Turbine stage

Superheaters and Reheaters A superheater raises the temperature of the steam generated above the saturation level. An important function is to minimize moisture in the last stages of a turbine to avoid blade erosion. With continued increase of evaporation temperatures and pressures, however, a point is reached at which the available superheat temperature is insufficient to prevent excessive moisture from forming in the low-pressure turbine stages. This condition is resolved by removing the vapor for reheat at constant pressure in the... 

Nozzle Area of the First Turbine Stage Expander Stage). This is a very eritieal parameter and limits the total airflow into the turbine seetion, thus this limits the amount of steam injeetion or the amount of the heated and humidified eompressed air injeetion. 

Figure 8-1. Infiuenoe of Reynoids number on turbine stage effieieney.

Figure 8-2. NsDs diagram for a turbine stage. Efficiency is on a total-to-total basis that is, it is related to inlet and exit stagnation conditions. Diagram values are suitable for machine Reynolds number Re > 10 . (Balje, O.E., A Study of Reynolds Number Effects in Turbomachinery, Journal of Engineering for Power, ASME Trans., Vol. 86, Series A, p. 227.)...

Sometimes, conventional techniques do not produce a satisfactory steam balance for all operating modes. Options are available for steam drives for flexibility, such as extraction and induction turbines. Extraction turbines are widely used. In these, an intermediate pressure steam is removed or extracted from an intermediate turbine stage with the extraction flow varying as required over preset limits. Induction turbines are not as widely used as extraction turbines, but are a very satisfactory application... 

Refer to Figure 7-12 for a diagram showing the various turbine stage losses. Efficiency, T)s, is defined as the actual enthalpy change divided by the isentropic enthalpy change or... 

Figure 7-12. Various turbine stage tosses. Courtesy of Elliott Company)...

It is also important to note that the lowest design pressure of any section of the casing must be specified to be no lower than the pressure which it may be subjected to under the PR valve relieving conditions. This is necessary to recognize pressure drop within the casing. The PR valve should be sized to pass the normal steam flow to the turbine, but credit may be taken for steam flow which is withdrawn from an intermediate turbine stage if it would not be blocked by the same contingency as closure of the exhaust. 

The argument developed in Section 4.2.1.2 can be extended for three or more steps of cooling, to give the same efficiency as the uncooled cycle. Indeed the efficiency will be the same for multi-step cooling, with infinitesimal amounts of air abstracted at an infinite number of points along the compressor to cool each infinitesimal turbine stage at the required pressures. 

Figure 4-193 is the side view of a single-turbine stage and describes the geometry of the rotor and stator. 

Utility plants primarily employ reheat condensing turbines, whereas cogeneration plants and larger process industries that produce their own electrical power tend to employ extraction turbines. Both types of turbine rely on a surface condenser to receive exhaust steam from the LP turbine stage and condense it to liquid for reuse. 

Figure 1. An abbreviated representation of a steam power plant cycle. The numbers 1,2,3,4,5,6, and 7 correspond respectively to the boiler, first turbine stage, second turbine stage, condenser, first feedwater pump, feedwater heater, and second feedwater pump. This first and second turbine stages may both lie inside a single-turbine (viewed as a single piece of equipment) with steam being tapped from a point near the middle of the turbine.

---