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8/12/2019 Pumps in Steam Power Plants
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Pumps in Steam Power Plants
Life Inducing Devices
P V SubbaraoProfessor
Mechanical Engineering Department
I I T Delhi
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Classification of Pumps
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Pumps in Steam Power Plants
Turbogenerator & Auxiliaries 3 sets.
Steam generator equipment 6 sets.
Chemical feed system 13 sets.
Fuel Oil systems
14 sets.
Lubricating oil systems 5 sets.
Fire Protection systems 6 sets.
Service water system 7 sets.
Miscellaneous around 4 sets.
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Pump services in Main Steam Cycle
Turbogenerator and auxiliaries Condenser circulating pumps
Screen wash-water pumps
Cooling tower make-up pumps
Steam generator equipment Condensate pumps
Condensate booster pumps
Boiler-feed pumps
Boiler-feed booster pumps
Deaerator make-up pumps
Heater drain pumps (low and high pressure)
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Boiler Feed Pumps
The boiler feed pump (BFP) is one of the most importantauxiliary equipments in coal-fired power plants.
With the increase in steam parameters of thermodynamic cycle
and the growth of unit capacity, the power capacity of boiler
feed pumps is also growing. The power consumption of BFP has been accounted for about
5% of power generation capacity in the large generating units.
The reasonable choice for boiler feed pump driving mode
plays an important role in the operation economy of the entirepower plant.
The type and number of BFP and the design of its water
supply system have a great impact on thermal efficiency and
operation cost
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Boiler-Feed Pump Capacity
The total boiler-feed pump capacity is established by adding tothe maximum boiler flow a margin to cover boiler swings and
the eventual reduction in effective capacity from wear.
This margin varies from as much as 20% in small plants to as
little as 5% in the larger central stations. The total required capacity must be either handled by a single
pump or subdivided between several duplicate pumps
operating in parallel.
Industrial power plants generally use several pumps. Central stations tend to use single full-capacity pumps to serve
turbo-generators up to a rating of 100 or even 200 MW and
two pumps in parallel for larger installations.
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Cross-section, single 65,000 hp boiler feed pump,
1300 MW fossil power plant
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Suction Conditions
The net positive suction head (NPSH) represents the net
suction head at the pump suction, referred to the pump
centerline, over and above the vapor pressure of the
feedwater.
If the pump takes its suction from a deaerating heater, the
feedwater in the storage space is under a pressure equivalent to
the vapor pressure corresponding to its temperature.
Therefore the NPSH is equal to the staticsubmergence
between the water level in the storage space and the pumpcenterline less the frictional losses in the intervening piping.
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Erection of Pump
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Theoretically, the requiredNPSH is independent of operating temperature.
Practically, this temperature must be taken into account when establishing the
recommended submergence from the deaerator to the boiler-feed pump.
A margin of safety must be added to the theoretical requiredNPSH to protect theboiler-feed pumps against the transient conditions that follow a sudden reduction
in load for the main turbogenerator.
The discharge pressure of the condensate pump or the booster pump must be
carefully established so the suction pressure of the boiler-feed pump cannot fall
below the sum of the vapor pressure at pumping temperature and the requiredNPSH.
Careful attention must be given to any strainer that might be installed in the
pump suction piping.
The pressure drop increase across the strainer is indicative of foreign material
and it reduces the net positive suction head available (NPSHA) to the pump.
Strainersin the pump suction pipe are most often removed following plant start-
up qualification testing.
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Pump with lower NPSH
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Transient Conditions Following Load Reduction
Following a sudden load reduction, the turbine governor reduces
the steam flow in order to maintain the proper relation betweenturbine and generator power and to hold the unit at synchronous
speed.
The consequence of this reduction is a proportionate pressure
reduction at all successive turbine stages, including the bleed
stage that supplies steam to the deaerator.
The check valve in the extraction line closes and isolates the
heater from the turbine.
As hot feedwater continues to be withdrawn from the heater and
cold condensate to be admitted to the heater, the pressure in the
direct-contact heater starts to drop rapidly.
The check valve reopens when the heater pressure has been
reduced to the prevailing extraction pressure and stable
conditions are reestablished.
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Alternate Means for Low Load Conditions
In the event that circumstances do not permit the provision ofsufficientNPSH margin to provide adequate protection to the
boiler-feed pumps during a sudden turbine load reduction, two
alternate means are available to compensate for these
circumstances:
A small amount of steam from the boiler can be admitted to
the direct-contact heater through a pressure-reducing valve, to
reduce the rate of pressure decay in the heater.
A small amount of cold condensate from the discharge of the
condensate pumps can be made to bypass all or some of the
closed heaters and be injected at the boiler-feed pump suction
to subcool the feedwater, thus providing additionalNPSH
margin during load reduction.
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BOOSTER PUMPS
The increasing sizes of modern boiler-feed pumps coupled
with the practice of operating these pumps at speeds
considerably higher than 3600 rpm have led toNPSH
requirements as high as 46 to 76 m.
In most cases, it is not practical to install the direct-contact
heaters from which the feed pumps take their suction high
enough to meet such requirements.
In such cases, it has become the practice to use boiler-feed
booster pumps operating at lower speeds, such as 1750 rpm, to
provide a greater availableNPSH to the boiler-feed pumpsthan can be made available from strictly static elevation
differences.
Such booster pumps are generally of the single-stage, double-
suction design.
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Axially split case multistage boiler feed pump, up
to 241 bar
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Radially split, segmental ring boiler feed pump
upto 240 bar
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Radially split, double-case, barrel boiler feed pump
above 250 bar
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High-Speed, High-Pressure Boiler Feed Pumps
As steam pressures rose to 200 to 310 barthe total head thatwas required to be developed by the pump rose to as high as
2140 and 3660 m.
The only means available of achieving these higher heads at
3000 rpm was to increase impeller diameter and the number ofstages.
The pumps had to have longer and longer shafts to
accommodate the larger number of stages.
This threatened to interfere with the long uninterrupted lifebetween overhauls to which steam power plant operators were
beginning to become accustomed.
The logical solution was to reduce the shaft span by reducing
the number of stages.
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Drives for Boiler Feed Pumps
There are many factors which affect the driving modes of
BFP, such as thermal economy and operational reliability,
amount of equipment investment and complexity of the system
structure, etc.
Among the factors above, thermal economy is one of the most
important factors when choosing the driving mode of BFP.
As is well known, there are two driving types that are motor-
driven and steam-driven for boiler feed pumps.
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International View : Motor Driven BFPs
The designers and owners of coal-fired power plants in
Western European countries tend to adopt motor-driven pumps
system to feed water for boiler.
The reasons for the choice are that internal efficiency of small
steam turbines which drive feed water pumps in their countries
is almost equivalent to the product of the efficiency of power
transmission and internal efficiency of low-pressure cylinder
of main steam turbine.
On this premise, an integrated investment of motor-driven feedwater pump system is lower than that of steam-driven feed
water pump.
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International View : Steam Driven BFPs
Other people such as American, Russian and Japaneseconsider that steam-driven mode is superior to motor-driven
mode.
The cause of this choice is that the internal efficiency of the
small steam turbine produced by companies in their countrieshas much higher than the product of the efficiency of power
transmission and internal efficiency of low-pressure cylinder
of main steam turbine.
In other word, the net output of generating unit which hassteam-driven feed water pumps is more than that of the same
generating unit which feed water system is driven by
electromotor.
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New Methods for Comparison
A new method called equivalent work efficiency rate to
evaluation thermal economy of the two main feed water pump
driving modes.
Thermal economy evaluation of the two main feed water pump
driving modes.
The heat consumption rate and comprehensive cost-based coal
consumption based on the principle of energy value analysis.
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COMPARISON OF THEIR HEAT CONSUMPTION
RATE
Description of heat consumption rate
Analysis and discuss of an example
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Description of heat consumption rate
Generally speaking, heat consumption rate (HR) is the key
indicator to determine thermal economy of thermodynamiccycle and operation of the turbine generator unit.
From different point of view, it has two expression forms, one
known as the gross heat rate, and the other called the net heat
rate. Heat consumption rate is defined as the amount heat which
generated 1kWh electricity by generating unit.
For different thermodynamic cycle, the formula of heat rate
has different expression forms. To the intermediate reheating unit whose boiler water is fed by
motor-driven pump, the gross heat consumption rate can be
expressed as
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Gen
crhhrhrhsteamfwmainsteam
P
hhmhhmHR
sup
The net heat consumption rate can be expressed as Formula
bfpGen
crhhrhrhsteamfwmainsteam
PP
hhmhhmHR
sup
To the intermediate reheating unit whose boiler water is fed by
steam-driven pump, the gross and net heat consumption rate are
formulated as
bfpGen
crhhrhrhsteamfwmainsteam
PPhhmhhmHR
sup
Gen
crhhrhrhsteamfwmainsteam
P
hhmhhmHR
sup
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Analysis of A Case Study
As the actual operation of generating unit and the configuration
parameters of motor-driven pumps were not exactly the same indifferent generating unit, the net heat rate of motor-driven pump was
calculated based on an average power consumption of a variety of
motor-driven pumps.
These thermal calculations were preformed for a plan of condensingturbine-driven pump, and then net heat consumption rates in
different operation conditions were obtained.
According to the average power of electromotor units and the
original design gross heat rates of the generator units, the net heat
consumption rates of motor-driven constant speed pumps and
motor-driven variable speed pumps in the sliding pressure modes
were calculated respectively after taken into account enthalpy rise in
feed water pump.
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NET HEAT CONSUMPTION RATES OF FEED
WATER PUMP DRIVEN BY STEAM AND ELECTRICITY OF
300MW UNIT IN SLIDING PRESSURE MODEKJ/KW
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Comparison : Steam & Constant Speed Motor
Thermal economy of steam- driven pumps is better than that of
motor-driven pumps in different operation loads.
In particular, thermal economy of constant speed electric pump
declines quickly in low loads.
As their operating speed is not adjusted, constant speed
electric pumps work in the variable load by reducing the pump
outlet pressure by the way of regulating flow which can be
performed by altering the pump speed through a throttle valve,
so that thermal efficiency of generating units in low-loaddeclines much.
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Comparison : Steam & Variable Speed Motor
Compared to that of the motor-driven mode of variable speed,
thermal economy of units which use steam-driven pumps to feed
water in full load has increased but not significantly.
Better at low load interval from 50% to 90%.
The main reason is that the efficiency of hydraulic coupler is
much lower than that of small steam turbine (SST) driving feed
water pump particularly in low load, and there are electro-
mechanical loss and power transmission loss.
The internal efficiency of SST changes slightly in variable load
conditions, although it is lower than that of main turbine in full
load.
At the same time SST can drive directly feed water pumps,
resulting in better thermal economy, because intermediate link of
energy conversion and transmission is few.
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EQUIVALENT WORK EFFICIENCY
Relative equivalent work efficiency rate is defined that the
ratio of power consumption of motor-driven pumps and
electricity which can be generated in steam turbine by the
equivalent enthalpy drops of the steam flow from extraction
point entering into SST.
This definition can reflect thermal economy of energy owned
by steam and electricity.
The calculation method by equivalent work efficiency is easy
to understand and be performed, simultaneously avoiding thecomputational precision difficulty of small steam turbine
exhaust enthalpy.
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Comprehensive Cost-based Coal Consumption rate
On the basis of the principle of energy value analysis,the term of comprehensive cost-based coal consumption
rate (CCCR) was brought forward, and it is defined as
the following expression.
Comprehensive power generation costs are made of the unitgenerating cost and the cost of plant electric consumption.
Unit generating cost can be express as the product of standard
coal consumption rate for generating and unit price of standard
coal
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The cost of plant electrical power consumption is equal to the
product of power consumption rate and pool purchase price.
So formula of CCCR can be expressed as:
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The physical meaning of CCCR
The physical meaning of CCCR is that the power consumptionof standard coal when 1kWh electricity generated according to
comprehensive generating cost.
Comprehensive cost-based coal consumption rate is a
corrected expression of standard coal consumption rate of
power supply considering monetary values of electricity and
coal.
It reflects main comprehensive cost of generating electricity
essentially.
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Comparison of CCCR
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CONCLUSIONS
With the increase of unit capacity, capacity of feed water
pump correspondingly will increase.
The steam-driven mode of the variable-speed pumps by small
steam turbine will be more and more acceptable to much more
people.
A steam-driven mode is better than motor-driven mode in
thermal economy.
Compared with motor-driven pumps, steam-driven pumps are
good to net electrical output increases for large units, reducing
the net heat rate of generating and CCCR.
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The small steam turbine driving variable-speed pumps does
well in declining of power consumption rate and rising of
operation efficiency, thus it could replace motor-driven pumps
in future.
The driving mode of boiler feed pump is mainly affected by
thermal economy of system.
Besides thermal economy of system, the driving mode of
boiler feed pump also depends on comprehensive combination
of investment income, operating reliability, complexity of
system structure.
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Head Vs Flow Rate & Selection of Operating Point
2
21 QKKHf
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PUMPS R i P ll l
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PUMPS Running Parallel
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Operation of Pumps at Low Flows
There are a number of unfavorable conditions which may occur
separately or simultaneously when the pump is operated at reducedflows. Some include:
Cases of heavy leakages from the casing, seal, and stuffing box
Deflection and shearing of shafts
Seizure of pump internals
Close tolerances erosion Separation cavitation
Product quality degradation
Excessive hydraulic thrust
Premature bearing failures
Each condition may dictate a different minimum flow low requirement. The final decision on recommended minimum flow is taken after
careful techno-economicalanalysis by both the pump user and themanufacturer.
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Cavitation
As the liquid flows onto the impeller of the pump it is acceleratedand initially its pressure falls (Bernoulli).
The pressure subsequently increases as the fluid leaves theimpeller and as the kinetic energy is recovered in the volutechamber.
If the pressure of the liquid falls below the vapour pressure, Pv, theliquid boils, generating vapour bubbles or cavities-cavitation.
The bubbles are swept into higher pressure regions by the liquidflow, where they collapse creating pressure waves and causemechanical damage to solid surfaces.
Moreover, pump discharge head is reduced at flow rates above thecavitation point.
Operation under these conditions is not desirable and damages theequipment.
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NPSH (Net Pressure Suction Head).
Net Positive Suction Head Required, NPSHr
NPSH is one of the most widely used and least understood termsassociated with pumps. Understanding the significance of NPSH isvery much essential during installation as well as operation of thepumps.
Pumps can pump only liquids, not vapors Rise in temperature and fall in pressure induces vaporization
NPSH as a measure to prevent liquid vaporization
Net Positive Suction Head (NPSH) is the total head at the suctionflange of the pump less the vapor pressure converted to fluid column
height of the liquid.
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P f f A D d I ll
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Performance of A Damaged Impeller
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Performance with Reduced Throat Area
P ith Mi W
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Pump with Minor Wears
Pump with Excessive Wear
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Pump with Excessive Wear
Pump with rough impeller & casing
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Pump with rough impeller & casing