Page 363 - Compression Machinery for Oil and Gas
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Drivers Chapter  7 347



                 High pressure steam
                            Load: driven equipment

                    Turbine  Load



                            Surface
              To boiler
                            condenser
                    P
             FIG. 7.32 Example of steam turbine exhausting to subatmospheric pressure.


             typically indicates a high inlet pressure and temperature. (See picture for VHP
             inlet/extraction/back pressure/double casing example.) Other designs have mul-
             tiple casings known as tandem casing arrangements or compound designs where
             HP, MP, and LP turbine stages are contained in separate casings. In these
             designs, the rotors may or may not be coupled together and rotating at same
             speed (Fig. 7.33).
                Blade design—Steam turbine blades and nozzles are primarily impulse or
             reaction designs (Fig. 7.34). With impulse designs, the pressure drop across
             each stage is taken over the fixed row. This corresponds to zero reaction of rotat-
             ing row and with this the rotor thrust is minimal. These designs are usually a
             disk/wheel and diaphragm construction. Fig. 7.34 shows the range of operation
             for each classification of turbine stage. This chart shows wide usage of impulse
             blades. This type of blading is generally less efficient however the design less
             susceptible to the effects of leakage flow around the stage. With Reaction
             designs steam expands in both the fixed and rotating blades. The pressure drop
             across each stage is split between fixed and rotating rows. This split results in
             higher reaction imparted on the rotating rows and therefore larger thrust loads
             which are counter balanced using balance pistons.
                Single or multistage—This indicates the number of stages of expansion the
             steam takes from high to LP. Single-stage turbines are mostly used for general

















             FIG. 7.33 Example of casing arrangement.
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