Boiler-Turbine Unit

Boiler-Turbine Unit


a steam power installation consisting of a steam boiler, a turbine, and auxiliary equipment; in normal operation such a unit has no steam or water connections to other installations. Since the turbine of a boiler-turbine unit in a power plant usually drives only one generator, which is not connected to any other generators, the unit is sometimes called a boiler-turbine-generator unit.

In a boiler-turbine unit, steam from the boiler enters the high-pressure cylinder of a condensing turbine and, after passing through it, returns to the boiler, entering through an intermediate superheater. The secondary superheated steam is fed into the medium-pressure cylinder of the turbine and then into the low-pressure cylinder and the condenser. The water is removed from the condenser by a pump. It then passes through the low-pressure and high-pressure feed-water heaters and a deaerator and enters the boiler. Usually, a boiler cannot operate at loads below a certain value for a number of reasons (for example, the conditions of cooling of the tubes of heating surfaces); therefore, some-times more steam is generated than is required for the turbine (for example, during the start-up of a unit). In such cases the excess steam is dumped into the condenser through a reduction device.

A boiler-turbine unit may have one boiler per turbine (single unit) or two boilers.per turbine (double unit). Single units are simpler and economically more advantageous. The advantage of a double unit is that in case of breakdown of one boiler the unit can be operated at half-load.

The technological processes in a boiler-turbine unit differ substantially from the processes in a nonmodular power plant. In a boiler-turbine unit the start-up of the boiler and turbine occurs simultaneously, thus making it possible to conduct the start-up at smoothly increasing steam pressure and temperature, which improves the warmup conditions for the turbine and steam pipes. In boiler-turbine units the load may be regulated by varying the pressure of the live steam (if the boiler design permits). The boilers, turbines, and auxiliary equipment are repaired simultaneously.

In heat and power engineering increased power levels are attained primarily by building large power units with condensing turbines. In the USSR boiler-turbine units now in operation have power ratings of 150 and 200 megawatts (MW) with a steam pressure of 13 meganewtons per sq m (MN/m2), or 130 kilo-grams-force per sq cm (kgf/cm2), and power ratings of 300, 500, and 800 MW with a steam pressure of 24 MN/m2 (240 kgf/cm2); a unit rated at 1,200 MW is planned.

Most boiler-turbine units now being constructed, including the unit rated at 1,200 MW, are single units. Nonmodular installations are mostly installed in central heat and power plants, where intermediate superheating is less frequently used. However, large heating units rated at 250 MW and using intermediate superheating have been introduced at central heat and power plants.