Foaming refers to the formation of froth or bubbles in the boiler.  Foaming is caused by high concentration of dissolved solids in boiler water.  When a bubble forms in water that has high levels of dissolved solids, the dissolved solids surround the bubble and make it tougher.  The bubble does not break.  When more bubbles collect, foam is formed. 

Foaming can be prevented by maintaining good water quality.  The level of dissolved solids should be maintained as low as possible. 

The presence of oil in the water can also cause foaming.

Anti-foaming agents are available which prevent the formation of bubbles and foam.




Feed water is the water that is fed to the Boiler.  The feed water is a mixture of the condensed water also known as return water and fresh water (makeup water) to compensate for its losses.  The make up water is treated to remove its minerals. 

Chemical treatments are carried out to bring the conductivity, pH and alkalinity within specific limits.  The water is deaerated to remove dissolved oxygen and carbon dioxide.

The makeup water that is fed into the boiler depends on the total water losses.  Water leaks from the boiler and the pipelines as steam.  Some of the water is deliberately released via blowdown to remove the impurities which have settled at the bottom of the tank.




Maintaining level is an important aspect of Boiler Control.  Automatic Boiler Controls work by sensing the level of water in the boiler based on conductivity and and taking the necessary action.  Level monitoring is done by means of conductivity probes which are inserted through the boiler wall.

These probes are placed at different levels of the boiler.  When the water reaches the particular level, the contacts are bridged and a current flows through the probes and a signal reaches the controller.  If the water level falls no current flows signalling that the level has fallen.

These probes are specially designed to withstand the temperature and pressure of the boiler.





Maintaining the proper level in a Boiler is a very important aspect of boiler control.  A boiler should have a reliable water level controller.  There are many automatic controllers which automatically keep the level of the water within the specified limits.  The boiler can be seriously damaged if the level of water is not maintained.


Boilers will also have alarms which alert the operator if the water level falls or exceeds the safe limits. If the water level rises above the safe limit, it can result in priming and carryover.  If the water level increases, it can exceed the horizontal limit of the boiler, the surface area for steam generation will then reduce.

If the water level falls below the safe limits, the tubes of the boiler will be exposed.  This will result in the tubes getting overheated and they can rupture.

The level of water in the boiler is also dependent on the pressure.  If the steam pressure is reduced, the water level rises.  If the pressure is increased, the level can fall. 




Level Indicators are very important instruments in any boiler.  Level indicators tell the operator how much water is there in the boiler.  The Level indicator is a vertical tube made of glass which can withstand high temperature.  This glass is mounted on the boiler drum.  One end of the level indicator is placed below the water level, the other end is placed above the water level.  This is called the steam end.

As the level of water rises and falls in the boiler, the level in the indicator also changes.  The glass tube of the indicator is covered by another protective layer of toughened glass.
The indicator has a drain cock which can be opened to drain the water in the level indicator.  This is used to check the functioning of the level indicator. 

The level indicator has two safety balls at each of the inlets, the water inlet and the steam inlet.  If the glass of the level indicator gets broken, the balls seal the two outlets and prevent leakage.
The Level indicator is a simple device which provides a direct reading of the level inside the boiler. 




The Fusible Plug is a safety device in a boiler.  When the temperature of the boiler reaches abnormal levels, the plug fuses (melts) and provides an exit for the steam.  The jet of steam which exits the boiler will alert the operator to the high temperature.

A typical scenario is when the water level in the boiler falls below the safe limit.  This can cause the boiler to overheat and ultimately fail due to the metal walls softening. 

The plug is placed below the water level.  If the water level falls below the plug, the boiler starts to heat and the plug operates, releasing steam.




The function of a throttling valve is to reduce the steam flow rate of the boiler.  This is necessary to prevent steam flashing. 

If a throttling valve is not used, a false temperature reading may reach the boiler blowdown controller.  This will result in wrong operation of the boiler blowdown.  The boiler throttling valve must be installed before the sensor. 




Boiler Mountings are the accessories which are mounted on the boiler for the effective control and safety of the boiler.  There are many accessories available today.


The mandatory boiler mountings are
1. Safety Valves - 2 Nos
2.  Water level Indicators - 2 Nos
3. Steam Stop Valve
4. Fusible plug
5. Blow Off Cock
6. Manholes and Mudholes




Stoichiometric Combustion is the ideal combustion where at the end of the combustion process no fuel or air is left behind.  Thus all the carbon in the fuel is converted to carbon dioxide and the sulphur is converted to sulphur dioxide.  If residues such as carbon, carbon monoxide or sulphur remain, then the combustion is not stoichiometric.

Stoichiometric combustion cannot be achieved practically.  By adjusting the air/fuel mixture, the actual combustion can be made as close to the stoichiometric combustion as possible. 




When fuel is burned in the combustion chamber of the boiler, the calculated amount of air which is required for the fuel to burn is never sufficient.  Excess air needs to be fed to the combustion chamber.  This air which is in excess of the calculated amount is known as excess air.


Excess Air refers to the additional air which is fed to the combustion chamber of the boiler to ensure that the fuel gets burnt properly.  Excess Air is provided by means of blowers.  in the case of supercharged boilers, it is provided by compressors. 

Different types of fuel have different requirements for excess air. Gas may require up to 10% of excess air.  Fuel will require up to 20 % and coal will require large amounts of excess air up to 60 %




A Condensate slug occurs when condensate which collects in a steam line and moves through it at high speed encounters a point of resistance such as a bend or a closed valve. 

The condensate travels at the speed of the steam in the line.  When this condensate meets a point of resistance such as a bend in the pipeline or a pressure reducing valve, a tremendous impact occurs.  The shock wave travels through the system getting reflected many times.  This results in hammering.  This can cause damage to the pipelines and even injury or death to personnel.

Hence, adequate precautions to prevent such a scenario. 

Steam Traps should be fitted at points where the line makes sharp turns.  Steam Traps should also be placed before pressure reducing valves and valves which are mostly closed and at the end of steam lines. 






A Velox boiler is a supercharged boiler.  The Velox boiler is based on the principle that when the velocity of the exhaust gases exceed the velocity of sound, the heat transfer increases greatly. 

The flue gases are used to drive a gas turbine coupled to a compressor which compresses the gas from the atmospheric pressure to the furnace pressure. 

The Velox Boiler has a very high combustion rate.  It can be quickly started. 




In a Super charged boiler, compressed air is fed to the furnace.  The Super charged boiler has better heat transfer capacity.  The heat transfer required is lesser compared to a conventional boiler.  The exhaust gases which come out with high velocity are used to rotate a gas turbine.  This gas turbine in turn can rotate other auxiliaries.

A compressor is used to produce the compressed air for the furnace.  This compressor is powered by the gas turbine. 

The boiler is easier to control. It is also easier to start and requires lesser personnel to operate it. 




Pulverised Coal is coal which has been powdered to a fine size.  Coal is powdered or pulverised by passing into through a pulveriser.  A Pulveriser is a machine which consists of rollers which crush coal into a fine size. Today, almost all coal fired plants used Pulverised Coal. 

The Pulverised coal is mixed with hot air.  This air and coal mixture is fed to a burner in the furnace which ignites it.  Modern pulverisers can grind coal to a very fine size of the order of microns.  When the pulverised coal mixes with air, it flows almost like a fluid stream.  

Pulverised coal burns more efficiently as it has a higher surface area.  Sometimes, pulverised coal is mixed with other fuels such as biomass. 




Draft or draught is an important factor in the combustion of fuel in the boiler.  Draft refers to the difference in pressure in the boiler furnace to the pressure on the top of chimney.

This pressure difference is necessary for the flow of fresh air into the boiler and for the removal of flue gases out of the boiler.  The draft should be optimal.  The draft has a direct influence on the fuel/air mixture in the furnace.  A higher draft will result in more air being sucked into the furnace.  This will result in higher combustion. 

A lower draft will not be able to remove the flue gases properly from the furnace.  This will result in less air entering the furnace.  This, in turn, will result in incomplete combustion which affects the efficiency.  Incomplete combustion will also result in more pollution.




The Stack temperature of the boiler is a very important parameter in Boiler design.  The stack temperature is the temperature of the flue gases when they reach the stack or the chimney.


A low temperature of the stack temperature indicates that little heat is carried away by the flue gases and that the boiler is operating efficiently.  The stack temperature is a very important specification at the time of boiler purchase.

If the temperature is too low, it can result in cold corrosion. 

The Stack temperature will vary with the time of the year as it is also dependent on ambient temperature. 




When the fuel is burnt in the furnace of a boiler, all the energy in the fuel is not available to heat the boiler.  Some of the energy is lost in the form of losses.
The Losses in combustion of a boiler are categorized in to the following types.

Loss in the Flue gases
The heat generated by burning the fuel is present in the flue gases.  When these gases escape into the atmosphere, some amount of heat also escapes with the gases.

Hydrogen Losses
This refers to the heat used in evaporating moisture or water present in the fuel.  This is particularly significant in coal-fired boilers.

Losses due to improper combustion.
The improper combustion of fuel due to poor quality or inadequate air also results in loss of potential energy

Losses due to Convection and Radiation
The furnace of the boiler is insulated.  Despite this, some heat escapes from the furnace to the atmosphere.




The Economizer in a boiler is used to preheat the water which is fed into the boiler by using the exhaust gases of the boiler.  In this way, it is able to "economize" or save energy.  The heat of the exhaust gases will be in the range of 380 to 550 degrees Celsius.  By utilizing this heat energy, the economizer increases the overall efficiency of the boiler. 

The Economizer is in the form of vertical tubes in which the water flows.  The gases on the way to the exhaust stack transfer their heat to the economizer.  The temperature of the inlet water to the economizer should not be too low as that can result in fouling and corrosion. 

The outlet temperature of the economizer is also below the boiling point of the water.




Steam Washing in boilers refers to the "washing" of the steam with fresh water or steam with condensate.  The objective of steam water is to remove the impurities in steam such as silica.  Silica in steam is mostly in the vapour state.  Silica can deposit in the blades of turbines and affect the efficiency and the operation.


When water which is at a colder temperature than steam is sprayed on the steam, the silica condenses and gets carried away by the water. 

The washing is usually carried in many stages for better efficiency. 




The steam that is generated from the boiler is wet steam.  This steam has water droplets suspended in it.  The water in steam can damage the components of a boiler system by way of corrosion.  It has lower energy carrying capacity.


Steam Separators are devices used to separate the suspended water from the steam.  There are many different methods to achieve this.  One method is the use of baffles which are placed in the path of the steam.  The baffles collect the water from the steam. 

Another method is by using the centrifugal principle.  The steam is passed through a chamber where there is a rotary device which spins the steam.  Due to the centrifugal principle, water which has a higher mass is separated from the steam and collected. 

Another method is by passing the steam through a wire mesh known as a demister.  The water particles tend to collect in the mesh while the steam alone passes. 

Steam separators usually use more than one method to separate the water droplets from the steam. 




Soot Blowers are devices used for cleaning in boilers.  The Soot Blower is a mechanical device which cleans the deposits due to fouling in the furnace.  The Soot Blower consists of a cleaning medium such as steam, compressed air or water.


It directs jets of the medium on the surfaces and tubes to be cleaned.  Special injectors which can precisely focus the jets on to the tubes are used.  Removing the soot deposits greatly improves the efficiency of the boilers. 

There are different types of Soot Blowers such as the insertable soot blowers which can be inserted into the tubes as well as the rotating kinetic type of soot blowers. 

The soot blower can be operated manually by an operated or it can be made automatic to operate at specific intervals.  Boilers will have a specific cleaning cycle which depends on the size of the boiler, type of coal used, operating load, etc.

Soot Blowers which are powered by compressed air will have a dedicated air compressor for soot blowing.  The soot blowing equipment is generally galvanized to withstand the corrosive effect of the gases




Fouling is a phenomenon where the hot flue gases and the ash precipitate and settle down in the places where the flue gases exit the boiler.

This layer which is formed reduces the gas flow into the Selective Catalytic Reduction tubes.  This can result in poor effluent treatment of the gases.
Fouling is generally removed by soot blowers. 




The Slag Screen is a bank of specialized Boiler tubes which is fitted at the entrance of the convection passes in a boiler furnace.  The slag screen cools the hot gases and the ash before the entry to the convection shaft.  This minimizes the risk of slag. 

The slag screen can be arranged in an inline arrangement where the slag screen tubes are parallel to the boiler tubes or a staggered arrangement where the tubes are at an angle to the boiler tubes.




Slag formation occurs when the temperature of the gases exiting the furnace is above the fusion temperature of the fly ash.  At these temperatures, the fly ash melts and gets deposits on the sides of the furnace.


Slag Formation can lead to problems such as
  • reduced heat transfer from the combustion gases inside the furnace to the water
  • It can lead to further overheating of the boiler gases which, in turn, leads to further deposition.
  • Leads to unpredictable behaviour in the boiler
It is necessary to maintain furnace temperature below the fusion temperature of the ash.  The fusion temperature of the ash can be obtained by testing ash samples at laboratories. 
Slagging can also be prevented or minimized by a Slag Screen Arrangement in Boilers.




Baffles are used in Boilers to reduce turbulence in the flow of the hot combustion gases over the boiler tubes.  The baffles maintain proper velocity of the gases which enables efficient energy transfer.


Baffles also guide the fly ash and slag to the proper place for deposition from where they an be easily removed.  

If the Baffles are damaged, it will result in overheating at certain places and poor heating in others.




Cyclone furnaces are used in boilers to burn poor quality coal which are not suitable for normal pulverized coal combustion.  These coal particles require higher temperatures and more oxygen to burn. 

Cyclone furnaces, as the name suggests, are able to provide a cyclone of air which results in greater turbulence which results in better mixing of the coal and air.  The cyclone of air also causes greater exposure of the surface area to the flame. 

The particles are whirled about in the air flow which results in greater exposure of the surface of the coal particle to the atmosphere. 




Thermal Spray is a protective coating made on the tubes of the boiler.  The Thermal spray prevents corrosion, damage to the tubes and unscheduled breakdowns.   The material used for coating is usually an alloy.

Alloys based on Iron with added Chromium are used.  Low carbon steel can also be used as a thermal spray as it resembles the weld overlay.  Aluminium based thermal sprays are also used. 

There are different methods of applying the Thermal spray.  The metal is melted by using an electric arc or a gas flame and sprayed on to the tubes.




Tangential Firing in Boilers is a widely used method in the combustion of fuel, usually, coal.  This method ensures efficient firing of the fuel.  The Tangential Firing method also results in reduced emissions.


In this method, finely powdered coal is blown into the combustion chamber along with air through a series of burner nozzles.  As the coal and air mixture enters the combustion chamber four burners placed in the wall tangential to the fireball are used to fire the mixture.

The method gets its name as the flame from the burners strikes the burning fireball at a tangent.  The burning mass rotates ensuring complete combustion of the fuel particles. 




Firing refers to the application of heat to a boiler.  Firing is done by different methods. 

Fuel fired Boilers
The most common method is by burning fuel such as wood, oil, coal or gas.


Waste Heat Recovery Boilers
These boilers function by recovering the heat from the gases or effluents of other industrial processes.  For instance, the exhaust gas from a power plant can be used to heat a boiler. 

Electrically heated Boilers
These boilers are used for heat generation.  They are not used for power generation.  They are generally of small capacity.  They can be used in hospitals for sterilizing equipment.  They can be used for laundry and for domestic heating purposes. 

Nuclear Powered Boilers
These boilers are used in Nuclear Power Plants.  The nuclear fission reaction which occurs in the reactor produces enormous amounts of heat.  This heat can be used to heat water and produce steam.  This steam is used in power plants to generate electricity or to drive submarines




The Flash tank is used to flash the condensate at high temperature into steam. The primary function of the Flash Tank is to reduce the pressure of the condensate.  Some of the condensate "flashes" into steam in the Flash Tank.This low pressure condensate can be reintroduced into the boiler or condensate through the low pressure lines.  The steam can be used in place of live steam in the boiler system.

The Flash Tank serves to preserve the heat content of the condensate while reducing the pressure to a safe value.  The Flash Tank is a very important component of closed systems.  In fact, the flash tank is what makes closed systems possible




Fired Boilers are boilers in which steam is generated by the direct application of heat.  Heat is generated by burning fuels such as coal, fuel or gas

In Unfired Boilers, steam is generated by an indirect source such as hot water or steam from another process.  No fuel is burnt in these kind of boilers. 




Supercritical Boilers are boilers in which the working fluid is above the critical pressure.  At this pressure, water changes into steam without boiling.  This intermediate state is known as a super critical liquid. 
Supercritical boilers are used in Turbine systems. 

The advantages of supercritical boilers over sub critical boilers are

Efficiency
Supercritical boilers are more efficient that sub critical boilers.  The efficiency rating of supercritical boilers is in the range of 32 - 38 % while that of ordinary boilers is in the range of 32% - 38%.

Reduced Operating Costs
As the efficiency increases, there is a natural reduction in fuel costs which translates into reduced operating costs

Lower Emissions
Due to less fuel being burnt, there are lower emissions.

Higher Initial Costs
The downside is that super-critical boilers have higher initial costs as the boiler and the systems have to be designed to withstand higher pressures. 

Advanced Water Chemistry
Supercritical boilers require very pure water.  Even small levels of impurities can cause deposits on the turbine blades. 




The Critical pressure in boiling a liquid is that pressure above which there is no clear change of state between the liquid and the vapour phases.  Simply put, water turns into vapour without boiling.  Above a pressure of 22.1 MPa, water reaches this state. 

In supercritical boilers, water is boiled at a very high pressure.  At that high pressure, there is no clear distinction between the water and vapour phases .  The fluid can  no longer be called liquid or vapour.  It becomes what is known as a super-critical fluid.

Supercritical Boilers are generally used in Turbine systems.  When the supercritical fluid drives the turbine, it loses pressure.  As the pressure drops below the critical point, the supercritical fluid becomes a mixture of water and steam which then passes through the condenser.

Supercritical Boilers consume less fuel as compared to Traditional Drum Boilers. 




Mobile Boilers are boilers which are not fixed to any particular location.  They can be moved from place to place.  A common example is the locomotive boiler in railway engines.  These boilers move along with the Engine. 

Mobile boilers  are available in a variety of capacities.  They are also available in a variety of pressure ranges.  The Boilers can be used to provide both steam and hot water.

Mobile Boilers are mounted on a trawler which can be towed from one place to another.  These boilers are handy and can be installed at the locations where steam is required on a temporary basis.  They can be used for heating, sterilizing in plants and in hotels.  Many mobile boilers are powered by electricity.

Mobile Boilers come in both fire tube and water tube versions.




A boiler, as the name suggests, boils the water before turning it into steam at subcritical pressure - the pressure at which bubbles can form.  Steam generators, on the other hand, convert water into steam into steam without boiling at a super-critical pressure.

There are also constructional differences between a boiler and a steam generator.  A boiler contains many tubes which carry the water.  A steam generator has, generally,  only one tube in which the heating occurs. 




A waste heat recovery boiler is a device which recovers the heat produced by another industrial
process or equipment such as a genset, incinerator, furnaces, etc.  Many industrial processes produce heat which is rejected into the environment as waste.  This heat can be captured by the boiler and used to generate steam. 

The steam which flows through the exhaust is diverted by means of a diverter to pass through the boiler and then to the exhaust. 

This steam generated by the boiler can be used for a variety of functions such as to generate power, for other application such as in the case of a textile mill, for heating, etc. 

Other benefits of Waste Heat recovery Boilers are that they reduce pollution and the temperature of the exhaust gas.  This reduces the maintenance requirements of the exhaust systems. 




Boilers are Constructed out of a variety of materials. 

The Boiler is a device which generates steam at very high pressure and temperature.  Thus, it should be designed to withstand the mechanical and thermal stress caused by the temperature and pressure.

While selecting materials or boilers, there are also issues such as cost and availability which determine the choice.

In general, Low Pressure Boilers are constructed out of cast iron or steel. 

Miniature Boilers can be made of stainless steel or copper while Power Boilers are designed out of special steels which can withstand very high pressure and temperature. 




The following are some of the factors which determine the type and Size  of boiler
  1. Power to be Generated
  2. Operating Pressure
  3. Type of Fuel and its quality.
  4. Load Factor
  5. Location
  6. Availability of Water
  7. Availability of Area for the Boiler
  8. Cost of Operation and Maintenance





The Burner of the Boiler is the source of heat in boilers which are powered by natural fuel such as gas or oil.  It is the place where the fuel is burnt to produce energy.


The burners in Boilers should combust the fuel with very low emissions.  They are sometimes provided with an air source such as a fan to ensure proper combustion with little residue.

Duel Fuel burners can burn both oil and gas.  Common fuels are Furnace Oil, Light Oil, Natural Gas and Liquefied Petroleum Gas. 

Boiler Burners can range in capacity from 200 kW to 15000 kW

Modern Burners in Boilers have sophisticated electronic controls which algorithms for precise air-fuel mixtures for optimum efficiency. 




Steam Traps are devices which are used to release condensate which may form in steam lines.  Steam traps allow only the condensate and prevent the useful steam from escaping. 

Condensate should be removed from steam lines.  Condensates can cause hammering in the pipelines and corrosion.

In addition to Steam, Steam traps also release air and other gases. 

There are many different types of steam traps.  The most simple can be a nipple in a pipeline.  Since condensate (water) is heavier than steam, it will collect at the lowest point.  The trap opens once sufficient amount of condensate has collected. 






In radiant super heaters, where the tubes of the superheater are placed directly in the furnace, special tubes are used to protect the superheater tubes from the high temperature of the furnace.


These tubes are called Screen Tubes as the screen the superheater tubes from the intense heat.




Water Walls Tubes are tubes which run along the inner surface of the boiler.  The function of the water wall tubes is to transfer the heat of the furnace to the water which flows in them.


Water Wall Tubes are more efficient than water tubes which were used earlier.  Water wall tubes help to better capture the heat of the boiler.  Since they are located in the walls, they prevent the heat from leaving the boiler and improve its efficiency.  They also keep the boiler surroundings cooler.

The water Wall Tubes are connected to the steam drum by means of risers. 




The Super heater Tubes serve to super heat the steam from the steam drum.  Steam from the steam drum is saturated steam.  It may still have droplets of water in it.  This steam is passed through tubes passing through the furnace to heat it.  Superheated steam has very high energy and is used to drive the turbines.


The capacity of a boiler is dependent on the amount of superheated steam it can generate.  Super heater tubes must be able to withstand very high temperature and pressure.  The steam-water ratio is designed such that it does not overheat.    Hence, super heater tubes are one of the most important parts of the boiler. 

Super heaters are classified into

  1. Radiant Super heaters which are placed directly in the furnace
  2. Convectional Super heaters which are placed in the path of the hot gases
  3. Separately fired super heaters are heated by a separate heat source.





Steam drum is the reservoir at the top of the water tube boiler.  The water tubes which carry water are connected to the Steam drum.  The steam drum serves to extract the steam from the water and send it for superheating.  The difference in density causes the steam to rise.


The pressure in the steam drum regulates the steam generation within the boiler.  As the steam is extracted, further steam is generated.

The water is sent back to the water drum for further heating. 




Boiler Capacities are often denoted in British Thermal Units.  One British Thermal Unit is defined as the amount of heat required to raise one pound of water by one degree Fahrenheit.  While the BTU has generally been replaced with the more popular unit, the Joule, Boilers and the Heating industry still use the British Thermal Unit.


One BTU is equal to 1.06 Joule

BTUs are also used for indicating the energy in fuels.  Oil has a BTU of 138000  per gallon.  Natural Gas has a BTU of 1075 per cubic foot.

A bigger unit is the MMBTU which stands for one million BTU.  The M  is the Roman number for thousand.  MM stands for a thousand thousand which is one million.  




Lagging in Boilers refers to the covering over the boiler.  The principal function of the lagging is to cover the insulation and prevent heat loss from the boiler.  It also provides a cool surface over the boiler for fitment of accessories.  It provides a formal shape to the boiler after the insulation has been fitted.


Lagging is a essential part of boiler design.  Lagging is usually made of aluminium or steel.  The Lagging should be adequately supported so that it maintains its shape. 

The surface of the Lagging should also be provided with drainage so as to drain the excess water in case of rain or other water leakage.  This is important particularly in outdoor design. 

Adequate allowance should be made in the design for expansion and contraction of the lagging during operation as the temperature varies. 

The expected lifetime of the insulation and lagging is 15 years.




Spray On Ceramic Insulation is a popular method of insulation in recent times. The insulation consists of a ceramic material which is sprayed on to the surface of the boiler and piping using a special equipment.

 Ceramic insulation is more expensive than fibreglass insulation. Once sprayed, the material solidifies into a foam type material. One advantage of this method is that it is easy to apply. The downtime required is very less.

It is lightweight and can also be easily applied to irregular surfaces and edges . Ceramic insulation is also a corrosion inhibitor. It can also withstand moisture better.

Special types of ceramic insulation can be sprayed on to the surfaces of hot equipment. This can reduce downtime.




Bottom blowdown in boilers is used to remove impurities which have fallen to the bottom as precipitates.  These impurities are in the solid phase.  Bottom blowdown is done by means of a valve connected to the bottom of the valve.  When the valve is opened, the impurities are flushed out by the boiler pressure.


The steam collected during the blowdown can be removed into a steam flasher and a heat exchanger.  The heat can be recovered and the steam can be recirculated after passing through the flash tank and the deaerator.

The duration and frequency of the blowdown is determined on factors such as size of the boiler, water quality and the location and the operating load.

A proper blowdown programme improves efficiency and reduces maintenance costs. 




When water is heated to the boiling point at a given pressure, its temperature rises.  However, once the boiling point is reached, the temperature stops to rise.  The energy which is fed to the water is used to convert the water to vapour. 

The heat which is given to the water converts it into the vapour state.  This is known as the latent heat of vaporization.  The steam produced in this stage contains water droplets.  The temperature does not rise till all the water has been converted into steam. 

The steam which contains suspended droplets of water is called saturated Steam.
Saturated Steam is used widely in the industry for drying, heating.  It is also used in cooking as it has a high energy transfer coefficient. 

If the steam is heated further, all the water is converted into steam.  The steam, at this stage, is called superheated steam. 




Carryover is the escape of moisture and impurities such as silica, copper, sodium, etc. along with the steam.  These impurities which are carried away by the steam can affect the piping and other equipment in the system.


Carryover occurs in two ways.  The first is due to mechanical action wherein the high velocity steam carries with itself small droplets of water which carry the impurities .  This is called mechanical carryover.  The second way in which carryover occurs is called vaporous carryover. 
In vaporous carryover, the impurities are carried over by steam. 

The sum of the mechanical and the vaporous carryover gives the total carryover. 
Causes of Carryover

The causes of carryover in boilers  are
  • Boiler Design
  • Fluctuating load on the boiler.  This causes the water level to rise and fall owing to the change in steam pressure.  This results in small droplets of water being ejected from the water surface into the steam above.
  • High quantity of impurities and dissolved substances
  • Overloading of the boilers
  • High water level
Use of efficient steam separation devices can minimize carryover.  A well designed blowdown schedule will help drain impurities and greatly reduce carryover.  Antifoam agents can also reduce carryover. 




Priming refers to the carryover of small droplets of water along with the steam.  Prime is undesirable as it causes corrosion to the steam line and other equipment.
 
Priming is caused due to many reasons such as high water levels, poor boiler design and fluctuating loads.

When the load in the boiler suddenly increases, the steam pressure in the boiler drops.  This causes the level of water to surge.   This surge of water level causes priming.  Small droplets of water are thrown up into the steam above the water.  This water can carry with it dissolved substances such as chloride, silica, copper, etc.  This is known as carryover.

Priming can be prevented by operating the boiler at steady loads and by maintaining good water chemistry which prevents foaming.




The Boiler Blowdown rate refers to the rate at which the blowdown should occur in an operating boiler.  It describes the blowdown in kilograms per hour.


The Boiler blowdown rate depends on the quantity of the impurities and the limits of tolerance for the employees.  The Blowdown rate is a product of the steam consumption and the ratio of the level of the TDS to the difference between the maximum allowable TDS and the actual TDS. 

qBD = qS fc / (bc - fc) 

where
qBD    the blowdown rate in kg/hour
qS is the rate of steam consumption in kg/hour
fc is the total dissolved substances in ppm
bc is the limit of the total dissolved substances in ppm





The Boiler Blowdown percentage refers to the amount of boiler water drained during a blowdown to the total quantity of the boiler feed water.  This is a very useful value

The formula is 





This value is a very important parameter.  The boiler blowdown percentage can range from 1% for high quality feed water to 20 % for low quality feed water. 




Vacuum Deaeration is a method of removing dissolved gases from water.  Removing dissolved gases from water is necessary as they can cause corrosion.

The principle on which Vacuum Deaerators are based is called Henry's Law.  Henry's Law states that the gas solubility in a solution reduces as the partial pressure of the gas above the solution decreases.
The Deaerator consists of a tower with baffles.  The tower is made of galvanized or reinforced
steel.

Water is drawn to the top of the tower and made to fall through the baffles.  The falling water is in the form of thin films.  This creates a large contact area between the water and the air. 

A vacuum pump creates a vacuum in the inside of the tower.  This lowers the air pressure on the inside.  The dissolved gases in the water are drawn to the vacuum and are removed.

In some vacuum deaerators the water is increased in lower the solubility.  The amount of vacuum to be created depends on the temperature of the water.  Vacuum deaerators are highly efficient and can deliver water with dissolved oxygen less than 5 ppb (parts of billion).




Deaeration refers to the process of removing dissolved gases such as oxygen and carbondioxide from the water.  Dissolved Oxygen in Water causes corrosion by the formation of rush on the surfaces of the boiler and the piping (rust).  Carbondioxide which is dissolved in the water forms carbonic acid which also causes corrosion. 

Hence, it is essential that these two gases are removed from water.

Deaerators can be classified into
Mechanical Deaerators
These Deaerators separate the gases by a mix of high temperature and mechanical action
Chemical Deaerators
Chemical deaerators work by passing the water through chemicals which absorb the oxygen and the carbondioxide.
Vacuum Deaerators
Membrane Contractors
Membrane Contractors work by passing the water through hollow fibres.  The water is made to pass on the outside of the hollow fibre.  A vacuum is created on the inside.  The gases pass through the membrane on to the inside and drawn into the vacuum pump.




Ordinary Silica is insoluble in water.  But when silica combines with other materials such as lime and soda, it can form scales which are very difficult to remove.  Soda and lime are used in softening units.


Use of silica based lubricants in the thermal plant as well can also result in silica entering the boiler water.  Another source is the presence of unreacted silicon in the feed water. 

If silica is not removed in time, it forms deposits in the turbine nozzles and change the direction of the steam.  The velocities and pressure drops are changed inside the turbine resulting in reduced efficiency.  Uneven nozzle flow can result in torsional vibration due to uneven loading of the blades.  This can result in vibrations. 

Silica deposits in the boiler are difficult to remove.  They equipment has to be dismantled and physically cleaned.  Blasting aluminium oxide on the surface is also a method used in the removal of silica deposits. 




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