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Mechanical Engineering


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  1. So far in this program on steam power we have examined steam production and the different types of boilers and fossil fuels which are used for this purpose. We are now going to shift the focus on to steam utilization by taking a detailed look at the steam turbine which drives the electrical generator. We already studied the various turbine cycles and factors affecting efficiency at the beginning of this program. In this module will look at the main features of turbine construction including the various components and support systems such as gland steam, lube and hydraulic oil and the condenser. The subject of turbine operation control and protection will be dealt with in the next module. The steam turbine is certainly one of the most popular prime movers especially for driving large power generators. The size of steam turbines built and installed runs all the way from about 5 megawatts up to 1000 megawatts or more so what is it that makes the steam turbine the preferred to prime movers in comparison with say the diesel engine. Well, there are quite a number of advantages the main factor being that of physical size: for example, a 30-megawatt steam turbine is probably smaller than a 5-megawatt diesel. Another advantage is that the steam turbine is far less complicated and contains very few moving parts. The moving parts are: the rotor which drives the generator and the steam control valves and control gear. The amount of vibration produced by a steam turbine generator and transmitted to the surrounding area is relatively low provided, of course, that the machine is in good condition and is correctly installed and the aligned. This makes for a quieter operating plant particularly when compared with the diesel engine or even the gas turbine. The steam turbine runs at high speed usually 3600 rpm in North America or 3000 rpm in Europe and can therefore be coupled directly to a two-pole generator. Remembering that the diesel engine operates at around 200 rpm we can see one of the reasons why the specific output of the steam turbine is much greater. One other advantageous factor is the relative ease of controlling the output of the steam turbine. This is achieved simply by adjusting the amount of steam admitted to and flowing through the turbine. As we'll see later admission valves also known as control valves are installed for this purpose. The turbine stop valve or valves is located upstream of the control valves. In case of an emergency the stop valve closes automatically cutting off all steam supplied to the turbine and bringing the machine to a halt. This sketch shoes the major components of a 150 megawatt reheat turbine. We see here the rotor and the inner and outer casing or shell. The inner casing surrounds the rotor leaving very fine clearances between the stationary and moving parts. The objective of the inner casing is to direct the steam through the turbine. The inner casing is fitted into the upper and lower half of the outer shell which in turn is supported on a heavy reinforced concrete base. The base must be firm enough to insure it rigidity that is it must permit no movement in the vertical plane which could upset the alignment of the machine. The support bearings will allow restricted movement in the axial plane to cope with the expansion of the turbine as we shall see later. Other components noticeable here are the governor pedestal which supports the front-end Bearing and the governor system and the stationary (part of the inner casing) and moving blades. The turbine admission valves are located in each steam chest to control the flow of steam into the turbine. The turbine rotor is directly coupled to the generator rotor to transfer the mechanical energy produced in the turbine to the generator where it is converted into electrical energy. Now as we progress through this module, we'll be discussing the function of all of these components and many others. At this point we're mainly concerned with identifying these items. The turbine outer casing. The generator. The turbine roto and the Associated coupling. The governor pedestal, the main steam stop valves, the steam chest and associated admission valves or control valves and the bearings which support the turbine rotor. These are the major components of any general-purpose steam turbine. There are different turbine arrangements: single stage, multistage and within this we have impulse and reaction blades. Also we have vacuum (condensing) and back pressure turbines according to the exhaust pressure, also we have extraction or no extraction. Now, let's take a closer look at a single cylinder machine in order to help us understand the principles of turbine operation how it actually functions and produces power. As we all know steam at high pressure and temperature say 1800 psi 8 and 1000°F is admitted at one end of the turbine. After passing through the turbine this team exits at a much lower pressure and temperature say at 50 psi a and 250°F for a typical back pressure turbine. However, if the turbine is of the condensing type and this is far more common, the steam exiting from the turbine will be at a pressure far below atmospheric say one psia and at a temperature of about 100°F. Clearly the amount of energy in the steam exhausted from the turbine is much less than in the steam entering the turbine. Low heat energy has been used to force the turbine rotor to rotate at high speed and consequently produce mechanical energy. But how does it do this? Well, the simple answer is that the turbine blades are designed to take advantage of the decrease in steam pressure and consequent increase in steam velocity. As we know the heart of the turbine the bit that makes it work is the relationship between the fixed (vanes) and moving blades. The fixed blades guide the steam onto the moving blades. As the steam passes through the moving blades it causes the disk to which they are attached to rotate and consequently the shaft rotates. Each pair of stationary and the associated moving blades are known as one stage. Most steam turbines contain many stages of blading. In this example of a single cylinder machine shown here we have 7 stages and do not forget the stationary blade is always ahead of the moving blade. Actually, each pair of stationary blades is shaped to form a convergent divergent nozzle. However, the form of the nozzle is bent to receive the steam exiting from the previous moving stage and then to turn and redirect the steam on to the next moving stage. Now before you raise the question but what about impulse and reaction type blades? let me say that in practice when you are operating a turbine it's not vitally important whether the blading is impulse or reaction this is really a design and construction feature however it will certainly be worth to take a look at this subject as the type of blading used does affect other structural features. Let's first examine impulse blading, as the steam passes through the first row of stationary blades or nozzles its pressure decreases and as a result the steam velocity increases. These changes are plotted on this graph. As this high velocity steam is directed onto the moving blade the impulse pushes the blade forward and consequently produces rotation of the shaft. By the time the steam leaves the moving blade it has lost much of its velocity edit then passes on through the next row of stationary blades; again the pressure falls and the velocity increases due to expansion of the steam and once again this is directed on to the next row of moving blades and so on.
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