Pumps in Steam Power Plants

1
Pumps in Steam Power Plants

• P M V Subbarao
• Professor
• Mechanical Engineering Department
• I I T Delhi

Life Inducing Devices
2
Classification of Pumps
3
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.

4
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)

5
Boiler Feed Pumps

• The boiler feed pump (BFP) is one of the most
  important auxiliary 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 entire power 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

6
Boiler-Feed Pump Capacity

• The total boiler-feed pump capacity is
  established by adding to the 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.

7
Cross-section, single 65,000 hp boiler feed pump,
1300 MW fossil power plant
8
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 static
  submergence between the water level in the
  storage space and the pump centerline less the
  frictional losses in the intervening piping.

9
Erection of Pump
10

• Theoretically, the required NPSH 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 required NPSH to protect the
  boiler-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 required NPSH.
• 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.
• Strainers in the pump suction pipe are most often
  removed following plant start-up qualification
  testing.

11
Pump with lower NPSH
12
Transient Conditions Following Load Reduction

• Following a sudden load reduction, the turbine
  governor reduces the steam flow in order to
  maintain the proper relation between turbine 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.

13
Alternate Means for Low Load Conditions

• In the event that circumstances do not permit the
  provision of sufficient NPSH 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 additional
  NPSH margin during load reduction.

14
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 to NPSH 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
  available NPSH to the boiler-feed pumps than can
  be made available from strictly static elevation
  differences.
• Such booster pumps are generally of the
  single-stage, double-suction design.

15
Axially split case multistage boiler feed pump,
up to 241 bar
16
Radially split, segmental ring boiler feed pump
upto 240 bar
17
Radially split, double-case, barrel boiler feed
pump above 250 bar
18
High-Speed, High-Pressure Boiler Feed Pumps

• As steam pressures rose to 200 to 310 barthe
  total head that was 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 of stages.
• The pumps had to have longer and longer shafts to
  accommodate the larger number of stages.
• This threatened to interfere with the long
  uninterrupted life between 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.

19
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.

20
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 feed water pump system is lower than
  that of steam-driven feed water pump.

21
International View Steam Driven BFPs

• Other people such as American, Russian and
  Japanese consider 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 countries has 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 has steam-driven feed water pumps is more
  than that of the same generating unit which feed
  water system is driven by electromotor.

22
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.

23
COMPARISON OF THEIR HEAT CONSUMPTION RATE

• Description of heat consumption rate
• Analysis and discuss of an example

24
Description of heat consumption rate

• Generally speaking, heat consumption rate (HR) is
  the key indicator to determine thermal economy of
  thermodynamic cycle 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

25
The net heat consumption rate can be expressed as
Formula
To the intermediate reheating unit whose boiler
water is fed by steam-driven pump, the gross and
net heat consumption rate are formulated as
26
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 in different
  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 condensing turbine-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.

27
NET HEAT CONSUMPTION RATES OF FEEDWATER PUMP
DRIVEN BY STEAM AND ELECTRICITY OF 300MW UNIT IN
SLIDING PRESSURE MODE(KJ/KW)
28
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-load declines much.

29
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.

30
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 the
  computational precision difficulty of small steam
  turbine exhaust enthalpy.

31
(No Transcript)
32
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 unit generating 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

33

• 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

34
The physical meaning of CCCR

• The physical meaning of CCCR is that the power
  consumption of 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.

35
Comparison of CCCR
36
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.

37

• 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.

38
Head Vs Flow Rate Selection of Operating Point
39
(No Transcript)
40
PUMPS Running Parallel
41
(No Transcript)
42
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 reduced flows. 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-economical analysis
  by both the pump user and the manufacturer.

43
Cavitation

• As the liquid flows onto the impeller of the pump
  it is accelerated and initially its pressure
  falls (Bernoulli).
• The pressure subsequently increases as the fluid
  leaves the impeller and as the kinetic energy is
  recovered in the volute chamber.
• If the pressure of the liquid falls below the
  vapour pressure, Pv, the liquid boils, generating
  vapour bubbles or cavities-cavitation.
• The bubbles are swept into higher pressure
  regions by the liquid flow, where they collapse
  creating pressure waves and cause mechanical
  damage to solid surfaces.
• Moreover, pump discharge head is reduced at flow
  rates above the cavitation point.
• Operation under these conditions is not desirable
  and damages the equipment.

44
(No Transcript)
45
NPSH (Net Pressure Suction Head).

• Net Positive Suction Head Required, NPSHr
• NPSH is one of the most widely used and least
  understood terms associated with pumps.
  Understanding the significance of NPSH is very
  much essential during installation as well as
  operation of the pumps.
• 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 suction flange of the pump less the
  vapor pressure converted to fluid column height
  of the liquid.

46
(No Transcript)
47
Performance of A Damaged Impeller
48
Performance with Reduced Throat Area
49
Pump with Minor Wears
50
Pump with Excessive Wear
51
Pump with rough impeller casing