Compressor surge limit

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

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... 

Compressor surge is a phenomenon of considerable interest, yet it is not fully understood. It is a form of unstable operation and should be avoided in both design and operation. Surge has been traditionally defined as the lower limit of stable operation in a compressor and involves the reversal of flow. This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually a part of the compressor is the cause of the aerodynamic instability, although it is possible that the system arrangement could be capable of augmenting this instability. Figure 6-36 shows a typical... 

A compressor surges at certain conditions of low flow, and the compressor map, a plot of head versus flow, has a surge line defining the limits. Surge controls help the machine avoid surge by increasing flow. For an air compressor, a simple spill to the atmosphere is sufficient. For a hydrocarbon compressor, recirculation from discharge to suction is used. 

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

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). 

The compressors operating range is between two major regions as seen in Fig. 10-74, which is a performance map of a centrifugal compressor. These two regions are. surge, which is the lower flow limit of... 

Fig ure 5-2. Compressor boost capacity is limited by the surge line. 

In all static surge-detection devices, the actual plienonemon of flow reversal (surge) is not directly monitored. What is monitored are other conditions related to surge. Control limits are set from past experience and a study of compressor characteristics. 

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