Valve characteristics, butterfly

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.)... 

The key to smooth operation of a CFB system is the effective control of the solids recirculation rate to the riser. The solids flow control device serves two major functions, namely, sealing riser gas flow to the downcomer and controlling solids circulation rate. Both mechanical valves or feeders (see Figs. 10.1(a) and (d)) and nonmechanical valves (see Figs. 10.1(b) and (c)) are used to perform these functions. Typical mechanical valves are rotary, screw, butterfly, and sliding valves. Nonmechanical valves include L-valves, J-valves (see Chapter 8), V-valves, seal pots, and their variations. Blowers and compressors are commonly used as the gas suppliers. Operating characteristics of these gas suppliers which are directly associated with the dynamics and instability of the riser operation must be considered (see 10.3.3.2). 

On many machines the base of the hopper is equipped with a valve to shut off material flow to the feeder if necessary. Depending on the nature of the granulation, the hopper valve can contribute to material bridging. For materials with very poor flow characteristics, a slide valve may be preferable to a butterfly valve. 

Pressure is commonly controlled in systems using diaphragm or piston pumps by adjusting the proportion of the output flow from the pump that is diverted into a return line back to tank. This can be actuated by a simple butterfly valve, an adjustable spring-loaded ball valve, or a valve based on the use of air pressure to control the line pressure. Measurements of the response characteristics of commercially available valve systems have shown that most are able to achieve within 10% of a set value in less than 10 s (Rietz el a/., 1997), and this then has formed the basis of environmental performance standards for agricultural crop sprayers that have recently been agreed within the European Community. 

Turndown devices are most often control valves (not shutoff valves) or dampers. The best valve turndown characteristic is usually accomplished with adjustable port valves or with characterized globe-type valves. Butterfly valves usually have very poor characteristics (not straight-line), but their characteristic curves can sometimes be improved by undersizing or selecting reduced port models. 

The internal design of process-control valves has a direct impact on the flow characteristics of the gas or liquid flowing through the valve. A fully open butterfly or gate valve provides a relatively straight, obstruction-free flow path. As a result, the product should not be affected. 

Other common valve types include the quick-opening (globe) and the butterfly. Their characteristics are more a result of accident than design. In other words, the nature of their construction is the reason for their characteristics, not vice versa. They will generally be employed regardless of, not because of, their nonlinearity. 

Scheme (3), while effective, has the disadvantage of requiring an extremely large valve. Because the valve is in parallel with a fixed resistance (the condenser), a quick-opening characteristic is desirable. A butterfly valve is often used, however, because of its low pressure drop. 

Figure 7 Installed flow characteristic curve and installed gain curve fora butterfly and a globe control valve. [1] Other factors affecting control valve perfonnafice ...

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