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Steam Turbine Calculations
Efficiency
The efficiency of the steam turbine may be specified using
one of several methods:
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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