Valves, control inherent flow characteristic

Most control valves today are either single-seated global types or rotary types. For both there are substantial, coercive stem forces when the valves are in flowing streams. Positioners compensate for this and maintain the valve s inherent flow characteristic expressed as a function of controller output signal. It should be noted, however, that some users prefer not to use positioners. 

FIG. 8-82 Installed flow characteristic as a function of percent of total system head allocated to the control valve (assuming constant head pump, no elevation head loss, and an R equal 30 equal-percentage inherent characteristic). 

Show how the valve effective characteristic is related to pressure drop. Figure 19.14 shows the inherent and effective characteristics of typical linear, equal-percentage, and on-off control valves. The inherent characteristic is the theoretical performance of the valve. If a valve is to operate at a constant load without changes in the flow rate, the characteristic of the valve is not important, since only one operating point of the valve is used. 

FLOW MEASUREMENT SIGNAL TO CONTROLLER LOCATION OF CONTROL VALVE IN RELATION TO MEASURING ELEMENT BEST INHERENT CHARACTERISTIC ... 

The inherent characteristics are those that relate flow to valve position in the situation where the pressure drop over the control valve is constant. These are the (APn/AP ) = 0 curves in Fig. 7.8. Instcdled characteristics are those that result from the variation in the pressure drop over the valve. 

Table 2.61 lists the inherent valve characteristics of the various types of control valves. The characteristics that are recommended are a function of the intended application, and are listed in the tabulation at the lower part of Figure 2.62. It should be noted that the listed valve characteristics assume that the valve pressure drop is constant. Unfortunately, in most applications (Figure 2.60), the available valve pressure differential is not constant but drops as the load (flow) increases. This is the reason why the recommended valve characteristics in Figure 2.62 are different if the ratio of the maximum to minimum pressure differential is above or below 2 1. 

The inherent characteristics of control valves allow classification into three main groups, based on the relationship between valve flow and valve position under constant pressure linear, equal-percentage, and quick opening (Seborg, Edgar, and Mellichamp, 2004 see Fig. 18.51). Usually in-plant testing is used to determine the actual valve characteristics because the dynamics of the valve can depend on other flow resistances in the process. 

There are, however, rules of thumb for selecting control valves and matching inherent valve characteristics to common process control loops or processes where the valve pressure drop is fairly constant. The equal percentage characteristic is the most common and is used where variations in pressure drop are expected or in systems where a small percentage of the total system pressure drop is taken across the valve, such as in pressure and flow control. More detailed recommendations are available from control valve vendors, e.g. [15,16]. 

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