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Stationary guide vanes direct the flow to the eye of the impeller in an orderly fashion. Depending upon the head requirements of an individualstage， these vanes may direct the flow in the same direction as the rotation or tip speed of the wheel， an action known as positive pre-swirl. This isusually done to reduce the relative Mach number entering theinducer， inorder to prevent shock losses.This， however， reduces the head delivered but improves the operating margin. The opposite action is known as counter-rotation or negative pre-swirl. This increases the head delivered but alsoincreases the inlet relative mach number. Negative pre-swirl is rarelyused， since it also decreases the operating range. Sometimes the guide vanes are set at zero degrees of swirl; these vanes are called radial guide vanes. Movableinlet guide vanes are occasionally employed on single-stage machines， or on the first stage of multi-stage compressors driven by electric motors at con-stant speed. The guide vane angle can be manually or automatically adjusted while the unit is on stream to accommodate off-design operating require-ments. Because of the mechanical complexity of the adjusting mechanism andphysical dimensional limitations， the variable feature can only be applied tothe first wheel in almost all machine designs. Hence， the effect of changing vane angle is diluted in the stages downstream of the first. Although the flowto the entire machine is successfully adjusted by moving the first stage vanes， the remaining stages must pump the adjusted flow at a fixed guide vane angle.
Incidentally， a butterfly throttle valve in the suction line to the machine will produce nearly the same effects as moving the first stage guide vanes.However， throttling is not as efficient as moving the guidevanes， so that inmanycases， the added cost of the movable vane mechanism can be justified by power savings.
Effects of Gas Composition
Figure 6-40 shows the performance of an individual stage at a given speed for three levels of gas molecular weight.
The heavy gas class includes gases such aspropane，propylene， andstandardized refrigerant mixtures.Air， naturalgases， and nitrogen are typ-ical of the medium class. Hydrogen-rich gases found in hydrocarbon proces-sing plants are representative of the light class.
The following observations can be made with respect to the curve for heavy gas:
1.  The flow at surge is higher.

2.  The stage produces slightly more head than that corresponding to medium gas.

3.  The right-hand side of the curve turns downward (approaches stone-wall) more rapidly.

4.  The curve is flatter in the operating stage.

 

 

 

INLET FLOW Q
Figure 6-4.. Effect of gas composition.
It is the last point (4) that often presents a problem to the designer of the antisurge control system. It should be noted that the flatness gets worse asstages are added in series. Since the RTS issmall， there is a large change inflow corresponding to a small change in Head. The controlsystem， there-fore， must be more responsive. It should be obvious that curves for lighter gases have a more desirable shape.
ExternalCauses and Effects of Surge
The following are some of the usual causes of surge that are not related to machine design:
1. Restriction in suction or discharge of a system.

2.Process
changes inpressure， temperature， or gas composition.

3. Internal plugging of flow passages of compressor (fouling).

4. Inadvertent loss of speed.

5. Instrument or control valve malfunction.

6. Malfunction of hardware such as variable inlet guide vanes.

7. Operator error.

8.  Maldistribution of load in parallel operation of two or more com-pressors.

9. 
Improper assembly of a compressor， such as a mispositioned rotor.

The effects of surge can range from a simple lack of performance to serious damage to the machine or to the connected system. Internal damage tolabyrinths， diagrams， the thrust bearing， and the rotor can be experienced. There has been a reported case of a bent rotor caused by violent surge. Surge often excites lateral shaft vibration and could produce torsional damage tosuch items as couplings and gears. Externally， devastating piping vibrationcan occur， causing structuraldamage， shaft misalignment， and failure of fittings and instruments.
　
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