Factors Affecting Single and Multi Stage Turbine Performance

Factors that differentiate performance between single-and multi-stage turbines include the effects of reheating between stages, steam quality, and nozzle changes at partial load or overload.

A reheating effect occurs between stages in a multistage turbine. With each stage, the steam energy in the enthalpy drop that is not converted to power remains in the steam so it can be used by subsequent stages. The portion of the available energy remaining in the fluid is termed the reheat. The reheat in a given stage is available to do work in the succeeding stage, except for the last stage. Therefore, more power is produced because the multi-stage turbine makes more efficient use of the available steam energy.

There is also a reheating effect on the exhaust steam of single-stage turbines. This is due to windage, or resistance losses resulting from the blades and discs passing through exhaust steam and the kinetic energy remaining in the steam leaving the blades.

The steam quality or moisture level is an important consideration for any turbine, affecting both performance and maintenance requirements. Steam quality is a measure of the amount of vapor in the two-phase liquid-vapor mixture and represented as a percentage ranging from 0 to 100%:

mass vapor

(massiiquid + massvapor) (11-19)

In all condensing and most non-condensing turbine applications, as the steam expands through the turbine, steam properties cross the saturation line. Seen on a T-s diagram, the process moves into the "vapor dome," the area under the saturation curve where liquid is present and, therefore, the steam has a lower enthalpy per lbm (hg) than the dry saturated steam. Figure 11-45 shows T-s diagrams using wet, dry saturated, and superheated steam.

If steam quality falls below 90 or 95% in the last low-pressure stage(s) of a steam turbine, liquid droplets can erode the turbine blades. Efficiency is also degraded by moisture because the presence of water drops increases friction losses in the steam itself and, since water drops tend to move more slowly than vapor, they strike the rotor blades at unfavorable velocities and exert a braking effect. Integral moisture separators and other design features can reduce the problem and, in multi-stage turbines, the reheating effect between stages can reduce moisture content. By superheating steam at the boiler, the quality of expanded steam in later turbine stages is improved.

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