Valve characteristic

Figure 4-143 represents a computer-generated plot that demonstrates the effectiveness of eddy current brakes in preventing overspeed of the string. The lower curve assumes the butterfly valve characteristic is linear from 60° open to the closed position. The rate of closure is 3.25 sec. (Butterfly valves are normally used to throttle the expander inlet gas.)... 

Code, Section S lII, and are tested at capacity by the National Board of Boiler and Pressure Vessel Inspectors. This type of valve characteristically pops full open and remains open as long as the overpressure exists. 

Relief Yalve normally selected for liquid relief service such as hydraulic systems, fire and liquid pumps, marine services, liquefied gases, and other total liquid applications. The valve characteristically opens on overpressure to relieve its rated capacity, and then reseats. 

Luyben (1973) (see simulation example RELUY) also demonstrates a reactor simulation including the separate effects of the measuring element, measurement transmitter, pneumatic controller and valve characteristics which may in some circumstances be preferable to the use of an overall controller gain term. 

In reality, the valve characteristic curve is likely nonlinear and we need to look up the technical specification in the manufacturer s catalog. After that, the valve gain can be calculated from the slope of the characteristic curve at the operating point. See Homework Problem 1.33 and the Web... 

Some general guidelines for the application of the proper valve characteristic are shown in Fig. 10-14. These are rules of thumb and the proper valve can be determined only by a complete analysis of the system in which the valve is to be used [see also Baumann (1991) for simplified guidelines]. We will illustrate how the valve trim characteristic interacts with the pump and system characteristics to affect the flow rate in the system and how to use this information to select the most appropriate valve trim. 

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. ... 

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

The installed valve characteristic. This depends upon the ratio of the pressure drop through the valve to the total pressure drop across the whole process line installation including the valve. If a valve of linear sensitivity is handling the entire system pressure drop, then its installed characteristic will also be linear. As the percentage of the pressure drop falls off, the installed characteristic rapidly changes to that of an on-off valve. The increasing-sensitivity valve also loses its characteristic in the same way, but to a far lesser extent. Hence, this type of valve is frequently chosen in preference to the linear type. Peters(U> has discussed the effects of choosing valves with incorrect characteristics. 

Note To emphasize the difference between overpressure (safety relief valve characteristic) and accu-mulation (code limitation on the pressure vessel), safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves, even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII. ... 

For fire case, basically the PED does not dictate any particular accumulation, and this must be reviewed by the designer of the pressure equipment and this engages his/her responsibility. This should not exclusively be decided by the safety valve characteristics, manufacturer or the user. 

This is a simple way to define control valve characteristics, including their capacity. Using this relationship to the K value derivation herein, the following equations derived by numerous vendors may be applied. 

Flow as a secondary cannot only overcome the effects of valve hysteresis, but also insures that line pressure variations or badly selected valve characteristics will not affect the primary loop. For these reasons, in composition control systems, flow is usually set in cascade. Cascade flow loops are also useful in feedforward systems. Flow controllers invariably have both proportional and integral modes. If their proportional band exceeds 100%, they must have an integral mode. 

An alternate method is to calculate the ratio R from the individual measurements of flows A and B, and use this calculated ratio as the measurement input into a manually set ratio controller (RIC). In this case the loop gain varies with both the ratio R and the flow B. Because the loop gain varies inversely with flow B, this can cause instability at low rates. Therefore, the use of equal-percentage valve characteristics is essential to overcome this danger (Figure 2.48). 

The most common control valve characteristics (top) and the recommended valve characteristics selections (bottom) for a variety of applications. 

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 2.63 illustrates the effect of the distortion coefficient (Dc) on the characteristics of a linear and an equal-percentage valve. As the ratio of the minimum to maximum pressure drop increases, the Dc drops and the equal-percentage characteristics of the valve shift toward linear and the linear characteristics shift toward QO. In addition, as the Dc drops, the controllable minimum flow increases, and therefore, the rangeability (the flow range within which the valve characteristic remains as specified) of the valve also drops. 

A flow control valve will be installed in a process system in which the flow may vary from 100 to 20 percent while the pressure drop in the system rises from 5 to 80 percent. What is the required rangeability of the control valve What type of control-valve characteristic should be used Show how the effective characteristic is related to the pressure drop the valve should handle. 

Select the type of valve characteristic to use. Table 19.1 lists the typical characteristics of various control valves. Study of Table 19.1 shows that an equal-percentage valve must be used if a rangeability of 20 is required. Such a valve has equal stem movements for equal-percentage changes... 

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