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As already stated, a turbine is driven by the flow of a high energy fluid - liquid, gas (or air). The kinetic energy of the fluid is converted into mechanical energy. In steam turbines, the thermal and pressure energy contained in superheated, high pressure steam is used to drive the shaft of the turbine. Steam turbines are generally used where there is a plentiful supply of water. The water must first be treated to remove impurities which will cause problems in the turbine. - Chlorides, other salts, Oxygen and solid particles. This is done to prevent corrosion, erosion and scale deposits in the system. When the water has been purified, it is then passed into a Steam Generation Plant where it is heated to produce steam. Steam at normal atmospheric conditions is Saturated (Wet) steam - i.e. 212 °F (100 °C) and is of no use for driving turbines. In the type of boiler used for steam generation, the system is maintained under high pressure - In this discussion we will use a steam system operated at 600 Psi. At this pressure, the water boils at 486 °F. However, at this pressure and temperature the steam is still saturated (wet steam). The use of this steam in a turbine will cause erosion of the turbine internals due to droplets of water contained in the steam. The boilers are therefore constructed with a 'Super-heater' section which takes the 600 Psi wet steam and adds more heat energy to it, to a temperature of 775 °F or higher depending on requirements. At this temperature, the steam cannot contain any water. When steam is super-heated, it contains much more heat energy than wet steam and can be piped long distances with little loss of energy or condensation taking place.
To re-cap, the steam used for driving steam turbines is produced from purified water to prevent corrosion and is produced at high pressure and super-heated to high temperature in order to prevent water erosion of the turbine parts. There are many types of steam turbine in use today which can produce many thousands of horse-power of energy for industrial uses.
PRINCIPLES & OPERATION
In the pin-wheel, the windmill and the water wheel, the action of the flowing fluid causes the wheel to rotate. This part of the machine is called the 'ROTOR'. In any turbine, the rotor is mounted on a shaft and consists of the 'Sails' or 'Paddles' which we will now refer to as 'Blades'. The blades are fitted into a wheel at an angle and are called 'Rotor Blades'. The wheel is then mounted on to the shaft. This arrangement of a single wheel is called 'one stage' or a 'Single Stage Rotor' and does not produce high power. (See Figure. 4)
Figure. 4 - Single Stage Turbine
For large processing and generation plants, very powerful turbines are needed for driving machines like compressors, large pumps or generators. In this case, 'Multi-Stage Turbines' are used. As stated earlier, the energy needed to drive these turbines, comes from high pressure, superheated steam. In order to get the steam to pass to the rotor blades, we need a means of directing the steam on to the blades. The piece of equipment used for this is called a ‘NOZZLE’. As the steam leaves a nozzle, its pressure decreases and its 'VELOCITY' increases. This high velocity steam jet is directed at the rotor blades and, as in the pin-wheel, the rotor and shaft begins to rotate. As more and more steam is released on to the blades, the speed of rotation increases. (As with a windmill, stronger wind, faster rotation).
MULTI-STAGE STEAM TURBINES
A Multi-stage turbine is one which has two or more wheels. The steam is directed on to the blades of the first stage wheel and, as it strikes the angled blades, they move away in an opposite direction to the flow of steam, causing rotation of the wheel and shaft. As the steam gives up energy to move the rotor blades, its pressure is decreased, its volume increases and it leaves the blades in an opposite direction to that taken by the wheel.
(See Figure. 5)
Some method of re-directing the steam on to a second wheel is now needed. To achieve this, a row of fixed, unmoving, angled blades is fitted into a ‘DIAPHRAGM’ which is mounted in the 'CASING' of the turbine. These stationary blades are called ' STATOR BLADES '.
Figure: 5 - Steam Flow Through the Rotor & Stator
The stator blades act like further nozzles and re-direct the steam on to the rotor blades of the second stage wheel. Because the steam pressure has dropped and its volume is greater, to get the same amount of energy out of it, the blades of the second stage are larger (greater area) than those of the first stage. This arrangement of alternating rotor and stator blades and increasing blade size, is continued through the turbine in order to obtain the required amount of power from the steam for operational needs. When the steam leaves a turbine, it may still contain a lot of energy - pressure and heat. This steam may be directed for use in another process system. (See Figure. 6)
(Figure. 7) - Shows a typical lube oil system for a steam turbine.
The lube oil system comprises a reservoir or oil tank in which three pumps are immersed. The main oil pump is driven by a shaft from the Steam Turbine accessory gear. The Auxiliary pump is driven by an A/C motor and is used for start-up, shut-down and other operating conditions necessitating its use. The 3rd pump is operated by a D/C motor (battery supplied) for use on main power failure - shut down of the complete system which will require lube oil while the units shut down. From the pumps the oil at the required pressure (controlled by PCV 1 that spills excess back to the reservoir), passes through 1 of 2 water cooled exchangers (1 operating & 1 standby) and temperature controlled by a TCV. After cooling the main oil flow passes through 1 of 2 filters (1 operating & 1 standby). The filters are fitted with a Differential Pressure (DP) gauge and alarm which, should the filter begin to get too dirty, at a pre-set DP will warn that the filters need changing over and the dirty elements changed out. From the filters the oil passes via a control valve (PCV 2) which maintains the desired lube oil pressure to the bearings of the turbine and possibly also to its driven machine - Compressor, Generator, pump .. etc. After lubricating and cooling the bearings, the oil returns to the reservoir. Any oil losses are made up via the oil make up line to the reservoir. In the oil systems, a number of alarms and shut-down devices are installed to ensure the safe operating conditions for the machine. Hydraulic Oil is provided from the lube oil system from between the coolers and the filters. This oil may be boosted in pressure, filtered and pressure controlled by PCV/A and is used for the control and shut-down systems of high power steam turbines.