what is compressor stall and surge ....and when does they occur in gas turbine axial compressor. what are the factors to cause compressor stall/surge in compressors?

 

> what is compressor stall and surge....and when does they occur in gas turbine axial compressor. what are the
> factors to cause compressor stall/surge in compressors?

Operating speed of the Gas Turbine is held constant and very little deviation takes place. The blades of the axial compressor are designed to achieve optimum efficiency at the synchronous speed. To achieve a good performance, the angle and aerofoil shape of the rotor and stator blades are precisely machined and set in place such that the compression of air through the compressor is smooth and efficient.

During Startup and shutdown of a gas turbine, the rotor and stator blades do not deliver a smooth progression of air through the compressor. This occurs, simply because the shape and the position of the blades is not conducive to low speed and low flow condition. Speeds less result would be separation of the flow at the airfoils, this is, and the air is not smoothly compressed. Such separation will cause the delivery to become unstable. The compressor will begin to surge. This characterized by rapid fluctuations of the compressor discharge pressure fluctuations with the compressor blades being endangered by the resulting high alternating bending stresses and high temperature Figure illustrates the effects of air flow through a gas turbine compressor during a compressor surge.

Since on startup and shutdown of the gas turbine and under frequency operation, the compressor is bound to run at speeds below the permissible value, air will have to be blown off at specific parts of the compressor as a result of which the volume of the flow is matched to the blades case cross-sections.