Another major factor affecting the theoretical head-capacity curve is choke or stonewall. The terms surge and stonewall are sometimes incorrectly used interchangeably, probably due to the fact that serious performance deterioration is observed in either case.
A compressor stage is considered to be in stonewall, in theory, when the Mach Number equals one. At this point the impeller passage is choked and no more flow can be passed. Industry practice normally limits the inlet Mach Number to less than 0.90 for any specified operating point.
We are concerned with two important items in defining stonewall: the inlet-gas velocity incidence angle, and the inlet-gas Mach Number.
The vector diagram (Figure 200-22) shows an inlet-gas velocity vector which lines up well with the impeller blade at design flow.
The ratio of the inlet gas velocity (relative to the impeller blade) to the speed of sound at inlet is referred to as the relative inlet Mach Number.
As flow continues to increase, the incidence angle of the relative gas velocity, with respect to the impeller blade, becomes negative as shown in Figure 200-23. The negative incidence angle results in an effective reduction of the flow area and impingement of the gas on the trailing edge of the blade, contributing to flow separation and the onset of choke.
It is important to note the choke effect is much greater for high molecular weight gas, especially at low temperatures and lower k values. For this reason, maximum allowable compressor speed may be limited on high molecular weight applications, with a corresponding reduction in head per stage.