Types of control valve

The control valve adjusts the flow rate by changing the size and shape of the flow area offered to the fluid passing through it. There are two main categories of control valve used on process plant globe valves and rotary valves. The flow area in a globe valve is dictated by the position of the valve plug relative to its seat, while the flow area in a rotary valve depends on the 

There are two classes of rotary valve butterfly and rotating ball. A disc is rotated to vary the flow area 

The control valve s relationship to a nozzle is important in understanding and modelling control valve behaviour, particularly for gas flow. 

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This symbol is used to represent all types of control valve, and both pneumatic and electric actuators. 

Table 1-5 gives critical pressures for miscellaneous fluids. Table 1-6 gives relative flow capacities of various types of control valves. This is a rough guide to use in lieu of manufacturer s data. 

Figure 10-21 Expansion factor (T) as a function of pressure drop ratio (X) for four different types of control valves. (From Hutchison, 1971.)...

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. 

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. 

The far left part of the spectrum deals with the control hardware and infrastructure required to operate a plant. We need to assemble the proper types of control valves and process measurements (for temperature, flow, pressure, composition, etc.). These are the sensory devices of the plant and are essential for any control system to function. Any control strategy, no matter how clever, will have severe difficulties without the right measurements and valves in the process. An Instrument Society of America (ISA) publication catalog (67 Alexander Drive, P.O. Box 12277, Research Triangle Park, NC 27709) contains many references that deal with control hardware. 

There are several types of control valve the type to be used depends on the characteristics of the process and characteristics of the measuring device. This will be discussed in a subsequent sectioa... 

There are essentially two types of control valve a fail open and fail closed type. They are... 

The putpose of using a control valve is to control a process parameter to an acceptable range, when the process is experiencing some disturbances, such as feed rate fluctuation, ambient temperature change, etc. A good control valve should be able to handle disturbance of 1% or less. There are two types of control valve performance need to be discussed. One is inherent control valve petformance and the other is installed control valve performance. 

If this is done for the common types of control valve over their whole range, then the gain curves shown in Figure 2.24 are obtained. 

Figure 2.24 Gain curves for common types of control valve...

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