9   Turbine applications

9.1   Power generation

 

The main concern of this volume is turbines for the generation of electric power via a constant speed generator. In this area of technology, as applied in large central power stations, the steam turbine has firmly established itself as pre-eminent during the 20th century.

The previous sections have mainly described the thermodynamic and mechanical features of turbines used for power generation. The condenser and feed-heating plant have been identified as essential features necessary to obtain a reasonable thermodynamic efficiency. Boiler feed pumps are required to pressurise the system. Cycles employing a variety of inlet steam conditions and options for reheating have been discussed and, depending on the source of steam and the prevailing economics, all these are possible. The mechanical features described in earlier sections have embraced those which are necessary to achieve plant capable of high efficiency and flexible operation.

It is helpful to examine briefly the history of turbine development, as it has led to the principal features of current machines. The earliest turbines had ratings of a few MW and were of the single cylinder type driving a DC generator. The speed of rotation was therefore not dependent on the electrical system and was purely a choice for the designer. Subsequently, fixed speed AC generators were developed, giving major advantages in the economy and ease of distribution of electrical power.

Higher unit ratings have been employed to reduce the capital cost per kW output and to improve the efficiency. Basically, doubling the length of the turbine blading gives four times the cross-sectional area through which the steam flows and the possibility of four times the output for double the cost. This simple example cannot be directly related to the practical case, as many other factors influence the precise ratio of the costs, for example the costs of assembly and manufacture, and the costs of buildings and construction work to house the turbine. The efficiency is also improved because the smaller turbine has a higher frictional loss (per kW output) due to the smaller blade height. Also, the blading clearances will not necessarily double for the larger size turbine, so the leakages (allowing steam to by-pass the blading) will be greatly reduced (per kW output) for the larger machine. Similar economies of scale also apply to condensing and feedheating plant, and to the auxiliary systems.

Apart from the difficulties of making and transporting larger components, the main limitation to this process is the extent to which the blade length can be increased without exceeding centrifugal and bending stress criteria. This is discussed in Section 5 of this chapter. Turbine ratings up to 1200 MW are now commonplace. However, the user must bear in mind the relative size of his power distribution system and the consequences arising if a large unit is tripped. Will the other plant on the system be able to pick up the load without interruption of the electricity supply to consumers or must a proportion of the consumer load be disconnected?

The development of higher turbine ratings and the increased steam inlet temperatures and pressures soon led turbine makers away from the single-cylinder designs to multiple-cylinder designs comprising high pressure (HP), intermediate pressure (IP) and low pressure (LP) cylinders. Because of the limitations in the length of the last-stage blading and the importance of having sufficient exhaust area to allow operation at an economically low exhaust pressure, double-flow cylinders were introduced as shown in Fig 1.128. This example shows a further development where two double-flow LP cylinders, connected on the same shaft, accept the exhaust flow from a double-flow IP turbine supplied by a single-flow HP turbine. This is often described as a 4-flow exhaust machine. Many other combinations are possible, with 6-flow and 8-flow turbines being constructed from three and four LP cylinders respectively.

A 4-flow exhaused turbine cycle with a motor-driven feed pump

The principle of improving turbine efficiency by reheating the steam in the boiler after it has completed an initial expanson was well known even at the beginning of the century. However, the first prototypes of commercial generating plant using reheat did not appear until the 1920s and, although now almost universal, it was only introduced gradually. Since the reheated steam returns to the turbine at a much higher temperature than that at which it was exhausted, there is a further strong incentive to provide a separate turbine cylinder to expand the reheated steam so that high temperature gradients are avoided on adjacent stages.

Manufacturers now have a wealth of experience behind them and some of the more fanciful options of the past have been eliminated. Development to improve efficiency and reliability continues on a broad front. The division of the overall turbine expansion between separate cylinders has enabled them to offer a modular design concept for a whole range of unit sizes for power generation. The modular concept centres around several standard cylinder designs which may be combined in a number of different ways to cover variations both in steam conditions and in unit output. Figure 1.4 illustrates this concept, as applied by both UK turbine manufacturers.

 

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