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					STUDY OF CONSTRUCTION OF
         A BOILER

               Introduction

A boiler is a closed vessel in which water, under
pressure, is transformed into steam by the
application of heat.

The primary function of a boiler is to generate steam
at pressures above atmospheric by the absorption of
heat that is produced by the combustion of fuel.
General construction of a Boiler
    Principle of Heat Transfer

   A boiler should be designed to absorb the maximum amount of
    heat released in the process of combustion.

    Heat flows from one body to another by virtue of a temperature
    difference, and it always flows in the direction of higher
    temperature to lower temperature.

 This heat is transmitted to the boiler by radiation, conduction,
  and convection, the percentage of each depending on the boiler
  design.
               Types of Boilers

 Boilers can be classified in various ways depending on firing
  method used, fuel fired, field of application, type of water
  circulation employed, and pressure of steam etc.

 In general there are two principle types of boilers.
      Fire tube boilers and
      water tube boilers
Fire Tube Boiler
    Contd..
    Fire tube boilers have been used in various early forms
     to produce steam for industrial purposes.
    The fire tube boiler is a special form of the shell-tube
     type boiler.
    A shell type boiler is a closed, usually cylindrical, vessel
     or shell that contains water.
    Hot gases pass through the tubes during the heat
     transfer process.
    The shell boiler evolved into modern forms such as the
     electric boiler, in which heat is supplied by electrodes
     embedded in the water.
Water Tube Boiler
  Contd..
 The difference between fire tubes and water tubes is simply,
  water flows through water tubes instead of fire.

The advantage of the water tube boiler is it can works in high
 steam pressure and capacities.

 With higher steam pressure and capacities, fire tube boilers
  would need large diameter shell.

 With such large diameter, the shell would have to operate under
  such extreme pressure and thermal stresses that their thickness
  would have been too large.
                            Working
 Energy is continuously added to water in a closed system array
  and the water evaporates to steam and the pressure and
  temperature increases as long as the strength of the containment
  is not exceeded.

 There will be a point where the energy supplied is just sufficient
  to maintain steam conditions and evaporation will stop.


 A vital part of any boiler system is the system designed to release
  the steam pressure to ensure safe conditions are maintained.
                   Boiler Efficiency
 The boiler efficiency is based on the calorific value of the
  fuel used and the enthalpy change from water to steam.




   m s = Mass flow rate of steam (kg/s)
   m f = Mass flow rate of fuel (kg/s)
   h = specific enthalpy of generated steam (kJ/kg)
   h w = specific enthalpy of feedwater (kJ/kg)
    Contd...
 To achieve maximum boiler efficiency the operator must

   Minimize excess air to reduce stack losses.

   Clean the gas side and water side of the boiler tubes to ensure
    maximum absorption of heat and reduce stack temperatures.

   Minimize blowdown to reduce blowdown losses.

   Perform maintenance on burners and controls to minimize
    unburned fuel.
                   Applications

 Boilers have many applications. They can be used in
  stationary applications to provide heat, hot water, or
  steam for domestic use or in generators and they can be
  used in mobile applications to provide steam
  for locomotion in applications such as trains, ships, and
  boats. Using a boiler is a way to transfer
  stored energy from the fuel source to the water in the
  boiler, and then finally to the point of end use.
       Construction of Boilers
 A boiler, which includes drums, tubes, flues, ducts, auxiliary
  equipment, and their associated supports, is subjected to
  continual stress resulting from expansion and contraction and
  from elevated temperatures when it is in service.

 Boilers must be made adequately strong and with suitable
  materials to withstand these forces and temperatures.

 Pressure-vessel construction codes adopted by federal, state,
  and local jurisdictional authorities play an important part in
  determining safety requirements and thus the construction
  features of pressure vessels.
   Contd..

 The most widely used of these is the ASME Boiler and Pressure
  Vessel Code, published by the American Society of Mechanical
  Engineers.

 This code sets rules of safety for the design, fabrication, and
  inspection of boilers, pressure vessels, and nuclear power plant
  components.
Contd..
             Major Components
   ECONOMIZER
   FEED WATER INLET PIPE
   DOWNCOMERS
   WATER DRUM
   GENERATING TUBES
   SUPERHEATER
   DESUPERHEATER
                      Economizer
 The economizer is a feed water heater deriving heat from the
  flue gases discharged from the boiler. The justifiable cost for
  economizer depends on the total gain in efficiency. In turn, this
  depends on the gas temperature out of the boiler and feed water
  temperature to the boiler.
         Feed Water Inlet Pipe
 These pipes are used for supplying boilers with hot feed
  water. An important benefit of these pipes is that most of the
  dissolved gases are liberated before the feed water enters the
  boiler.

 The feed water flow passes through tubes which are exposed to
  partially expanded steam brought from, typically, one or more
  points in the turbine system. The turbine exhausted steam is
  thus used to some advantage.
                     Superheater
 Steam that has been heated above the saturation temperature
  corresponding to its pressure is said to be superheated.

 This steam contains more heat than does saturated steam at the
  same pressure and the added heat provides more energy for the
  turbine for conversion to electric power, or in the case of
  process steam, more energy contained in a pound of steam for a
  more efficient process.

 The convection superheater is placed somewhere in the gas
  stream, where it receives most of its heat by convection.
Contd..
   Major Components Contd…
 Downcomer:
         A tube or pipe in a boiler or water wall circulating system
through which fluid flows downwards.

 Water Drum:
      It is a cylindrical shell enclosed at both ends designed to
withstand water and internal pressure.

 Generating Tubes:
     A tube in which steam is generated.

 Desuperheater:
      Apparatus for reducing and controlling of temperature of a
superheated vapour or of a fluid.
                   Boiler Design
Requirements of Boiler Design:

 Operational Safety.
 Generation of clean system or hot water at the desired rate,
  pressure, and temperature.
 Economy of operation and maintenance.
 Conformance to applicable codes.
    Contd..
 A set of rules for the construction and operation of boilers,
  known as the American Society for Mechanical Engineers (ASME)
  Boiler and Pressure Vessel Code, has been widely adopted by
  insurance underwriters and government agencies.

    To meet the above listed requirements, a boiler must have
    following characteristics:

    Adequate water or steam capacity.
    Property sized steam/water separators for steam boilers.
    Rapid, positive, and regular water circulation.
    Heating surfaces which are easy to clean on both water and gas sides
    Parts which are accessible for inspection and repair.
    Correct amount of land, proper arrangement of heating surface.
    A furnace of proper size and shape for efficient combustion and
    for directing the flow of gases for efficient heat transfer.
                        Materials
 The pressure vessel in a boiler is usually made of steel (or alloy
  steel), or historically of wrought iron.

 Stainless steel is virtually prohibited (by the ASME Boiler Code)
  for use in wetted parts of modern boilers, but is used often in
  superheater sections that will not be exposed to liquid boiler
  water.

 In live steam models, copper or brass is often used because it is
  more easily fabricated in smaller size boilers.

 Cast iron may be used for the heating vessel of domestic water
  heaters.
                       Fabrication
Working Procedure:

   Marking & Assembly
   Bending Procedure
   Welding Procedure
   Welding consumable control procedure
   Temporary cleat fixing and removal procedure
   Shrink fitting of gills procedure
   Pre-machining procedure
   Rolling and forming procedure
   Panel welding and repair procedure
   Casting and duct fabrication
   Water wall assembly
    Contd..

   Steam drum production procedure
   Economizer & Superheater production procedure
   Pre-heating & PWHT procedure
   Hydrostatic Testing procedure
   Quality control procedure
   NDT procedures
     Non Destructive Testing
 Nondestructive testing (NDT) is the development and application
  of technical methods for the detection, location, measurement,
  and evaluation of discontinuities, defects and other
  imperfections, the assessment of integrity, the assessment of
  properties and composition, or the measurement of geometrical
  characters without impairing the intended use or application of
  the test object.
    Contd..
The 6 most commonly used methods in boiler manufacturing
are:

    Liquid Penetrant Testing (PT)
    Magnetic Particle Testing (MT)
    Ultra Sonic Testing (UT)
    Radiographic Testing (RT)
    Visual Testing (VT)
    Leak Testing (LT)
  Contd..
 Boiler design requires a detailed study of the forces that are
  exerted on the various parts and of the temperatures to which
  they will be exposed.



 After a boiler has been completed, it must be subjected to a
  hydrostatic test to make sure it can operate safely within the
  maximum allowable pressure and after that NDT process is
  applied.
THANK YOU

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posted:8/31/2011
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