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Boiler

Heat Transfer, Heat Exchanger, Refinery

boiler banner 3A boiler, abbreviated as BLR, is a closed vessel in which water or other fluid is heated under pressure for generating steam or other hot fluids.

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When sizing a boiler you must have enough capacity to handle the maximum hourly load of the system.  You should always install a boiler with future expansion in mind.  More than one may be required depending how critical it is for continuous service during shutdown and maintenance.  All boilers should be equipped with fully automatic controls to provide adequate safety at all times.  All equipment in the system should be rated at least to the maximum allowable working pressure.  Equipment and controls should meet if not exceed ASME, UA, local, state and all applicable codes.  

Boilers require fresh air and ventilation and must be installed in areas where both can be attained.  When in confined rooms permanent openings for air transfer should be provided.  Vents and draft creating equipment may be needed to achieve proper movement.  The venting system should be wired to the boiler to assure proper air when the boiler is fired up.  At least two doors at a minimum are recommended for ingress and egress to insure proper safety for personnel.

Do not use galvanized pipe or fittings on any of the boiler piping: feed, steam, or water lines.  A proper blowdown system must be provided for safety.          

When boilers are not used for periods of time, such as seasonal or storage in freezing weather the system should be drained so not to damage the equipment.

Boiler Design Classification

Fire-tube Boiler  -  In this type of boiler, gases are directed through vertical or horizontal steel tubes that are surrounded by the water for heating.  The fire tube boilers have a smaller capacity and can not handle high pressures over 250 psig.  This boiler can be classified by its size, steam pressure, and steam temperature.  A single-drum or multi-drum boiler is determined by the number of drums it has.  Most boilers have just one drum but some may have 2 or more.  These boilers are used in heating applications and are frequently found in hospitals and schools as well as in industrial process applications such as food and chemical plants.

  • Cochran Boiler  -  This is a low pressure type of boiler arranged in the vertical direction that has multi tubes.  This is a modified version of a basic vertical boiler with different fire tubes that can exchange heat by convection to water.  The efficiency of this boiler is much higher than the efficiency of simple vertical boiler.
  • Cornish Boiler  -  This kind of boiler includes a plane cylindrical shell, as well as a tiny flue pipe holding the heating system, flows through it.
  • Horizontal Boiler  -  Has a horizontally cylindrical shell that has many flue tubes arranged in a horizontal direction.
  • Lancashire Boiler  -  This boiler is enclosed in brickwork.  It consists of fire tubes surrounded by water and the fire tubes are conical towards the rear end.  A brick arch is placed after the furnace in order to increase the flow of hot gases.
  • Locomotive Boiler  -  It is a horizontal, multi-tubular, internally fired, and naturally circulated type boiler.  This type of boiler is mounted horizontally in direction and has multi tubes.
  • Scotch Marine Boiler  -  This type of boiler is different from other because of using a large number of small diameter tubes.  The furnace remains a single large diameter tubes with a number of small tubes arranged above it.  They connect each other with a combustion chamber.  So the flow of flue gas into the pipes is from the back to the front.  An enclosed smokebox covering the front of these tubes leads up to a chimney or a funnel.
  • Simple Vertical Boiler  -  Installed in a vertical direction, and it doesn’t remain in a stationary condition, unlike other fire tube boilers.  It has a low working steam pressure and is not best for high steam pressure applications.

Water-tube Boiler  -  The water-tube boiler is a type of boiler in which the water is present inside the tubes and fire or hot gases surround these tubes.  Watertube boilers are safer by design and generally considered to last much longer than the firetube boiler.  They are available in much larger sizes and recover faster than firetube boilers, can handle pressures up to 5000 psig, and have the ability to reach very high temperatures with the use of superheaters.

  • Benson Boiler  -  A high pressure super critical water boiler in which no bubble is formed because the water is compressed at super critical pressure and it directly converts into steam.
  • Babcock and Wilcox Boiler  -  A horizontal type drum axis, stationary, high pressure, natural circulation, solid fuel-fired water tube boiler in which coal is burned to heat the water for changing the phase into steam and later that steam is used for power generation.
  • Lamont Boiler  -  This is a high pressure type of water tube boiler in which steams are generated from water by the burning of coal for fuel, and later that steam is used for the generation of electricity and so on.
  • Loeffler Boiler  -  This boiler is used to evaporate the water by taking superheated steam from the superheater.  In this boiler, 70% of the superheated steam is useed for the water evaporation in the evaporating drum, and 30% of the superheated steam is used to run the steam turbine.
  • Yarrow Boiler  -  This high pressure boiler design is characteristic of the three-drum boiler.  Two banks of straight water-tubes are arranged in a triangular row with a single furnace between them.  A single steam drum is mounted at the top between them, with smaller water drums at the base of each bank.  Circulation, both upwards and downwards, occurs within this same tube bank.

Boiler Advantages and Disadvantages

AdvantagesDisadvantages
  • Boilers are known for their high efficiency in converting fuel into heat.  They can provide a consistent and even heat distribution throughout a space, making them effective for both residential and industrial heating.
  • Boilers can use a variety of fuels, including natural gas, oil, coal, and biomass.  This flexibility allows them to be used in different applications and industries.
  • Well-maintained boilers can have a long operational life, providing reliable heating for many years.  Regular maintenance can help extend the lifespan of a boiler.
  • Boilers offer precise control over the temperature and pressure of the generated steam or hot water, allowing for customization to meet specific heating requirements.
  • Boilers can be designed to produce clean and efficient combustion, reducing the emission of pollutants compared to some other heating systems.
  • Boilers are generally quieter than some other heating systems, providing a more comfortable and peaceful environment in residential and commercial spaces.
  • In some cases, boilers can have lower operating costs, especially when they are well maintained and operated efficiently.  This is particularly true in areas where the cost of the chosen fuel is relatively low.
  • The installation of a boiler system, including the purchase and installation of the boiler unit and associated components, can involve a significant upfront cost.
  • Boilers, especially larger industrial boilers, may require substantial space for installation.  This can be a limitation in some settings where space is at a premium.
  • Boiler installation can be complex, involving various components such as piping, pumps, and controls.  Professional installation is often required to ensure proper functioning.
  • Boilers require regular maintenance to ensure efficient and safe operation.  Neglecting maintenance can lead to reduced efficiency, increased fuel consumption, and potential safety hazards.
  • Boilers can have a slower heating response compared to some other heating systems.  It may take some time for the boiler to reach the desired temperature, especially in large industrial applications.
  • Depending on the fuel source, boilers can contribute to carbon emissions and other environmental concerns.  Use of cleaner fuels or integration with renewable energy sources can help mitigate this issue.
  • Boilers can be susceptible to leaks, which may pose safety risks and require immediate attention.  Regular inspections and preventive measures are necessary to address this potential issue.

Boiler Design Consideration 

Look at the Boiler Glossary for other design considerations.
 
Boiler Safety  -  Boiler systems require knowledgeable and experienced personnel for maintenance, diagnostics, and service.  Standard electrical, mechanical, and other safety practices apply to these systems. High temperatures and pressures are present in all boiler systems, and high voltage is present in electric boilers.
Burner Controllers  -  Controllers that provide the proper sequencing for burner operation including ignition, flame monitoring, purging, cut-off, and typically include safety limit and operational limit circuits.
Corrosion  -  Corrosion is one of the main causes of reduced reliability in steam generating systems.  Many corrosion problems occur in the hottest areas of the boiler, the water wall, screen, and superheater tubes.  Other common problem areas include deaerators, feedwater heaters, and economizers.  Methods of corrosion control vary depending upon the type of corrosion encountered.  The most common causes of corrosion are dissolved gases, primarily oxygen and carbon dioxide, under deposit attack, low pH, and attack of areas weakened by mechanical stress, leading to stress and fatigue cracking.
  • Acidic Corrosion  -  The mishandling of chemicals during acid cleaning or the boiler pH being run too low.  This will passivate the carbon steel surfaces of the boiler.
  • Caustic Corrosion  -  Caustic stress corrosion cracking is a failure affecting both carbon and austenitic steels.  Corrosion and gouging occurs when alkalinity of boiler water increases.
  • Crevice Corrosion  -  This is a localized form of corrosion usually results from a crack in the boiler that does not get good circulation to rinse away caustic corrosion.
  • Galvanic Corrosion  -  Accelerated corrosion of a metal because of an electrical contact with a more noble metal or nonmetallic conductor in a corrosive electrolyte.
  • Oxygen Corrosion  -  In the presence of oxygen, steel breaks down into either insoluble or soluble iron compounds.  Oxygen will cause pitting in the preboiler section and in the tubes.  Dissolved oxygen refers to the volume of oxygen that is contained in water.  The amount of oxygen that can be held by the water depends on the water temperature, salinity, and pressure.
  • Pitting Corrosion  -  Pitting is a localized form of corrosion.  Either a local anodic point or more commonly a cathodic point, forms a small corrosion cell within the surrounding normal surface.
Corrosion Allowance  -  A designer shall consider whether to add corrosion allowance when designing thickness of steam boiler parts.
Combustion Efficiency  -  Efficiency is expressed in percentage and always less than 100%.  Depending on how well the fuel is being used in combustion, high pressure boilers are highly efficient and all of the fuel is converted into thermal energy.
Maximum Allowable Working Pressure  -  The maximum gage pressure or the pressure above the atmospheric pressure that is permitted in the steam boiler.  It’s based on the lowest design pressure of any steam boiler part.
Number of Tubes  -  Depending on the total number of tubes, a boiler can be classified as either single tube or multitubular.
Operating Pressure  -  The actual pressure that be operated in steam pressure parts.
Pressure  -  As the pressure of steam reduces, turbulence at the water surface increases, thereby increasing the moisture carryover in the steam.  To ensure high dryness fraction of steam, the boiler should be operated close to design pressure.  Design pressure shall be based on the expected maximum pressure drop at the top of the vessel under normal operation.  When the vessel is venting to the atmosphere, the minimum design pressure shall be full of water or liquid, whichever is greater.
Steam  -  As water is heated and approaches its boiling point, some of the molecules attain kinetic energy enough to escape into the space above the surface of the liquid.  The more the water is heated the more molecules excapes.  When more molecules leave the liquid than enter the liquid, the saturation point is reached.  As the temperature continues to rising it reaches superheated steam where no liquid exists.
Tempereature  -  The effective temperature driving force is a measure of the actual potential for heat transfer that exists at the design conditions.

Boiler Standards 

API Standards

  • API RP 535 - Burners for Fired Heaters in General Refinery Services
  • API RP 573 - Inspection of Fired Boilers and Heaters

ISO Standards

  • ISO 16528-1 - Boilers and pressure vessels -- Part 1: Performance requirements
  • ISO 16528-2 - Boilers and pressure vessels -- Part 2: Procedures for fulfilling the requirements of ISO 16528-1

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