Surge is the most important process design consideration for centrifugal compressors. The surge condition occurs when the compressor does not have enough flow to produce sufficient head. At this point, the gas in the discharge piping flows back into the compressor momentarily. This lowers the back-pressure of the system, establishing forward flow at a temporarily low head. The cycling from zero, or even backward flow to forward flow, is called “surge” and is very detrimental to the compressor
bearings and seals. Most compressors can only sustain a very few cycles of surge before severe mechanical problems develop.
Surge may be caused by an increase in head requirement or a loss in throughput. Figure 10-14 shows the capacity curves for a typical compressor. The surge line for this particular compressor is shown. Any combination of speed, pressure, and flow rate to the right of the surge line is acceptable. Typically, a surge control line offsetting the theoretical surge limit given by the manufacturer is used to establish set points for a control system adjusting speed and recycle as shown in Figure 10-16.
A stonewall or choked flow condition occurs when sonic velocity is reached at the exit of a compressor wheel. When this point is reached, flow through the compressor cannot be increased even with further increase in suction pressure. If this occurs, the suction pressure will rise, Operation in this region will cause excessive use of horsepower, occasionally to the point of overload, and frequent flaring.
If higher flow rates are desired, modifications to the impeller must be made, as shown in Figure 10-15.