Feed pumps, Feed Injectors, Feed Regulators, Feed heaters, Air Heaters and Steam Accumulators

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Feed pumps, Feed Injectors, Feed Regulators, Feed
heaters, Air Heaters and Steam Accumulators

• A. Ranganath

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Contents

• Feed Pumps
• Feed Injectors
• Feed Regulators
• Feed Heaters
• Air heaters
• Steam Accumulators

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• FEED PUMPS

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Boiler feed pump

• Pump is a device which converts mechanical
  energy into pressure energy due to which the
  fluid moves from one point to another.
• A Boiler Feed Pump is a specific type of pump
  used to pump feed water into a steam boiler.
• The water may be freshly supplied or returning
  condensate produced as a result of the
  condensation of the steam produced by the boiler.
• These pumps are high pressure units that take
  suction from a condensate return system and can
  be of the centrifugal type or positive
  displacement type.
• The pump also supplies water for attemperation
  sprays that control the Superheat/Reheater steam
  temperature.

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BFP Design Considerations

• Features of BFP
• Capital cost, running cost and maintenance costs
  to be optimised.
• Single pump option generally not considered.
• Satisfactory operation during start up and
  continuous operation.
• Should have adequate margins on NPSH.
• Should be capable of taking care of the pressure
  decay during turbine load variations resulting in
  decay in de-aerator pressure.
• Flow head margins shall be provided.
• Generally the optimum values are for flow margin
  5 above the total flow and Head margin of 3
  above the total head.

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BFP Design Considerations

• Max. availability with a design life of 45,000
  hrs
• Rugged high speed machine
• Designed for rapid replacement of complete shaft
  /rotor assembly
• Substantially stiffened shaft with the number of
  stages, giving improved rotor rigidity and lower
  shaft deflections
• Use of balance drum to oppose axial hydraulic
  thrust with residual unbalance being carried by
  external oil cooled thrust bearing.

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• PUMP CHARACTERISTICS
• - It is the relationship between Capacity,
  Head, Power and Efficiency.
• - The graphs, showing the inter-relationship
  between Capacity, Head, Power and Efficiency,
  are called Pump Characteristic Curves.
• Capacity
• - It is the quantity of fluid flowing through
  the Pump for a given time of period.
• - It is expressed in m3/hr.
• - It is measured by weight method, volumetric
  method, orifice plate or by weirs.
• Head
• - It is the measure of energy to move the fluid
  from one point to another.
• - It is expressed in metres of liquid column.

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• Power
• - The horse power produced by the liquid is
  called as Water Horse Power (WHP) or Liquid
  Horse Power which is expressed as
• WHP (? Q H) / 75
• where Q m3/sec , H mlc ?
  kg/m3
• - The power required to drive the pump is called
  as Brake Horse Power (BHP) which is expressed as
• BHP (? Q H) / 75 ?
• where ? is the efficiency of Pump.
• Efficiency
• - It is the measure of the Pump performance.
• - It is the ratio of WHP to BHP.

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Characteristics of a Pump
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Optimisation of sizing design margins

• Design margins are provided on equipment /
  systems to cater for ageing, wear tear,
  uncertainties etc
• Conservative designs with large margins ( e.g. on
  flow and head of pumps) and specifying
  suitability for abnormal operating conditions
  result in lower efficiency and higher auxiliary
  power consumption
• Proper standby philosophy based on efficiency of
  operation, availability reliability, like
  following are considered.
• 1x100 Working 1x100 Standby or
• 1x100 Working 1x30 Startup or
• 2x50 Working 1x50 Standby etc.

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Pumps parallel operation
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Types of Boiler feed pumps

• Based on the Casing splitting type, Pumps are
  classified as

• AXIALLY-SPLIT CASING TYPE
• RADIALLY-SPLIT CASING TYPE

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• AXIALLY-SPLIT CASING
• - It refers to a Casing split in a plane
  parallel to the axis of rotation.
• - Both Suction Discharge nozzles are located
  on bottom half of the Casing so that the top half
  may be removed for inspection repair without
  disturbing the Pump proper and Suction
  Discharge piping.

• Advantages
• The pump internal can be inspected by simply
  removing the top case no need to remove its
  rotor
• It is relatively inexpensive than a radial split
  case pump.
• Disadvantages
• Typically limited to 204 deg C operating
  temperature due to thermal expansion
  considerations
• Typically limited to 248 bar maximum working
  pressure due to the difficulty in bolting with a
  flat, unconfined, and irregular case gasket, and
  due to the non-symmetrical volute and suction
  areas between the upper half and lower half
  casing.

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• RADIALLY-SPLIT CASING
• - It refers to a Casing split in a plane
  perpendicular to the axis of rotation.
• - It contains two Casings, the inner casing
  encloses the rotating parts of Pumps and the
  outer casing encloses the inner casing.
• - Suction Discharge nozzles are an integral
  part of outer casing and the internal pump
  assembly can be removed without disturbing the
  piping connections.

• Advantages
• Typically suitable for operating at very high
  temperature of up to 426 deg C as the centerline
  support design ensures equal case thermal
  expansion in radial direction
• The case and cover design is suitable for higher
  working pressure than an axial split case pump
  due to its smaller bolting area, symmetrical
  bolting pattern.
• Full cartridge pull out for rapid changeover
• Disadvantages
• The pump internal cannot be inspected without
  removing its rotor assembly from inside the
  casing.
• In some multistage pumps the rotor assembly
  cannot be removed from the casing without
  removing the driver to clear the way for the
  rotor assembly.
• It is very expensive as the pump will have to be
  of double barrel construction.

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BFP TRAIN WITH COMMON FOUNDATION FRAME
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General Problems/Fault finding of BFP
Problem Possible cause
Pump fail to start Motor problem / Seizure of pump set / De-aerator level low low
Pump performance low Problem in motor, suction strainer choked, position of suction valve, excessive wear of pump internals, etc.
Bearings overheating Defective lube oil system, bearings worn or misaligned, misalignment of pump
Mechanical seals overheating Insufficient cooling water, mechanical seal damaged
Excessive noise and / or vibration Misalignment of pump, bearing misalignment, excessive clearance of pump internals, rotating assembly out of balance
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• FEED INJECTOR

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Feed Injector

• Why an Injector
• Theyre a simple device that uses no extra power
  source.
• The design is such that, live steam can inject
  water into the boiler that supplies the steam.
  They are used to replace mechanical driven pumps,
  as injectors are very reliable and very
  efficient.
• Where it is used
• Since very early 1900s the primary water source
  for putting water into locomotive boilers.
• What are Injectors
• They are a device which is used by locomotive
  crew to take water from the water tender and
  combine it with live boiler steam and inject it
  into the boiler via a check valve.
• It is achieved by opening the steam and water
  valves in their correct order, they will pick up
  and inject the water into the boiler.

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Feed Injector

• How does the Injector work
• A injector has several pipe connections, live
  steam supply, water supply, overflow and delivery
  to the boiler. Inside, it has several conical
  shaped cones being, steam, combining and
  delivery.
• Steam injector works on the principle of steam
  nozzle.
• It utilises the kinetic energy of a steam jet for
  increasing the pressure and velocity of feed
  water.
• It is used for forcing the feed water into steam
  boiler under pressure.

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Feed Injector Operating Principle

• The explanation is based on an injector operating
  at 12.41 bar,
• When the steam valve is opened, steam flows
  through the steam cone. It is here where the cone
  reduces in size and this in turn, throttles the
  steam until it reaches a speed of approximately
  1856 kph.
• At this speed it is admitted to the water space
  or combining cone. Where the steam is condensed
  and carries forward by the force of its momentum
  about twelve times its own weight of water at
  speed of about 144 kph.
• The speed attained is sufficient to carry the
  combined jet across the space to the delivery
  tube, and through the check valve into the
  boiler.
• When the steam mixes with the water and
  condenses, this is when it forms the vacuum. This
  then allows atmospheric pressure to push the
  water from the tender up into injector (a very
  important action).

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Feed Injector- Types

• Lifting type
• They are capable of lifting water from a lower
  into the injector then forcing it past the check
  valve into the boiler. They can be placed above
  the water supply on the locomotive.
• It works when the steam is turned on first, this
  allows for a vacuum to be formed causing the
  water to fill the void, under the influence of
  atmospheric pressure.
• Non-lifting type
• Very similar to lifting type, the main difference
  is that they cannot lift water into the injector.
  That means that the water supply must be above
  the injector so that the water flows through the
  injector freely.
• The non-lifting injector must have the water on
  first. Then seeing the water flowing from the
  overflow, turn on the steam valve and it will
  work.
• Turning off the injectors is the same for both.
  Turn off the steam first, then the water.

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Feed Injectors- Advantages and Disadvantages

• Advantages
• No extra power source required
• Very much suitable for small boilers like
  steam-driven locomotive boilers.
• The addition of heat to the flow of water lessens
  the effect of the injected water in cooler the
  water in the boiler compared to the case of cold
  water injected via a mechanical feed pump.
• They are thermally efficient, as most of the heat
  energy in the condensed steam is returned to the
  boiler increasing the thermal efficiency of the
  process.
• No moving parts as in case of pump.
• Disadvantages
• Limited to very small boilers

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• FEED REGULATORS

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Feed Regulators

• A boiler feed water regulator valve used in many
  power plants is required to transition from
  feedpump recirculation to operation of the unit.
• Not only is the valve used to initially fill the
  steam drum, it is also used to control flow
  during normal operation when the steam drum is
  under pressure. This valve, therefore, must
  address cavitation during initial operation and
  provide adequate rangeability to address the
  entire feedwater requirements.

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Feed Regulators

• The regulator valve will begin to transition the
  flow from the recirculation valve and will open
  as the recirculation valve closes.
• The valve must have adequate cavitation
  protection during initial filling of the drum and
  then transition to flow control mode.

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Feed Regulators

• Feed regulators are used to control the drum
  level in power plant.
• It is critical within power plant operation in
  drum boilers (or flow in once-through boilers) to
  maintain the quantity of feed water to drum to
  match with steam generation.
• Drum or boiler level control is crucial at plant
  start-up, when the pressure differential between
  the BFP and boiler is very high and control is
  difficult.
• Boiler feed water control valves must achieve a
  smooth start-up and maintain required drum level
  for safe, reliable and efficient plant operation.
  
• The high pressure differential at
  start-up/low-load, and sensitive control
  requirement, requires a high-performance severe
  service control valve.

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Feedwater Control Valve Requirements

• During Start-up and Low-load Operation
• Operate at high pressure differentials of up to
  240 bar (Drum Boilers), without damaging the trim
  components, and maintaining good control
• Smooth and quick transition from start-up to
  normal operation.
• Consistent and reliable operation.
• Tight shut-off to prevent leaks and subsequent
  valve erosion
• During Normal Operation
• A valve with a high capacity is required at
  normal operation to minimize friction losses in
  the system to minimise Boiler feed pump power
  requirements.
• During Load Change (assuming fixed speed boiler
  feed pump)
• Load changes are often experienced and this will
  result in a lower steam pressure and drum
  pressure, but feed water pressure will remain
  similar, resulting in a higher pressure
  differential. So the control valve used must be
  able to meet a wide range of capacity
  requirements to provide full flexibility to the
  plant.

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Feedwater Control Valve Problems

• Erosion damage Caused by
• Insufficient number of trim stages, creating
  excessive trim velocities
• Poor seat design and insufficient seat force
• Plug or stem breakage Typically caused by high
  trim velocities, and subsequent trim vibration
  and fatigue failure
• Vibration and noise Caused by cavitation and
  excessive internal velocities.

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Consequences of Feedwater CV Problems

• Cavitation/flashing Insufficient pressure
  reducing stages will cause high velocity flows,
  leading to valve/trim damage owing to
  cavitation/flashing.
• Lost Production Poor control at low flows can
  lead to plant trips and/or an extended start-up
  process.
• High maintenance costs Frequent replacement and
  repair of valve components adds to maintenance
  costs.

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Problems caused by applying the wrong Boiler
Feedwater Control Valve
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• FEED HEATERS

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Feedwater heaters

• A Regeneration process in steam power plants is
  accomplished by extracting steam from the turbine
  at various points. This steam, which could have
  produced more work by expanding further in the
  turbine, is used to heat the feed water instead.
• The device where the feedwater heated by
  regeneration is called a Regenerator or a
  Feedwater Heater (FWH).
• A feedwater heater is basically a heat exchanger
  where heat is transferred from the steam to the
  feedwater either by mixing the two streams (open
  feedwater heaters) or without mixing them (closed
  feedwater heaters).

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Why feed water heating?
Above figure shows the basic Rankine cycle with
water heated only in the boiler. The carnot cycle
diagram for the same steam conditions is
superimposed and indicates the maximum
efficiency, (that is the greatest area of useful
work) that can be achieved in any power plant
operating between the temperature T1 and T2. But
practically this cannot be achieved.
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Why feed water heating?
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No. Of feed heaters in a regenerative cycle
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No. Of feed heaters in a regenerative cycle
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No. Of feed heaters in a regenerative cycle
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No. Of feed heaters in a regenerative cycle
It is clearly seen that the efficiency improves
with each additional heater but the incremental
gain with each becomes progressively smaller.
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Types of Feedwater Heaters

• Open Feedwater Heaters
• An open (or direct-contact) feedwater heater is
  basically a mixing chamber, where the steam
  extracted from the turbine mixes with the
  feedwater in a chamber. Ideally the mixture
  leaves the heater as a saturated liquid at the
  heater pressure.
• Eg Deaerator
• The advantages of open heater are simplicity,
  lower cost, and high heat transfer capacity.
• The disadvantage is the necessity of a pump at
  each heater to handle the large feedwater stream.

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Types of Feedwater Heaters

• Closed Feedwater Heaters
• In closed feedwater heater, the heat is
  transferred from the extracted steam to the
  feedwater without mixing taking place.
• The feedwater flows through the tubes in the
  heater and extracted steam condenses on the
  outside of the tubes in the shell. The heat
  released from the condensation is transferred to
  the feedwater through the walls of the tubes. The
  condensate (also called as drip) formed passes to
  the next lower pressure heater. This, to some
  extent, reduces the steam required by lower
  pressure heater.

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Temperature rise of feed water in heaters
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Feed water heater
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Feed water heater performance

• Feed water temperature rise is the difference
  between the feed water outlet temperature and
  feed water inlet temperature.
• Terminal Temperature Difference (TTD) provides
  feedback on the feedwater heaters performance
  relative to heat transfer and is defined as the
  saturation temperature of the extraction steam
  minus the feedwater outlet temperature. An
  increase in TTD indicates a reduction in heat
  transfer while a decrease a improvement.
• Drain Cooler Approach (DCA) is a method used to
  infer feedwater heater levels based on the
  temperature difference between the drain cooler
  outlet and the feedwater inlet. An increase in
  DCA temperature indicates the level is
  decreasing whereas, a decreasing DCA indicates
  rise in level.

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• AIR HEATER

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Air Heater

• Requirement of Air Pre-heater
• Pre Heating Combustion air using Heat in out
  going Flue gas
• Flue gas leaves Economiser at a temperature of
  approx. 3800C
• Every 400C drop in Flue gas Temp. improves Boiler
  Efficiency by 2 to 3
• Air pre-heater is an heat recovery unit used in
  the last stage in boiler.
• It absorbs heat from exit flue gas in boiler and
  transfers the heat to the incoming cold air.
• In utility boilers it is used to heat the air
  required for combustion purpose as well as dry
  and transport coal

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Advantages of using Air heater

• BOILER EFFICIENCY IS INCREASED.
• ENABLES EFFICIENT BURNING OF LOWER GRADE FUELS.
• SAVINGS ON FUEL COSTS.
• MORE STABLE AND EFFICIENT COMBUSTION OF FUEL.
• PREHEATS AIR FOR COAL DRYING AND TRANSPORTING
  THE PULVERISED COAL TO BURNERS.
• REDUCED FLUE GAS VOLUME LEADS TO SIZE
  REDUCTION IN POLLUTION CONTROL EQUIPMENT.

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Air Heater types

• Based on Operating Principle
• Recuperative and
• Regenerative

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Recuperators

• IN RECUPERATORS GAS AND AIR ARE ALLOWED TO
  FLOW IN SEPARATE CHANNELS AND HEAT IS
  CONTINUOUSLY TRANSFERRED FROM GAS TO AIR THROUGH
  THE WALLS OF THE FLOW CHANNELS.
• TUBULAR AIR PREHEATER
• PLATE TYPE AIR PREHEATER
• STEAM COIL AIR PREHEATER

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Tubular Air Heater
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Plate type Air Heater
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Steam Coil Air Pre-Heater (SCAPH)
HOT AIR
COLD AIR
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Advantages of Recuperative Air Heaters

• Advantages
• No moving parts
• No possibility of Fly ash carry over by air
• Disadvantages
• Occupies more area
• Tube puncture results in air mixing with Flue gas
• Soot deposits reduce heat transfer
• Less effective cross flow heat transfer
• More material cost
• High pressure drop
• Severely affected by cold end corrosion

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Regenarators

• IN REGENERATOR TYPE GAS AND AIR ALLOWED TO FLOW
  ALTERNATIVELY IN THE SAME CHANNELS TO STORE AND
  RETRIEVE HEAT RESPECTIVELY.
• CHANNELS ARE MADE IN MATRIX FORM TO ACHIEVE THE
  HEAT TRANSFER.
• TYPES
• Rotating matrix (Ljungstrom RAPH)
• Stationary matrix (Rothemuhle RAPH)

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Ljungstorm Air Heater Working Principle

• As the rotor revolves, waste heat is absorbed
  from the hot exhaust gas passing through one-half
  of the rotor.
• This accumulated heat is released to the incoming
  air as the same surfaces pass through the other
  half of the rotor.
• The heat transfer cycle is continuous as the
  surfaces are alternately exposed to the outgoing
  gas and incoming air streams.

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Advantages, Disadvantages and Frequently
encountered problems of RAPH

• Advantages
• Compact and hence save space and structure cost.
• Can be effectively cleaned when in service
• Economically suitable for high capacity boiler.
  As the boiler size increases heat transfer area
  required in air heater also increases and hence
  Regenerator is better in comparison with
  Recuperative as it assumes a greater size.
• Disadvantages
• Moving parts need operators attention.
• Frequently encountered problems
• Fouling, plugging and corrosion
• Erosion (normally encountered in tubular air
  heaters)

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Air heater fire

• All types of air heaters are a potential fire
  hazard particularly at start-up of the boiler and
  shut-down especially if the heater is having a
  thick deposit of soot.
• Finely divided particles of combustible matter is
  deposited on the low temperature air heater
  surface when the combustion is poor in the
  furnace due to various reasons.
• If the ignition temperature at combustible matter
  is reached and sufficient oxygen is available,
  fire occurs and may sometime destroy the whole
  air heater, duct etc., if not noticed earlier and
  put off.
• The outlet gas and air temperatures from the air
  heater will rise above normal in case of fire and
  is the best indication to detect fore and to take
  necessary step, for fighting.
• Use of on-load cleaning at frequent intervals
  during boiler starting, at low load or during
  shutting down periods will reduce the hazard to a
  great extent.
• Cutting out of fuel automatically on fire out,
  and automatic combustion monitoring are essential
  features of modern boiler which may eliminate
  this hazard.

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Significance of Acid Dew Temperature
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• STEAM ACCUMULATORS

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Steam Accumulators

• Generally used in pulp and paper industry,
  chemical plants or breweries, combined heat and
  power generation process, etc.
• It can be noted that, in the processes, the usage
  of steam sometimes is very rapid and may cross
  the limits of boiler capacity and this leads into
  rapid depressurisation of boiler.
• A rapidly depressurised boiler can suffer poor
  steam quality and nuisance of trips on high/low
  water levels. And bringing back the boiler into
  service takes its own time.
• To mitigate these consequences of high steam
  demand, the engineers must oversize the boiler,
  design in back pressure regulators to control
  depressurisation or use a steam accumulator.

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Steam Accumulators

• Generally, oversized of boiler means additional
  cost, and back pressure regulation will starve
  the steam-using equipment, causing high cycle
  times.
• One solution to this challenge is to incorporate
  steam accumulation equipment into the steam
  system design.

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Steam Accumulators
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• To meet these instantaneous steam load demands,
  usage of a dry accumulator or usage of a wet
  accumulator are preferred.
• Both design enhancements increase the mass of
  stored steam. Either type of steam accumulation
  will not create steam, but rather, create a means
  to store steam.
• Only adding fuel energy to a boiler will create
  more steam.

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Dry Accumulator
This extra amount of steam will slow down the
depressurisation rate of the boiler and help
mitigate water carryover from the boiler
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Wet Accumulator

• If more instantaneous steam is required than a
  dry accumulator can supply, then a wet
  accumulator can be used.
• A wet accumulator is a pressurised vessel in line
  with the boiler steam line. This vessel is
  pressurised to the boiler operating pressure and
  will discharge stored steam when the header is
  depressurised.
• Once depressurised, the boiler will recharge the
  accumulator when the load equipment no longer
  requires steam.
• Therefore, during idle periods of the steam use
  cycle, the accumulator can be fully recharged and
  be readied for the next cycle.
• The amount of stored steam is proportional to the
  water volume and change in pressure (based on the
  flash steam charts).
• This wet accumulator will store significantly
  more steam than the same size dry accumulator.

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References

• Google
• Central Electricity Generating Board Manuals
• BHEL Manuals
• Steam Accumulators and Steam Boiler Response to
  Load Changes, by C. Merritt, Fulton Thermal Corp.
• Friends and Colleagues

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Thank You