Once the valve type is selected, the next task is to choose the valve characteristics, and size the valve. Three of the most common valve characteristics are described in Figure 1 below, and the tabulation below lists the recommended selections of valve characteristics for some of the most common process applications.
The characteristic curves were drawn, assuming that the pressure drop through the valves remain constant while the valves throttle. The three valve characteristics differ from each other in their gain characteristics.
The gain of a control valve is the ratio between the change (=%) in the control signal, which the valve receives and the resulting change (=%) in the flow through the valve. Therefore, the valve gain (Gv) can be expressed as GPM/% stroke. The gain (Gv = GPM/%) of a linear valve is constant, the gain of an equal percentage (=%) valve is increasing at a constant slope, and the gain of a quick opening (QO) valve is dropping as the valve opens.
The valve characteristics are called linear (straight line in Figure 1), if the gain is constant (Gv=1), and a 1% change in the valve lift (control signal) results in the same amount (GPM) of change in the flow through the valve, no matter how open the valve is. This change is the slope of the straight line in Figure 1, and it can be expressed as a percentage of maximum flow per a 1% change in lift, or as a flow quantity of say 5 GPM per % lift, no matter how open the valve is.
FIGURE 1: VALVE CHARACTERISTICS
The most common control valve characteristics for common applications.
If a 1% change in the valve stroke results in the same percentage change (not quantity, but % of the flow that is occurring!), then the valve characteristic is called equal percentage (=%). If the valve characteristics is =%, the amount of change in flow is a small quantity, when the valve is nearly closed, and it becomes larger and larger as the valve opens.
As shown in Figure 1, in case of quick opening (QO) valves, the opposite is the case; at the beginning of the stroke, the valve gain is high (the flow increases at a fast slope) and towards full opening, the slope is small.
The recommended choice of the valve characteristic is a function of the application. For common applications, the recommendations are tabulated at the bottom of Figure 1. It should be noted that Figure 1s valve characteristics assume that the valve pressure drop is constant. Unfortunately, in most applications, it isnt constant, but drops off as the load (flow) increases. This is why the valve characteristics recommended in Figure 1 are different if the ratio of maximum to minimum pressure differential across the valve is above or below 2:1.
One approach to characterizing an analog control signal is to insert either a divider or a multiplier into the signal line. By adjusting the zero and span, a complete family of curves can be obtained. A divider is used to convert an air-to-open, equal-percentage valve into a linear one, or an air-to-close linear valve into an equal-percentage one. A multiplier is used to convert an air-to-open linear valve into an equal-percentage, or an air-to-close equal-percentage valve into linear one.
Distortion of Valve Characteristics
Figure 2 below shows the effect of the distortion coefficient (Dc, defined in Figure 2) on the characteristics of an =% valve. As the ratio of the minimum to maximum pressure drop increases, the Dc coefficient drops and the =% characteristics of the valve shifts towards linear.
FIGURE 2: DISTORTION COEFFICIENT
The characteristic curve of an equal percentage valve shifts towards linear as the Distortion Coefficient (Dc) drops. This drop also reduces the rangeabillity of the valve.
Similarly, under these same conditions, the characteristics of a linear valve would shift towards quick opening (QO, not shown in Figure 2). In addition, as the Dc coefficient drops, the controllable minimum flow increases, and therefore the rangeability of the valve also drops.
The conventional definition of rangeability is the ratio between the maximum and minimum controllable flows through the valve. Minimum controllable flow (Fmin) is not the leakage flow (which occurs when the valve is closed), but the minimum flow that is still controllable, and can be changed up or down as the valve is throttled.
Using this definition, manufacturers usually claim a 50:1 rangeability for equal-percentage valves, 33:1 for linear valves, and about 20:1 for quick-opening valves. These claims suggest that the flow through these valves can be controlled down to 2%, 3%, and 5% of maximum. However, these figures are often exaggerated. In addition, as can be seen in Figure 2, the minimum controllable flow (Fmin) rises as the distortion coefficient (Dc) drops. Therefore, at a Dc of 0.1, the 50:1 rangeability of an equal-percentage valve drops to about 10:1.
Consequently, the rangeability should be defined as the flow range over which the actual installed valve gain stays within ±25% of the theoretical (inherent) valve gain (in the units of GPM per % stroke). To illustrate the importance of this limitation, Figure 3 shows the actual gain of an equal percentage valve starts to deviate from its theoretical gain by more than 25%, when the flow reaches about 65%.
Therefore, in determining the rangeability of such a valve, the maximum allowable flow should be 65%. Actually, if one uses this definition, the rangeability of an =% valve is seldom more than 10:1. In such cases, the rangeability of a linear valve can be greater than that of an =% valve. Also, the rangeability of some rotary valves can be higher because their clearance flow tends to be lower, and their body losses near the wide open position also tend to be lower than those of other valve designs.
To stay within ±25% of the theoretical valves gain, the maximum flow should not exceed 60% of maximum in a linear valve or 70% in an =% valve. In terms of valve lift, these flow limits correspond to 85% of maximum lift for =% and 70% for linear valves.
PICKING THE RIGHT CONTROL VALVE
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When it comes to selecting and sizing control valves, this unique and completely non-commercial valve selection chart not only helps you pick the right control valve for the job, but also serves as a fantastic reference tool you can download!
>> CLICK HERE to open an enlarged pdf version for easy viewing chart, or click the Download Now button at the end of this article to save the chart for future reference.
[Editor's Note: How to Select Control Valves, Part 3, will appear in the November 2006 issue of Control magazine, and here on ControlGlobal.com.]
|About the Author|
Béla Lipták is editor of the Instrument Engineers Handbook, and former chief instrument engineer at C&R (later John Brown). He is a recipient of ISAs Life Achievement Award (2005) and member of the CONTROL Process Automation Hall of Fame (2001).