A 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.

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.

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boiler Types

  • Condensate Boiler  -  Also known as a high efficiency gas boiler, recycles extra heat from the exhaust gases, thereby wasting less energy.
  • Electric Boiler  - Is very similar to a gas or oil boiler, but it uses electricity to heat water rather than gas or oil.
  • Hot Water Boiler  -  A pressure vessel, fired or unfired, used to heat water.  They can be classified by: low pressure boiler, high pressure or temperature power boiler, or miniature boiler depending on the various codes.
  • Oil Boiler  -  Uses oil rather than natural gas to heat water rather.
  • Natural Gas Boiler  -  Uses natural gas rather than oil to heat water rather.
  • Steam Boiler  -  A closed pressure vessel, fired or unfired, used to create steam by applying heat.  They can be classified by: low pressure boiler, high pressure or power boiler, small power boiler or miniature boiler depending on the various codes.
  • Water Boiler  -  A boiler whose primary heating surface is composed of many tubes filled with water.  Tubes of 3 inch diameter and above are termed "large tube boilers", and water tube designs using smaller tubes of 2 inches or less diameter are termed "small tube boilers".


Boiler Components

  • Burner  -  The burner nozzles spray and ignite the fuel, heating the water inside the boiler, controlled by a thermostat.
  • Combustion Chamber  -  The fuel is burned inside the combustion chamber.
  • Heat Exchanger  -  A device used to transer heat from one medium to another at different temperatures.  The heat transfer can be air or a liquid such as water or oil.
  • Plumbing System  -  A system that collects, distributes, stores, and treats water for.


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


Boiler Glossary


  • Air-fuel ratio  -  The ratio of air weight to fuel weight consumed in a furnace.
  • Allowable working pressure  -  The maximum pressure for which the boiler was designed and constructed.
  • Ambient temperature  -  When outdoors the ambient temperature is the current surrounding environment air temperature.  This temperature has nothing to do with high or low forcasts.


  • Blastpipe  -  Part of the exhaust system that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.
  • Boiler crown  -  The upper round plates of a boiler of the shell type boiler.
  • Boiler horsepower  -  The evaporation of 34-1/2 lbs of water per hour from a temperature of 212\(^\circ\)F into dry saturated steam at the same temperature.
  • Boiler plate  -  A mild steel plate, generally produced by the open hearth process, used mainly for the shells and drums of steel boilers.
  • Boiler pressure  -  The pressure at which steam is generated in a boiler.
  • Boiler rating  -  The heat capacity of a boiler expressed in boiler horsepower, Btu/hr, or pounds of steam/hr.
  • Boiler shell  -  The outer cylindrical portion of a pressure vessel.


  • Check valve  -  These valves are designed to allow the process fluid to flow in only one direction to prevent backflow.
  • Cladding  -  The layer of insulation and outer wrapping around a boiler shell.
  • Combustion  -  A reaction called rapid oxidation or burning produced with the right combination of a fuel, oxygen, and heat.
  • Corrosion  -  The thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.


  • Design pressure  -  The pressure the vessel is designed for.
  • Design temperature  -  The temperature the vessel is designed for.
  • Downed Tube  -  Either a fire tube or water tube that is entirely below the water level of the operating boiler.  As corrosion and scaling is most active in the region of the water level, this reduces wear and maintenance requirements.
  • Dry steam  -  Does not contain water held in suspension.  It is similar to superheated steam.


  • Enthalpy  -  Measures the sum of internal energy changes in heat under constant pressure of the system.
  • Exhaust gas damper  -  Used to control the pressure in an exhaust gas boiler.
  • Expansion joint  -  Heat and cold causes piping systems to expand and contract. 


  • Feedwater  -  The water supplied to a boiler to replace that evaporated as steam or blown off.
  • Feedwater heater  -  An apparatus for raising the temperature of boiler feedwater by abstracting some of the heat from exhaust steam or from the hot gases of combustion.
  • Feedwater line  -  The pipeline through which the water is fed to the boiler from the feedwater tank with the help of a pump.
  • Furnace  -  That part of the boiler designed for burning the fuel.


  • Gauge Glass  -  Part of the water level gauge, which normally consists of a vertical glass tube connected top and bottom to the boiler backplate.  The water level must be visible within the glass at all times.


  • Head loss  -  A pressure loss due to the resistance of the fluid and obstructions along the way in a pipe.
  • Heat  -  A form of energy that causes physical change in what is being heated.
  • Hotwell  -   A tank for the condensate returns and from which the feed water pump takes its suction.


  • Insulation  -  A material of low thermal conductivity used to reduce heat loss.


  • Joint efficiency  -  The ratio of the strength of a section of riveted or welded joint to the strength of a related section of solid plate.



  • Latent heat  -  The energy absorbed or released by a substance during a constant temperature or phase change from a solid to liquid, liquid to gas or vise versa.
  • Live steam  -  Steam available directly from a boiler under full pressure.
  • Lowest Possible Water Level  -  The lowest water level at which the boiler can be safely operated without damaging or overheating any part of the boiler.
  • Low Water Fuel Cutoff  -  A safety device which cuts off the fuel supply to the burner if the boiler water level drops below a safe level.


  • Makeup water  -  Raw water that has been filtered and softened and then introduced into the hotwell to compensate for the water loss in the system.
  • Motive pressure  -  Line velocity pressure in steem piping.


  • Natural circulation  -  The circulation of water in a boiler caused by the differences in density.



  • Pressure  -  It is the force exerted perpendicular to the surface of an object and is expressed as force per unit area.
  • Pressure indicating transmitter  -  An instrument for measuring, controlling and indicating positive, negative or differential pressures of different fluids.
  • Pressure loss  -  The difference in pressure between two points, usually caused by friction resistance in the pipe, but moisture can also affect it.



  • Raw water  -  Water supplied to the plant before any treatment.
  • Rupture Disk  -  A safety device which acts like a safety valve to protect against excessive pressure buildup in a system.  The disk shatters when its maximum pressure is reached and must be replaced each time it activates.


  • Scale  -  Dissolved minerals from hard water that precipitate out in the steam space around the water level.  Where this scale falls to the bottom of the boiler and mixes with other contaminants, it is called mud.
  • Scum Valve  -  A blowdown valve mounted at the water level of a boiler, used to blow down lighter oily or foamy deposits within a boiler that float on the water-level.
  • Steam  -  The invisible vapor (gas) when water is heated to its boiling point and passes from a liquid to a gaseous state.
  • Steam gauge  -  A device for indicating gauge pressure.
  • Stop-check valve  -  Essentially, two valves built into one.  It can act as a globe valve to isolate or control the flow rate.  It also acts as a check valve by preventing reverse flow.
  • Stop Valve  -  Stop valves are used to shut off or partially shut off the flow of liquids.
  • Syphon -  A protective device used to prevent steam from entering the internal works of a steam gage.  Often a syphon only consists of a single coil of high pressure pipe with threaded ends.


  • Temperature  -  Normally described as the amount of heat or cold, but it is neither heat or cold.
  • Thermal conductivity  -  The ability to transfer heat within a material without any motion of the material.




  • Waste heat  -  Sensible heat in non-combustible gases discharged to the environment.
  • Windbox  -  The box surrounding the burner damper on a boiler.  The fan blows combustion air into the box and the damper regulates and directs the air into the burners.

  • Water hammer  -  Occurs when a valve is suddenly opened or closed.
  • Water line  -  The level at which water stands in the boiler.
  • Water softener  -  Used to remove dissolved minerals from water.  Hard water can leave mineral scale after evaporation.



  • Yield point  -  The point where an elastic material is permanent change in length with no extra load force.
  • Yield strength  -  The minimum stress that leads to permanent deformation of the material.



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General Information about Boilers

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