9.2 Mechanical drive
Figure 1.128 shows a typical cycle diagram of a unit for power generation. The turbine exhaust flow is ducted to the condenser where the condensation temperature fixes the exhaust pressure of the turbine. The con-densate is routed back to the boiler by LP feedheating plant followed by one or more feed pumps which pump the condensate back to the boiler, often through HP feedheating plant.
The boiler outlet pressure is determined by the feed pump, and the pre-heating of the feedwater by the feedheating plant improves the thermal efficiency by using low-grade heat from bled-steam tappings on the main turbine rather than high grade heat obtained by burning more fuel.
One of the options for driving the feed pump is to use a small auxiliary turbine, supplied by steam from the main turbine and exhausting either to the condenser or back into the main turbine, as shown in Fig 1.129. The advantages of these forms of feed pump drive, compared with the electric drive shown in Fig 1.128, are fully discussed in Section 4.3 of this chapter. In practice, there are numerous examples of turbine-driven feed pumps, usually in the power range of 10-20 MW. The characteristic curves of the pumping system were shown on Fig 1.71. The system resistance is made up of a static component set by the required boiler outlet pressure, modified by the relative suction level and water level within the boiler. There is also a frictional component gen¬erated by the pressure drops through the boiler and feed water system following the pump. This frictional component of the pressure drop is proportional to the square of the flow. The operating point for the system is determined by superimposing the pump characteristics of the system resistance curve. From the pump efficiency curve and pump power versus speed curve, the power required by the pump is deter¬mined. The maximum power available from the drive turbine over the flow range (proportional to main turbine load) follows a linear characteristic if its throttle valves are kept wide open (see Section 3 of this chapter). The proximity of the pump power re¬quired and the turbine power available curve shows good matching of their characteristics over most of the flow range, with very little throttling of the turbine required. Furthermore, control of the feed flow is accomplished by varying the speed of the drive system so that the throttling losses of feed regulating valves in a constant speed drive system are avoided. At low feed flows, however, the variable-speed turbine drive is incapable of providing sufficient output. Electrically-driven feed pumps may be used to provide start-up and operation at low loads. An alternative system, often used in the UK, is to provide a live steam stage to the feed pump turbine so that it can be used over the full load range. Live steam is normally reduced to a warming flow only at high load on the main turbine. The maximum pumping power requirement for a fossil-fired supercritical plant (with a once-through boiler) will be approximately 3.5% of the main unit output. For a fossil-fired superheat unit or an AGR nuclear application, this will reduce to 2.5%, while the low pressure operation of PWR tur¬bines will only require 1.5%. Typical speed ranges are 5000-7000 r/min for fossil-fired plant and 3500-5000 r/min for nuclear, wet steam plant.
Back-pressure boiler feed pump turbines are usually arranged to exhaust into the LP inlets of the main turbine. This poses operational problems when the main turbine has not been started or if the main turbine is tripped. Under these conditions, the feed¬pump turbine exhaust flow must be routed to the condenser. This disadvantage is partly offset by their facility for supplying bled-steam to feedheaters which would otherwise need to be fed from tappings on the main turbine IP cylinder. It is generally simpler to arrange this with shorter pipe runs from the feed pump turbine (usually a single-cylinder, single-flow unit) rather than from a double-flow IP turbine. However, as discussed in Section 4 of this chapter, the option of tapping points on feed pump turbines is not being pursued for future plant.
Condensing-type boiler feed pump turbines are also of single-cylinder single-flow design and, because they exhaust into the condenser, their inlet steam conditions are much lower than the back pressure type, often being supplied from the IP exhaust. They are favoured in applications where the exhaust area of the main machine may be a limiting factor as they provide a ready means of increasing the total effective exhaust area.