Editing Centrifugal compressor (section)

=== Surge ===

Surge - is a low flow phenomenon where the impeller cannot add enough energy to overcome the system resistance or backpressure.<ref>
{{cite book
 |author=Pampreen, Ronald C.
 |title=Compressor Surge and Stall
 |edition=
 |publisher=Concepts ETI
 |year=1993
 |isbn=978-0-933283-05-3
}}</ref> At low flow rate operation, the pressure ratio over the impeller is high, as is back system backpressure. Under critical conditions, the flow will reverse back over the tips of the rotor blades towards the impeller eye (inlet).<ref>{{cite journal |last1=Semlitsch |first1=Bernhard |last2=Mihăescu |first2=Mihai |title=Flow phenomena leading to surge in a centrifugal compressor |journal=Energy |date=May 2016 |volume=103 |pages=572–587 |doi=10.1016/j.energy.2016.03.032|url=https://www.repository.cam.ac.uk/handle/1810/301108 }}</ref> This stalling flow reversal may go unnoticed as the fraction of mass flow or energy is too low. When large enough, rapid flow reversal occurs (i.e., surge). The reversed flow exiting the impeller inlet exhibits a strong rotational component, which affects lower radius flow angles (closer to the impeller hub) at the leading edge of the blades. The deterioration of the flow angles causes the impeller to be inefficient. A full flow reversal can occur. (Therefore, surge is sometimes referred to as axisymmetric stall.) When reversed flow reduces to a low enough level, the impeller recovers and regains stability for a short moment at which point the stage may surge again. These cyclic events cause large vibrations, increase temperature and change rapidly the axial thrust. These occurrences can damage the rotor seals, rotor bearings, the compressor driver, and cycle operation. Most turbomachines are designed to easily withstand occasional surging. However, if the machine is forced to surge repeatedly for a long period of time, or if it is poorly designed, repeated surges can result in a catastrophic failure. Of particular interest, is that while turbomachines may be very durable, their physical system can be far less robust.

====Surge line====
[[File:StallFormation.svg|thumb|upright=1.3|Figure-6.2.1 Stall formation]]
The surge-line shown in Figure 5.2 is the curve that passes through the lowest flow points of each of the four speed-lines. As a test map, these points would be the lowest flow points possible to record a stable reading within the test facility/rig. In many industrial applications, it may be necessary to increase the stall line due to the system backpressure. For example, at 100% RPM stalling flow might increase from approximately 0.170&nbsp;kg/s to 0.215&nbsp;kg/s because of the positive slope of the pressure ratio curve.

As stated earlier, the reason for this is that the high-speed line in Figure 5.2 exhibits a stalling characteristic or positive slope within that range of flows. When placed in a different system those lower flows might not be achievable because of interaction with that system. System resistance or adverse pressure is proven mathematically to be the critical contributor to compressor surge.