4.2.2 Reheat in Steam Turbine
The steam turbine forms part of the steam-water circuit of a power plant. Thus, the turbine rating is mainly influenced by the steam conditions. As an example, a steam-water circuit is described by means of heat balance diagram as shown in Figure (4.12).
The live steam, coming from the boiler superheater, with a pressure of 120 bar and temperature of 560 oC expands in the HP turbine to an HP exhaust pressure of approx. 40 bar at 250 oC and is then led to the boiler re-heater, where it is heated-up again to approx. 540 oC. Re-heating of steam improves substantially the generation of water drops is shifted to lower pressures which results in minimizing the phenomenon of erosion in the turbine.
The hot re-heat steam is led from the re-heater to the IP turbine which can be of single or double flow design, depending on the turbine size. After being expanded in the IP turbine, the steam is led through cross over pipes to the LP turbine. For units larger than 200 MW, the LP turbines are of double flow design. Cold cooling water and, consequently, low exhaust steam pressures result in large volume flows and, therefore, two or three double Flow LP turbines are arranged at the shaft train.
After being expanded in the LP turbine, the steam is condensed in the condenser. The condensate is then pumped through the LP feed water heaters into the deaerator/feed water storage tank. Boiler feeds pumps pump the feed water through the LP feed water heaters back to the boiler. LP and HP feed water heaters improve the thermal efficiency of the steam cycle.
4.2.3 Lube Oil System for Steam Turbines
During normal operation, the lube oil system is supplied by a gear type main oil pump, which is driven via a gear train from the turbine rotor. It is located in the front bearing pedestal. This self-priming pump has normally a maximal suction head of 5.5 m and takes the oil from the oil tank on an intermediate floor. Before being pumped to the bearings, the oil passes through the oil coolers. Valve on the oil and water-side allow the coolers to be changed over without interrupting the oil flow by regulating the oil flow through the coolers, the required bearing inlet oil temperature is maintained. Downstream of the coolers, two 100% capacity oil filters fitted in parallel are installed. Constant pressure valve controls the oil pressure upstream of the bearings. During start-up, shut down and turning gear operation, an auxiliary centrifugal pump Driven by an A/C. motor supplies oil. This pump will automatically be started when the oil Pressure drops below 60% of its design value or the turbine speed is below 90% of rated Speed. If the normal oil supply fails, an emergency centrifugal oil pump driven by a D/C motor.
Even if the auxiliary and emergency oil pumps should fail simultaneously, the danger of bearing damage due to lack of oil is extremely low, since the main oil pump which works in accordance with the positive displacement principle will continue to supply oil virtually until the rotor comes to rest. The oil vapor exhauster maintains a vacuum in the oil tank, the oil drain pipes and bearing pedestals. This not only effectively remotes the oil vapor from the tank, but also prevents oil from leaking past the bearing pedestal oil baffles.
4.2.4 Gland Steam System for Steam Turbines
The task of the gland steam can be summarized as follows it has to prevent: that air is sucked into those turbine parts, which are under vacuum
that steam from the turbine glands is blown into the enclosure
that gland steam temperature is kept within allowable limits
4.2.5 Condenser of Steam Turbine
The primary purpose of the condenser is to condensate the exhaust steam from low pressure turbine and thus recovers the high quality feed water for reuse in the cycle. If the circulating cooling water temperature is low enough it creates a low back pressure (vacuum) for the turbine to exhaust, this pressure is equal to the saturation pressure that corresponds to condensing steam temperature, which in turn is a function of cooling water temperature. There are primary two types of condensers: direct contact and surface contact which used in Cairo north power station as shown in Figure (4.13).
4.2.5.1 Large Surface Condensers
Large surface condensers are shell-and-tube heat exchangers of modular design, in which the primary heat transfer mechanisms are the condensing of saturated steam on the outside of the tubes and the forced convection heating of the circulating water inside the tubes. A Number of tube bundles, each resembling a church window in shape, are built into mostly a single housing. The bundle was configured on the basic of comprehensive tests with analogue models.
The steam turbine forms part of the steam-water circuit of a power plant. Thus, the turbine rating is mainly influenced by the steam conditions. As an example, a steam-water circuit is described by means of heat balance diagram as shown in Figure (4.12).
The live steam, coming from the boiler superheater, with a pressure of 120 bar and temperature of 560 oC expands in the HP turbine to an HP exhaust pressure of approx. 40 bar at 250 oC and is then led to the boiler re-heater, where it is heated-up again to approx. 540 oC. Re-heating of steam improves substantially the generation of water drops is shifted to lower pressures which results in minimizing the phenomenon of erosion in the turbine.
The hot re-heat steam is led from the re-heater to the IP turbine which can be of single or double flow design, depending on the turbine size. After being expanded in the IP turbine, the steam is led through cross over pipes to the LP turbine. For units larger than 200 MW, the LP turbines are of double flow design. Cold cooling water and, consequently, low exhaust steam pressures result in large volume flows and, therefore, two or three double Flow LP turbines are arranged at the shaft train.
After being expanded in the LP turbine, the steam is condensed in the condenser. The condensate is then pumped through the LP feed water heaters into the deaerator/feed water storage tank. Boiler feeds pumps pump the feed water through the LP feed water heaters back to the boiler. LP and HP feed water heaters improve the thermal efficiency of the steam cycle.
4.2.3 Lube Oil System for Steam Turbines
During normal operation, the lube oil system is supplied by a gear type main oil pump, which is driven via a gear train from the turbine rotor. It is located in the front bearing pedestal. This self-priming pump has normally a maximal suction head of 5.5 m and takes the oil from the oil tank on an intermediate floor. Before being pumped to the bearings, the oil passes through the oil coolers. Valve on the oil and water-side allow the coolers to be changed over without interrupting the oil flow by regulating the oil flow through the coolers, the required bearing inlet oil temperature is maintained. Downstream of the coolers, two 100% capacity oil filters fitted in parallel are installed. Constant pressure valve controls the oil pressure upstream of the bearings. During start-up, shut down and turning gear operation, an auxiliary centrifugal pump Driven by an A/C. motor supplies oil. This pump will automatically be started when the oil Pressure drops below 60% of its design value or the turbine speed is below 90% of rated Speed. If the normal oil supply fails, an emergency centrifugal oil pump driven by a D/C motor.
Even if the auxiliary and emergency oil pumps should fail simultaneously, the danger of bearing damage due to lack of oil is extremely low, since the main oil pump which works in accordance with the positive displacement principle will continue to supply oil virtually until the rotor comes to rest. The oil vapor exhauster maintains a vacuum in the oil tank, the oil drain pipes and bearing pedestals. This not only effectively remotes the oil vapor from the tank, but also prevents oil from leaking past the bearing pedestal oil baffles.
4.2.4 Gland Steam System for Steam Turbines
The task of the gland steam can be summarized as follows it has to prevent: that air is sucked into those turbine parts, which are under vacuum
that steam from the turbine glands is blown into the enclosure
that gland steam temperature is kept within allowable limits
4.2.5 Condenser of Steam Turbine
The primary purpose of the condenser is to condensate the exhaust steam from low pressure turbine and thus recovers the high quality feed water for reuse in the cycle. If the circulating cooling water temperature is low enough it creates a low back pressure (vacuum) for the turbine to exhaust, this pressure is equal to the saturation pressure that corresponds to condensing steam temperature, which in turn is a function of cooling water temperature. There are primary two types of condensers: direct contact and surface contact which used in Cairo north power station as shown in Figure (4.13).
4.2.5.1 Large Surface Condensers
Large surface condensers are shell-and-tube heat exchangers of modular design, in which the primary heat transfer mechanisms are the condensing of saturated steam on the outside of the tubes and the forced convection heating of the circulating water inside the tubes. A Number of tube bundles, each resembling a church window in shape, are built into mostly a single housing. The bundle was configured on the basic of comprehensive tests with analogue models.
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