Page 288 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Steam Turbine Best Practices
Fig 5.1.4 Mollier steam diagram (Courtesy of Elliott Company) A:S:R:
determined by dividing the theoretical steam rate (TSR) by the estimating purposes.
turbine efficiency. Alternately, if the turbine efficiency is not Refer to Figures 5.1.7 and 5.1.8 for typical efficiency values
known, and the turbine inlet and exhaust conditions are given for multistage and single stage steam turbines as a function of
(P 2 ,T 2 or % moisture), the actual steam rate can be obtained in steam conditions, power and speed.
Be st Practice 5.1
the same manner as theoretical steam rate, but substituting
DH actual for DH isentropic .
Turbine efficiency
As shown in Figure 5.1.5, turbine efficiency can be 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-
ating turbine in the field if the exhaust conditions are super-
heated. All that is required are calibrated pressure and
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
2. For inlet condition with DS ¼ 0, determine h 2ideal
3. For outlet conditions, determine h 2actual
4. Determine Dh ideal ¼ h 1 h 2ideal
5. Determine Dh actual ¼ h 1 h 2actual
6. Determine efficiency
DH actual
Efficiency ¼
DH ideal
However, for turbines with saturated exhaust conditions, the
above procedure cannot be used because the actual exhaust con-
dition cannot be easily determined. This is because the % moisture
must be known. Instruments (calorimeters) are available, but re-
sults are not always accurate. Therefore the suggested procedure
for turbines with saturated exhaust conditions is as follows:
1. Determine the power required by the driven equipment or
record turbine power if a torquemeter is installed. This is
equal to the power produced by the turbine.
2. Measure the following turbine parameters using calibrated
gauges:
- P in - P exhaust
- T in - Steam flow (in lbs/hr)
3. Determine the theoretical steam rate by plotting P in ,T in ,
P exhaust @ DS ¼ 0, and dividing Dh isentropic into the
constant.
4. Determine the actual steam rate of the turbine as follows:
Steam Flow ðlb=hrÞ
Actual Steam Rate (A.S.R.) ¼
BHP required by driven equipment
5. Determine efficiency
T:S:R:
Efficiency ¼
Figure 5.1.6 presents the advice and values concerning steam
turbine efficiencies. The efficiencies presented can be used for
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