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Steam Turbines:
• A steam turbine is a turbo-machines (prime mover), its purpose is extract maximum energy in which
heat energy is transformed into in to kinetic/velocity/mechanical energy in the form of rotary motion for
generating electricity. A steam turbine basically an assembly of nozzles and blades.
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• Steam turbine may be classified based on the Basis of Principle of Operation :
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Classification of steam turbine
• Based on action of steam or type of expansion:
1. Impulse or velocity or De Laval turbine
2. Reaction or pressure or Parson’s turbine
3. Combination turbine
• Based on number of stages:
1. Single stage turbine
2. Multi-stage turbine
• Based on type of steam flow:
1. Axial flow turbine
2. Radial flow turbine
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• Comparison of steam turbines based on the
method of energy transformation taking place
inside the turbine or how the steam is expanded.
• The two methods are impulse and reaction
turbine, the differences are discussed below
• Impulse turbines:
• Tend to be smaller than reaction turbine of
comparable power
• Have longer time between overhaul than reaction
turbines.
• Are more durable and
• Reaction turbines have a slightly higher operating
efficiency but are usually used in low pressure
steam environments.
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• On the Basis of Means of Heat Supply:
i. Single pressure turbine: there is single source of steam supply
ii. Mixed or dual pressure turbine: use two sources of steam, at different pressures.
iii. Reheated turbine: the turbine steam may be taken out to be reheated in a reheater to rise its temperature.
• On the Basis of Means of Heat Rejection :
i. Pass-out or extraction turbine: a considerable proportion of the steam is extracted from some suitable
point in the turbine where the pressure is sufficient for use in process heating; the remain continuing.
ii. Regenerative turbine: small proportions of steam continuously extracted for the purpose of heating the
feed water in a feed heater to increase efficiency of the plant. Now a days, all steam power plants are
equipped with reheating and regenerative arrangement.
iii. Condensing turbine: the condensing turbine allows the exhaust steam to expand to the lowest possible
pressure before being condensed. All steam power plants use this type of turbine.
iv. Noncondensing turbine: the exhaust steam coming out from the turbine is not recovered or condensed for
feed water in boiler but exhausted in the atmosphere.
v. Back pressure or topping turbine: the steam rejected at lowest pressure for heating purpose.
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• On the Basis of Number of Cylinder:
i. Single cylinder: When all stages of turbine are housed in one casing, then it is called single cylinder single
shaft.
ii. Multi-cylinder: In large output turbine, the number of the stages needed becomes high that additional
bearings are required to support the shaft. Hence, multi-cylinders are used.
• On the Basis of Arrangement of Cylinder Based on General Flow of Steam.
i. Single flow: the steam enters at one end, flows once through parallel to the axis. High pressure cylinder
uses single flow. This is also common in small turbines.
ii. Double flow: the steam enters at the center and divides, the two portions passing axially away through
separate sets of blading on the same rotor
iii. Reversed flow: sometimes used in h.p, cylinder where higher temperature steam is used on the larger sets
in order to minimize differential expansion.
The use of single, double and reversed flow is shown in the layout below.
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• On the Basis of Number of Shaft
i. Tandem compound: Most multi-cylinder turbines drive a single shaft and single generator Such turbines
are termed as tandem compound turbines.
ii. Cross compound: two shafts are used driving separate generator. The turbine house arrangement, limited
generator size,
• On the Basis of Rotational Speed
i. Constant speed turbines: Requirements of rotational speed are extremely rigid in turbines which are
directly connected to electric generators.
ii. Variable speed turbines: These turbines have geared units and may have practically any speed ratio
between the turbine and the driven machine so that the turbine may be designed for its own most
efficient speed.
• Such turbines are used to drive ships, compressors, blowers and variable frequency generators.
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STEAM TURBINE GOVERNING
• Governing of steam turbine means to regulate the supply of steam to the turbine in order to maintain
speed of rotation sensibly constant under varying load conditions. Some of the methods employed are :
• Bypass governing: In case of loads of greater than economic load a bypass valve opens and allows steam to
pass from the first stage nozzle box into the steam belt.
• Nozzle control governing: In this method of governing the supply of steam of various nozzle groups N1, N2,
and N3 is regulated by means of valves V1, V2 and V3 respectively.
• Throttle governing: control the flow of steam through a throttle by valves.
• In such system the steam enters the turbine controlled by governor. When the load on the turbine
decreases, its speed will try to increase. This will cause the fly bar to move outward which will in return
operate the lever arm and thus the double beat valve will get moved to control the supply of steam to
turbine. In this case the valve will get so adjusted that less amount of steam flows to turbine.
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• The function of governor is to maintain the shaft
speed constant as the load varies
• The steady-state speed regulation
is given by;
−
= 100
Where: = speed at no load
=
=
• Types of governor:
• Centrifugal flyball
• Throttling
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ADVANTAGES OF STEAM TURBINE OVER STEAM ENGINE
• The various advantages of steam turbine are as follows :
• It requires less space.
• Absence of various links such as piston, piston rod, cross head etc. make the mechanism simple. It is quiet
and smooth in operation,
• Its over-load capacity is large.
• It can be designed for much greater capacities as compared to steam engine. Steam turbines can be built in
sizes ranging from a few horse power to over 200,000 horse power in single units.
• The internal lubrication is not required in steam turbine. This reduces to the cost of lubrication.
• In steam turbine steam consumption does not increase with increase in years of service.
• In steam turbine power is generated at uniform rate, therefore, flywheel is not needed.
• It can be designed for much higher speed and greater range of speed.
• The thermodynamic efficiency of steam turbine is higher.
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STEAM TURBINE CAPACITY
• The capacities of small turbines and coupled generators vary from 500 to 7500 kW whereas large turbo
alternators have capacity varying from 10 to 90 mW. Very large size units have capacities up to 500 mW.
• Generating units of 200 mW capacity are becoming quite common. The steam consumption by steam
turbines depends upon steam pressure, and temperature at the inlet, exhaust pressure number of bleeding
stages etc.
• The steam consumption of large steam turbines is about 3.5 to 5 kg per kWh.
• Turbine kW = Generator kW / Generator efficiency
• Generators of larger size should be used because of the following reasons:
• Higher efficiency.
• Lower cost per unit capacity.
• Lower space requirement per unit capacity. 3.45.1 Nominal rating.
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STEAM TURBINE PERFORMANCE
• Turbine performance can be expressed by the following factors :
• The steam flow process through the unit-expansion line or condition curve.
• The steam flow rate through the unit.
• Thermal efficiency.
• Losses such as exhaust, mechanical, generator, radiation etc.
• Mechanical losses includes
• Bearing losses,
• Oil pump losses and
• Generator bearing losses. Generator losses include electrical and mechanical losses.
• Exhaust losses include the kinetic energy of the steam as it leaves the last stage and the pressure drop from the exit
of last stage to the condenser stage.
• For successful operation of a steam turbine it is desirable to supply steam at constant pressure and
temperature.
• Steam pressure can be easily regulated by means of safety valve fitted on the boiler.
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• The steam temperature may try to fluctuate because of the following reasons :
• Variation in heat produced due to varying amounts of fuel burnt according to changing loads.
• Fluctuation in quantity of excess air.
• Variation in moisture content and temperature of air entering the furnace.
• Variation in temperature of feed water.
• The varying condition of cleanliness of heat absorbing surface.
CHOICE OF STEAM TURBINE
• The choice of steam turbine depends on the following factors :
• Capacity of plant .
• Thermal efficiency
• Reliability
• Plant load factor and capacity factor
• Location of plant with reference to availability of water for condensate.
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STEAM TURBINE SPECIFICATIONS
• Steam turbine specifications consist of the following:
• Turbine rating. It includes:
a) Turbine kilowatts d) Phases
b) Generator kilovolt amperes e) Frequency
c) Generator Voltage f) Power factor
• Steam conditions. It includes the following:
a) Initial steam pressure, and Temperature
b) Reheat pressure and temperature
c) Exhaust pressure.
• Steam extraction arrangement such as automatic or non-automatic extraction.
• Accessories such as stop and throttle valve, tachometer etc.
• Governing arrangement. 122
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STEAM TURBINE TESTING
• Steam turbine tests are made for the following:
• Power . Running balance.
• Valve setting . Speed regulation & Over speed trip setting .
• Steam condition is determined by pressure gauge & thermometer where steam super heated.
• Acceptance test as ordinarily performed is a check on (a) Output, (b) Steam rate or heat consumption, (c) Speed
regulation, (d) Over speed trip setting.
• Periodic checks for thermal efficiency and load carrying ability are made. Steam used should be clean. Unclean
steam represented by dust carry over from super heater may cause a slow loss of load carrying ability.
• Thermal efficiency of steam turbine depends on the following factors:
• Steam pressure and temperature at throttle valve of turbine.
• Exhaust steam pressure and temperature.
• Number of bleedings.
• Lubricating oil should be changed or cleaned within the recommendation.
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The impulse principle
• If high velocity, pressure & temperature steam is expand in nozzles and comes out and blow to a curved
blade at high velocity, then steam direction will be changed as shown in figure. Hence, high velocity
steam jet will impact a force on turbine blades as a direction shown. Then continuous power can be
produced if a series of blades were attached on the circumference of a turbine wheel.
• The steam jet is inclined to the blade at an angle of ( ), so that as the wheel rotates, the blades will
continually face the jet as shown in figure.
• Single unit of steam turbine can develop power ranging from 1 mW to 1000 mW.
• In general, 1 mW, 2.5 mW, 5 mW, 10 mW, 30 mW, 120 mW, 210 mW, 250 mW, 350 mW, 500 mW, 660 mW,
1000 mW are in common use.
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The reaction principle
• A high pressure steam is passed through nozzle, as shown in figure. When steam comes out from these
nozzles, its velocity increases relative to the rotating disc.
• In practice, we hardly find any pure reaction turbine. The common type is impulse-reaction turbine
known as “Reaction Turbine”.
• In reaction turbine, the pressure drop occur in both
the stationery and moving blades.
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VELOCITY DIAGRAM FOR IMPULSE TURBINE BLADE
• In an impulse turbine, the rotor drives by the force from the steam jet impact force (the direct push of
steam on the blades). It was first built in 1883 by the Swedish engineer De Laval.. The following symbols
are used to designated the velocity conditions of the turbine blade.
Special case : U
• if the blade is symmetrical, then 1 = 2
• if no friction effects are to be considered, then 1 = 2
• however, friction exist in actual conditions where, (Vr2) is reduced by a factor called “blade velocity
coefficient (K) “ , i.e. 2 = . , then 126
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FORCES (MOMENTUM) ON BLADES
• Force = mass x acceleration;
RATE OF WORK DONE-POWER
• Power = FV
STEAM TURBINE BLADE EFFICIENCY
• The total energy available for mechanical work is given by
• The efficiency of the blade
•
• or 128
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AXIAL THRUST ON ROTOR
• Axial trust or force on the rotor is given by
Faxial = change of momentum in the axial direction
• For negligible blade friction
EFFECT OF FRICTION ON BLADE EFFICIENCY
• The total energy converted to heat energy due to friction in the passages of the turbine blades of given by
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• How steam turbine works
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Example
1. The velocity of steam leaving the nozzle of an impulse turbine is 900 m/s and the nozzle angle is 200.
the blade velocity is 300 m/s and the blade velocity coefficient is 0.7. for a steam mass flow rate of
1kg/s, and symmetrical blading, Determine the following.
a. The blade inlet angle ( 1). d. The driving force on the wheel (FT)
b. The axial trust (Fa) e. The diagram Power (P)
c. The diagram efficiency ( )
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Exercise
1. The velocity of steam entering a simple impulse turbine is 1000 m/s and the nozzle angle is 200. the
mean velocity of the blade is 400 m/s. if the steam is to enter the blade without shock, what will be the
blade angles?
a. Neglecting friction effects on the blade, calculate the tangential force on the blades and the
diagram power for a mass flow rate of 0.75 kg/s. Determine also the axial thrust and the diagram
efficiency.
b. If the relative velocity at exit is reduced by friction to 80% of the at inlet, determine the axial
thrust, diagram power and diagram efficiency.
2. In a stage of an impulse turbine provided with a single row of wheel, the mean diameter of the
blade ring is 800 mm and the speed of rotation is 3000rpm. The steam issues from the nozzles
of angle 200 with a velocity of 300 m/s. if the blade velocity coefficient is 0.86, what is the power
developed in the blades. When the axial thrust on the blade is 140N.
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3. The following data refer to a two-row velocity compounded impulse wheel.
• Steam velocity at nozzle exit = 600 m/s
• Nozzle angle = 160
• Mean blade velocity = 120 m/s
• Exit angles:
• First row of moving blades = 180
• Fixed guide blades = 220
• Second row of moving blades = 360
• Steam flow rate = 5 kg/s
• Blade velocity coefficient = 0.85
Determine a) The tangential thrust
b) The power developed
c) The diagram efficiency.
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