Inherent valve characteristic

The inherent valve characteristic. This is a measure of the theoretical performance of the valve and is divided into ... 

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. 

FIGURE 15.18 Inherent valve characteristics for a quick opening (QO), linear, and equal percentage valve (=%). 

Figure 2.20 Examples of inherent valve characteristic curves (reproduced by permission of Emerson...

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]. 

An inherent valve flow characteristic is defined as the relationship between flow rate and travel, under constant pressure conditions. Since the last two terms in Eq. (8-115) are zero in this case, the inherent characteristic is necessarily also the relationship between flow coefficient and travel. 

The relation between the valve stem position and the flow through the valve at constant pressure drop is termed the valve characteristic. Two characteristics must be evaluated for valve selection, the inherent and the installed characteristics 67. ... 

Theoretical gain characteristics of equal % valve Actual, inherent gain characteristics of equal % valve... 

Actual, inherent gain characteristics of linear valve... 

FIGURE 19.14 (a) Inherent flow characteristics of valves at constant pressure drop (b) effective characteristics of a linear valve (c) effective characteristics of a 50 1 equal-percentage valve. 

Problem from a work object The work object itself such as a valve or pump has an inherent vulnera-biUty to human error due to its inherent structural characteristics. 

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. 

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. 

For pumped systems rapidly increasing energy costs have aroused considerable interest in designing for very low valve-pressure drops. This pushes us toward line-sized valves. If piping and equipment pressure drops are large in comparison with valve-pressure drops, valve turndown sui rs (becomes smaller). The installed flow characteristic also becomes very different ftom the inherent flow characteristic. A linear valve tends toward an installed square-root flow characteristic, while an equal-perccntJ e valve tends toward an installed linear characteristic. At the hi -flow, high-lift end of the plot, however, all curves, r ardless of y alve inherent characteristic, level off at a maximum flow. This is so because the valve has run out of pressure drop. 

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). 

Control Valve Pressure Drop Beat Inherent Characteristic... 

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. 

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