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Centrifugal compressors are designed to operate near their rated flow conditions. Operations away from the design point can result in unstable flow phenomena. Figure 1.4 defines some of the operating conditions typically defined by centrifugal compressor manufacturers. Surge and stonewall conditions will be briefly touched on here.

Figure 1.4 Centrifugal compressor operating limits. This compressor curve represents performance at a single speed.

Centrifugal Compressor Surge is defined as the operating point where the centrifugal compressor peak head capability and minimum flow limits are reached. Surge can be a detrimental condition in compression systems because it causes the compressor to vibrate violently due to rapid internal flow reversals, potentially damaging internal compressor parts. Surge control systems are specifically designed to prevent compressors from operating below the surge flow point or line. Figure 1.5 depicts a typical surge control system, which is composed of a flow measurement element, a controller with a surge control algorithm program, and a recycle valve. If, at any time, the inlet flow approaches the surge control line flow programmed into the surge controller, the recycle valve will open to increase the total compressor flow and prevent surge from occurring.

Centrifugal Compressor stonewall or choked is a flow condition that occurs when the sonic velocity of gas is reached at the exit of a compressor wheel. When stonewall flow is reached, the discharge pressure and flow drop precipitously. Flow through the compressor cannot be increased any further without an increase in the suction pressure. At the stonewall (or choke) flow the pressure versus volume curve becomes essentially vertical, and it is not possible to develop head or pressure at any greater flow. When the required operating flow exceeds the stonewall limit, the only remedy is to reconfigure the compressor with impellers (and matched stationary hardware) designed for larger flow rates.

Figure 1.5 Typical centrifugal compressor surge control system.

Note: Axial compressors have been known to fail catastrophically at high flows due to blade vibration and high temperatures generated when operated beyond the stonewall line.