Steam turbine power plant is a generating station which converts heat energy of coal combustion into electrical energy. Shikalbaha 60MW power plant is steam turbine power station. In this type of generating system, steam is produced in the boiler by utilizing the heat of coal combustion and this steam is then expanded in the steam turbine. Finally, the steam turbine drives the generators and the generator converts the mechanical energy of the turbine into electrical energy.
In Shikalbaha 60MW power plant, there are different types of pump have used such as circulating water pump, feed water pump and lube oil pump, demy water pump etc .
The circulating water pump supply large amount of river water in condenser is to minimize the steam temperature. It is mainly used in cooling system because it circulates water within a closed circuit. The circulating water extracts heat from demi water and drop into the river. There are two pump houses are used in Shikalbaha 60MW power plant. One is open system and another is closed system. Open system uses when water is available in Karnafully River. Closed system uses when the water is not available i.e. during dry season. There are t wo 4MW pumps used; one is used for closed system and another is used for open system.
It is a boiler feed water pump that is used to supply demi-water in the boiler tank from water treatment plant to produce continuous steam in the boiler at Shikalbaha 60MW power plant . The water freshly supplied from treatment plant or returning condensate produced as a result of the condensation of the steam produced by the boiler. Feed water pumps are normally high pressure units (»11 bars) that take suction from a condensate return system.
In Shikalbaha 60MW power plant, lube oil pump is used. The using purpose of this pump is pumping lube oil for lubrication of turbine and generator bearing. Lube oil supplies to the bearing for cooling also. The pump supplies more lube oil in the system with the falling of the pressure. The lube oil pump needs to be able to operate over a wide range of temperature and liquid viscous conditions.
Water treatment plant
The raw water is collected from the river. This water cannot be used to produce steam. The raw water carries some small portion of mineral which can break or harm the turbine blades. Some process is applied to demineralize the raw water which is fed to the boiler. All the process has finished in the water treatment plant. Figure-3.2 shows a block diagram that describes the whole water treatment system.
Demineralized water flows into the condenser through the demi water pump. This pump controls the water flowing in the condenser.
Boiler is a device that is used to produce steam by applying heat energy from the water. There are two types of boilers. One is fire tube boiler and another is water tube boiler. Water tube boiler is used in this power plant. The purpose of the boiler is to produce necessary and quality steam continuously. It is used to run the turbine.
Different parts of the boiler
Furnace is the chamber inside the boiler. In this chamber natural gas and air is burned for producing flue gas by keeping gas and air ratio of 1:10. In Shikalbaha 60MW power plant, the temperature inside the furnace chamber is almost 1800°-2000° C. The treated water enters into the furnace through tubes and the flue gas passes around the tubes. Flue gas releases heat and water convert into saturated steam. Temperature of this saturated steam is about 350° C and pressure is about 158kgf/cm2. The forced draught fan forcefully supplies air to the furnace and the saturated steam from the furnace goes to the boiler drum.
Burner is a fire making component in steam power plant. Burner needs ignition gas to initiate the operation of burner. The ignition gas comes to the burner through ignition pipes. In Shikalbaha steam power station, each furnace has 9 burners and each burner produces 7 MW of power.
The water is reserved in the boiler drum. This water comes to the boiler drum from economizer. Water pressure in the boiler drum maintains a level by level transmitter. This system is controlled by an automatic safety valve. From the boiler drum the saturated steam is supplied to the super heater.
The feed water is fed to the economizer before supplying to the boiler. Economizer is essentially a feed water heater and drives heat from the flue gases. It extracts some heat of flue gases which increases the feed water temperature. It helps to increase the boiler efficiency, reduce fuel costs and also reduce stresses in the boiler. It is basically consist of a large number of closely spaced parallel steel tubes which is connected by headers of drums. The temperature of feed water rises because a part of the heat of flue gases is transferred to feed water.
After vaporization steam boilers can produce low quality steam that requires additional superheating of the steam. So the steam is superheated by the heat of flue gases in super heater. In Shikalbaha steam turbine power station, the superheated temperature of the steam is 540°C and the pressure 130 kgf/cm2
A condenser condenses the exhaust steam of the turbine. There are two major functions of the condenser. First, it helps to convert the heat energy of steam into mechanical energy in turbine. Second, the condensed steam of the condenser can be used as feed water into the boiler. It consists of a bank of horizontal tubes enclosed in a cast iron shell.
ID fan & FD fan
Induced Draught Fan (ID fan) is located between the radiant air heater and chimney. ID fan delivered the hot flue gases to chimney from the radiant air heater. Forced Draught Fan (FD fan) is used basically for providing the requirement quantity of hot air to the furnace for combustion of fuel. ID fan creates negative pressure in the furnace to remove the flue gases from furnace and push the hot flue gas to the chimney. The temperature of the negative forced flue gas is 128°C which is removed by the chimney. Furthermore, FD fan produced positive pressure inside the furnace. The temperature of the hot air is 50°C and pressure is 510 kgf/cm2
Steam turbine is a mechanical device that converts thermal energy into mechanical energy. In Shikalbaha 60MW power plant steam turbine is used. The thermal energy extracted from pressurized steam. Steam turbine is attached to a generator that acts as a prime mover. The turbine basically consists of rotary blades and stator blades wheels. Stator blades are fixed and rotors blades are attached to the rotor of the steam turbine. The rotor gets rotated by exerting a force on the blades with the steam. Figure-3.4 shows the outside of the steam turbine.
High pressure turbine
Firstly, the high pressure steam from the super heater enters to the high pressure turbine. Blades of the high pressure turbine are smaller than other blades of the turbine. In this part of the turbine, the incoming steam has very high energy having low volume. The blades are fixed to a shaft and as the steam hits the blades, it causes the shaft to rotate. In the high pressure turbine, the temperature is 5300C and the pressure is 28 kg-cm-2
Intermediate pressure turbine
The steam from the re-heater enters to the intermediate pressure turbine. The turbine blades of the intermediate pressure turbine are bigger than that of high pressure turbine. This is because the steam has expanded and lost energy. After hitting the blades, steam goes straight to the next section of turbine set. In the intermediate pressure turbine the temperature is 5300C and the pressure is 25 kg-cm-2
Low pressure turbine
The steam from intermediate pressure turbine enters into the low pressure turbine. The steam gets expanded in this section. The blades of the low pressure turbine are larger than the other two sections and the energy of steam is lesser than other two sections of the turbine. In the low pressure turbine, the temperature is about 5300C and the pressure is about 1.34 kg-cm-2
Generator is an electrical device that converts mechanical energy to electrical energy. In Shikalbaha 60MW power plant, AC Generators are used which produces alternating current. Main two parts of that generator are rotor and stator. An external force rotates the rotor. Rotating part of the steam turbine is connected with the generator’s rotor. So, when the rotor of steam turbine starts moving, the rotor of the generator also starts moving. As a result, we get electricity. The generating voltage of this power plant is 15.75 KV. The running capacity of the plant is 40-42 MW. The rotation speed of the rotor of this power station is 3000 RPM.
For exciting, the alternator field needs DC current. DC current comes from another DC source. A DC generator is connected to the alternator. That DC generator is called exciter. Alternator and exciter are connected in the same shaft.
Step up transformer is used to step up the voltage of generator. The step up transformer increases the generating voltage from 15.75 KV to 132 KV. Power Grid Company of Bangladesh (PGCB) controls this 132 KV transmission line.