surge point

surge point

surge pointclick for a larger image
The value of the airflow and the pressure ratio at which surge occurs. Such a point can be decided for each combination of each value of the engine RPM. The line joining these points and plotted on a graph of the pressure ratio against the airflow is known as the surge line. This line defines the volume of stable airflow that can be obtained at various RPM.
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At surge point, flow separation occurs inside the impellers making it unsteady and changes its direction.
The surge point is dependent on multiple parameters such as the molecular weight and flow into the compressor.
Additionally, timing of the compressor ESD signal, the fuel gas shutoff signal, fuelgas manifold size (in the case of gas-turbine drivers), power-train inertias and compressor aerodynamic characteristics close to surge point, all contribute to the complexity of the problem.
Once a surge occurs, the reversal of flow reduces the discharge pressure or increases the suction pressure, thus allowing forward flow to resume again until the pressure rise again reaches the surge point.
Both systems operate close to the surge point, and the HGBP opens to avoid surge, whenever required.
Opening of the recycle valve in the surge-control system effectively avoids surge by providing more flow and reducing compressor head, to move the compressor away from its surge point.
While the gas flow corresponding to this surge point is fixed for a constant-speed compressor, it changes at each operating speed for a variable-speed compressor.
In any event, good industry practice dictates that compressor gas flow should never drop below 5 percent in excess of the surge point for any operating speed.
Recycle valves perform two basic functions: react quickly during startup, shut-down and, emergency situations; and modulate recycle flow during normal system operation to avoid compressor operation near or below the critical surge point.
A collection of surge points during varying compressor speed or varying inlet gas angle is fitted as surge line.