Steam Turbines Fundamentals

CONTENT

NAME OF THE TOPIC -------------------------------------------PAGE NO. 1.

INTRODUCTION------------------------------------------------ 01- 09

2.

CHAPTER-1

-------------------------------------------------- 10- 15

3.

CHAPTER-2

---------------------------------------------------- 16- 17

4.

CHAPTER-3

------------------------------------------------- 18- 27

5.

CHAPTER-4

------------------------------------------------- 28- 35

6.

CHAPTER-5

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36 -39

7.

CHAPTER-6

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40 -41

8.

CHAPTER-7

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42-43

INTRODUCTION A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. It has an emergency stop valve (ESV),control valve(CV), and high & low pressure turbines. Steam turbines has almost completely replaced the reciprocating piston steam engine because of its greater thermal efficiency and higher power-to-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 these of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process. Many industrial plants like sugar, pulp, paper, chemicals, fertilizers, steel and petroleum refineries require steam at low and medium pressures for process purposes power

for driving

incorporated

in

the

numerous

machines, like turbo generators, compressors,

plant. Normally steam is

generated either

and

pumps etc .,

in separate boilers that

utilize the heat provided by exothermic reactions of the chemical process, depending on the nature of the industry. Steam is generated at pressures and

temperatures higher than that

needed for the process, in order to keep the size of boilers and steam carrying piping small and also to realize a reasonably good efficiency during steam generation. This high pressure steam is expanded in steam turbine upto the pressure levels required for process. The power developed by the steam turbine is utilized for driving either compressors, pumps, blowers etc ., or turbo generators for generating electric power. Most of the driven machines are run at comparatively high speeds except pumps, blowers and turbo generators . Especially centrifugal compressors used in chemical, fertilizer, petro-chemical and other plants run at very high speeds so as to achieve maximum efficiency.

When these turbines are used to drive generators and other low speed machines, such as reciprocating compressors and cooling water pumps, a gear box must be incorporated in order to reduce the speed.

To fit into varying operational requirements of energy balance, a wide range of steam turbines are required. 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.

PRINCIPLE OF WORKING: Steam turbines works on the principle of rankine cycle.

RANKINE CYCLE: The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after William John Macquorn Rankine, a Scottish polymath.

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram to the right. •

Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.

•

Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.

•

Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.

•

Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change. In an ideal Rankine cycle the pump and turbine would be isentropic, i.e., the pump

and turbine would generate no entropy and hence maximize the net work output. Processes 1-2 and 3-4 would be represented by vertical lines on the Ts diagram and more closely resemble that of the Carnot cycle. The Rankine cycle shown here prevents the vapor ending up in the superheat region after the expansion in the turbine , which reduces the energy removed by the condensers.

STEAM TURBINE POWER PLANT CYCLE: Steam turbine power plants are based on the Rankine cycle investigated by a Scotch Engineer and Scientist William Rankine (1820 -1872). Rankine cycle for Steam turbine power plant

with ideal turbines and pumps and superheated and saturated steam as a working fluid respectively as shown below. A conventional power plant steam for such a consideration is also shown:

Fig.1.1 Ideal Rankine cycle for superheated steam on T-S axes.

Fig.1.2 Ideal Rankine cycle for saturated steam on T-S axes

The steam turbine is fed with steam under temperature t1, pressure p1, and enthalpy h1. Expanding within the turbine, steam produces work Wt and goes into the condenser under conditions p2 and h2. Hence its rejects heat Qr to cooling water and the resulted condensate with enthalpy h3