Centrifugal Compressor Performance and Surge Phenomenon The performance characteristic of a centrifugal compressor is graphically presented in the form of a family of curves that collectively are known as a performance map or operating envelope. The “approximate surge limit”, depicted at the left side of the map, defines the minimum flow necessary to avoid a potentially damaging surge condition. At the extreme right portion of the map is the “stonewall” (choke) limit. Each of the family of curves from the surge limit to the stonewall represents the flow vs. pressure characteristic curve at a given compressor speed. The elliptical curves (dashed lines) denote compressor efficiency. Centrifugal compressor performance map Surge The surge limit defines the flow at which, for a given speed, the operation of the compressor becomes unstable. At flow rates below the surge limit the characteristic curve actually droops toward zero flow after having reached its maximum point at the surge limit. Because operation below the surge limit is unstable, this portion of the curve is not shown in above figure “Centrifugal compressor performance map”. When the flow is reduced below the surge limit, the pressure at the discharge of the compressor exceeds the pressure-making capability of the compressor, causing a momentary reversal of flow. When this flow reversal occurs, the pressure of the discharge system is reduced, allowing the compressor to resume delivering flow until the discharge pressure again increases, and the surge cycle repeats. Surging usually creates a clearly audible noise. Prolonged operation in this unstable mode can cause serious mechanical damage to the compressor. When operating in a surge condition, the compressor discharge temperature 1 increases significantly and the compressor experiences erratic and severe vibration levels that can cause mechanical damage (particularly to the internal seals). Surge Prevention A compressor can be brought out of surge in a number of ways. The most obvious is to increase flow by the installation of an Anti-surge (recycle) Control Valve (ASCV) and its associated control devices. The antisurge valve is located in a recycle line connecting the compressor discharge to the inlet. For multisection compressors, it is good practice to install a separate recycle line with an antisurge valve for each of the compressor sections. Instrumentation is required to measure the flow to each section, and a surge controller must initiate the opening of the recycle valve when reduced capacity approaches the surge limit. The capacity at which the antisurge valve begins to open is usually set to be about 10% larger than the actual surge limit. The gas recycled through the antisurge valve also must be cooled because its source is the compressor discharge. If uncooled, the suction temperature will increase by mixing the hotter recycled gas with the main process inlet gas. Decreasing discharge pressure and/or increasing speed are other ways to move out of a surge condition. Compressor manufacturers usually perform an aerodynamic performance test before delivering the compressor. Determination of the compressor’s actual surge limit is a very important aspect of the manufacturer’s shop testing program. Stonewall (choke) The stonewall limit of the performance curve defines the flow at which the gas velocity at one of the impellers approaches the velocity of sound for the gas at the conditions within the compressor where this sonic condition is first encountered. At the stonewall (or choke) flow the pressure vs. 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. 2
