Figure 6-.6. Typical compressor performance map.
performance map for a centrifugal compressor with efficiency islands and constant aerodynamic speed lines. The total pressure ratio can be seen to change with flow and speed. Compressors are usually operated at a working line separated by some safety margin from the surge line.
Surge is often symptomized by excessive vibration and an audible sound;however， there have been cases in which surge problems that were not audible have caused failures. Extensive investigations have been conducted on surge. Poor quantitative universality of aerodynamic loading capacities of differentdiffusers and impellers， and an inexact knowledge of boundary-layer behavior make the exact prediction of flow in turbomachines at the design stagedifficult.However， it is quite evident that the underlying cause of surge is aerodynamic stall. The stall may occur in either the impeller or the diffuser.
When the impeller seems to be the cause ofsurge， the inducer sectionis where the flow separation begins. A decrease in the mass flowrate， anincrease in the rotational speed of the impeller， or both can cause the com-pressor to surge.
Surge can be initiated in the diffuser by flow separation occurring at the diffuser entrance. A diffuser usually consists of a vaneless space with the prediffuser section before the throat containing the initial portion of the vanes in a vaned diffuser. The vaneless space accepts the velocity generated by the centrifugal impeller and diffuses the flow so that it enters the vaneddiffuser passage at a lower velocity， avoiding any shock losses and resultantseparation of the flow. When the vaneless diffuserstalls， the flow will notenter the throat. A separationoccurs， causing the flow to finally reverse and surge the compressor. Stalling of the vaneless diffuser can be accomplished in two ways.by increasing impeller speed or decreasing the flow rate.
Whether surge is caused by a decrease in flow velocity or an increase inrotationalspeeds， either the inducer or vaneless diffuser can stall. Whichstalls first is difficult to determine， but considerable testing has shown thatfor a low-pressure-ratio compressor， the surge initiates in the diffuser sec-tion. For units with single-stage pressure ratios above3:1， surge is probably initiated in the inducer.
Most centrifugal compressors have for the most part impellers with back-ward leaning impeller blades. Figure 6-37 depicts the effects of impeller blade angle on the stable range and shows the variance in steepness of the slope of the head-flow curve.
The three curves are based on the same speed and show actual head. The relationship of ideal or theoretical head to inlet flow for different bladeangles would be represented by straight lines. For backward leaning blades， the slope of the line would be negative. The line for radial blades would be horizontal. Forward leaning blades would have a positively sloped line.For the average petrochemical process plant application， the compressor industry commonly uses a backward-leaning blade with an angle (.2)of between about 55.750 (or backward leaning angle of 15.350)， because it provides a wider stable range and a steeper slope in the operating range. This impeller design has proven to be about the best compromise betweenpressuredelivered， efficiency， and stability. Forward leaning blades are notcommonly used in compressordesign， since the high exit velocities lead to large diffuser losses. A plant air compressor operating at steady conditionsfrom day to day would not require a wide stable range， but a machine in a processing plant can be the victim of many variables and upsets. So morestability is highly desirable. Actually， the lower curve in Figure 6-37 appears to have a more gentle slope than either the middle or upper curve. This
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