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ABSTRACT
TITLE OF THE PROJECT
Manufacturing of Steam Turbines.
The main aim of project is to study about the assembly and manufacturing of steam turbine and its parts. In the era of Mechanical Engineering, Turbine, A Prime Mover (Which uses the Raw Energy of a substance and converts it to Mechanical Energy) is a well known Machine most useful in the field of Power Generation. This Mechanical energy is used in running an Electric Generator which is directly coupled to the shaft of turbine. From this Electric Generator, we get electric Power which can be transmitted over long distances by means of transmission lines and transmission towers. Mostly steam turbines are used because of its greater thermal efficiency and higher power-to-weight ratio. About 80% of all electricity generation in the world is by use of steam turbines. In my Industrial Training in B.H.E.L., we go through various sections in Turbine Manufacturing. We studied about the various Steam Turbine types, parts like Blades, Casing and Rotor etc. The efficiency and reliability of steam turbine depends on the proper design of blades, casing and rotor. Rotor is considered as the heart of the steam turbine and it affects the efficiency of the steam turbine. Here in BHEL, Steam turbines are made in a variety of sizes ranging from small 250 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 200 MW turbines used to generate electricity.
Standard Turbines from BHEL operate on 50% reaction principle and are therefore fitted with reaction blading, preceded by a single row impulse wheel as governing stage.
The inverted T- root grooves are made on all reaction stages except on low pressure last stage. The inverted T- root blades with shrouding are milled from solid material and are bottom caulked with the brass after insertion in the rotor groove. The roots are so designed that after assembly the specified blade spacing is obtained without the need of fitting spacers. A lock blade, which is secured to the rotor by means of threaded screws, closes off the blading gate without forming a gap.
Absolute and incremental programs are being used along with the machining cycles for roughing and finishing on CNC lathe. The programmer is required to put efforts in writing number of sub programs for the new groove details and should keep the track of information. And Any changes in the existing groove dimensions leads to the modification of the available programs. The efforts are continuous.
Modern CNC systems offer the manual programmer increased computing power. The Machining cycles ( Canned cycles ) designed in the CNC system are uniquely written for the programmer and allows the programmer to use cycles to save time. Variables ( Parameters ) along with Conditional jump functions (@ functions ) allows the programmer to design his own machining cycles to develop full pledged dedicated programs for a component.
For 2 Dimensional machining like lathe / milling operations where the component shapes are same and the dimensions vary, CNC system based dedicated programs are more convenient and easy to use in the shop floor than individual part programs, APT programs or CAD / CAM based programs.
Development of variable CNC programs for the machining of steam turbine rotors, mainly blade grooves ( Inverted T- shape ) are taken up as a project, efforts are put in leaning the CNC programs and no of exercises are practiced to standardize the machining activity utilizing the CNC system features to the possible extent. Graphic simulation for the tool paths are verified and carried out the machining on regular rotor.
INTRODUCTION
Generation of power in abundance is needed for any country to develop into an industrial nation. It is possible to generate power using thermal, tidal or nuclear energy. Thermal power plants play a key role in the generation of power in view of the abundant coal supplies in our country. Steam turbine, mechanical rotating equipment which consists of stationary and moving blade rows, is the main component of a thermal power plant. A steam turbine based power plant involves raising high-pressure steam in a boiler from the thermal energy and expanding the steam in a turbine to generate shaft power, which in turn is converted into electricity in a generator.
In the power generation industry gas turbines and steam turbines are widely used for generating power. These industrial machines are capable of producing power in hundreds of megawatts. Industrial applications need mechanical energy to drive the generator..
Introduction to the Steam Turbine
De Laval, Parsons and Curtis developed the concept for the steam turbine in the 1880's.Steam turbines are used in all of our major coal-fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam generated in 'Boilers' or 'Steam Generators' as they are sometimes called.
Working principle of Steam Turbine
A steam turbine is a prime mover, which converts the energy stored in the steam into rotational mechanical energy. In general, nozzle and blade are the two most important elements of a steam turbine. As the steam passes through the nozzle, the steam pressure falls and due to this fall in the pressure certain amount of heat energy is converted into kinetic energy i.e. steam is given a high velocity. The rotor blades (moving part of the turbine) change the direction of motion of this high velocity steam and thus give rise to change in momentum and therefore a force. The force acts on the blades and thus rotates the shaft on which the blades are mounted. Thus the energy stored in the steam is converted into rotational mechanical energy and then this energy is used to drive the generators coupled to the shaft. The turbine shaft is connected to a generator, which produces the electrical energy. The rotational speed is 3,000 rpm for Australian (50 hertz (Hz)) systems and 3,600 for American (60 Hz) systems.
The Turbine is a rotary device that affects an exchange of energy between a flowing fluid and a rotating shaft. In a steam turbine, the energy transfer takes place in two steps:
1. The available energy in the hot and high pressure steam is first converted into kinetic energy by the expansion of steam in a suitably shaped passage known as nozzle from which it issues as a high velocity jet having a high tangential component.
2. Then a part of this kinetic energy and sometimes part of the pressure energy are converted into mechanical energy by directing the jet at a proper angle, against curved blades mounted on a rotating disc. The rotor compiled to generator produces the electricity.
A turbo machine consists of a rotating part having a series of vanes or buckets arranged around its periphery and a set of stationary vanes used to control the angular momentum of the fluid passing through it. The pressure falls in the flow through a turbine stage and a high loading may not lead to excessive boundary layer growth or separation. The same level of energy exchange in a turbine can be affected with fewer stages.
Many of the utility steam turbines are of three cylinder construction i.e. High pressure cylinder in which pressure is maximum with minimum specific volume so that blade height is minimum, Intermediate pressure cylinder in which pressure is intermediate so that the blade height is intermediate and subsequently low pressure cylinder which has a minimum pressure level and maximum specific volume hence maximum blade height is maximum.
After the steam has passed through the HP stage, it is returned to the boiler to be re-heated to its original temperature although the pressure remains greatly reduced. The reheated steam then passes through the IP stage and finally to the LP stage of the turbine.
Of all heat engines and prime movers the steam turbine is nearest to the ideal and it is widely used in power plants and in all industries where power and/or heat are needed forprocesses. These include: pulp mills, refineries, petro-chemical plants, food processing plants, desalination plants, refuse incinerating and district heating plants.
Classification of Steam Turbines:
Steam Turbines can be broadly categorized as follows:
According to principle of action of steam:
i. Impulse turbine: As the name indicates, this turbine runs by the impulse of jet. In the nozzle, the pressure of the steam jet is reduced while the velocity of steam is increased. While the steam flows over the moving blade, the pressure remains constant but velocity is decreased. e.g. De-Laval Turbine.
ii. Reaction Turbine: In the reaction turbine, the high pressure steam from the boiler is passed through the nozzles at the inlet of the turbine. There exists a pressure drop over both moving blade and fixed blade. When the steam comes through these nozzles, the velocity of the steam increases relative to the rotating wheel. e.g.:- Parson’s turbine.
According to the direction of steam flow:
i. Axial Turbine: It is the turbine in which the steam flows in a direction parallel to the axis of the turbine.
ii. Radial turbine: The turbine in which the steam flows in a direction perpendicular to the axis of the turbine is known as Radial turbine.
According to the heat drop process:
i. Condensing turbine: In a condensing turbine, steam at vapour pressure is directed to a condenser where the heat of exhaust steam is given up to cooling water. Besides the steam is also extracted from intermediate stages for feed water heating, the number of such extraction being from 2-3 to as much as 8-9.
ii. Back pressure turbine: In this turbine, the exhaust steam is utilized for industrial or heating purpose. The pressure of steam at exit is always greater than atmosphere.
iii. Topping turbine: These turbines are also of the backpressure turbine type with the difference that the exhaust steam is further utilized in medium and low pressure condensing turbines.
According to the number of Stages:
i. Single stage turbine: The expansion of steam takes place in a single row each of guide and moving blades.
ii. Multi stage turbine: The expansion of steam takes place in a multiple row of guide and moving blades depending upon the inlet and exit pressures.
According to the Steam condition at inlet:
i. Low Pressure Turbine: The turbine in which the pressure of steam used is in between 1.2 to 2 Ata is known as low-pressure turbine.
ii. Medium Pressure Turbine: It is the turbine in which the steam used has a pressure up to 40 Ata.
iii. High Pressure Turbine: If the utilized steam has a pressure above 40 Ata, then such a turbine is known as high-pressure turbine.
iv. Very High Pressure Turbine: It If the turbine in which the pressure of steam used is 170 Ata or greater.
v. Super Critical Pressure Turbine: If the steam used in a turbine has a pressure equal to or greater than 225 Ata, then such a turbine is known as critical pressure turbine.
Other general classifications of turbines:
i. Condensing Turbines: With the condensing turbine, the steam exhausts to the condenser and the latent heat of the steam is transferred to the cooling water. The condensed steam is returned to the boiler as feedwater.
ii. Condensing-Bleeder Turbines: The condensing-bleeder turbine reduces the condenser losses, as steam is bled off at several points of the turbine. The bleed-steam is used for feedwater heating; up to 20% of the total steam flow may be bled off.
iii. Back-Pressure Turbines: Back-pressure turbines are often used in industrial plants, the turbine acts as a reducing station between boiler and process steam header. The process steam pressure is kept constant and the generator output depends on the demand for process steam. The backpressure turbine may also have bleed points and is then called a back-pressure-bleeder-turbine.
iv. Extraction Turbines: Extraction turbines are turbines where steam is extracted at one or more points at constant pressure. Extraction turbines may be single or double-extraction-condensing turbines or single-or double-extraction back-pressure turbines. The extraction turbines may, besides extraction points, have bleed points for feedwater heating.
v. Topping Turbines: Topping turbines have been used when old boilers are replaced with new high pressure boilers. The turbine is a backpressure turbine exhausting to the old boiler header still supplying steam to the old lower pressure turbines.
vi. Mixed Pressure Turbines: Mixed pressure turbines are used where excess steam from process is available for the low pressure part of the turbine, while steam at boiler pressure may be added to the high pressure part of the turbine when more loads are applied to the turbine.
vii. Cross Compound Turbines: Cross compound turbines are large turbines with parallel shafts with a generator on each shaft. The steam flows through the high pressure turbine, then is crossed-over to the low pressure turbine.
viii. Tandem Compound Turbines:Tandem compound turbines are large turbines consisting of two or more turbines in series coupled together as one shaft and applied to one generator.
1.5 Operation Principles:
1.5.1. Impulse Turbine
In principle the impulse steam turbine consists of a casing containing stationary steam nozzles and a rotor with moving or rotating buckets.
The steam passes through the stationary nozzles and is directed at high velocity against the rotor buckets causing the rotor to rotate at high speed.
The following events take place in the nozzles:
• The steam pressure decreases.
• The enthalpy of the steam decreases.
• The steam velocity increases
• The volume of the steam increases.
The nozzles may be convergent nozzles or they may be convergent-divergent nozzles. Convergent nozzles are used for smaller pressure drops where the minimum exit pressure is 0.577 x the inlet pressure (the critical pressure for nozzles.)
If the exit pressure is less than 0.577 x inlet pressures, eddy-currents are developed and the exit velocity will be less than calculated. The convergent-divergent nozzles prevent eddy-currents and the calculated velocity will be obtained even at large pressure drops.
The purpose of the bucket or moving blade on the rotor is to convert the kinetic energy of the steam into mechanical energy. If all kinetic energy is converted the steam exit velocity will be 0 m/s. This is not possible but it shows that the rotor blades must bring the steam exit velocity near 0 m/s.
i. The Impulse Principle
If steam at high pressure is allowed to expand through a stationary nozzle, the result will be a drop in the steam pressure and an increase in steam velocity. In fact, the steam will issue from the nozzle in the form of a high-speed jet. If this high velocity steam is applied to a properly shaped turbine blade, it will change in direction due to the shape of the blade. The effect of this change in direction of the steam flow will be to produce an impulse force on the blade causing it to move. If the blade is attached to the rotor of a turbine, then the rotor will revolve.
Force applied to the blade is developed by causing the steam to change direction of flow (Newton's 2nd Law - change of momentum). The change of momentum produces the impulse force. In an actual impulse turbine there are a number of stationary nozzles and the moving blades are arranged completely around the rotor periphery. Note that the pressure drops and the velocity increases as the steam passes through the nozzles. Then as the steam passes through the moving blades the velocity drops but the pressure remains the same. The fact that the pressure does not drop across the moving blades is the distinguishing feature of the impulse turbine.
ii. The Reaction Principle:
If the moving blades of a turbine are shaped in such a way that the steam expands and drops in pressure as it passes through them, then a reaction will be produced which gives a force to the blades. This reaction effect can be illustrated by considering a container filled with high-pressure steam. If there is no escape opening or nozzle for the steam, then the pressure will be the same on all walls of the container and the container will remain at rest. If, however, the container has an escape opening or nozzle, then steam will expand through the opening and drop in pressure. Therefore there will be an unbalanced pressure on the wall opposite to the opening and a reaction force R will be produced causing the container to move reaction Effect of this principle applied to a turbine drive.
A reaction turbine has rows of fixed blades alternating with rows of moving blades. The steam expands first in the stationary or fixed blades where it gains some velocity as it drops in pressure. It then enters the moving blades where its direction of flow is changed thus producing an impulse force on the moving blades. In addition, however, the steam upon passing through the moving blades again expands and further drops in pressure giving a reaction force to the blades.
This sequence is repeated as the steam passes through additional rows of fixed and moving blades. The blade arrangement and the pressure and velocity changes of the steam in a reaction turbine. Note that the steam pressure drops across both the fixed and the moving blades while the absolute velocity rises in the fixed blades and drops in the moving blades. The distinguishing feature of the reaction turbine is the fact that the pressure does drop across the moving blades. In other words there is a pressure difference between the inlet to the moving blades and the outlet from the moving blades.
Special Aspects of Reaction Turbines:
• There is a difference in pressure across the moving blades. The steam will therefore tend to leak around the periphery of the blades instead of passing through them. Blade clearances therefore must be kept to a minimum.
iii. Impulse Turbine Staging
In order for the steam to give up all its kinetic energy to the moving blades in an impulse turbine, it should leave the blades at zero absolute velocity. This condition will exist if the blade velocity is equal to one half of the steam velocity. Therefore, for good efficiency the blade velocity should be about one half of the steam velocity. If the steam was expanded from admission pressure down to final exhaust pressure in a single set of nozzles (single stage) then the velocity of the steam leaving the nozzles might he in the order of 1100 m per second. In order to have good efficiency the blade velocity would have 10 be about 550 m per second, which would require excessively high rev/mm of the turbine rotor and failure due to centrifugal force could result. In addition to this objection, excessively high steam velocity will cause high friction losses in nozzles and blading.
In order to reduce steam velocity and blade velocity, the following methods may be used:
• Pressure compounding.
• Velocity compounding.
• Pressure-velocity compounding.
a) Pressure Compounding:
The expansion of steam from boiler pressure to exhaust pressure is carried out in a number of steps or stages. Each stage has a set of nozzles and a row of moving blades. The rows of moving blades are separated from each other by partitions or diaphragms, into which the nozzles are set. As only a portion of the velocity available is developed in each set of nozzles, the blade velocity is kept down to a reasonable amount