A feature of the steam pipework is a high-pressure system that allows the superheated steam to bypass the high-pressure turbine when necessary, to flow directly into the reheat pipe work. From there, it bypasses the intermediate pressure and low pressure turbines and flows into the main condenser. This system means that the boiler can be operated independently of the turbine and the correct steam temperature and pressure can be obtained before the generator is started up. Also, if a turbine trips, the system allows the boiler to be operated at roughly 40 per cent of its maximum load, ready for a rapid restart; and, if a turbine sheds a large part of its load, the turbine can be run at a low load while the boiler is maintained at a higher load. In both instances, the boiler is maintained at a higher load. In both instances, the bypass system prevents, within the limits of its capacity, sudden pressure rises from exceeding the settings of the safety valves in the re-heater.

Each boiler is fitted with 24 burners arranged in six rows of two in each of the front and rear walls of the furnace. Each coal mill supplies four burners – two in the front wall and tow in the rear wall. Mounted in the centre of each burner is an oil burner, and these are used for starting up and for stabilising the combustion of coal at low loads.

Forced draught fans supply secondary air to the burner windboxes while primary air fans transport pulverised coal from the mills to the burners. Combustion gases are drawn from the furnace, by two induced draught fans, over the surfaces of the steam superheater, the re-heaters, the economiser, and air preheaters, through the electrostatic precipitators and into the chimney. The precipitators collect more than 99 per cent of the dust, or fly ash.

Each boiler has a capacity of 507 kg/s of steam, and the feedwater is supplied, at a pressure of 22 Mpa, by two electric pumps rated at 13 MW each or one steam-driven pump. At maximum continuous rating the efficiency of the boilers is 93,9 per cent. The total mass of one boiler and its supporting structure is 9 800 tons, and its water content, when cold, is 185 tons. The volume of the furnace is approximately 13 000 m².

Fabric Filter Plants (Unit 1,2 and 3)

This plant replaces the precipitators for Units 1,2 and 3 and consists of an ABB fläkt Optipulse pulsejet fabric filter, utilising acrylic dralon T bags. The filter is divided into four isolatable compartments. Each compartment contains a total of 6 724 bags, each eight metres long, giving a total of 26 896 bags per boiler. The plant is designed to handle 1 290 cubic metres of boiler gas per second, which gives a filter velocity of 90,015 metres/sec.

TURBINES

There are six turbines at Duvha power station, manufactured by GEC Turbo-Generators of Rugby in the United Kingdom. Each turbine unit has high-pressure and intermediate pressure cylinders and twin double-flow low-pressure cylinders. The high pressure and intermediate pressure cylinders are constructed with an internal and external casing to allow for fast starting up and rapid variation in load. Foundations and supports for one turbine include 8 000 m³ or 20 000 tons of concrete.

FEED HEATING PLANT

The condensate temperature in the condenser hotwell is about 37°C. In order to obtain maximum system efficiency it is necessary to reheat the condensate before returning it to the boiler. To achieve this, the condensate is passed through numerous heaters before feeding it into the boiler. All the heaters are of the surface type in which the water flows through tube nests. Steam extracted from different stages of the turbine heats the water by conduction through the tube metal.

A de-aerator, situated between the low- and high-pressure heaters, removes entrained oxygen in the condenstate and heats the water through direct contact with steam extracted from the turbines. The condenstate is pumped from the condenser to the deaerator through the low-pressure heaters by means of the main condensate extraction pumps. The condensate is then pumped from the de-aerator feedwater tank to the boiler through the high-pressure heaters by the boiler feedpumps. The final feedwater temperature at 600 MW is 247ºC.

CONDENSERS

The condensers are of the dual-pressure, surface type. The vacuum inside the condensers is established by steamjet air ejectors and maintained in normal operation by waterjet air ejectors. The steam exhausted from the low-pressure cylinder condenses over 22 552 brass tubes, which have a surface area totalling 23 400m². Each boiler-turbine set has a condenser with a heat exchange capacity of approximately 400 MW.

GENERATORS

Each generator is rated at 667 MVA, and produces 600 MW at full load. The sent-out capacity is 575 MW, the difference of 25 MW being used to run the auxiliary equipment of the turbine and boiler plant. Generator cooling is achieved through two systems. The stator and rotor are cooled by hydrogen at a pressure of 400kPa, which is circulated through water coolers of the surface type. Circulation of the hydrogen is by two fans that are locked to the rotor shaft. The circulation of demineralised water-cools the stator windings and core. The generators operate at 22 kV, and their output is transformed to 400 kV for distribution via the national grid.