Steam turbine part-2,Compounding of steam turbines
SIMPLE
VELOCITY-COMPOUNDED IMPULSE TURBINE
What is velocity compounding?
In this type
of turbine, the compounding is done for velocity of steam only i.e. drop in
velocity is arranged in many small drops through many moving rows of blades
instead of a single row of moving blades. It consists of a nozzle or a set of
nozzles and rows of moving blades attached to the rotor or wheel and rows of
fixed blades attached to casing as shown in Fig. 6.8.
The fixed blades are guide blades which
guide the steam to succeeding rows of moving blades, suitably arranged between
the moving blades and set in a reversed manner. In this turbine, three rows OT
rings of moving blades are fixed on a single wheel or rotor and this type of
wheel is termed as the three Tow wheel. There are two blades or fixed blades
placed between Lint first and the second and the second and third rows of
moving blades respectively.
The whole
expansion of steam from the steam chest pressure to the exhaust pressure takes
place in the nozzles only. There is no drop in either in the moving blades or
the fixed i.e. the pressure remains constant in the blades as in the simple
impulse turbine. The steam velocity from the exit of the nozzle is very high as
in the simple impulse turbine. Steam with this high velocity enters the first
row of moving blades and on passing through these blades, the Velocity slightly
reduces i.e. the steam gives up a part of its kinetic energy and reissues from
this row of blades with a fairly high velocity. It then enters the first row of
guide blades which directs the steam to the second row of moving blades.
Actually, there is a slight drop in velocity in the fixed or guide blades due
to friction. On passing through the second row of moving blades some drop in
velocity again occurs i.e. steam gives up another portion of its kinetic energy
to the rotor. After this, it is redirected again by the second row of guide
lades to the third row of moving blades where again some drop in velocity
occurs and finally the steam leaves the wheel with a certain velocity in a more
or less axial direction. compared to the simple impulse turbine, the leaving
velocity is small and it is about 2 percent of initial total available energy
of steam.
So we can say that this arrangement is nothing but splitting up the velocity gained from the exit of the nozzles into many drops through several rows of moving blades and hence the name velocitycompounded This type of turbine is also termed as Curtis turbine. Due to its low efficiency the three row wheel is used for driving small machines The two row wheel is more efficient than the three-row wheel.
velocity compounding is also possible with only one row of moving blades. The whole pressure drop takes place in the nozzles and the high velocity steam passes through the moving blades into a
reversing chamber where the direction of the steam is changed and the same steam is arranged to pass through the moving blade of the same rotor. So instead of using two or three rows of moving blades, only one row is required to pass the steam again and again; thus in each pass velocity decreases.
PRESSURE AND VELOCITY COMPOUNDED IMPULSE TURBINE
This type of
turbine is a combination of pressure and velocity compounding and is
diagrammatically. There are two wheels or rotors and on each, only two rows of
moving blades are attached cause two-row wheel are more efficient than
three-row wheel. In each wheel or rotor, velocity drops i.e. drop in velocity
is achieved by many rows of moving blades hence it is velocity compounded.
There are two sets of nozzles in which whole pressure drop takes place i.e.
whole pressure drop has been divided in small drops, hence it is
pressure-compounded.
In the first
set of nozzles, there is some decrease in pressure which gives some kinetic
energy
od there is no drop in pressure in the
two rows of moving blades of the first wheel and w of fixed blades. Only, there
is a velocity drop in moving blades though there is also a slight drop
ity due to friction in the fixed blades. In second set of nozzles, the remaining pressure drop lan but the
velocity here increases and the drop in velocity takes place in the moving blades of a wheel or rotor. Compared to the pressure-com-pounded impulse turbine this arrangement was popular due to its simple construction. It is, however, very rarely used now due to its low efficiency.
IMPULSE-REACTION
TURBINE
As the name
implies this type of turbine utilizes the principle of impulse and reaction
both. Such pe of turbine is diagrammatically shown. There are a number of rows
of moving blades attached to the rotor and an equal number of fixed blades
attached to the casing.
In this type
of turbine, the fixed blades which are set in a reversed manner compared to the
moving blades, corresponds to nozzles mentioned in connection with the impulse
turbine. Due to the Tow of fixed blades at the entrance, instead of the
nozzles, steam is admitted for the whole circumference and hence there is
all-round or complete admission. In passing through the first row of fixed
lades, the steam undergoes a small drop in pressure and hence its velocity
somewhat increases. After is it then enters the first row of moving blades and
just as in the impulse turbine, it suffers a change in action and therefore in
momentum. This momentum gives rise to an impulse on the blades.
But in this type of turbine, the passage of the moving blades is so designed (converging) that ere is a small drop in pressure of steam in the moving blades which results in a increase in kinetic lergy of steam. This kinetic energy gives rise to reaction in the direction opposite to that of added clocity. Thus, the gross propelling force or driving force is the vector sum of impulse and reaction es. Commonly, this type of turbine is called Reaction Turbine. It is obvious from the Fig. 6.10 that a oradnal dron in nressure in both moving blades and fixed blades.
As the
pressure falls, the specific volume increases and hence in practice, the height
of blad increased in steps i.e. say upto 4 stages it remains constant, then it
increases and remains constant for next two stages.
In this type of turbine, the steam
velocities are comparatively moderate and its maximum value about equal to
blade velocity. In general practice, to reduce the number of stages, the steam
velocitud arranged greater than the blade velocity. In this case the leaving
loss is about 1 So 2 per cent of the tot initial available energy. This type of
turbine is used mostly in all power plants where it is great succes An example
of this type of turbine is the Parsons-Reaction Turbine. The power plants 30 MW
and ahe are all impulse-reaction type.
ADVANTAGES OF
STEAM TURBINE OVER STEAM ENGINE
The various advantages of steam turbine are as follows:
(i) It requires less space.
(ii) Absence of various links such as
piston, piston rod, cross head etc. make the mechanism simple. It is quiet and
smooth in operation,
(i) Its over-load capacity is large.
(iv) 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.
(v) The internal lubrication is not
required in steam turbine. This reduces to the cost of lubrication.
(vi) In steam turbine the steam
consumption does not increase with increase in years of service. (vii) In steam
turbine power is generated at uniform rate, therefore, flywheel is not needed.
(vii) It can be designed for much higher speed and greater range of speed. (ix)
The thermodynamic efficiency of steam turbine is higher.
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: (1) Higher efficiency. (ii) Lower cost per unit capacity. (iii) Lower
space requirement per unit capacity. 3.45.1 Nominal rating. It is the declared
power capacity of turbine expected to be maximum load.
CAPABILITY
The capability of set 9 under specified throttle The difference be practice is to design are absorb rated power at o turbine capability, ability of steam turbine is the maximum continuous out put for a clean turbine operate a throttle and exhaust conditions with full extraction at any openings if provided. once between capability and rating is considered to be overload capacity. A com in a turbine for capability of 125% nominal rating and to provide a generator that was war at 0.8 power factor. By raising power factor to unity the generator will absorb the full turbine capability
STEAM TURBINE
GOVERNING
Owning of
steam turbine means to regulate the supply of steam to the turbine in order to
need of rotation sensibly constant under varying load conditions. Some of the
methods maintain speed of reproved are as follows:
Bypass
governing.
(ii) Nozzle
control governing.
(iii)
Throttle governing. In this system the steam enters the turbine chest
(C) through a
valve
(V) Controlled
by governor.
of loads of
greater than economic load a bypass valve (Vi) opens and allows steam to pass
from the first stage nozzle box into the steam belt (S).
In this method of governing the supply
of steam of various nozzle groups N , N2, and N3 is regulated by means of
valves V1, V2 and V3 respectively.
In this method of governing the double
beat valve is used to regulate the flow of steam into the urbane. 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.
STEAM TURBINE PERFORMANCE
Steam as it leaves the Turbine performance can be expressed by the following factors :
(i) The steam flow process through the unit-expanse online or condition curve.
(ii) The steam flow rate through the unit.
(ii) Thermal efficiency.
(iv) Losses such as exhaust, mechanical, generator, radiation etc. Mechanical losses include bearing losses, oil pump losses and generator bearing losses. Generics include will electrical and mechanical losses. Exhaust losses include the kinetic energy of the 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 aperture. Steam pressure can be easily regulated by means of safety valve fitted on the boiler.
Temperature may try to fluctuate because of the following reasons:
Variation in heat produced due to
varying amounting amounts of fuel burnt according to changing loads.
(ii) Fluctuation in quantity of excess air.
(iii)
Variation in moisture content and temperature temperature of air entering the
furnace.
(iv) Variation in temperature of feed
water.
(v) The varying condition of
cleanliness of heat absorbing surface. The efficiency of steam turbines can be
increased:
(1) By using super heated steam.
(ii) Use of
bled steam reduces the heat rejected to the condenser and this increa
efficiency.
is increases the tur
STEAM TURBINE
TESTING
Steam turbine
tests are made for the following:
(i)
Power
(ii)
Valve setting
(iii) Speed regulation
(iv) Over speed trip setting
(v) Running balance.
Steam condition is determined by
pressure gauge, and thermometer where steam is super he The acceptance test as
ordinarily performed is a check on (a) Output, (b) Steam rate or heat cons
tion, (c) Speed regulation, (d) Over speed trip setting.
Periodic checks for thermal efficiency
and load carrying ability are made. Steam used should clean. Unclean steam represented
by dust carry over from super heater may cause a slow loss of carrying ability.
Thermal efficiency of steam turbine
depends on the following factors:
(i) Steam pressure and temperature at throttle
valve of turbine.
(ii) Exhaust steam pressure and temperature.
(iii) Number of bleedings. Lubricating oil
should be changed or cleaned after 4 to 6 months.
CHOICE OF
STEAM TURBINE
The choice of
steam turbine depends on the following factors:
(i) Capacity of plant
(ii) Plant load factor and capacity factor
(iii) Thermal efficiency
(iv) Reliability
(v) Location
of plant with reference to availability of water for condensate.
STEAM TURBINE
GENERATORS
A generator
converts the mechanical shaft energy it receive from the turbine into electrical
energy. Steam turbine driven a.c. synchronous generators (alternators) are of
two or four pole are These are three phase measuring machines offering
economic, advantages in generation and transmission. Generator loss Lager
generators has Ad ventilation or losses appearing as heat must be
constantly removed to avoid damaging the windstorms have cylindrical
rotors with minimum of heat dissipation surface and so they have Orion to
remove the heat. Large generators generally use an enclosed system with air of
plant. The gas picks up the heat from the generator any gives it up to the
circulating water in
Organ
coolant. The tee heat
STEAM TURBINE
Steam turbine specifications consist of the following:
(1) Turbine rating. It includes:
(a) Turbine kilowatts
(b)Generator kilo volt amperes
(c) Generator Voltage
(d) Phases
(e) Frequency Power factor
(g) Exciter characteristics.
(ii) Steam conditions. It includes the following:
(a) Initial steam pressure, and Temperature
(b) Reheat pressure and temperature
(c) Exhaust pressure.
(iii) Steam extraction arrangement such as automatic or non-automatic extraction
(1) Accessories such as stop and throttle valve, tachometer etc.
(v) Governing arrangement.
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