Page 293 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 5 .4 Steam Turbine Best Practices

Fig 5.4.2 Impeller with side plate removed
Fig 5.4.4 Impeller with side plate removed

impeller passages that reduces flow area and roughens surface
finish. It reduces impeller head capacity and efficiency.
Note that the surge margin actually increases slightly in the
fouled condition. This is because the cause of surge is low gas
velocity. Since the area of the flow passage is reduced, the gas
velocity increases; thus increasing the surge margin. The surge
margin is defined as the flow at surge divided by the impeller
design flow. However, the stage head produced by the impeller
at any flow rate is reduced. Therefore, for the same process
system head required, the impeller flow rate will be reduced,
thus forcing the operating point closer to the surge line.
While this example considers a centrifugal compressor stage,
fouling has the same effect on a steam turbine stage. Steam
velocity in the blades is increased, efficiency is reduced and less
power is produced.
Fig 5.4.3 Impeller with side plate removed

Fig 5.4.5 Impeller fouling

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