Compressors surging

Compressor surging is prevented by opening the bypass of the expander and the blow-off valves of the low- and high-pressure compressors... 

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... 

Flashback usually happens during unexpected engine transients, e.g., compressor surge. The resultant change of air velocity would almost certainly result in flashback. Unfortunately, as soon as the flame-front approaches... 

Compressor. surging. Espeeially in double-flow maehines. 

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. 

It is not a bad idea for the process engineer to familiarize himself with compressor surge controls. The interaction of the compressor surge controls with downstream process control valves can become a problem area later, and this study phase is not too early to put such items on a checklist. An LNG plant example comes to mind where such an operating problem existed. 

Boyce, Meherwan P., et al. Practical Aspects of Centrifugal Compressor Surge, and Surge Control, Proceedings of the 12th Turbomachinery S ni-posium, Purdue University, West Lafayette, IN, 1983, pp. 147-173. 

Figure 12-61D. Centrifugal compressor surge control schematic diagram shows instrumentation required when primary flow-measuring device is located in centrifugal compressor discharge line. Symbols T = temperature P = pressure A = differential across compressor outlet to inlet. See Reference 89 for a detailed discussion. (Used by permission White, M. H. Chemical Engineering, p. 54, Dec. 25,1972. McGraw-Hill, Inc. All rights reserved.)...

Friedli, F, H. Lenz, and S. Ross, Controlling Compressor Surge, Instruments and Control Systems, p. 36, Feb. (1973). 

A) Compressor surge is prevented by using a low flow controller that opens the valve in the spill-back line from compressor discharge to compressor suction. 

Compressor surging. This problem occurs especially in doubleflow machines. 

Surge is quite similar to aerodynamic stall. Of course, when a compressor surges, its rotor does not stop spinning. The rotor is spun by the motor. But when the flow of gas through the rotor falls below a certain rate, the forward velocity of the gas stops. With no flow, there is no velocity to convert to feet of head. Then, the AP developed by the compressor falls to zero. 

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