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Steam Turbine Calculations 



The efficiency of the steam turbine may be specified using one of several methods:

  • GE Published Data

Efficiency may be taken from the published General Electric Co. data for fossil turbines. This method is an excellent choice for preliminary work since it takes into account the effect of size, percent load, and steam conditions on turbine efficiency, using a minimum of data. For design point calculations, the user may request FCYCLE/CCYCLE/NCYCLE to size the turbine at full load for the heat balance being calculated.    The user may enter "fudge faactors" to change teh efficiency from that given in these papers, either to accounts for advances in design since the papers were written in the 1970's, or to account for a turbine's degradation in service.

Nuclear Tturbines  (NCYCLE)

Baily, F.G., Booth, J.A., Cotton, K.C., and Miller, E.H., "Predicting the Performance of Large Steam Turbine generators Operating with Light Water Cooled Reactors", Paper GE6020, Schenectady, New York, General Electric Company, 1973.

Baily, F.G., Cotton, K.C., and Spencer, R.C., "Predicting the Performance of Large Steam Turbine Generators Operating with Satuated adn Low Superheat Steam Conditions"  Paper GER2454A, Schenectady, New York, General Electric Company, 1967.

Fossil Type Turbines (FCYCLE and CCYCLE)

Spencer, R.C., Cotton, K.C., and Cannon, C.N., "Predicting the Performance of Steam Turbine Generators,...... 16,500 kw and Larger", Paper GER2007C, Schenectady, New York, General Electric Company, 1974.

Richardson, P.W. "A Method for Predicting the Performance of Marine Turbine-Gear Sets", Paper LTP-106, Lynn, Massachusetts, General Electric Co., 1971.
  • Turbine Vendor Data ("Thermal Kit" data)
Efficiency may be taken from a set of turbine vendor performance data.  This data should comprise vendor heat balances (a minimum of four is ideal. one is essential).  FCYCLE/CCYCLE/NCYCLE request curves of pressure and enthalpy for various points in the turbine as functions of throttle flow.  This method will result in a near perfect match to any vendor's data.  When calculating heat balances at points away from the design conditon, FCYCLE, CCYCLE, and NCYCLE will adjust the efficiency to accurately account for changes in steam velocity, governor valve operation, and changes in steam conditions and moisture.

  • Efficiency as input Constants and Curves
Efficiency may be input as either a curve or constant for each group of stages. This method is excellent for special applications for which the GE method or the "thermal kit" method are imperfect, such as for "automatic extraction/admission" turbines.    

  • Efficiency from Measured Data
    • Efficiency may be determined from measured data. For HP and IP turbines, the efficiency is determined by enthalpy drop using input inlet and exhaust temperatures. For the LP turbine, efficiency is determined by calculating a complete heat balance around the turbine based on measured data that includes both the generator output and a primary flow measurement (such as feedwater flow, condensate flow, etc.)                                      

Throttle, Inlet, and Extraction Pressures

    • Throttle pressure may be either a fixed constant, or calculated from the main steam flow abd a user specified valve opening,  This second method  iallows "sliding pressure" or "hybrid sliding pressure" operation to be easily and accurately simualted.  (For PWR plants with saturated type steam generators, NCYCLE bases the throttle pressure steam generator outlet pressure, minus the main steam pressure drop)
    • Extraction pressures and IP turbine inlet pressures may either be fized constant (for design point calcs or when test data is available) or they may be determined for any operaying conditon by FCYCLE CCYCLE or NCYCLE from user entered data for a single known condition (normally data from a vendor heat balance) using the simplified version of St Laurent's formula: W= Const * Sqrt(P/v)

Exhaust Losses


Regardless of which method is used to determine efficiency of the main stages of the steam turbine, the user may select from several methods for calculating last stage exhaust loss.Exhaust losses can be approximated if the physical dimensions (blade length, tip diameter, and RPM) are known.                                                                              
  • Exhaust loss curves may be input in the GE, Westinghouse, or European format. 
  • Exhaust loss curves for the standard GE last stage buckets are stored in the program and may be used.
  • Exhaust losses may be specified as a constant. 
  • Exhasut losses will be estimated from last stage blade dimensions (root diameter, tip diameter, and RPM)

Extraction Enthalpy

Extraction conditions are normally the same as steam on the expansion line. However, the extraction enthalpy can be offset from the expansion line, or if the extraction temperature is known, (for example, when a heat balance based on measured data is calculated) the known extraction temperature can be used by the program to determine extraction enthalpy.


Cross Compund Turbines

FCYCLE and CCYCLE allow the user to specify a cross compound turbine of either the 3600/3600 RPM type, or 3600/1800 RPM type. (3000 and 1500 RPM for 50 Cycle countries)

Leakages and Gland Steam System


The complete steam turbine gland steam system and leakage flows may be modeled. The user inputs the flow as either a curve, a constant or a percent of throttle flow, or defines the gland packing constant. A steam seal regulator for supplying steam to the LP turbine glands may also be modeled. 

Moisture Separator Reheaters 

NCYCLE will model a non-reheat, single reheat, or double reheat moisture separator reheater. The user defines reheater TD, moisture separator effectiveness (or outlet moisture), and pressure drops. NCYCLE allows for moisture separator preseparation and venting flows, and scavenging steam and venting flows from the reheater. The user specifies the flow and enthalpy of these flows.


Variable Pressure Calculations

In addition to specifying the unit load, the user may specify either throttle pressure, or valve position. If the latter is specified, the program will determine the steam pressure necessary for the turbine to pass the required steam flow.

Auxiliary Turbines

The user may specify a turbine driver for the feedwater pump, a fan (FCYCLE only), a booster pump, and/or a turbine driving a separate generator. The user specifies the source of steam, the destination of exhaust steam, inlet and outlet pressure drops (as either percentages, constants, or scaled from a known point) and the efficiency. If the auxiliary turbine uses main steam to supplement normal steam at low loads, the user enters data to allow the program to determine the flow changeover point.  The auxilary turbines may have up to two extractions for feedwater heating or other purposes.

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