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Be st Practice 5 .13 Steam Turbine Best Practices

Uses:
Determine the amount of steam required per hour
Determine the amount of potential kW (horsepower)
Required:
Steam conditions
Theoretical steam rate table or Mollier diagram
Thermal efficiency of turbine
Formula:
Metric Units U.S. Units
Theoretical steam rate
3600 kJ/kW-hr 2545 BTUS/HP-hr
TSR (kg/kWhr) = TSR (lb/HP-hr) =
ΔH ISENTROPIC ΔH ISENTROPIC
Actual steam rate
T.S.R. 3600 kJ/kW-hr T.S.R 2545 BTU/HP-hr
A.S.R. (kg/kWhr) = = A.S.R. (lb/HP-hr) = =
Efficiency ΔH ACTUAL Efficiency ΔH ACTUAL HP/hr
Turbine efficiency
T.S.R. ΔH ACTUAL
Efficiency = =
A.S.R ΔH ISENTROPIC
Fig 5.13.5 Determining steam rate

5. Determine efficiency Performance curves
T:S:R:
Efficiency ¼
A:S:R: The performance curve format for steam turbines is to plot
steam flow on the y axis and produced shaft horsepower on the
Figures 5.13.6 to 5.13.8 present the advice and values
concerning steam turbine efficiencies. The efficiencies x axis. Figure 5.13.10 presents important facts concerning steam
presented can be used for estimating purposes. turbine performance curves.
In Figure 5.13.11, a typical performance curve is presented
for a single stage turbine with manual hand valves.
Note that this turbine contains three manual hand valves (x,
Quoted turbine efficiencies are external efficiencies; they y, and z). Closing hand valves for low horsepower loads increases
include mechanical (bearing, etc.) and leakage losses the efficiency of the turbine. However, please note that closed
Turbine efficiency at off load conditions will usually be lower hand valves limit the steam flow through a turbine, and
than rated efficiency
Typical efficiencies are presented for impulse turbine: therefore the horsepower produced. Hand valves are not
• Condensing multi-stage modulating e that is, they are either fully open or fully closed.
• Non condensing multi-stage Throttling a hand valve will destroy the valve seat and may
• Non condensing single state damage the valve stem, thus rendering it immovable. Normally
hand valves are manually actuated, however, modern electronic
governor systems provide outputs to open or close hand valves
Fig 5.13.6 Typical steam turbine efficiencies
based on power requirements.
Figure 5.13.12 shows a performance curve for a typical
Why steam turbines are not extraction steam turbine. This performance curve plots inlet
performance tested flow and extraction flow vs. turbine horsepower produced.
When selecting an extraction turbine, care must be taken to be
When purchasing large steam turbines that do not use proven sure the turbine produces the horsepower required during the
components, keep in mind that it is not cost effective to per- start-up of the process. The cost of an extraction steam tur-
formance test the turbine prior to field installation. If the tur- bine can be significantly reduced if the size of the exhaust
bine does not meet predicted output horsepower values, the section (LP steam section) is reduced. Figure 5.13.12 shows an
field modifications will be lengthy and costly in terms of lost extraction turbine capable of producing 100% power with 0%
product revenue resulting from reduced output horsepower. In extraction flow. Usually, extraction turbines are sized to only
some cases, the output power predicted may never be attained. provide the process start-up horsepower with 0%
Figure 5.13.9 presents the reasons why steam turbines are not extraction. These values may be as low as 50-60% of full load
performance tested. horsepower.

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