Basics of Supercritical Steam Generators

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Basics of Supercritical Steam Generators
P M V Subbarao Professor Mechanical Engineering
Department I I T Delhi
Thermodynamically Obvious Choice but . Why
Arent There More Supercritical Units?
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Reheat Supercritical Rankine Cycle
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5
2s
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2
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1
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Double Reheat Ultra Super Critical Cycle
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Historical Development in Steam Generators
Furnace Heat Release Rate
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Effect of Boiler Steam Conditions on Net Plant
Efficiency
250bar 565/5900C
250bar 565/5650C
250bar 556/5650C
170bar 556/5560C
250bar 556/5560C
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Towards Matured Sustainable SC Technology
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Why So Late?

• If supercritical units are more efficient
  supercritical the right answer for any new
  coal-fired unit?
• First, there is history.
• Most coal-fired plants in the World are
  subcritical.
• Second, initial supercritical units suffered more
  from the rapid increase in unit size than from
  technology.
• Supercritical systems demanded considerable
  advances in design and operation .
• Heavy Slagging Undersized furnace and inadequate
  coverage by soot blowing system.
• Circumferential cracking of waterwall tubes High
  Metal Temperatures.
• Frequents requirement of acid cleaning.
• Low Availability and Reliability.
• Lower efficiency than expected.

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Historical trend of furnace heat release rates
for coal-fired boilers
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Expectations from Steam Generator for Higher
Efficiency

• High Main Steam Pressure.
• High Main Steam Temperature.
• Double Reheat Higher Regeneration.
• Metal component strength, stress, and distortion
  are of concern at elevated temperatures in both
  the steam generator and the steam turbine.
• In the steam generators heating process, the
  tube metal temperature is even higher than that
  of the steam, and concern for accelerated
  corrosion and oxidation will also influence
  material selection.

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Role of SG in Rankine Cycle
Perform Using Natural resources of energy .
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Thermal Structure of A SG
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The Basics of Flow Boiling.
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Boiling process in Tubular Geometries
Water
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Religious to Secular Attitude
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Flow Boiling
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Tube Wall Temperature Sub-critical Flow Boiling
Newtons Law of Cooling
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Heat Transfer in Flow Boiling
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Auto Control Mechanism in Natural Circulation
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Religious to Secular Attitude
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Tube Wall Temperature Super-critical Flow
Boiling
Newtons Invention
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Heat Transfer in Liquid Region

• The liquid in the channel may be in laminar or
  turbulent flow, in either case the laws governing
  the heat transfer are well established.
• Heat transfer in turbulent flow in a circular
  tube can be estimated by the well-known
  Dittus-Boelter (subcritical ) equation.

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Thermo physical Properties at Super Critical
Pressures
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Heat Transfer Coefficient
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Actual Heat Transfer Coefficient of SC Water
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Specific heat of Supercritical Water
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Pseudo Critical Line
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Study of Flow Boiling
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Selection of Flow rate in Flow Boiling

• This process may either be forced convection or
  gravity driven.
• At relatively low flow rates at sufficient wall
  superheats, bubble nucleation at the wall occurs
  such that nucleate boiling is present within the
  liquid film.
• At high qualities and mass flow rates, the flow
  regime is normally annular.
• As the flow velocity increases, convection in the
  liquid film is augmented.
• The wall is cooled below the minimum wall
  superheat necessary to sustain nucleation.
• Nucleate boiling may thus be suppressed, in which
  case heat transfer is only by convection through
  the liquid film and evaporation occurs only at
  its interface.

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Pressure drop Religious to Secular Attitude
Dphydro Dpfriction
Dphydro Dpfriction
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Pressure Drop in Tubes

• The pressure drop through a tube comprise several
  components friciton, entrance loss, exit loss,
  fitting loss and hydrostatic.

Exact prediction of wall temperature, it is
important to know the pressure Variation along
the flow
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Selection of Steam Mass Flow rate

• A once-through forced circulation furnace with
  high mass flow
• If any tube receives more heat than the average,
  then it will accept receives less flow.
• This can result in further increasing
  temperatures, potentially leading to failures.
• Thus the mass flow per tube must start very high
  to ensure adequate remaining flow after the heat
  upsets.
• Designs with medium mass flow
• These were attempted in once through forced
  circulation boilers with moderate success.
• These exhibit worse consequences than the high
  mass flow designs.
• When the mass flow is degraded during load
  reduction in a tube receiving more heat than the
  average, the remaining flow will have less margin
  to provide acceptable cooling.
• Medium mass flow designs can experience heat
  upsets and/or flow excursions that result in
  flows in individual tubes that are lower than the
  low mass flow design.

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Destructive Mechanisms in Forced Circulation/Onec
through
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• Both the multi-pass and the spiral designs use
  high fluid mass flows.
• High fluid mass flow rates result in high
  pressure losses as well as a once-through .
• Means that strongly heated tubes have a reduction
  in fluid mass flow and a correspondingly high
  increase in fluid and therefore metal temperature
  which can result in excessive tube-to-tube
  temperature differentials.
• This type of behavior is sometimes referred to as
  a "negative flow characteristic.
• In the Vertical design, the furnace enclosure is
  formed from a single, upflow pass of vertical
  tubes.

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• The tube size and spacing is selected to provide
  a low fluid mass flow rate of approximately 1000
  kg/m2-s or less.
• With low mass flow rates, the frictional pressure
  loss is low compared to the gravitational head,
  and as a result, a tube that is heated strongly,
  i.e., absorbs more heat, draws more flow.
• With an increase in flow to the strongly heated
  tube, the temperature rise at the outlet of the
  tube is reduced which limits the differential
  temperature between adjacent tubes.
• This is known as the "natural circulation or
  "positive flow" characteristic.
• Minimize peak tube metal temperatures.
• To minimize peak tube metal temperatures,
  multiple pass and spiral types designs use high
  fluid mass flow rates to achieve good tube
  cooling.
• This results in the "once-through characteristic
  noted above.

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The State of the Art The Market

• In the past both the multi-pass and the spiral
  designs were using high fluid mass flows.
• High fluid mass flow rates result in high
  pressure losses as well as a once-through.
• Means that strongly heated tubes have a reduction
  in fluid mass flow and a correspondingly high
  increase in fluid and therefore metal temperature
  which can result in excessive tube-to-tube
  temperature differentials.
• This type of behavior is sometimes referred to as
  a "negative flow characteristic.
• In the Vertical design, the furnace enclosure is
  formed from a single, upflow pass of vertical
  tubes.

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• The tube size and spacing is selected to provide
  a low fluid mass flow rate of approximately 1000
  kg/m2-s or less.
• At low mass flow rates, the frictional pressure
  loss is low compared to the gravitational head.
• A tube that is heated strongly, i.e., absorbs
  more heat, draws more flow.
• With an increase in flow to the strongly heated
  tube, the temperature rise at the outlet of the
  tube is reduced.
• This limits the differential temperature between
  adjacent tubes.
• This is known as the "natural circulation or
  "positive flow" characteristic.
• Minimize peak tube metal temperatures.

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Heat Transfer Deterioration

• At optimal mass flow heat flux the wall
  temperature behaves smoothly and increases with
  increasing bulk temperature.
• The difference between wall temperature and fluid
  bulk temperature remains small.
• At high heat flux the wall temperature increases
  sharply and decreases again.
• This large increase in temperature is referred to
  Heat Transfer Deterioration.
• There is no unique definition for onset of Heat
  Transfer Deterioration.
• This puts a great challenge to boiler water wall
  design.

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Supercritical Benson Steam Generator

• Supercritical units use a once-through design,
  also referred to as the Benson cycle.
• In a once-through boiler the fluid passes through
  the unit one time, and there is no recirculation
  as takes place in the water walls of a typical
  drum-type boiler.
• Since there is no thick-walled steam drum, the
  startup time and ramp rates for a once-through
  unit can be significantly reduced from that
  required for a drum-type unit.

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Circulation Vs Once Through
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Special Features of Supercritical Steam
Generators

• Wall thicknesses of the tubes and headers need to
  be designed to match the planned pressure level.
• At the same time, the drum of the drum-type
  boiler which is very heavy and located on the top
  of the boiler can be eliminated.
• Once-through boilers can be operated at variable
  steam pressure.
• Once-through boilers have been designed in both
  two-pass and tower type design.
• Large once-through boilers have been built with a
  spiral shaped arrangement of the tubes in the
  evaporator zone.
• The latest designs of once-through boilers use a
  vertical tube arrangement.

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Examples of Once-through Boilers
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BENSON BOILER ARRANGEMENTS

• Benson boilers are designed and constructed in
  two basic arrangements, the two-pass or tower
  type.
• Both perform equally well.
• The selected arrangement is generally driven by
  customer preference and site-specific factors.
• Some particular advantages of the two-pass design
  are
• Small plant profile (height)
• Lower cost construction
• More optimized heating surface size because of
  decoupling back-pass from furnace section
• Smaller stack height requirement, depending on
  regulations.
• The tower arrangement also has certain
  advantages. They are
• Small plant footprint, especially if fitted with
  SCR
• Even flow distribution of flue gas and
  particulates
• Lower flue gas velocity and erosion potential
• Direct load transmission to the boiler roof and
  free expansion
• No temperature differences between adjacent wall
  systems
• Ease of extreme cycling operation.

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1000MW BABCOCK-HITACH
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Main Specifications
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Once Through SG Furnace Arrangements
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Furnace Arrangements

• In a rectangular furnace, the heat flux imposed
  upon the furnace walls is not evenly distributed
  as shown below.

• To accommodate the differences in heat flux
  around the periphery of the furnace and the
  upsets inherent in the combustion process,
  sufficient fluid flow must be provided to each
  tube to maintain tube temperatures within
  acceptable limits over the load range.

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Vertical Tube Furnace

• To provide sufficient flow per tube, constant
  pressure furnaces employ vertically oriented
  tubes.
• Tubes are appropriately sized and arranged in
  multiple passes in the lower furnace where the
  burners are located and the heat input is high.
• By passing the flow twice through the lower
  furnace periphery (two passes), the mass flow per
  tube can be kept high enough to ensure sufficient
  cooling.
• In addition, the fluid is mixed between passes to
  reduce the upset fluid temperature.

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Spiral Tube Furnace

• The spiral design, on the other hand, utilizes
  fewer tubes to obtain the desired flow per tube
  by wrapping them around the furnace to create the
  enclosure.
• This also has the benefit of passing all tubes
  through all heat zones to maintain a nearly even
  fluid temperature at the outlet of the lower
  portion of the furnace.
• Because the tubes are wrapped around the
  furnace to form the enclosure, fabrication and
  erection are considerably more complicated and
  costly.

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Riffled Tubes

• The advanced Vertical technology is
  characterized by low fluid mass flow rates.
• Normally, low fluid mass flow rates do not
  provide adequate tube cooling when used with
  smooth tubing.
• Unique to the Vertical technology is the use of
  optimized rifled tubes in high heat flux areas to
  eliminate this concern.
• Rifled in the lower furnace, smooth-bore in the
  upper furnace.
• The greatest concern for tube overheating occurs
  when the evaporator operating pressure approaches
  the critical pressure.
• In the range 210 to 220 bar pressure range the
  tube wall temperature required to cause film
  boiling (departure from nucleate boiling DNB)
  quickly approaches the fluid saturation
  temperature.

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• DNB will occur in this region and a high fluid
  film heat transfer coefficient is required to
  suppress the increase in tube wall temperature.
• Standard rifled tubing will provide an
  improvement in heat transfer.

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High main steam temperature even at part loads

• Main steam temperatures in the once through
  boiler are independent of load.
• This results in a higher process efficiency for
  the power plant over a wide load range.
• The fuel/feed water flow ratio is controlled in
  the once through boiler such that the desired
  steam temperature is always established at the
  main steam outlet.
• This is made possible by the variable evaporation
  endpoint.
• The evaporation and superheating surfaces
  automatically adjust to operating conditions.
• In dynamic processes, desuperheaters support
  maintenance of constant main steam temperature.

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• Minimum output in once-through operation at high
  main steam temperatures is 35 to 40 for furnace
  walls with smooth tubes and is as low as 20 if
  rifled tubes are used.

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Fuel Flexibility

• Compare the behavior of a natural circulation
  boiler and a Benson boiler with a hypothetical
  10 deviation of furnace heat absorption due to
  varying combustion and slagging characteristics.
• In the natural circulation boiler, the result of
  reduced furnace heat absorption is an increased
  furnace exit gas temperature and a higher steam
  attemperation rate.
• In the case of enhanced furnace heat absorption,
  the attemperation rate is gradual reduced to zero
  and the superheater outlet temperature drops.
• In contrast, the Benson boiler balances
  differences in heat absorption via the variable
  evaporation end-point.
• The allocation of the evaporative and superheat
  duties to the various surfaces may vary according
  to differential heat absorption of the furnace as
  well as with changing load.
• By controlling the firing rate according to final
  steam temperature, rather than to drum pressure,
  the Benson boiler achieves the desired steam
  temperature, at nearly constant attemperation
  rate, largely independent of shifts in heat
  absorption of the heating surfaces.

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Fuel Flexibility
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Furnace Design Vs Ash Content
Ash Content
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Issues with High Ash Coals

• Severe slagging and/or fouling troubles that had
  occurred in early installed coal fired utility
  boilers are one of the main reasons that led to
  their low availability.
• Furnace dimensions are determined based on the
  properties of coals to be burned.
• Some coals are known to produce ash with specific
  characteristics, which is optically reflective
  and can significantly hinder the heat absorption.
  
• Therefore an adequate furnace plan area and
  height must be provided to minimize the slagging
  of furnace walls and platen superheater sections.

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• The furnace using high ash coal need to be
  designed such that the exit gas temperature
  entering the convection pass tube coils would be
  sufficiently lower than the ash fusion
  temperatures of the fuel.
• For furnace cleaning, wall blowers will be
  provided in a suitable arrangement.
• In some cases as deemed necessary, high-pressure
  water-cleaning devices can be installed.
• As for fouling, the traverse pitches of the tubes
  are to be fixed based on the ash
  content/properties.
• An appropriate number and arrangement of steam
  soot blowers shall be provided for surface
  cleaning.

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Countermeasures for Circumferential Cracking

• There have been cases of waterwall tube failures
  caused by circumferential cracking in older
  coal-fired boilers.
• It is believed that this cracking is caused by
  the combination of a number of phenomena,
• the metal temperature rise due to inner scale
  deposits,
• the thermal fatigue shocks caused by sudden
  waterwall soot-blowing, and
• the tube wastage or deep penetration caused by
  sulfidation.
• Metal temperature rise due to inner scale
  deposits can be prevented by the application of
  an OWT water chemistry regime.

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7-year OWT experience in 1,000MWsupercritical
coal firing boiler in Japan
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• In high sulfur coal firing boilers, the tube
  wastage from sulfidation is to be controlled by
  applying protective coatings in appropriate areas
  of the furnace, as well as by the selection of
  optimum burner stoichiometry.
• The sudden metal temperature change due to
  periodic de-slagging can be minimized by
  optimized operation of wall blowers or
  high-pressure water cleaning systems.

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Closing Remarks on Super Critical Steam Generators

• Critical to the design of a supercritical
  once-through boiler is the design of the furnace
  steam/water evaporator circuitry, the associated
  start-up system.
• How they are integrated with the firing and heat
  recovery area (HRA) systems.
• To provide safe and reliable operation of a
  once-through supercritical
• boiler requires minimizing peak tube metal
  temperatures and limiting the temperature
  differential between adjacent enclosure tubes.
• The Vertical boiler addresses these issues in the
  following unique and effective ways

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• Limit tube-to-tube temperature differentials.
• In a once-through boiler which operates at
  supercritical pressure, there is no distinction
  between liquid and vapor phases, and there is a
  continual increase in fluid temperature.
• Radial unbalances in heat absorption occur due
  to,
• tube geometric position,
• burner heat release pattern,
• Furnace cleanliness,
• variations in flow rate, caused by
• hydraulic resistance differences from
  tube-to-tube,
• All the lead to variations in tube temperatures
  occur.
• High differential temperatures induce high
  thermal stresses, which, if not limited, can
  result in tube failure.

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• Historically, this issue has been addressed in
  two different ways
• In units with multiple passes in the furnace
  evaporator, the differential temperature is
  limited by the fact that each pass picks up a
  fraction of the total evaporator duty.
• This limits the magnitude of the unbalance and
  intermediate mixing occurs before the fluid is
  distributed to the next downstream pass.
• However, with multiple passes, the furnace must
  operate at supercritical pressure throughout its
  load range to avoid the difficulties of uniformly
  distributing a steam-water mixture to the down
  stream passes.

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• In units with a spiral tube configuration, the
  unbalance issue is addressed by having each
  inclined tube pass through the varying heat
  absorption zones so that each tube absorbs
  approximately the same amount of heat.
• With a single up-flow pass, the spiral design can
  operate with variable pressure steam, which
  minimizes part load auxiliary power requirements
  and allows matching of steam and turbine metal
  temperature for extended steam turbine life.
• However, the spiral tube evaporator configuration
  requires a special support system for the
  inclined tubes, which are not self-supporting.

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Present Status of Technology

• Many of the "supercritical-related" problems with
  the early supercritical units have been resolved
  with new designs.
• New units are also designed to operate with
  sliding pressure, which improves load change
  characteristics.
• Once-through design is now available with faster
  responding controls and adaptive tuning over the
  entire load range.

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Sliding Pressure Operation
P M V Subbarao Professor Mechanical Engineering
Department I I T Delhi
High Efficiency at Part Loads
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Sliding pressure operation

• Variable pressure operation (sliding pressure
  operation) is desired in many modern power plants
  because it provides more efficient part load
  operation.
• The loss due to constant pressure operation at
  low load is always a concern for the utility.
• The vertical tube supercritical boiler can
  provide variable turbine pressure operation to
  gain the thermodynamic advantage of variable
  pressure.
• Since the boiler furnace must be operated in the
  supercritical region, this is accomplished by a
  pressure-reducing valve located between
  superheater stages.
• Thus the turbine efficiency advantages are
  obtained but without the savings in boiler feed
  pump power associated with true variable pressure
  operation.

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• The spiral tube furnace design can be operated in
  the subcritical region thus also providing the
  pump power savings.
• The advanced SC boiler with vertical internally
  ribbed tube furnace design may also be operated
  in the subcritical region.
• This provides the same pump power savings in
  variable pressure operation plus the advantage of
  a lower full load pressure loss for additional
  power savings.
• Temperature changes occur in the boiler and in
  the turbine during load changes.
• These can cause thermal stresses in thick walled
  components.
• These are especially high in the turbine during
  constant-pressure operation. They therefore limit
  the maximum load transient for the unit.

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• By contrast, in sliding pressure operation, the
  temperature changes are in the evaporator
  section.
• However, the resulting thermal stresses are not
  limiting in the Once through boiler due to its
  thermo elastic design.

In fixed pressure operation , temperature change
in the turbine when load changes, while in
sliding-pressure operation ,they change in the
boiler
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• The enthalpy increase in the boiler for
  preheating, evaporation and superheating changes
  with pressure.
• However, pressure is proportional to output in
  sliding pressure operation.
• In a uniformly heated tube, the transitions from
  preheat to evaporation and from evaporation to
  superheat shift automatically with load such that
  the main steam temperature always remains
  constant.

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• While sliding pressure is beneficial for the
  turbine, it can cause difficulties for the
  boiler.
• The following adverse effects can occur
• As the pressure falls, the boiling temperature
  (boiling point) changes.
• The boiler is divided into zones in which the
  fluid is expected to be entirely water, mixed
  steam / water or dry steam.
• A change in the boiling point can change the
  conditions in each zone.
• The heat transfer coefficient in each zone
  depends upon the pressure.
• As the pressure falls, the heat transfer
  coefficient reduces.
• This means that the steam may not reach the
  correct temperature.
• Also, if heat is not carried away by the steam,
  the boiler tubes will run hotter and may suffer
  damage.

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• The heat transfer coefficient also depends upon
  the velocity of the steam in the boiler tubes.
• Any change in pressure causes a change in steam
  density and so alters the steam velocities and
  heat transfer rate in each zone.
• Pressure and temperature cause the boiler tubes
  to expand. If conditions change, the tubes will
  move. The tube supports must be capable of
  accommodating this movement.
• The expansion movements must not lead to adverse
  stresses.
• The ability to use sliding pressure operation is
  determined by the boiler

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• Boilers can be designed to accommodate sliding
  pressure.
• When it is used, coal fired boilers in the 500 to
  1000 MW class normally restrict sliding pressure
  to a limited load range, typically 70 to 100
  load, to minimize the design challenge. Below
  this range, the boiler is operated at a fixed
  pressure.
• This achieves an acceptable result because large
  units are normally operated at high load for
  economic reasons.
• In contrast, when sliding pressure is used in
  combined cycle plant, the steam pressure is
  varied over a wider load range, typically 50 to
  100 load or more.

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• As stated, in coal-fired plant, sliding pressure
  is normally restricted to a limited load range to
  reduce design difficulties.
• In this range, the boiler pressure is held at a
  value 5 to 10 above the turbine internal
  pressure. Consequently, the governor valves
  throttle slightly.
• The offset is provided so that the unit can
  respond quickly to a sudden increase in load
  demand simply by pulling the valves wide open.
• This produces a faster load response than raising
  the boiler firing rate alone.The step in load
  which can be achieved equals the specified margin
  ie 5 to 10.

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• The throttling margin is agreed during the
  tendering phase and then fixed.
• A margin of 5 to 10 is usually satisfactory
  because most customers rely upon gas turbines,
  hydroelectric or pumped storage units to meet
  large peak loads.
• The throttling margin means that the full
  potential gain of sliding pressure is not
  achieved.
• Nevertheless, most of the throttling losses which
  would otherwise occur are recovered.

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Sliding Pressure
24.1 Mpa
6.9 Mpa
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• During such operation, the pressure is determined
  by the turbine system and the boiler is operated
  by fully opening the turbine inlet valve.
• At constant-pressure operation, on the contrary,
  the turbine inlet pressure is controlled by
  throttling the turbine inlet valve, which results
  simultaneously in the control of the flow rate.
  This induces reduction in efficiency owing to the
  irreversibility of the throttling.
• On the other hand the efficiency drop at partial
  load is small because of the nonexistence of
  throttle loss.
• At loads of 100-77, the system pressures are in
  the supercritical region.
• In contrast, at 77-25 loads, the system
  pressures are in the sub critical region and
  boiling occurs in the water-wall tubes, while the
  state of the fluid at the outlet of the water
  wall is superheated steam.

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• There are no operational limitations due to
  once-through boilers compared to drum type
  boilers.
• In fact once-through boilers are better suited
  to frequent load variations than drum type
  boilers, since the drum is a component with a
  high wall thickness, requiring controlled
  heating.
• A principle advantage of once through boiler is
  that it does not require circulating pumps or
  drums.
• Energy required for circulation is provide by the
  feed pump.