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The systems impacted by the introduction of T-turbine are stated within the green frame of Figure 3.
A U.S. patent for a similar cycle but with a different T-turbine is available. [6] The inventor was George J. Silvestri, from Westinghouse Electric Corp.
The T-turbine could work alone delivering all of its power to a generator, but it is ideal to let the T-turbine partly replace the conventional turbine drive for a 100 percent feed pump for the following reasons:
- Balancing the power generated by the T-turbine and taken up by the feed pump is done by a small variable speed motor, named the balancing motor, feeding excess power into the main feed pump via gear and frequency converters. The motor would work as a generator at part load.
- Enthalpy drop of the T-turbine is relatively large, meaning it should be a high-speed turbine to improve blading efficiency. It seems that a speed range of 5,000-5,500 rpm is ideal to both T-turbine and feed pump.
- The T-turbine has many similarities with a feed pump turbine (FPT), but steam expansion through
the T-turbine ends in an LP heater, thus avoiding the complicated condenser and cooling water systems of the conventional FPTs.Advertisement - The T-turbine is operating with the inlet valves wide open and a control stage is not needed.
The steam path of the T-turbine is unconventional, since the first stages experience a relatively high volume flow compared to a conventional feed pump turbine. However, as steam is extracted for the regenerative heaters, the blades get shorter and the final stage group only experiences the steam flow for one heater. This also means that a major problem in relation to T-turbine design is not limiting the length of the last-stage rotating blades, as for a conventional FPT, but getting sufficiently long and effective turbine blades.
Thermodynamics
Additional reheat means more efforts to optimize main steam and reheat steam pressures than for the single reheat cycles. Simply saying that the main steam parameters should be identical for single and double reheat cycles would block the road to the optimum use of the additional investment cost of more advanced double reheat cycles. Therefore, specifying same thermal flexibility for SR and MC plant optimum main steam pressure would be around 4,650 psia, and additional concerns on oxidation of it would limit main and reheat steam temperatures to 1,090°F -1,110°F.
The introduction of the HP turbine steam bleed in the early 1990s decoupled optimization of reheat pressure and final feedwater temperature. In the same way, the T-turbine decouples optimization of reheat pressure and pressures of the steam bleeds for the regenerative heaters.
In other words, the T-turbine can tune the cycle into its optimum configuration.
This section presents a method of how to select the optimum reheat pressures and final feedwater temperature of the MC. All optimization efforts are constrained by economic and operational demands, such as fuel cost, boiler fuel flexibility, unit operational flexibility and boiler tube material for furnace walls and superheaters.
Reheat pressures
Calculation of the thermodynamic optimum combination of reheat pressures is a classic engineering task. The result is presented in Figure 4, showing the change of the half net heat rate on the vertical axis vs. the two hot reheat pressures along the two horizontal axes.
The diagram is called the egg shell diagram, and it shows a thermodynamic optimum combination of the two hot reheat pressures in the range of 1,190 psi for reheat 1 and 405 psi for reheat 2.
