limit line thus determined is used within the surge limit control the compressor is always sufficiently protected from operating in the unstable range of the performance characteristic even if the gas composition is subject to greater variations the process becomes somewhat more complicated when the compressor is operated with variable speed or with variable geometry guide vanes inlet guide vane or throttling fitting and variable gas composition a surge limit line or a surge limit control line is already obtained in the case of compressors of such a design only in the case of constant gas composition as is known the compressor must never be operated beyond i e to the left of the surge limit line to make it possible to ensure this a control line is positioned to the right of the surge limit with a sufficient safety margin such that the surge limiter can always operate the compressor outside the surge limit range even under extreme operating conditions there are many turbocompressors especially multi stage machines in which especially the course of the surge limit line in the performance characteristic depends on the gas composition a surge limit line or a surge limit control line of a different shape may be obtained for each gas composition in the case of variable geometry or variable speed and variable gas composition the surge limit line or the surge limit control line becomes a family of surge limit lines and surge.