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Turbine for Unit 4

The Siemens UB KWU is a multistage, axial, extraction condensing reactive turbine with steam reheating. It comprises a high, intermediate and low pressure part and a system of auxiliary devices.

The high-pressure part comprises the high-pressure casing, rotor, emergency shutdown valves, regulating valves and check valves on the outlet of the high-pressure part. An emergency shutdown valve is fitted on both sides of the turbine and particle filters are installed before the valves.

The emergency shutdown valves are opened hydraulically and closed by spring tension. The turbine has four regulating valves, positioned in a common casing together with the trip valves. An electro-hydraulic regulator and controlling oil regulators control the aperture of the regulating valves. To achieve a proper operation of the turbine the regulation valves need to open in a determined order. The check valves are fitted on the exhaust side of the turbine with the function of preventing influx of steam from the reheater into the exhaust chamber. They are opened hydraulically via oil driven regulators.

The high-pressure casing is composed of an external and an internal part. The external part of the casing is a single-piece design, while the internal part is a two-part horizontal design. The internal structure comprises the primary regulating guide vane that extends from the external casing to the rotor, and has four segmental groups of leading channels installed. The 18 stages of guide vanes are attached to the internal high-pressure casing. For the shaft seals both sides have external labyrinth seals installed while internal seals are present between the guide vanes. The rotor of the high-pressure part has a regulating stage and reaction blades that comprise the following stages; the first is a regulation impulse stage, also known as a “Curtis wheel”, 18 reactive stages whose guide vanes are attached to the rotor shaft. Labyrinth sealing strips seal the gaps between the stages.

The intermediate pressure casing comprises an external and internal part, the latter being a two-part horizontal design. The internal casing has 18 asymmetrically attached sets of guide vanes. For the shaft seals both sides have external labyrinth seals installed while internal seals are present between the guide vanes. This casing has steam conduits attached for the collection of steam from the seals, steam that used to heat the feedwater and the feedwater reservoir. They are installed on the appropriate locations after individual turbine stages. The rotor of the intermediate part turbine is a twin design and consists of 18 reactive blade sets, inserted asymmetrically into the shaft and 16 reactive blade sets that are inserted into the rotor shaft symmetrically. Labyrinth sealing strips provide the sealing. Two regulating valves and two emergency shutdown valves are installed to control the flow of the steam.

The turbine also has an internal and external low-pressure casing, of a two-part horizontal design. Only the low-pressure turbine has two parts and hence two external casings, of which one contains an internal structure with 21 sets of guide vanes and a second contains an internal two-part structure with 11 two-part guide vane sets. The turbine is sealed with external labyrinth seal straps. Steam outlet conduits are built into the structure at the individual stages and the steam collected here is used to heat the main condenser. The turbine has a twin low-pressure rotor with 22 reactive rotor sets, (two times 11 sets) which are mounted on the shaft of the low-pressure rotor.

The low-pressure turbine also has four emergency shutdown and regulating valves with hydraulic control and safeties. The low-pressure steam conduits convey the excess steam to the condenser in case the reheated steam pressure breaches the set value. The shafts connections of the turbine rotors and the generator transmit the forces directly. All the bearings on the turbines and the generator are, because of their special purpose, separable sliding bearings*.

The turbine unit has an oil supply system which is used for bearing lubrication, lifting of the shafts, spinning the rotors of the turbine during the shutdown procedure, controlling the regulating and emergency valves, and the operation of the excess steam conduit and the seal system. It comprises of an oil reservoir, two auxiliary oil pumps, an emergency oil pump, oil filters, oil pumps dedicated to shaft lifting and rotor spinning during shutdown and the main pump, these are driven by the rotor shaft of the high-pressure part of the turbine. The lubricating oil system is cooled by auxiliary systems from the main generating unit’s cooling system.

To minimise steam loss from the high and medium pressure parts of the turbine, and entry of air into the condenser through the gaps on the shafts of the low-pressure part, the turbine operates a seal system with a pressure of approximately 0.01 bars. To maintain an optimum condensation of the steam, gasses are sucked out of the condenser with special vacuum pumps. The main coolant for the condensation of steam is decarbonised cooling water.

For the safe and efficient operation of a steam turbine, they are equipped with a large number of measurement devices which convert the physical values into usable electrical values. Along with the monitoring of the bearing temperatures, the deformation of the turbine casings, bearing vibration, shaft and casing movement and protection from wet steam, the turbine has the most important casing temperature sensor. It monitors temperatures in the high-pressure casing and medium-pressure shaft.

The calculated values on the turbine allow for loading the turbine at start-up and the variation of the generating unit power output during normal operation. Along with the electric safeties of the turbine, which set off the emergency shutdown valves, the turbine also has primary, mechanical safety mechanisms that set off the emergency valves as well.

The steam turbine of generating unit three is similar to the turbines in units one and two; the only difference is that it has a power output of 75MW.

The auxiliary turbine devices are designed according to its power. The high-pressure rotor has thirteen impulse blade stages and a high-pressure casing with thirteen lead vane sets installed.

The low-pressure twin rotor has two sets of five impulse blade stages on the shaft of the rotor.

The low-pressure turbine casing is a twin horizontal design. It is assembled from three parts, the middle section end the two exit sections, which contain the two sets of five two-part stators with guide vanes installed.

All auxiliary devices are the same as on the turbines in generating units one and two and so is the control and monitoring systems.