Question one:

Compressor Operation Characteristics
 
A compressor operates over a large range of flow and speed, delivering a stable head/pressure ratio. During start up, the compressor must be designed to operate in a stable condition at low rotational speeds. There is an unstable limit of operation known as 'surging', and it is shown on the performance map as the surge line. The surge point in a compressor occurs when the compressor back pressure is high and the compressor can not pump against this high head, causing the flow to separate and reverse its direction. Surge is a reversal of flow and is a complete breakdown of the continuous steady flow through the whole compressor. It results in mechanical damage to the compressor due to the large fluctuations of flow, which results in changes in direction of the thrust forces on the rotor, creating damage to the blades and the thrust bearings. The phenomenon of surging should not be confused with the stalling of a compressor stage. Stalling is the breakaway of the flow from the suction side of the blade aerofoil, causing an aerodynamic stall. A multi-stage compressor may operate stably in the unsurged region with one or more of the stages stalled, and the rest of the stages unstalled.
 
Compressor Surge
 
Compressor surge is a phenomenon of considerable interest, yet it is not fully understood. It is a form of unstable operation and should be avoided. It is a phenomenon that, unfortunately, occurs frequently and sometimes with damaging results. Surge has been traditionally defined as the lower limit of stable operation in a compressor, and it involves the reversal of flow. This reversal of flow occurs because of some kind of aerodynamic instability within the system. Usually, a part of the compressor is the cause of the aerodynamic instability, although it is possible for the system arrangement to be capable of augmenting this instability. Compressors are usually operated at a working line, separated by some safety margin from the surge line. Extensive investigations have been conducted on surge. Poor quantitative universality or aerodynamic loading capacities of different blades and stators, and an inexact knowledge of boundary-layer behavior make the exact prediction of flow in the compressor at the off-design stage difficult.
 
A decrease in the mass flow rate, an increase in the rotational speed of the impeller or both can cause the compressor to surge. Whether surge is caused by a decrease in flow velocity or an increase in rotational speeds, the blades or the stators can stall. One should note that operating at higher efficiency implies operation closer to surge. It should be noted here that total pressure increases occur only in the rotational part of the compressor, the blades.
 
The surge line slope on multistage compressors can range from a simple single parabolic relationship to a complex curve containing several break-points or even “notches.” The complexity of the surge line shape depends on whether or not the flow limiting stage changes with operating speed from one compression stage to another; in particular, very closely matched stage combinations frequently exhibit complex surge lines. In the case of compressors with variable inlet guide vanes, the surge line tends to bend more at higher flows than with units which are speed controlled.
 
Usually surge is linked with excessive vibration and an audible sound, yet, there have been cases where surge not accompanied by audible sound has caused failures. Usually, operation in surge and, often, near surge is accompanied by several indications, including general and pulsating noise level increases, axial shaft position changes, discharge temperature excursions, compressor differential pressure fluctuations and lateral vibration amplitude increases. Frequently, with high pressure compressors, operation in the incipient surge range is accompanied by the emergence of a low frequency, asynchronous vibration signal which can reach predominant amplitudes, as well as excitation of various harmonics of blade passing frequencies. Extended operation in surge causes thrust and journal bearing failures. Failures of blades and stators are also experienced due to axial movement of the shaft causing contact of blades and stators. Due to the large flow instabilities experienced severe aerodynamic stimulation at one of blade natural response frequencies is caused leading to blade failure.

For more detailed information, see chapter seven in my handbook, "Gas Turbine Engineering Handbook"
 
Question two:

Higher sulfur content in the oil are called "sour" as opposed to low-sulfur "sweet" oil
 
Question three:

It is the speed of the shaft which coincides with the natural frequency of the system. A shaft that operates above its critical speed is known as a flexible shaft, while a shaft operating below its critical speed is known as a rigid shaft. The operating speed of the shaft must be at least 15% away from its critical speed.
 
I hope this helps you.