Page 330 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 5 .13 Steam Turbine Best Practices
Theoretical steam rate
The steam conditions are the pressure and temperature The theoretical steam rate is the amount of steam, in LBS per
conditions at the turbine inlet and exhaust flanges hour required to produce one (1) horsepower if the isentropic
They define the energy per unit weight of vapor that is
converted from potential energy to kinetic energy (work) efficiency of the turbine is 100%. As shown in Figure 5.13.5,itis
determined by dividing the theoretical enthalpy Dh isentropic into
the amount of BTUs/HR in horsepower.
Fig 5.13.1 Steam conditions
Actual steam rate
to do this will affect power produced, and can cause mechanical The actual steam rate is the amount of steam, in LBS per hour,
damage to turbine internals resulting from blade erosion and/or required to produce one (1) horsepower based on the actual
corrosion. Figure 5.13.2 presents these facts. turbine efficiency. As shown in Figure 5.13.5, it is determined
by dividing the theoretical steam rate (TSR) by the turbine ef-
ficiency. Alternately, if the turbine efficiency is not known and
the turbine inlet and exhaust conditions are given (P 2 ,T 2 or %
Inlet steam conditions should be as close as possible (+ / 5%) to moisture), the actual steam rate can be obtained in the same
specified conditions because: manner as theoretical steam rate but substituting DH actual for
Power output will decrease DH isentropic .
Exhaust end steam moisture content will increase, causing
blade, nozzle and diaphragm erosion
Turbine efficiency
As shown in Figure 5.13.5, turbine efficiency can be
Fig 5.13.2 Steam condition limits determined either by the ratio of TSR to ASR or Dh actual to
DH isentropic .
It is relatively easy to determine the efficiency of any oper-
A Mollier diagram or steam tables allow determination of the
energy available in a pound of steam for a specific pressure and ating turbine in the field if the exhaust conditions are super-
temperature. Figure 5.13.3 describes the Mollier diagram and heated. All that is required are calibrated pressure and
the parameters involved. temperature gauges on the inlet and discharge, and a Mollier
diagram or steam tables. The procedure is as follows:

1. For inlet conditions, determine h 1
Describes the energy per unit mass of fluid when pressure and 2. For inlet condition with DS ¼ 0, determine h 2ideal
temperature are known. 3. For outlet conditions, determine h 2actual
Enthalpy (energy/unit mass) is plotted on Y axis 4. Determine Dh ideal ¼ h 1 h 2ideal
Entropy (energy/unit mass degree) is plotted on X axis
5. Determine Dh actual ¼ h 1 h 2actual
Locating P 1 ,T 1 gives a value of enthalpy (H) horizontal and
entropy (S) vertical 6. Determine efficiency
Isentropic expansion occurs at constant entropy ( S = 0) and
represents an ideal (reversible) expansion DH actual
Efficiency ¼
DH ideal
Fig 5.13.3 The Mollier Diagram However, for turbines with saturated exhaust conditions, the
above procedure cannot be used because the actual exhaust
condition cannot be easily determined. This is because the %
Refer to Figure 5.13.4 e an enlarged Mollier Diagram. As an
exercise, plot the following values on the Mollier Diagram in this moisture must be known. Instruments (calorimeters) are avail-
section and determine the corresponding available energy in able, but results are not always accurate. Therefore the
BTUs per pound. suggested procedure for turbines with saturated exhaust con-
ditions is as follows:
BTU
1. P 1 ¼ 600 PSIG, T 1 ¼ 800 F h 1 ¼ 1. Determine the power required by the driven equipment.

LB M
This is equal to the power produced by the turbine.
BTU
2. P 2 ¼ 150 PSIG, T 2 ¼ 580 F h 2 ¼ 2. Measure the following turbine parameters using calibrated

LB M
gauges:
BTU

3. P 1 ¼ 1500 PSIG, T 1 ¼ 900 F h 1 ¼ - P in - P exhaust
LB M - Steam flow (in lbs/hr)
- T in
BTU
4. P 2 ¼ 2 PSIG, % moisture ¼ 9% h 2 ¼ 3. Determine the theoretical steam rate by plotting P in ,T in ,
LB M
P exhaust @ DS ¼ 0, and dividing Dh isentropic into the constant.
Having plotted various inlet and exhaust conditions on the
Mollier diagram to become familiar with its use, please refer to 4. Determine the actual steam rate of the turbine as follows:
Figure 5.13.5, which presents the definitions and uses of steam Steam Flow ðlb=hrÞ
Actual Steam Rate (A.S.R.) ¼ BHP required by driven equipment
rate.
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