent flow characteristic is the relation between valve opening and flow under constant pressure conditions. The gain of a valve is defined as the change in flow per unit % change in the valve opening.
Characteristics of Control Valve
The linear flow characteristic has a constant slope, meaning that valves of this type have constant gain through complete range of flows. These valves are often used for liquid level control and certain flow control operations requiring constant gain.
Equal percentage valves are known by that name because whenever the valve opening is changed, the percentage change in flow is equal to percentage change in the valve opening. This means the change in flow proportional to the flow just before the incremental valve opening is performed. This can also observed in the following figure. This type of valves is commonly used for pressure control applications. They can be considered for applications where high variations in pressure drop are expected.
Quick opening type of valves does not have a specific mathematical definition. These valves give a large increment in flow for relatively smaller valve opening, as can be observed in the following figure. These valves usually find use for on-off service applications.
Modified parabolic valves lie somewhere between the linear valves and equal percentage valves. As can be observed in the following figure, they can be used for throttling at low flow levels and have almost linear characteristics at higher flows.