Power Plant Engineering

What is Babcock And Wilcox Boiler? How Does it Work?


Babcock and Wilcox boiler is a kind of water tube boiler, why? In this type of boiler, the hot combustion or flue gases are present outside the water-filled tubes. In a water tube boiler, almost any solid, liquid, or gaseous fuel can be burnt.

Today in this article, we will cover coal-fired Babcock and Wilcox water tube boilers with various parts, accessories, working principles, advantages, and disadvantages respectively.

Read More: Water tube Boiler

What is Babcock and Wilcox Boiler

In 1856, Stephen Wilcox, Jr an American inventor, introduced a new variant of water tube boiler. Later his friend George Herman Babcock joined him and introduced the Babcock & Wilcox boiler. In this version of the water tube boiler, the water circulation system was improved and also provided more heating surface area.

In one word we can say that Babcock and Wilcox Boilers are horizontal single drum, natural circulation, balanced draft, radiant, externally fired water tube boiler which is used to generate superheated steam for the production of electricity.

The water tubes of the boiler are inclined and are connected with the water spaces at the front and rear part, with a steam container above. Because of safety and reliability, this water tube boiler is quite popular in the industry. 

Babcock and Wilcox Boiler Diagram


Parts of Babcock and Wilcox Boiler

Generally, the parts and components of Babcock and Wilcox Boiler are classified into two categories, which are given below.

Babcock and Wilcox Boiler Mountings

These are preinstalled safety devices for which a Babcock and Wilcox boiler works smoothly.

  • Burner: A gun is attached to the burner which injects atomized air and fuel (i.e. High-Speed Diesel) in a proper ratio and a lighter ignites the mixture by supplying LPG for initial burning. In this way, it helps in burning the solid fuel i.e. coal inside the furnace of a Babcock and Wilcox Boiler. The pressure of atomizing air and oil supply should be maintained properly to avoid burner failure. Pilot, Impeller, Spud, and Ignitor are some important parts of a burner.
  • Heat Exchanger: A heat exchanger is a piece of mechanical equipment where heat transfer takes place means heat energy is transferred from one system to another without any direct contact. Basically, heat exchangers are a series of multiple tubes present inside a closed vessel. The heat energy generated by the combustion of solid fuel inside the furnace is carried out by the feedwater through the water-filled tubes. Finally, the Economizer supplies this feedwater to the boiler steam drum.
  • Pressure Gauge: This instrument measures the pressure of a fluid (water or steam) present inside a media-like vessel or tube in a system. High indication shows there is an excess pressure and if there is any leakage it shows low indication. Generally, the Bourdon-tube pressure gauge is used widely in thermal power plants. The unit is expressed in Kg/cm2.
  • Supply Lines and Return Lines: Supply lines are provided to deliver hot water or steam to distribution headers like steam drums and superheater lines. Return lines like drain points and steam traps are provided to bring the excess hot water and steam into the system like Blowdown Tank to minimize heat loss.
  • Combustion Chamber Or Furnace: In the Babcock and Wilcox boiler, this chamber is made up of a series of multiple numbers of vertical tubes and each tube is welded with each other by a membrane bar to form a closed chamber, which is also called a furnace. Generally burning of solid fuel i.e. coal is taking place here.
  • Safety Valve: A safety valve is a mechanical safety device and also an important mounting of a Babcock and Wilcox boiler. When the pressure inside the steam drum or superheater line exceeds the maximum allowable working pressure (MAWP), it opens automatically and releases the excess pressure into the atmosphere. Similarly, it closes automatically after reaching the working pressure. It protects human life and the company’s assets of a thermal power plant.
  • Circulator Pump: It delivers hot water to the distribution channels through supply tubes and similarly brings it back through return lines. Generally in the case of forced circulation circulator pump is used. Due to free-flow circulation, the water tube boiler does not require any external pump.
  • Temperature Gauge: This instrument measures the temperature of a homogenous substance in a system. Generally, it consists of a capillary tube, a temperature sensor, and a Bourdon tube. The measuring system itself is filled with fluid. The unit is expressed in oC.
  • Exhaust Stack or Chimney: This is an open chamber that is attached to the Induced Draft Fan (ID Fan). This ID Fan pulls out the flue gases from the Babcock and Wilcox boiler furnace via Air Preheater and electrostatic precipitator (ESP) where the remaining ash is collected over a hopper and combust gases are extracted out to the atmosphere through this chimney.
  • Gauge Glass: A gauge glass is a level indicator device that is used to measure the fluid level (water or steam) inside a steam drum of a Babcock and Wilcox Boiler. The top part of the gauge glass is connected to the steam side of the steam drum and the bottom part of the gauge glass is connected to the waterside of the steam drum respectively.
  • Startup Vent: During startup conditions, the temperature and pressure inside the Babcock and Wilcox boiler are quite low. So to remove the low-temperature moisturized steam as well as to raise the steam pressure slowly, venting is required. It also carries away the heat released from the flue gases into the atmosphere to protect the superheater tubes from overheating. During emergencies like island operation, the boiler load is reduced suddenly for which the steam flow inside the boiler is more than the load generation. Accumulation of excess steam could overheat the metal and pressurize the main steam line that will causing back pressure inside the steam drum which results in the steam drum level being low. So to protect it from overheating and maintain the steam flow, the startup vent released the excess steam into the atmosphere.
  • Thermocouple: A thermocouple is a thermoelectric instrument that is normally used to measure the temperature of a system. Because of this device, the temperature of a system could be measured through a Distributed Control System (DCS).
  • Electro-mechanical Relief Valve (EMRV): This is a pilot-operated safety valve that uniformly maintains the main steam line pressure, and balances the main steam pressure during peak and off-peak loading respectively. Generally, an EMRV opens faster than a startup vent. In a Babcock and Wilcox Boiler, EMRV can remove the entire steam in the main steam line into the atmosphere within a second.
  • Main Steam Stop Valve (MSSV): The main steam stop valve is present at the discharge part of the main steam line of a Babcock and Wilcox boiler through which steam flows to the next system i.e. steam turbine for work done. This safety device is used to shut off or regulate the steam from the main steam line area. Generally, the main body of the steam stop valve is made up of cast steel and the valve and nuts are made up of brass with excellent resistance and limited dimensional tolerance.
  • Blowdown Valve: This is a mechanical device that is used for periodical cleaning by extracting the dissolved solids and sediments from the bottom part of a boiler. It also minimizes the corrosion property of the material or shell of a boiler. Another use of this device is for emptying the boiler whenever the boiler is not in operation.
  • Manhole: A manhole is a small covered opening that is used for inspection, cleaning, and maintenance work while the Babcock and Wilcox boiler is in shutdown condition. Normally a man enters into a boiler furnace area through this hole and easily identifies the integral defects such as water tube leakage, refractory work bed material filling, etc.
  • Non-return Valve (NRV): It is also called a check valve that allows fluid to flow in a one-directional way and ensures the fluid flows in the right direction. If the fluid tries to flow in the reverse direction, the closing member is forced back over the entrance of the valve preventing any flow. The flow through this valve can create relatively a large pressure drop, which has to be taken into consideration when designing the system.

Babcock and Wilcox Boiler Accessories

The accessories are used to increase the efficiency of a Babcock and Wilcox boiler which are listed below.

  • Economizer: An economizer is a closed chamber where multiple numbers of horizontal small tubes are placed. Feedwater that flows inside these tubes gains the heat energy from the hot flue gases present inside the shell and supplies it to the steam drum to get saturated steam. Also, it reduces the firing rates which are needed for the boiler output calculation. Thus it increases the efficiency of a water tube boiler.
  • Air preheater (APH): An air preheater is a closed chamber that is filled with multiple numbers of vertical tubes where atmospheric air flows towards the Babcock and Wilcox boiler furnace area that helps in the combustion process. It recovers the heat energy from the boiler’s hot flue gases which are present outside the tubes (means inside the shell). To avoid heat loss from the combustion gases and increase the Boiler efficiency, this arrangement is provided.
  • Superheater: Superheaters like Primary, Secondary, Slinger, and In-bed Superheaters are used to increase the temperature of saturated steam without raising its pressure. Normally a boiler utilizes two types of heat energy i.e.(sensible heat and latent heat) to convert feedwater into superheated steam. A superheater utilizes only sensible heat to superheat the steam to increase the thermal efficiency and enthalpy of a boiler.
  • Deaerator: A deaerator is an open-type heat exchanger having a deaeration chamber and a storage tank. Generally, in the deaeration chamber the dissolved gases like oxygen and carbon dioxide are removed from the boiler feedwater to protect it from corrosion and finally stored in the storage tank which is then ready to supply into the boiler drum for generating steam.
  • Forced Draft Fan: A forced draft fan or FD fan is a centrifugal fan that is used to pull out atmospheric air at high pressure and force it into the boiler furnace through an air preheater (APH). Inside the furnace, it fluidizes the bed material and mixes it with the fuel that helps in burning. Normally it is located at the suction part of a boiler. In a thermal power plant, an FD fan is used as a primary and secondary air fan which helps in the combustion process and to increase fuel efficiency.
  • Induced Draft Fan: An Induced draft fan or ID fan is a centrifugal fan that is used to pull out the flue gases from the furnace through an air preheater and Electrostatic precipitator (ESP) and extract them out into the atmosphere by the chimney. Normally it is located at the discharge part of a boiler between APH and ESP. This fan creates negative pressure to pull the hot flue gases out after burning from the furnace.
  • Primary Air Fan: A Primary air fan or PA fan is a centrifugal fan that supplies air for conveying the fuel i.e. pulverized coal from the coal bunker to the Babcock and Wilcox boiler furnace area. This fan produces a high pressure that allows the fuel to move into the furnace. PA Fan suctions air from the air preheater 2nd pass to remove the moisture of the pulverized coal. The air temperature should be maintained below the coal’s volatile ignition temperature to avoid coal burning in the PA line area.
  • Ash Handling System: To maintain the temperature, storage, transportation, disposal, and reuse of the ash, an Ash handling system is required. In thermal power plants, two types of ash are found fly ash and bedash. Flyash can be collected from the flue gases of the boiler through an economizer hopper, air preheater hopper, and Electrostatic precipitator which is then finally stored in a silo. This can be used for making bricks. Similarly, bed ash can be collected from the furnace drain and bedash cooler drain. This ash can be reused in the boiler.
  • Dosing System: Two types of dosing systems: HP dosing used in the boiler and Lp dosing used in the Deaerator. In the HP dosing system, a high-pressure dosing pump is used to supply a chemical like Tri-sodium Phosphate (TSP) which delivers this chemical to the pressurized parts like tubes and pressure gauge lines, etc. To protect the inner surface of the pressurized tube from corrosion this chemical maintains the p.H, TDS, and conductivity of the feedwater respectively.
  • Coal Feeding System: To supply pulverized coal to the furnace area coal feeding system is required. It includes a Bunker, surge hopper, plate gate, rod gate, Drag chain feeder (DCF) or Coal feeder, Distribution chute, and Mixing chamber. The pulverized coal is then supplied by the coal feeder and mixed with the primary air which pushes it into the furnace of the Babcock and Wilcox boiler for combustion.
  • Feed Control Station: Boiler Feed Pump is a centrifugal pump that supplies feedwater at high pressure to the boiler (through an economizer) for generating steam. Normally the pump has its suction from the Deaerator storage tank and then discharge the feedwater to a High-pressure Heater (HP Heater) for reheating purpose. In case of any disturbances in the HP heater line, the feedwater can be bypassed by the economizer before proceeding to the boiler. That’s why this feed pump is considered the heart of a thermal power plant.
  • Blowdown Tank: To reduce the concentration of the boiler feedwater by removing unwanted solids, floating impurities, and residual sludge that settled at the bottom part, a continuous blowdown system is required. A blowdown tank is used to store continuous blowdown and flash steam from the boiler at different pressures and temperatures. So four individual lines Continuous Blowdown Line (CBD), Emergency Blowdown Line (EBD), Drain line of Furnace Drain Header, and Drain line of Superheater Drain Header are connected with this tank. This tank stores the hot water which usually takes 6-12 hours for cooling. By quenching with the cooling water inside the tank, the hot gases or flash steam are released from the vent, and the water is then drained to a blowdown pit for reuse. Basically, this tank is used to conserve water and minimize heat loss.
  • Penthouse: The penthouse is present at the top portion of the boiler i.e. above the furnace area and near to steam drum. Basically, all water and steam line headers are located inside it and are connected to the steam drum. It minimizes the heat loss due to radiation. Also, it maintained the internal pressure during Babcock and Wilcox boiler expansion at the time of start-up or in continued operation. The overhung support of the penthouse is provided at each corner with a groove, which permits expansions of the boiler without forming any leakage in the penthouse.
  • Downcomers: These are the large tubes that connect with the steam drum and circulate water to the bottom header of the furnace water-wall tube which is present at the lower part of the boiler. It is divided into two parts i.e. Main downcomer which supplies water to the front, Divisional, left, and right side water-wall ring header. Similarly, the Rear downcomer supplies water to the rear water-wall header for heating the feedwater. These large tubes are converted into multiple numbers of tubes and are joined with respective furnace water-wall tube headers. During operation, it is important to maintain the flow velocity in the downcomers to avoid circulation problems. These tubes are located outside the furnace water-wall tubes and are enclosed with insulation and casing to avoid accidents.
  • Steam Drum: A steam drum is a horizontal closed vessel of a tank where both water and steam are stored. Feedwater with gaining some temperature from the Economizer flows towards the steam drum and the hot water is again circulated by the downcomers to the furnace water-wall bottom header which is located above the bed. The water inside the evaporative coils gets heated by the combustion of fuel and due to a change in its density, the steam flows inside the furnace wall tube top header through natural circulation and enters into the steam drum through riser tubes. A cyclone separator separates water droplets from the steam and supplies saturated steam to the superheater zone through supply tubes. As this is a closed-loop pressurized vessel two numbers of safety valve, two pressure gauges, a level transmitter, vents, and gauge glass is attached for safety operation.

Specification of Babcock and Wilcox Boiler

The specifications of the Babcock and Wilcox boiler are listed below.

  • Working/ operating pressure of Babcock and Wilcox Boiler: 140 bar or 142 kg/cm(max)
  • Size of water tube coils: 7.62 to 10.16 cm
  • Size of superheater tubes: 3.84 to 5.71 cm
  • Steam drum diameter: 1.22 to 1.83 m
  • Steam drum length: 6.096 to 9.144 m
  • Steam generation capacity: 113400 kg/hr or 125 TPH
  • Overall Efficiency: 60-80%

Application of Babcock and Wilcox Boiler

The applications of Babcock and Wilcox Boiler are given below.

  • Babcock and Wilcox’s boiler produced high-pressure superheated steam that is used in Thermal power plants for generating electricity.
  • To deliver or transfer oil in Tanker ships cargo pumps are used and are driven by the Babcock and Wilcox boiler respectively.

Babcock and Wilcox Boiler Working

The solid fuel i.e. coal is used as a burning agent and is fed over the grate surface of the Babcock and Wilcox boiler. The hot flue gases that are generated during combustion move upward and also strike with the baffle plates, it gets deflected towards the superheater and water tubes zone. If you see the flue gas path then you will find a zig-zag pattern. Finally transferring the heat energy, the flue gas is released out to the atmosphere through a damper control stack or chimney. These flue gases that carry fly ash are collected through a duct collector or Electrostatic Precipitator (ESP).

Heat energy is released during the combustion of fuel which is then utilized to convert feedwater into steam. Normally convection type of heat transfer takes place in water-filled tubes or evaporative coils zone. These tubes are located just above the combustion chamber or furnace and are heated comparatively to a higher temperature than the superheater tubes. Continuous supply of feedwater is done by the feed control station through a Boiler Feed Pump (BFP). The economizer raised the temperature of the feedwater and circulated it into the steam drum respectively. The hot feedwater is then supplied to the bed coils or evaporative coils through downcomers. Phase transfer takes place in this zone.

The feed water is converted into steam (moisture) flows upward (due to differences in density) and enters into the steam drum through riser tubes. Basically, natural flow circulations are seen in this type of boiler. Both steam and water are stored in the steam drum. A cyclone separator which is present inside the steam drum separates water droplets from the steam and supplies saturated steam to the superheater tubes. Again the steam is heated through these radiant-type superheater tubes to get superheated steam and is controlled by the main steam stop valve (MSSV) for rotating the blade of a steam turbine. To protect the closed vessel i.e. steam drum, various mountings like a safety valve, Gauge glass, level transmitter, pressure gauge, vents, and drain lines are provided. A continuous Blowdown (CBD) line is present to keep the concentration of the impurities within the permissible limit by continuous blowdown of feedwater from the drum. An emergency Blowdown (EBD) line is present to protect the steam drum from water high-level conditions which may lead to the carryover of water by steam and the separators may be submerged into the water.

Advantage of Babcock and Wilcox Boiler

The advantages of Babcock and Wilcox boiler are given below.

  • The steam generation capacity of the boiler is high approx. 2000 to 40000 kg/hr.
  • During shutdown, this boiler can be easily repaired and cleaned.
  • According to its structure, this is a vertical type boiler, so it requires less space as compared to other boilers.
  • This boiler shows high overall efficiency.
  • The evaporative coils, primary superheater, secondary superheater, and In-bed superheater can be easily replaced during tube failure.
  • In this boiler, draught losses are very low.
  • Due to the large heating surface area, the solid fuel i.e. coal is completely burnt inside the furnace area.

Disadvantage of Babcock and Wilcox Boiler

The disadvantages of Babcock and Wilcox boilers are given below.

  • As the water-filled tubes i.e. the evaporative coils are located just above the furnace bed, a continuous supply of feedwater is needed for generating steam. If the feedwater supply is discontinued for a short period, then the water tube will be overheated due to a shortage of feedwater. Thus it will impact the steam generation and can trip the boiler at drum level low.
  • The maintenance cost of the Babcock and Wilcox boiler is high.

This is all about the “Babcock and Wilcox Boiler”. Thanks for giving your valuable time. If you have any doubts related to this boiler please don’t hesitate, to ask in the comments section below. If you have any prior experience related to this boiler and know more about it, please share it with us. We promise that we will solve your doubts as soon as possible. And if you really like this article, don’t forget to share it with your colleagues and friends. Good Luck!

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