Tutorial on Steam Turbine Power Plants

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A steam turbine based power plant consists of raising high pressure steam in a boiler from the thermal energy and expanding the steam in a turbine to generate shaft power which in turn is converted into electricity in the generator.

Axial flow steam turbines consist of circularly distributed stationary blades called nozzles which direct steam on to rotating blades or buckets mounted radially on a rotating wheel. Typically, the blades are short in proportion to the radius of the wheel, and the nozzles are approximately rectangular in cross section. Several stages of expansions are obtained by using a series of nozzles and buckets, with the exhaust from the buckets of one stage flowing directly into the nozzles of the following stage. A compact machine can be built economically with ten or more stages for optimum use of high pressure steam and vacuum exhaust by mounting the wheels of a number of stages on a single shaft, and supporting the nozzles of all stages from a continuous housing. Large axial turbines must be operated under such conditions that the exhaust steam does not contain more than 10 to 13% of liquid since condensate droplets could seriously erode the high velocity nozzles and blades. The moisture content of the exhaust is dependent upon the inlet steam pressure/temperature combination. Special moisture removal stages may be incorporated in the design when the steam superheat temperature is limited.

Steam may be utilized directly in the steam turbine without any superheat as may be done with low pressure steam, or superheated to increase the cycle efficiency. Reheat may also be included to further increase the ef ficiency of converting heat to power by superheating the steam after partial expansion and admitting the steam thus reheated back into the turbine


Turbine types

When people began to use water power to win mechanical work, they looked first for the best forms of impellers. Three types were established thereby and variations of them are used today in various applications, among other in steam turbines in power stations, as marine propellers, as compressors in gas turbines etc. These three types are introduced here:

The pelton turbine

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The pelton turbine (also free-jet turbine) was invented 1880 by L.A. Pelton. It possesses spoon-shaped shovels, the jet hits the impeller tangentially, gets divided by the two shovels and transfers an impulse. The pelton turbine is used in storage power stations with downward gradients up to 2000 meters and can contain up to 6 nozzles.

The francis turbine

he reaction turbine invented by J.B. Francis 1849 is hit by the jet almost axially (toward the axle) and radially (away from the center). The rotor blades can be adjusted, in order to ensure an even run. It looks similar to the type shown below as Steam turbine.

The Kaplan turbine

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The Kaplan turbine, developed around 1915 by the Austrian V.Kaplan, looks like a marine propeller. The jet is led thereby axially on the freely adjustable shovel pages.

Mode of operation of the steam turbine

Since it is a steam jet and no more a water jet who meets the turbine now, the laws of thermodynamics are to be observed now. The modern steam turbine is an action turbine (no reaction turbine), i.e. the steam jet meets from a being certain nozzle the freely turning impeller. There's a high pressure in front of the turbine, while behind it a low pressure is maintained, so there's a pressure gradient: Steam shoots through the turbine to the rear end. It delivers kinetic energy to the impeller and cools down thereby: The pressure sinks.

Steam is produced in a steam boiler, which is heated in power stations by the burn of coal or gas or by atomic energy. Steam doesn't escape then, but after the passage through the turbine it is condensed in a condensor and then pushed back into the steam boiler again by a pump. This has the advantage that for example in nuclear power stations work- and cooling water are clearly separated.

Multi-level steam turbines

In modern steam turbines not only one impeller is propelled, but several being in a series. Between them idlers are situated, which don't turn. The gas changes its direction passing an idler, in order to perform optimally work again in the next impeller. Turbines with several impellers are called multi-level. The principle was developed 1883 by Parsons. As you know, with the cooling gas expands. Therefore it is to be paid attention when building steam turbines to a further problem: With the number of passed impellers also the volume increases, which leads to a larger diameter of the impellers. Because of that, multi-level turbines are always conical.

Coupling of several turbines

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In power stations today, different types of turbines are used in a series, e.g. one high pressure -, two medium- and four low pressure turbines. This coupling leads to an excellent efficiency (over 40%), which is even better than the efficiency of large diesel engines. This characteristic and the relatively favorable production make the steam turbine competitionless in power stations. Coupled with a generator and fired by an atomic reactor, they produce enormously much electric current. The strongest steam turbines achieve today performances of more than 1000 megawatts