Page 347 - Steam Turbines Design, Applications, and Rerating

Elliott Shortcut Selection Method for Steam Turbines 321

The extraction turbine can substantially reduce the energy charged
to the driven machine if process steam would otherwise be supplied
through pressure-reducing valves. Even though back-pressure tur-
bines can be used to supply process steam, they are rather inflexible
since the shaft power and process steam requirements must be closely
matched. An extraction turbine, however, can cope with changes in
these variables and satisfy the requirements of each over a broad
range.
Figures 15.9 and 15.10 show performance maps for a typical extrac-
tion turbine. Determining the shape of this diagram is a problem that
often arises. Here is an example that demonstrates the procedure to
follow in drawing an approximate extraction diagram, Fig. 15.9:
Assume: Shaft hp and speed: 25,000 hp
at 4500 r/min
Steam conditions: 600 psig/750°F/4
Hga
Extraction requirements:
150,000 lb/h at 250 psig
First tabulate the TSR.
TSR (inlet-to-extraction),
600 psig/750°F/250 psig = 35.4 lb/kWh

TSR (inlet-to-exhaust),

600 psig/750°F/4-in HGA = 7.64 lb/kWh)

Now assume that the efficiency of the entire turbine (inlet-to-exhaust) is 75 per-
cent and the efficiency of the inlet-to-extraction section is 70 percent. Therefore:

Approximate SR (inlet-to-extraction) will be:

(35.4 lb/kWh × 0.746 kW/hp) ÷ 0.70 = 37.8 lb/hph
Approximate SR (inlet-to-exhaust) will be:

(7.64 lb/kWh × 0.746 kW/hp) ÷ 0.75 = 7.60 lb/hph
Point A is the first point to be located on the diagram by multiplying:

Total hp × approximate SR (inlet-to-exhaust) = 25,000 hp × 7.60 lb/kWh
= 190,000 lb/h

Locate Point A at 25,000 hp and 190,000 lb/h throttle flow.
Point B is located at zero hp and a throttle flow of 5% of A. This 5% flow is cool-
ing steam going to the extraction section.