Factors Affecting Performance

Most of the change in steam enthalpy between inlet and exhaust emerges as shaft work. Examination of the Mollier Diagram shows that at high pressure, incremental temperature increases enthalpy proportionally more than incremental pressure. Note that lines of constant pressure are steeply inclined and closely bunched whereas lines of constant temperature have shallow inclination. However, at low steam pressure, it is the pressure lines that have shallow inclination.

Therefore in order to provide the maximum differential between inlet and exhaust enthalpy, inlet temperature should be maximized and exhaust pressure minimized. The steam line from boiler to turbine should well insulated, but even then there is likely to be temperature loss depending on the length of line. It is better to adjust the boiler de -superheater set point so that steam arrives at the turbine at the turbine's maximum safe temperature.

Many steam systems are designed at nominal pressure levels, for example, 40 bar, 10bar and 3 bar. These pressures may not necessarily be the best for the plant concerned and so slow adjustments while monitoring fuel usage can often result in a net reduction.

Condensing turbines should be avoided if possible since even the best reject up to 60% of the steam energy to the turbine's condenser. For an existing condensing turbine, the most important way to reduce steam consumption is to achieve the best possible exhaust vacuum. The only limit is the amount of moisture in the turbine exhaust which, if too great, risks LP blade erosion. Obviously, higher inlet temperature not only improves performance but also reduces this risk.

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