Page 287 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 5 .1 Steam Turbine Best Practices

Best Practice 5.1Practice 5.1
Best
Accurately define steam conditions to ensure maximum Low steam energy conditions in the field (low pressure and tem-
turbine power output and reliability. perature differentials) will reduce power output.
Ensure that inlet and exhaust pressure and temperature ranges High vacuum conditions (closer to a perfect vacuum) increase the
(maximum, normal and minimum) at the turbine flange are confirmed moisture in the exhaust steam. Moisture content in excess of 12% will
prior to data sheet preparation. result in reduced exhaust end blading life in rotor and diaphragm
Lower than specified pressure and temperature differential conditions blading assemblies.
during field operation will limit produced power and affect plant revenue.
For condensing turbines, be sure that condenser conditions are Benchmarks
maintained properly to prevent power reduction or exhaust end blade This best practice has been used since the mid 1970s, and has
erosion (high vacuum conditions). resulted in optimum steam turbine performance and reliability (in
excess of 99.7%).
Lessons Learned
Failure to properly specify steam conditions has led to low
turbine power output and erosion of the last stages of
blading in condensing turbines.

B.P. 5.1. Supporting Material
Describes the energy per unit mass of fluid when pressure and
temperature are known.
Steam conditions Enthalpy (energy/unit mass) is plotted on Y axis
Entropy (energy/unit mass degree) is plotted on X axis
Steam conditions determine the energy available per pound of Locating P 1 ,T 1 gives a value of enthalpy (H) horizontal and
entropy (S) vertical
steam. Figure 5.1.1 explains where they are measured, and how
Isentropic expansion occurs at constant entropy ( S = 0) and
they determine the energy produced.
represents an ideal (reversible) expansion
Fig 5.1.3 The Mollier Diagram
The steam conditions are the pressure and temperature
conditions at the turbine inlet and exhaust flanges. As an exercise, plot the following values on the Mollier dia-
They define the energy per unit weight of vapor that is gram in this section and determine the corresponding available
converted from potential energy to kinetic energy (work). energy in BTUs per pound.
BTU
1. P 1 ¼ 600 PSIG, T 1 ¼ 800 F h 1 ¼

Fig 5.1.1 Steam conditions LB M
BTU
2. P 2 ¼ 150 PSIG, T 2 ¼ 580 F h 2 ¼

Frequently, proper attention is not paid to maintaining cor- LB M
rect steam conditions at the flanges of a steam turbine. Failure to 3. P 1 ¼ 1500 PSIG, T 1 ¼ 900 F BTU

do this will affect the power produced, and can cause me- h 1 ¼ LB M
chanical damage to turbine internals resulting from blade ero- BTU
sion and/or corrosion. Figure 5.1.2 presents these facts. 4. P 2 ¼ 2 PSIG, % moisture ¼ 9% h 2 ¼
LB M
Having plotted various inlet and exhaust conditions on the
Mollier diagram to become familiar with its use, please refer to
Figure 5.1.5, which presents the definitions and uses of steam rate.
Inlet steam conditions should be as close as possible ( / 5%) to
specified conditions because:
Power output will decrease Theoretical steam rate
Exhaust end steam moisture content will increase, causing The theoretical steam rate is the amount of steam, in kg or lb per
blade, nozzle and diaphragm erosion. hour, required to produce one (1) horsepower, if the isentropic
efficiency of the turbine is 100%. As shown in Figure 5.1.5, it is
determined by dividing the theoretical enthalpy, Dh isentropic , into
Fig 5.1.2 Steam condition limits
the amount of kJ/hr (btu/hr in one (1) unit of power (kW or hp)).
Mollier diagrams or steam tables allow determination of the
energy available in a pound of steam for a specific pressure and Actual steam rate
temperature. Figure 5.1.3 describes the Mollier diagram and the The actual steam rate is the amount of steam, in kg or lb per
parameters involved. hour, required to produce one (1) unit of power based on the
Refer to Figure 5.1.4, an enlarged Mollier diagram. actual turbine efficiency. As shown in Figure 5.1.5,itis

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