The surge margin (SM) is defined as the distance between the surge line and the operating point on a vertical line for a constant corrected mass flow value. Several definitions are in use to define the SM. From the compressor map it is clear that the distance to the surge line can be defined using multiple definitions. The most commonly used definition is defined by SAE, where the SM is defined as the distance from the operating point to the surge line at constant flow:
This definition for SM is used by GSP.
An alternative definition, that is more oriented at the physical surge process is defined at constant speed (thus following the constant speed line to obtain the PR surge), is not used by GSP:
Note that there are definitions around where the pressure ratio is corrected by subtracting 1 from the PR to account for pressure ratios being larger than 1 (see reference ) e.g.:
The pressure ratio used in the SM calculation is the PR wherein scaling effects for deterioration and variable geometry have been taken into account. This means that the SM for operating points that use these effects is not very accurate.
The required SM varies between different applications and is highly depending on engine configuration, accel and decel times, inlet distortion, compressor type (LP, HP, centrifugal, axial), etc. Typical values for the SM are in the order of 10 - 20% for fans,10 - 25% for LP compressors, and 15 to 30% for HP compressors, depending on the type of application. For a fan the inlet distortion alone contributes with 5% to the total fan SM.
Surge margin is scaled in scaled maps corresponding to the scaling of pressure ratio, i.e. proportional to PR-1.