стр. 41 |

FT

99

Liquid water

Steam

Product Stream

FT

TT

98

100

FSt FC

98

T

T set

set

FSt TC

100

Figure 5-5.19 Temperature and ratio control for reactor of Example 5-5.3.

110 RATIO, OVERRIDE, AND SELECTIVE CONTROL

R FA

MUL

76

FTw

FT

97

FA + Stream A

set

FLw

SUM

75

- FC

99

+

FTw FLw

DIV SUM

73 74

FT

+ 99

FTw Liquid water

FA

Steam

Product Stream

FT

TT

98

100

FSt FC

98

T

RFB RFB

T set

Y RC TC

LS

95 100

101 set

set

FSt

FSt

Figure 5-5.20 Complete control scheme for reactor of Example 5-5.3.

LS101 and the lowest selected as the set point to FC98. Under normal conditions

TC100 will be selected. Only when the ratio of total water to stream A is above the

set point to RC95 will the ratio controller reduce its output enough, in an effort to

cut the steam, and thus, be selected. Note the reset feedback signals to RC95 and

TC100.

Notice that the ratio of total water to stream A will be at, or close to, Y only after

the liquid water п¬‚ow has been reduced to zero; that is, the only water entering is

the steam. Using this fact, Fig. 5-5.21 shows a simpler control scheme. In this case,

FA is multiplied by Y to obtain the maximum п¬‚ow of water that could be fed,

FTW max. This scheme is simpler because there is no need to tune a controller. The

reader may want to write the software program to implement the scheme shown in

Fig. 5-5.21.

5-6 SUMMARY

In this chapter we have introduced the computation tools provided by manufac-

turers. An explanation for the need for scaling was given. A brief discussion of

the signiп¬Ѓcance, and importance, of п¬Ѓeld signals was also presented. We also pre-

sented the concepts, and applications, of ratio control, override control, and

111

REFERENCES

R FA

MUL

76

FTw

FT

97

FA + Stream A

set

FLw

SUM

75

- FC

99

FLw

MUL

Y

73

FT

99

FTW max

Liquid water

Steam

Product Stream

FT

TT

98

100

FSt FC

98 T

RFB

T set

TC

LS

100

101 set

FSt

Figure 5-5.21 Another complete control scheme for reactor of Example 5-5.3.

selective control. These techniques provide a realistic and simple method for

improving process safety, product quality, and process operation. Finally, the chapter

concluded with three examples, to provide some hints on the design of control

schemes.

REFERENCES

1. C. A. Smith and A. B. Corripio, Principles and Practice of Automatic Process Control, 2nd

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