High Pressure Boilers

HIGH PRESSURE
BOILERS
VANITA THAKKAR
ASSOCIATE PROFESSOR,
MECHANICAL ENGINEERING DEPARTMENT,
BABARIA INSTITUTE OF TECHNOLOGY,
VARNAMA, VADODARA

INTRODUCTION
 A Power Plant / Power Station is an industrial
facility for generation of Electric Power.
 It is a set-up consisting of systems and sub-systems,
equipments and auxiliaries required for the
generation of Electricity, which involves conversion
of energy forms like chemical energy, heat energy or
gravitational potential energy into Electrical Energy.
2VANITA N THAKKAR BIT, VARNAMA

ENERGY CONVERSION
PROCESS IN POWER PLANT
The energy content in a primary source of energy, like
 Chemical Energy of a Fossil Fuel,
 Potential Energy of water stored at a height,
 Renewable / Non-conventional sources, like Solar
Thermal Energy, Wind energy, Geothermal Energy,
Tidal Energy, Wave Energy, etc.
is converted stage-wise to Mechanical Energy
(Rotational Energy) to obtain Electricity by creating
relative motion between a magnetic field and a
conductor.

THERMAL POWER PLANTS
 In Thermal Power Plants, mechanical power is produced by a
Heat Engine that transforms Thermal Energy, often from
Combustion of a Fuel, into Rotational Energy.
 Most Thermal Power Stations produce steam, and these are
sometimes called Steam Power Plants / Stations.
 Not all thermal energy can be transformed into mechanical
power, according to the Second Law of Thermodynamics.
Therefore, there is always heat lost to the environment.
 If this loss is employed as useful heat, for industrial processes
or distinct heating, the power plant is referred to as a
Cogeneration Power plant or CHP (combined heat-and-power)
plant. 4VANITA N THAKKAR BIT, VARNAMA

RANKINE CYCLE
 A Thermal Power Plant is a
power plant in which the prime
mover is steam driven.
 Water is heated, turns into steam
in Boiler and spins a Steam
Turbine which either drives an
Electrical Generator or does some
other work, like Ship Propulsion.
 After it passes through the turbine,
the steam is condensed in a
Condenser and recycled to where it
was heated.
 This is known as a Rankine cycle –
as shown in the figure. 5VANITA N THAKKAR BIT, VARNAMA
G2
G3
G1

G2 A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern
manifestation was invented by Sir Charles Parsons in 1884.
Guest, 01-01-2002
G3 Electrical Generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction.
Guest, 01-01-2002
G1 A machine that transforms energy from thermal or pressure form to mechanical form; typically an engine (A mechanical device used to produce
rotation to move vehicle or otherwise provide the force needed to generate kinetic energy.) or turbine (any of various rotary machines that use
the kinetic energy of a continuous stream of fluid - a liquid or a gas - to turn a shaft).
Guest, 01-01-2002

MORE ABOUT RANKINE
CYCLE
 The Rankine cycle is a
thermodynamic cycle which
converts heat into work.
 The heat is supplied externally to a
closed loop, which usually uses
water as the working fluid.
 This cycle generates about 80% of
all electric power used throughout
the world, including virtually all
solar thermal, biomass, coal and
nuclear power plants.
 It is named after William John
Macquorn Rankine, a Scottish
polymath. 6VANITA N THAKKAR BIT, VARNAMA
G4

G4 A polymath (Greek polymathēs, πολυμαθής, "having learned much") is a person whose expertise fills a significant number of subject areas. In
less formal terms, a polymath (or polymathic person) may simply refer to someone who is very knowledgeable.
Guest, 01-01-2002

WILLIAM RANKINE
 The Rankine Cycle is named after William Rankine. Trained as a
civil engineer, William Rankine was appointed to the chair of civil
engineering and mechanics at Glasgow in 1855. He developed
methods to solve the force distribution in frame structures.
 He worked on heat, and attempted to derive Sadi Carnot's law from
his own hypothesis. His work was extended by Maxwell.
 Rankine also wrote on fatigue in the metal of railway axles, on
Earth pressures in soil mechanics and the stability of walls. He was
elected a Fellow of the Royal Society in 1853.
 Among his most important works are Manual of Applied Mechanics
(1858), Manual of the Steam Engine and Other Prime Movers
(1859) and On the Thermodynamic Theory of Waves of Finite
Longitudinal Disturbance.

PROCESSES IN RANKINE
CYCLEThere are Four processes in the Rankine cycle, each
changing the state of the working fluid.
 Process 1-2: Working fluid is PUMPED from low to
high pressure, as the fluid is a liquid at this stage the
pump requires little input energy.
 Process 2-3: The high pressure liquid enters a
BOILER where it is heated at constant pressure by an
external heat source to become a dry saturated
vapour (or wet vapour).
 Process 3-4: The dry saturated vapour expands through
a TURBINE, generating power. Due to decrease in
temperature and pressure of the vapour, and some
condensation may occur.
 Process 4-1: The wet vapour then enters a
CONDENSER where it is condensed at a constant
pressure and temperature to become a saturated
liquid. The pressure and temperature of the
condenser is fixed by the temperature of the cooling
coils as the fluid is undergoing a phase-change. 8VANITA N THAKKAR BIT, VARNAMA

RANKINE CYCLE : PRACTICAL
CARNOT CYCLE
 The Rankine cycle is sometimes referred to as a
Practical Carnot cycle as, when an efficient
turbine is used, the TS diagram begins to resemble
the Carnot cycle.
 The main difference is that a pump is used to
pressurize liquid instead of gas. This requires
about 1/100th (1%) as much energy than that in
compressing a gas in a compressor (as in the
Carnot cycle).

Thus, BASIC COMPONENTS OF
THERMAL POWER PLANT
 BOILER
 STEAM TURBINE : The Prime Mover
 CONDENSER
 FEED PUMP
Supported by various sub-systems / accessories /
equipments required for their proper, efficient
working and to ensure their proper working in co-
ordination with each other.

SCHEMATIC DIAGRAM OF A
STEAM POWER PLANT

BOILER
 A boiler is a closed vessel in which water or other
fluid is heated.
 The heated or vaporized fluid exits the boiler for use
in various Processes or Heating applications or for
Power generation.
 In a Steam Power Plant, typically called Thermal
Power Plant, Water is heated to get Steam for
Power Generation.

STEAM PRODUCTION

STEAM PRODUCTION (contd.)

STEAM PRODUCTION (contd.) : From
Steam Tables ….

STEAM PRODUCTION (contd.) : From
Steam Tables ….
Boiling point

STEAM PURITY

STEAM QUALITY

BASIC COMPONENTS OF BOILER
In a conventional steam power plant, a boiler consists of :
BOILER
FURNACE :
FUEL IS BURNT
Fuel : Fossil Fuel /
Waste Fuel /
Nuclear Fuel
SURFACES :
TO TRANSMIT
HEAT FROM
COMBUSTION
PRODUCTS
TO WATER.
SPACE (DRUM) :
WHERE STEAM
CAN
FORM AND
COLLECT 21VANITA N THAKKAR BIT, VARNAMA

HISTORICAL BACKGROUND
 Boilers were built as early as the 1st century AD by Hero
of Alexandria but were used only as toys.
 Not until the 17th century was serious consideration given
to the potential of steam power for practical work.
 Denis Papin of France designed the first boiler with a
safety valve in 1679;
 Earlier Boilers were made of wrought iron.
 As the advantages of high pressure and temperature were
realized, manufactures turned to steel.
 Modern boilers are made of alloy steel to withstand high
pressures and extremely high temperatures. 22VANITA N THAKKAR BIT, VARNAMA

TYPES OF BOILERS

FIRE-TUBE BOILERS
 Water surrounds the steel tubes through which hot gases
from the furnace flow.
 The steam generated collects above the water level in a
cylindrically shaped drum.
 A safety valve is set to allow escape of steam at pressures
above normal operating pressure; this device is necessary
on all boilers, because continued addition of heat to water in a
closed vessel without means of steam escape result in a rise in
pressure and ultimately, in explosion of the boiler.
 Fire-tube boilers have the advantage of being easy to install
and operate.
 They are widely used in small installations to heat buildings
and to provide power for factory processes and in steam
locomotives.

FIRE-TUBE BOILERS (contd.)

WATER-TUBE BOILERS
 Water is inside tubes with the hot furnace gases circulating
outside the tubes.
 When the steam turbo generator was developed early in the
20th century, modern water tube boilers were developed in
response to the demand for large quantities of steam at high
pressures and temperatures, far exceeding those possible
with fire-tube boilers.
 The tubes are outside the steam drum, which has no heating
surface and is much smaller than that in the fire-tube boiler.
 So, drum of water tube boiler is better able to withstand
higher pressures and temperatures.
 A wide variety of sizes and designs of water tube boilers are
used in ships and factories. 26VANITA N THAKKAR BIT, VARNAMA

WATER-TUBE BOILERS (contd.)
 The Express Boiler is designed with small water
tubes for quick generation of steam.
 The Flash Boiler may not require a steam drum,
because the tubes operate at such high temperatures
that the feed water flashes into steam and
superheats before leaving the tubes.
 The largest units are found in the central-station
power plants of public utilities.
 Units of substantial size are used in steel mills,
paper mils, oil refineries, chemical plants, and
other large manufacturing plants. 27VANITA N THAKKAR BIT, VARNAMA

HIGH PRESSURE BOILERS
Boilers used for :
Steam capacities – 30 tons/hr. to 650 tons/hr.
and above,
Pressure – up to 160 bar (can be more),
Maximum steam temperatures – typically
about 540oC (can be more).

UNIQUE FEATURES OF HIGH
PRESSURE BOILERS
 Method of water circulation : Forced
circulation, using pump.
 Type of Tubing : Water Tube Boilers, with
flow through several sets of parallel system
of tubing – to reduce pressure loss occurring
in single tube system and to have better
control over quality of steam.

UNIQUE FEATURES OF HIGH
PRESSURE BOILERS (contd.)
 Improved Method of Heating :
 Saving of heat by evaporation of water above
critical pressure of steam.
 Heating of water by mixing with superheated
steam, to give high heat transfer coefficients.
 Increase in overall heat transfer coefficient by
increasing water velocity inside the tubes and
increasing gas velocity above sonic velocity.

LA MONT BOILER

LA MONT BOILER (contd.)
45 – 50 Tons of
superheated steam
at a pressure of
about 160 bar and
a temperature of
500oC.

BENSON BOILER
One of the main difficulties in La Mont Boiler :
Formation and attachment of bubbles on inner
surfaces of heating tubes, which reduce heat flow
and steam generation due to high thermal
resistance than water film.
If boiler pressure is raised to critical pressure (225
atm.), density of steam and water would be the
same, hence bubble formation would be prevented.
Benson got this idea in 1922 and the first Benson
Boiler became operational in West Germany in 1927.

BENSON BOILER (contd.)

Typical Parameters :
Temperature Range :
upto 650oC
Pressure : upto 500
atm
Steam Generating
Capacity : 150 tons/hr.

First Benson Boiler - 1927

VELOX BOILER
 Uses Pressurized Combustion, i.e. When
gas velocity exceeds sound velocity, heat is
transferred from the gas at much higher rates
than those achieved with sub-sonic flow. This
fact is made use of to obtain large heat
transfer rates from smaller surface area in
Velox boilers.

VELOX BOILER
Gas Turbine runs
Axial Compressor :
raises pr. of
incoming air from
atmospheric pr. to
furnace pr.
Feed
Water
Steam
separated
here flows to
superheater
and then to
prime mover
39
VANITA N THAKKAR BIT, VARNAMA

VELOX BOILER (contd.)

LOEFFLER BOILER
One of the main difficulties in La Mont Boiler :
Deposition of sand and sediment on inner surfaces of heating
tubes, which reduce heat transfer and steam generation
due to high thermal resistance than water film.
This difficulty is solved in Loeffler Boiler by preventing flow
of water in boiler tubes.
Principle : Evaporation of feed water by means of Superheated
steam from Superheater, hot gases from furnace being
primarily used for superheating purposes.

LOEFFLER BOILER (contd.)
Loeffler boiler can manage higher
concentrations of salt than any other type.
It is more compact than indirectly heated boilers
having natural circulation.
Hence it is very useful for land and sea
transport power generation.
Typical specifications : 100 tons/hour
generating capacity, 140 bar pr.

This boiler can carry
higher salt
concentrations than
any other boiler.
Hence more compact
than indirectly
heated boilers
having natural
circulation.
So, it is useful for land
and sea transport
power generation.

SCHMIDT HARTMANN BOILER
Working similar to that of a transformer.
Two pressures are used to effect interchange of
energy.

(contd.)

SUPER-CRITICAL BOILERS

SUPER-CRITICAL BOILERS (contd.)
Advantages of Super-ciritical Boilers (contd.) :

SUPER-CHARGED BOILER

SUPER-CHARGED BOILER (contd.)

ADVANTAGES :

DISADVANTAGES :

FLUIDIZED BED COMBUSTION
BOILERS

PRINCIPLE : FBC BOILERS

(CONTD.)
 When an evenly distributed air or gas is passed
upward through a finely divided bed of solid
particles such as sand supported on a fine mesh,
the particles are undisturbed at low velocity.
 As air velocity is gradually increased, a stage is
reached when the individual particles are
suspended in the air stream – the bed is called
“fluidised”.

(CONTD.)
 With further increase in air velocity, there is
bubble formation, vigorous turbulence,
rapid mixing and formation of dense defined
bed surface.
 The bed of solid particles exhibits the
properties of a boiling liquid and assumes the
appearance of a fluid – “bubbling fluidized
bed”.

(CONTD.)
 At higher velocities, bubbles disappear, and
particles are blown out of the bed. Therefore,
some amounts of particles have to be
recirculated to maintain a stable system -
"circulating fluidised bed".
 Fluidization depends largely on :
 particle size
 air velocity.

(CONTD.)
 The mean solids velocity increases at a slower rate than does the
gas velocity (Refer Fig 2.).
 Mean Gas Velocity – Mean Solid Velocity = Slip velocity.
 Maximum slip velocity is desirable for good heat transfer and
intimate contact. 60VANITA N THAKKAR BIT, VARNAMA

(CONTD.)
 Sand particles in a fluidised state heated to
ignition temperatures of coal + coal
injected continuously into the bed  coal
will burn rapidly + bed attains a uniform
temperature.
 The F B combustion (FBC) takes place at
about 840°C to 950°C – much below the ash
fusion temperature  melting of ash and
associated problems are avoided. 61VANITA N THAKKAR BIT, VARNAMA

(CONTD.)
 Lower combustion temperature is achieved
due to high coefficient of heat transfer by :
 rapid mixing in the fluidised bed
 effective extraction of heat from the bed through in-
bed heat transfer tubes and walls of the bed.
 Gas velocity is maintained between minimum
fluidisation velocity and particle entrainment
velocity to ensures stable operation of bed and
to avoid particle entrainment in gas stream.

(CONTD.)
 Combustion process requires the three “T”s :
Time, Temperature and Turbulence.
 In FBC, turbulence is promoted by fluidisation.
Improved mixing generates evenly distributed
heat at lower temperature.
 Residence time is many times greater than
conventional grate firing. Thus an FBC system
releases heat more efficiently at lower
temperatures. 63VANITA N THAKKAR BIT, VARNAMA

(CONTD.)
 Since limestone is used as particle bed, control
of sulfur dioxide and nitrogen oxide emissions
in the combustion chamber is achieved without
any additional control equipment.
 This is one of the major advantages over
conventional boilers.

AFBC / BUBBLING BED FBC BOILER

CFBC BOILER

PFBC BOILER

SUPER-HEATERS
 A super-heater is a device used to convert
saturated steam or wet steam into dry steam
used for power generation or processes.
 There are three types of super-heaters
namely: radiant, convection, and separately
fired.
 A super-heater can vary in size from a few
tens of feet to several hundred feet (a few
meters or some hundred meters).

TYPES OF SUPERHEATERS
Based on mode of heat transfer :
 A Radiant Super-heater is placed directly in
the combustion chamber.
 A Convective Super-heater is located in the
path of the hot gases, before economizer.
 A Separately-fired Super-heater, as its name
implies, is totally separated from the boiler.
 Combined Radiant-Convective Superheater
(Pendant Superheater.) 69VANITA N THAKKAR BIT, VARNAMA

TYPES OF SUPERHEATERS
(contd.)
Based on position in furnace with respect to
water tubes :
 Over-deck Superheater.
 Inner-deck Superheater.
 Inner tube Superheater.
 Inner bank Superheater.

SUPERHEATER : IMAGES

SUPER-HEATERS (contd.)
The Super-heater :
 Increases capacity of the plant.
 Eliminates corrosion of the steam turbine.
 Reduces steam consumption of the steam turbine.
 To resist metal temperatures above 600oC
different types of materials are used for the tubes
: M.S., seamless carbon steel tubes to chromium
molybdenum seamless alloy steel tubes to
Stainless Steel Tubes.

RE-HEATERS
For reheating steam going to lower turbine
stages to :
 To improve efficiency of the plant.
 To prevent steam in the lower stages from
becoming wet – so, erosion and maintenance
problems can be reduced.

RE-HEATERS (contd.)
74

ECONOMIZERS
Feed-water heaters in which heat from waste
gases is recovered to raise the temperature
of feed-water supplied to the boiler.
Advantages :
 Fuel economy.
 Longer life of the boiler.
 Increase in steaming capacity.

ECONOMIZERS (contd.)
Types :
 Based on construction :
 Finned / Gilled Tube Economizers : C.I. Gilled Tube
Economizers.
 Plain Tube Coil Economizers.
 Based on part of steam generation :
 Steaming type Economizers (5-7% feed water is converted into
steam; used in large steam power plants.).
 Non-steaming type Economizers.
 Based on location :
 Independent Economizers
 Integral Economizers.

AIR PRE-HEATERS
 Device designed to heat air before
another process (for example, combustion
in a boiler) with the primary objective of
increasing thermal efficiency of the
process.
 Air preheater recovers heat from boiler
flue gases which increases thermal
efficiency of the boiler by reducing useful
heat lost in flue gases.
 Flue gases are sent to the flue gas stack
(or chimney) at a lower temperature,
allowing simplified design of the ducting
and the flue gas stack.
 It also allows control over the
temperature of gases leaving the stack
(to meet emissions regulations, for
example).
Types :
Recuperative : Tubular Type
and Plate Type
Regenerative : Storage Type77VANITA N THAKKAR BIT, VARNAMA

TYPICAL PLANT LAYOUT SHOWING
DIFFERENT ACCESSORIES

METHODS OF SUPERHEAT
CONTROL
 Dampers in the flue gas circuit : operated manually.
 Instrumentation for monitoring and controlling Boiler
superheat.
 Combined Radiant-Convective Superheaters.
 Desuperheating.
 Pre-condensing the steam.
 By-passing furnace gas around superheater.
 Gas recirculation.
 Tilting burners in furnace
 Auxiliary burners.
 Twin furnace. 79VANITA N THAKKAR BIT, VARNAMA

CORROSION IN BOILERS
CORROSION : Conversion of metal into oxides and
salts, causing loss of material, which if not prevented,
leads to failure of metal parts.
In boilers, corrosion occurs in :
 Inner surfaces (water / steam side) : due to –
 Acidity of water (low pH)
 Presence of O2 (enters through leakages – condenser,
condenser pump, etc.), CO2 (released by heating of
bicarbonates) and chlorides dissolved in feed water.
 External surfaces (flue gases side) : due to coal ash. 80VANITA N THAKKAR BIT, VARNAMA

PREVENTION OF CORROSION IN
BOILERS
INNER SURFACE CORROSION PREVENTION :
1. Addition of adequate amount of scavengers like hydrazine sodium
sulphite – to remove dissolved oxygen for protection against pitting
of inner tube surface.
2. Deaerator in water treatment plant to remove dissolved O2 and CO2.
3. Addition of ammonia for neutralizing amines in water and lowering
pH value of water  Effect of CO2 gets neutralized.
4. Addition of alkali salts in water  neutralizing acids.
5. Addition of ammonium hydroxide in water  reacts with CO2 to
form ammonium carbonate and water  Effect of CO2 gets
neutralized.
6. Applying protective coating of amines on boiler tube inner surfaces.

BOILERS (contd.)
OUTER SURFACE CORROSION PREVENTION :
1. For Air Preheater – Flue gases should not be cooled below dew
point of corrosion species by :
1. Passing some air around preheaters.
2. Recirculating air from preheater outlet to forced draught fan
inlet.
2. Removing deposits of soot regularly from surfaces of
economizer, superheater, evaporator tubes, air preheaters,
reheaters, etc.
3. Heating feed water with steam from boiler in shell and tube
heat exchanger to prevent low temperature corrosion by feed
water. 82

BOILERS (contd.)
OUTER SURFACE CORROSION PREVENTION :
4. Using low sulphur coal – to enable use of low temperature
feed water, as chances of carrying sulphur trioxide with flue
gases is less, leading to acid formation (feed water temperature
has to be maintained above acid dew point temperature).
5. High temperature corrosion in superheaters, reheaters, etc. can
be prevented by :
1. Using good quality coal.
2. Providing stainless steel tube shields.
3. Replacing damaged / old tubes with tubes containing high chromium
content.

THANKS !!!
VANITA THAKKAR
ASSOCIATE PROFESSOR,
MECHANICAL ENGINEERING DEPARTMENT,
BABARIA INSTITUTE OF TECHNOLOGY,
VARNAMA, VADODARA

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