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Surge points are the peak points on the characteristic curves (as in Figure 5.) left of which the pressure generated by the compressor is less than the pipe pressure and these points initiates the surge cycle. These points on the curves are shown in the fig. by point S. [4]
When compressor surge happens, the operating point of a compressor, which is usually denoted by the pair of the mass flow rate and pressure ratio, orbits along a surge cycle on the compressor performance map. The unstable performance caused by compressor surge is not acceptable to machines on which a compressor is mounted to ventilate or dense air.
Comparison of normal and distorted airflow into the compressor section [1] A compressor stall is a local disruption of the airflow in the compressor of a gas turbine or turbocharger. A stall that results in the complete disruption of the airflow through the compressor is referred to as a compressor surge. The severity of the phenomenon ranges ...
Compressor surge is a particular problem during slam-accelerations and can be overcome by suitable adjustments to the fueling schedule and/or use of blow-off (bleeding air off the compressor, for handling purposes). In the particular example shown, a slam-acceleration from ground idle would cause a high-pressure compressor surge.
Various points on the performance curve depending upon the flow rates and pressure difference. In the plot of pressure-flow rate the line separating graph between two regions- unstable and stable is known as the surge line. This line is formed by joining surge points at different rpms.
If the turbocharger's compressor wheel is spinning at high speed when the throttle is suddenly closed (such as during a gear change), the flow reduces beyond the surge line of the compressor. At this point the change in pressure across the compressor reduces, leading to a collapse in flow and possibly even flow reversal and a collapse in plenum ...
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On civil turbofans, the first stage of the LP compressor is often a single-stage fan. If the HP compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry (i.e. variable stators) on the first few stages, to make the surge line on the compressor map more shallow, to accommodate the shallow working line.