Surge limit

The essence of surge protection is determining when and how much to open or close the recycle valve. In the simple application of Figure 6-45, the antisurge controller (UIC) will position the recycle valve using a control algorithm based on AP, AP, and data on the location of the surge limit. 

The set point of the antisurge eontroller is represented on the eompressor map by a line parallel to the surge limit line. The eontroller ealeulates the deviation of the operating point from this surge eontrol line by the equation ... 

Operating close to the surge limit in steady-state with small values of bj requires a fast control loop. This allows the valve to open rapidly to prevent surge when necessary. 

Compressors driven by steam mrbines, gas turbines, hot gas expanders, or variable speed electric motors usually employ variable rotation speed for capacity control. Generally, the surge limit slope varies witli rotation... 

Figure 3.22. Characteristic curves of a centrifugal compressor at different speeds, showing surge limits.

Continue to pinch on the suction throttle valve until the compressor surge limit is approached, or until the spill-back is shut. 

Normal operation of the system is possible in a wide power range from approximately 17% of the rated power. The average temperature of the cell varies in the range 720-800°C and the temperature difference in the stack is in the range 300-340°C, which can be regarded as acceptable. The operation line is located preferably on the characteristics of the compressor—far from the surge limit (see Fig. 5.68). 

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