Steam Turbine Report

Acknowledgeme nt Many lives & destinies are destroyed due to the lack of proper guidance, directions & opportunities. It is in this respect I feel that I am in much better condition today due to continuous process of motivation & focus provided by my parents & teachers in general. The process of completion of this training was a tedious job & requires care & support at all stages. I would like to highlight the role played by individuals towards this. I am eternally grateful to BHEL, BHOPAL for providing us the opportunity & infrastructure to complete the training as a partial fulfillment of B.E. degree. I am very thankful to Mr. A.K. NIMJE, SR.DGM. of STE Deptt. For his kind support & faith in us. I would like to express my sincere thanks, with deep sense of gratitude to my training officer Mr.D.C.NIRMAL; Dy.Manager of STE Deptt. for their keen interests my project. I also thankful to Mr.P.K.GOYAL & Mr.AJAY KUMAR & to all visible & invisible hands which helped us to complete this training with a feeling of success.

INDEX 1.

Introduction

2.

History

3.

Types

Steam Supply and Exhaust Conditions 5. Casing & Shaft Arrangement 6. principle of Operation & design 7. Turbine Efficiency 8. Impulse Turbines 9. Reaction Turbines 10.Operation & Maintenance 11.Speed Regulation 12.Direct Drive 13.Speed Reduction 14.Working 15.Steam Turbine Principle 16. Impulse Blading 4.

17. 18.

Reaction Blading Rankine Cycle

INTRODUCTION A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884. It has almost completely replaced the reciprocating piston steam engine (invented by Thomas Newcomen and greatly improved by James Watt) primarily because of its greater thermal efficiency and higher powerto-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process. History The first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Hero of Alexandria in Roman Egypt. A thousand years later, the first impact steam turbine with practical applications was invented in 1551 by Taqi-alDin in Ottoman Egypt, who described it as a prime mover for rotating a spit. Similar smoke jacks were later described

by John Wilkins in 1648 and Samuel Pepys in 1660. Another steam turbine device was created by Italian Giovanni Branca in 1629. The modern steam turbine was invented in 1884 by the Englishman Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW of electricity. Parson's steam turbine, making cheap and plentiful electricity possible and revolutionizing marine transport and naval warfare, the world would never be the same again.. His patent was licensed and the turbine scaledup shortly after by an American, George Westinghouse. A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. This was good, because the turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with any pressure of steam. It is also, however, considerably less efficient. The Parson's turbine also turned out to be relatively easy to scale-up. Parsons had the satisfaction of seeing his invention adopted for all major world power stations. The size of his generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity. He knew that the total output from turbo-generators constructed by his firm C. A. Parsons and Company and by their licensees, for land purposes alone, had exceeded thirty million horse-power.. Within Parson's lifetime the generating capacity of a unit was scaled-up by about 10,000 times.

Types Steam turbines are made in a variety of sizes ranging from small 1 hp (0.75 kW) units (rare) used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 hp (1,500,000 kW) turbines used to generate electricity. There are several classifications for modern steam turbines. Steam Supply and Exhaust Conditions These types include condensing, non-condensing, reheat, extraction and induction. Non-condensing or backpressure turbines are most widely used for process steam applications. The exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found at refineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of low pressure process steam are available. Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam in a partially

condensed state, typically of a quality near 90%, at a pressure well below atmospheric to a condenser. Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits from a high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steam then goes back into an intermediate pressure section of the turbine and continues its expansion. Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from various stages of the turbine, and used for industrial process needs or sent to boiler feed water heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled. Induction turbines introduce low pressure steam at an intermediate stage to produce additional power. Casing or Shaft Arrangements These arrangements include single casing, tandem compound and cross compound turbines. Single casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem compound are used where two or more casings are directly coupled together to drive a single generator. A cross compound turbine arrangement features two or more shafts not in line driving two or more generators that often operate at different speeds. A cross compound turbine is typically used for many large applications. Principle of Operation and Design An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies

ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage. Turbine Efficiency

Schematic diagram outlining the difference between an impulse and a reaction turbine To maximize turbine efficiency, the steam is expanded, generating work, in a number of stages. These stages are characterized by how the energy is extracted from them and are known as impulse or reaction turbines. Most modern steam turbines are a combination of the reaction and impulse design. Typically, higher pressure sections are impulse type and lower pressure stages are reaction type. Impulse Turbines

An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle its pressure falls from steam chest pressure to condenser pressure (or atmosphere pressure). Due to this relatively higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss". Reaction Turbines In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. Operation and Maintenance When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed

to heat up the lines in the system along with the steam turbine. Also a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 RPM to slowly warm the turbine. Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. It is, however, essential that the turbine be turned with dry steam. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine. Speed regulation The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials.

Direct drive Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralized stations are of two types: fossil and nuclear power plants. The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. In installations with high steam output, as may be found in nuclear power stations, the generator sets may be arranged to operate at half these speeds, but with four-pole generators.

Speed Reduction

The Tu rbinia - the first steam turbine-powered ship Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. (Steam turbine locomotives were also tested, but with limited success.) A steam turbine is only efficient when operating in the thousands of RPM range while application of the power in propulsion applications may be only in the hundreds of RPM and so requiring that expensive and precise reduction gears must be used, although several ships, such as Turbinia, had direct drive from the steam turbine to the propeller shafts. This purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power, except for diesel engines which are capable of higher efficiencies. Steam turbine efficiencies have yet to break 50% yet diesel engines routinely exceed 50%, especially in marine applications.

Working

–

Introduction A steam turbine is a mechanical device that converts thermal energy in pressurized steam into useful mechanical work. The original steam engine which largely powered the industrial revolution in the UK was based on reciprocating pistons. This has now been almost totally replaced by the steam turbine because the steam turbine has a higher

thermodynamic efficiency and a lower power-to-weight ratio and the steam turbine is ideal for the very large power configurations used in power stations. The steam turbine derives much of its better thermodynamic efficiency because of the use of multiple stages in the expansion of the steam. This results in a closer approach to the ideal reversible process. Steam turbines are made in a variety of sizes ranging from small 0.75 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 1,500,000kW turbines used to generate electricity. Steam turbines are widely used for marine applications for vessel propulsion systems. In recent times gas turbines , as developed for aerospace applications, are being used more and more in the field of power generation once dominated by steam turbines.

Steam Turbine Principle The steam energy is converted mechanical work by expansion through the turbine. The expansion takes place through a series of fixed blades (nozzles) and moving blades each row of fixed blades and moving blades is called a stage. The moving blades rotate on the central turbine rotor and the fixed blades are concentrically arranged within the circular turbine casing which is substantially designed to withstand the steam pressure. On large output turbines the duty too large for one turbine and a number of turbine casing/rotor units are combined to achieve the duty. These are generally arranged on a common centre line (tandem mounted) but parallel systems can be used called cross compound systems.

Two Turbine Cylinders Tandem Mounted There are two principles used for design of turbine blades: The Impulse Blading and The Reaction Blading. Impulse Blading The impulse blading principle is that the steam is directed at the blades and the impact of the steam on the blades drives them round. The day to day example of this principle is the pelton wheel. In this type of turbine the whole of the stage pressure drop takes place in the fixed blade (nozzle) and the steam jet acts on the moving blade by impinging on the blades.

Blades of an impulse turbine

Velocity diagram impulse turbine stage z represents the blade speed , V r represents the relative velocity, V wa & V wb- represents the tangential component of the absolute steam in and steam out velocities The power developed per stage = Tangential force on blade x blade speed. Power /stage= (V

wa

- V

wb

).z/1000 kW per kg/s of steam

Reaction Blading The reaction blading principle depends on the blade diverting the steam flow and gaining kinetic energy by the reaction. The Catherine wheel (firework) is an example of this principle. For this turbine principle the steam pressure drop is divide between the fixed and moving blades.

Velocity diagram reaction turbine stage z represents the blade speed , V r represents the relative velocity, V wa & V wb- represents the tangential component of the absolute steam in and steam out velocities The power developed per stage = Tangential force on blade x blade speed. Power /stage= (V

wa

- V

wb

).z/1000 kW per kg/s of steam

The blade speed z is limited by the mechanical design and material constraints of the blades. Rankine Cycle The Rankine cycle is a steam cycle for a steam plant operating under the best theoretical conditions for most efficient operation. This is an ideal imaginary cycle against which all other real steam working cycles can be compared. The theoretic cycle can be considered with reference to the figure below. There will no losses of energy by radiation, leakage of steam, or frictional losses in the mechanical components. The condenser cooling will condense the steam to water with only sensible heat (saturated water). The feed pump will add no energy to the water. The

chimney gases would be at the same pressure as the atmosphere. Within the turbine the work done would be equal to the energy entering the turbine as steam (h1) minus the energy leaving the turbine as steam after perfect expansion (h2) this being isentropic (reversible adiabatic) i.e. (h1- h2). The energy supplied by the steam by heat transfer from the combustion and flue gases in the furnace to the water and steam in the boiler will be the difference in the enthalpy of the steam leaving the boiler and the water entering the boiler = (h1 - h3).

Basic Rankine Cycle The various energy streams flowing in a simple steam turbine system are as indicated in the diagram below. It is clear that the working fluid is in a closed circuit apart from the free surface of the hot well. Every time the working fluid flows at a uniform rate around the circuit it experiences a series of processes making up a thermodynamic cycle. The complete plant is enclosed in an outer boundary and the working fluid crosses inner boundaries (control surfaces). The inner boundaries defines a flow process.

The various identifiers represent the various energy flows per unit mass flowing along the steady-flow streams and crossing the boundaries. This allows energy equations to be developed for the individual units and the whole plant. When the turbine system is operating under steady state conditions the law of conservation of energy dictates that the energy per unit mass of working agent ** entering any system boundary must be equal to the rate of energy leaving the system boundary. **It is acceptable to consider rates per unit mass or unit time whichever is most convenient

Steady Flow Energy Equations Boiler The energy streams entering and leaving the boiler unit are as follows: F+A+h

Turbine

d

=h

1

+ G + hl

b

hence hl b

F+A=G+h

1

-h

d

+

The energy streams entering and leaving the turbine are as follows: h

1

=T+h

2

+ hl

hence 0 = T - h

t

1

+h

2

+ hl

t

Condenser Unit The energy streams entering and leaving the condenser unit are as follows: Wi+h

2

=Wo+h

w

+ hl

c

hence hl c

Wi=Wo+h

w

-h

2

+

Feed Water System The energy streams entering and leaving the Feed Water System are as follows: h

w

+ d e + d f= h

d

+ hl

f

hence d e + d f = - h

w

+h

d

+ hl

The four equations on the right can be arranged to give the energy equation for the whole turbine system enclosed by the outer boundary. That is the energy of the fuel (F) per unit mass of the working agent (water) is equal to the sum of - the mechanical energy available from the turbine less that used to drive the pumps (T - (d e+ d f) - the energy leaving the exhaust [G - A] using the air temperature as the datum. - the energy gained by the water circulating through the condenser [W o - W i] - the energy gained by the atmosphere surrounding the plant Σ hl

The overall thermal efficiency of a steam turbine plant can be represented by the ratio of the net mechanical energy available to the energy within the fuel supplied as indicated in the expressions below-

RAW MATERIALS USED AND THEIR SUPPLIERS

Bar material:

Suppliers:



AA 19331 Ghaziabad

kissan steel,



AA 19332 ispat,Ghaziabad

Ghaziabad



AA 10108

CFFB ,Howrah



AA 10112



AA 10622

MUSCO ,Bombay



AA 10723

SAIL ,Durgapur

Plate material:

Suppliers:

AA 10119

•

SAIL ,durgapur

( higher thk)

Thysen, germany Mobar , germany m.tayllor,u.k.

Pipe material:

suppliers:

AA 10455 Mumbai

•

sandoz metal,

( SA 106GrB )

Customers

of

the

Public sector customer

product

:

•

National thermal power corporation ( NTPC )

•

Nuclear power corporation of india limited ( NPCIL)

•

State electricity board ( SEB )

•

Power development corporation limited ( PDCL )

•

Damodar valley corporation

Private sector customers: •

Arunachal Pradesh sugar mills

Technology collaboration Earlier (1976 -77):G.E.C Now: ALSTOM , U.K .

Reference 1.

STEAM AND GAS TURBINE DEPTT. BHEL BHOPAL

2. WWW.GOOGLE.COM