Selecting a valve with the correct flow characteristic (the relationship between valve opening and flow capacity) can be as important as the selection of the valve size. Actually, a control valve has two characteristics, an inherent characteristic and an installed characteristic.
The inherent characteristic of a valve is the characteristic published by the manufacturer, based on tests performed in a system where great care is taken to ensure that the pressure drop across the test valve is held constant at all valve openings and flow rates. The inherent characteristic, therefore, represents the relationship between valve flow capacity and valve opening when there are no system effects involved. Figure 1 shows the ideal linear and equal percentage characteristics. The source of the name of the linear characteristic is self evident from the graph.
The name of the equal percentage characteristic comes from the definition: "Equal changes in valve position causes equal percentage changes in flow." The result, as shown in the graph in Figure 1, is a characteristic where a small increment of valve position at small openings results in a small increase in flow capacity, while the same increment of valve position at large openings results in a larger increase in flow capacity.
Figure 1. Linear and equal percentage inherent characteristics.
Figure 2. Typical actual inherent characteristics of common control valve types.
Globe valves are available with either linear or equal percentage inherent characteristics, giving them versatility not generally available with the rotary valves. In fact, most globe control valve designs can be changed from linear to equal percentage (and vice versa) by changing the trim. The ball valves (both full ball and segmented ball) exhibit a nearly perfect (more so than most equal percentage globe valves) equal percentage inherent characteristic. High performance butterfly valves exhibit an inherent characteristic that is approximately midway between equal percentage and linear.
Most control systems give the best performance when they behave in a linear manner. In light of this fact, the question might be asked, "why the equal percentage valve?" which is not at all linear. The answer is because of the installed characteristic. The installed characteristic is the relationship between valve position and flow in the specific system being considered, taking into account any changes in the pressure differential available to the control valve due to the approximately flow squared relationship between flow and piping pressure losses and/or a centrifugal pump head curve.
A large percentage of process systems include a significant amount of pipe and a number of fittings (elbows, Tee's, isolation valves, etc.) resulting in a relationship between flow in the system and pressure drop available to the control valve similar to Figure 3.
Figure 3. Valve pressure drop vs. flow in a typical system with a significant amount of pipe.
Figure 4. Installed characteristic of an equal percentage valve in the system of Figure 3.
Refer to Figure 4 and imagine a control valve with an inherent equal percentage characteristic installed in a system that has a characteristic like that of Figure 3. When the control valve is wide open, the flow will be at its maximum value. Moving the control valve in the closing direction of course causes the flow to decrease. As soon as the flow starts to decrease, it can be seen from Figure 3 that the pressure drop across the valve tends to increase, resisting the decrease in flow through the valve. The result of this interaction between control valve and system produces a nearly linear installed characteristic.
As a general rule, systems with a significant amount of pipe and fittings (the most common case) are best suited to equal percentage inherent characteristic valves. Systems with very little pipe (where the pressure drop available to the control valve remains nearly constant and as a result the inherent characteristic of the valve is also the installed characteristic) are better suited to linear inherent characteristic valves.
At least one control valve sizing computer program that is available can graph the actual installed flow characteristic based on the valve manufacturer's published inherent characteristic and user input actual process conditions.
Jon Monsen, Ph.D., P.E.
is a Control Valve
at Valin Corporation.