Sulphur Corrosion in Steam Boiler

All fuels (gaseous fuel, liquid fuel, and solid fuel) has comprise some sulphur if it is burned will generate compounds which can cause corrosion. Corrosion must be prevented because it would damage the pressure part in the steam boiler. Sulphur may be separated from the fuels but it would require an expensive process. When in burning process of sulphur, two products of sulphur such as sulphur dioxide (SO₂) and sulphur dioxide (SO₃) will be generated. Water that be produced from combustion of hydrogen will combine with these compounds (SO₂ and SO₃) to generate acids.

At acid dew point, these acids will be condensed between temperature 120^o C and 175^o C depending on the number of sulphur content in the fuels. These acids will be absorbed by some soot deposits contained in stack, economizer, air heater, and other pressure part also in flue gas will be fused and formed acid smut. The water dew point will be achieved when the temperature down to reach 55^o C. and these acids will melt. It is the process which will cause corrosion.

The corrosion can be prevented if the metal temperature in the steam boiler is maintained in high temperature. The flue gas temperature exit of steam boiler is usually maintained at minimum 200^o C to avoid the formation of acid in air heater, economizer, stack and other pressure part also flue gas. For best design, the stack must be insulated to avoid cooling and corrosion. If the steam boiler is shut down, all parts should be cleaned to remove all acids in the steam boiler.

Rules Regarding the Use of Material on Steam Boiler

All of materials used for construction of the steam boiler for best design should refer to ASME Code (American Society of Mechanical Engineers) because ASME Code takes high care to ensure the quality of the material. All of specification materials with classification of temperature design of material, allowable pressure, corrosion, manufacturing tolerance, erosion, wear and tear can be seen in ASME Boiler and Pressure Vessel (ASME BPV) Code Section II Parts A, B, C, and D. While for design and do calculation steam boiler refers to ASME BPV Section I.

ASME BPV Section II consists of four separate books:

1.      ASME BPV Section II Part A: Ferrous Material Specifications

This code provides ferrous material specification that is safe for use on pressurized equipment.

2.      ASME BPV Section II Part B: Nonferrous Material Specifications

This code provides nonferrous material specification that is safe for use on pressurized equipment.

3.      ASME BPV Section II Part C: Specification for Welding Rod, Electrodes, and Filler Metals

This code provides filler metal procurement guidelines, specification for carbon, stainless and low alloy steel electrodes for shielded metal arc welding, specification for carbon and low alloy steel rods for oxyfuel gas welding, welding shielding gases, and so on.

4.      ASME Section II Part D: Properties

This code provides properties of material such as stress tables, physical properties tables, chats and tables for determining shell thickness of components under external pressure.

Demineralization Process

Feed water before it is supplied to the steam boiler should be treated first to be suitable water for feeding into steam boiler. One of water treatment is demineralization process. Demineralization process is the process to remove dissolve matter in the form of positive ion and negative ion by treating water with demineralizer.

Demineralizer consists of two exchanger, anion exchanger and cation exchanger. Demineralization process is divided by two steps, in the first step; water is distributed to cation exchanger. Cation exchanger has function to exchange cations of sodium, magnesium and calcium to hydrogen ions. In the second step, water from cation exchanger is distributed again to anion exchanger. Anion exchanger has function to remove anions of silica, carbonate, chloride, and sulfate. The result is output pure water. Demineralizer are generally used in steam boiler or power plant that has high or medium pressure and need high quality of pure water.

Operator of boiler or power plant should maintain demineralizer equipment to prevent condensation on both cation exchanger and anion exchanger. Corrosion will happen to these equipments if constant formation of moisture with acces to air. Corrosion can be prevented with the use of good ventilation in the room containing equipments and coating to be as insulator.

The advantages of using demineralization process are reduce water blowdown in the steam boiler, with pure water can produce high quality of steam, and reduce requirement of make up water. 

Design Buckstay in the Steam Boiler

Fig. 1: Design Buckstay in the Steam Boiler

The furnace of steam boiler has internal pressure which is usually designed ±200 mm wg. The internal pressure is resisted by attaching buckstay in every approximately 2-3 m along the height of furnace wall. Buckstay is support that consist of four girder which be installed in the four sides of furnace wall; right side wall, left side wall, front side wall and rear side wall. At the corner design buckstay is hinged to permit expansion on the furnace wall of steam boiler.

Bucstay is designed to resist longitudinal load such as internal and external pressure, normal load such as internal pressure, moment bending because of support bracket of pressure part, and temporary load such as explosion, wind and seismic forces. Buckstay has important roles to keep the furnace remain in its position and no bending occurs when the steam boiler operates continuously.

Spacing of buckstay a long the height of furnace is calculated to avoid wall tubes deflection and prevent vibration on tubes which has same frequency same with natural frequency of furnace wall tubes. Deflection of buckstay is usually calculated as L/360.  At the time of application buckstay is attached to tie bar with slip connection to give limitation of deflection inward and outward of furnace wall tubes. Tie bar is rectangular bar and form continuous band around the furnace wall tubes (see figure 1).

For more details about calculation of design buckstay, please look this spreadsheet: "Calculation of Design Buckstay"

Classification of Circulation Water Steam Boiler

A steam boiler has system circulation of water to converts it into steam continuously. In the Water Tube Boiler, water starts from water treatment system will be distributed to steam boiler through economizer to steam drum. From steam drum water flow to lower header through downcomer then be distributed again to water wall tubes to generate saturated steam. Saturated steam is the generated steam and mixture water-steam in the same temperature with water. The mixture will be separated in steam drum, steam tends to move upward to be heated again in the superheater then to be sent to turbine generator and water move downward to downcomer to be evaporated again. Based on circulation of water in the steam boiler, circulation can be commonly classified as follow:

1.      Natural or thermal circulation

Water from steam drum is distributed to water drum and lower drum through down comer. Water in down comer is unheated water. Evaporation occurs when water flow to water wall then get heat transfer from combustion process in the furnace. Heat transfer generates steam and mixture water-steam in water wall. Steam and mixture steam-water has less density than water, so gravity can cause the water will flow downward to downcomer, and steam/mixture will move upward into steam drum of steam boiler. The difference of density between water and mixture water-steam will influence the rate of circulation. The rate of circulation is affected by:

• The height of steam boiler; if the steam boiler is taller will cause higher difference of pressure between unheated segment (downcomer) and heated segment (water wall tubes), so this condition will increase the rate of circulation.
• The operating pressure of steam boiler; if the steam drum has high pressure will cause the density of steam and mixture water-steam is higher and cause the difference pressure between water and steam/mixture is less and decrease the rate of circulation.
• The rate of heat input: if the rate of heat input is increased will generate a lot of steam and reduce density of mixture steam-water, so increase the difference of density and the rate of circulation.
• The free flow areas of component; the increase of the free flow area will increase the rate of circulation.

2.      Forced Circulation

High pressure pump is installed in downcomer pipe of steam boiler to force and control the water flow to water wall tubes. Forced circulation may be used at high pressures when natural circulation forces are small or maybe used at lower pressure if designer want to design tube and other pressure part layout as he likes.

Auxiliaries of Steam Boiler

Auxiliaries of steam boiler are devices that be installed to the steam boiler, and can make it operates efficiently. These devices should be maintained and controlled, so steam boiler can run in good condition. Some of auxiliaries which are installed in steam boiler are:

·          Air heater

Air heater is device of heat exchanger to increase temperature of air before supplied to steam boiler usually located between the boiler and stack. Primary air and secondary air are heated by air heater before enter the combustion process, so the combustion process will be more efficient.

·          Gas system

Gas system is a group of devices that consist of control valve, solenoid valve, gas regulator, and mechanism control to arrange mixture ratio between air and gas in the furnace of steam boiler, so combustion can take place completely.

·          Oil system

Oil system is a group of devices that consist of oil pre heater, storage tank, piping, fuel strainer, and relief valve to control mixture between oil and air can be combusted perfectly.

·          Economizer

Economizer is device of heat exchanger to increase temperature of feedwater before to be supplied in steam boiler, efficiency of steam boiler is expected higher than without using economizer.

·          Feedwater regulator

Feedwater regulator is device to control feedwater that be supplied and maintain water level in the steam boiler at normal condition, so reduce excess of fuel consumption and thermal shock.

·          Feedwater heater

Feedwater is device of heat exchanger to increase temperature of feedwater before entering boiler feed pump.

Appurtenances of Steam Boiler

Appurtenances are devices which are installed to the steam boiler readily to make steam boiler can be performed safely. Appurtenances can also be known as mounting and fitting. Safety should be paramount in designing steam boiler. The following below are some of appurtenances that are installed to steam boiler:

1. Safety Valve
Safety valve is protective device that be used in the steam boiler to avoid overpressure; pressure that occurred in the steam boiler exceeds the Maximum Allowable Working Pressure (MAWP). If the pressure increases until it reaches MAWP, the safety valve will pops up until pressure drops.

2. Pressure Gage
Pressure gage is measuring device that be mounted in the steam boiler to look working pressure in the steam boiler at various point. The pressure is maintained in allowable pressure range.

3. Water Column
A water column has some fitting such as gage glass (a device as primary indicator of water level in the steam drum), and try cocks (a device as secondary indicator ofwater level). Water column is used to decrease turbulence in water gage, so water gage can be read accurately.

4. Blowdown Devices
Blowdown devices are device to discharge scale and sludge from the steam boiler.

5. Fusible Plug
Fusible Plug is device to prevent inadequate water in the steam boiler; it gives warning if condition in the steam boiler is low water.

6. Soot blower
Soot blower is device to remove soot and slag around pressure part and maintain heat transfer on the surface of pressure part.

7. Steam Boiler Venting
Steam boiler venting is device to remove air when filling water and warming the steam boiler.

Classification of Superheater in Steam Boiler

Superheater is a device consists of tubes which get the heat from the product of combustion to add additional heat into steam, so the temperature of the steam passing through the tubes raise higher than its saturation temperature. The superheater can help steam boiler increase its capacity because help turbine generator get its required steam both in terms pressure and temperature.

Classification of superheater can be classified based on design of flow:

1.      Parallel Flow  Superheater

A superheater which has direction of steam flow in superheater same with the direction of flue gas flow

2.      Counter Flow Superheater

A superheater which direction of steam flow in superheater opposite with the direction of flue gas flow

3.      Mix Flow Superheater

A superheater which direction of steam flow in superheater same and opposite with the direction of flue gas flow

Classification of superheater can be classified based on heat transfer:

1.      Convection Superheater

The convection superheater is located somewhere in the flue gas flow, where it absorbs heat transfer by convection.

2.      Radiant Superheater

Radiant superheater is located in or near furnace of steam boiler and receives heat transfer from combustion process by radiation.

.

Classification of superheater can be classified based on arrangement:

1.      Vertical Superheater

The superheatar are arranged vertically. This type has is easy to suspend and has free moving to expand. But the disadvantage, it is not drainable.

2.      Horizontal Superheater

The superheatar are arranged horizontally. This type has is more difficult to suspend and should give special support to give moving for expand. But the advantage, it is drainable.

Maintenance Tools in Steam Boiler

Steam boiler needs specialized tools to do maintenance properly, so steam boiler can operate continuously and efficiently.  The required tools are depending on type of maintenance; these are some variety of tools and equipments:

1.      Tube Cleaning Tool, tubes in steam boiler if it is not cleaned will cause scale, sludge, slag, and deposit and other chemical substances on the surface of the tubes, will can disturb flow through the tubes and make corrosion. Steam boiler tubes should be cleaned by tube cleaning tool such as air/water tube cleaning kits, buffing tools, cutter heads, cutter bits, flexible shafts and tube cleaning extension rods.

2.       Electronic Instrument Tool, steam boiler need this tool like O₂ Analyzer to observe excess air in the flue gas that will be discharged to atmosphere, tube thickness calculator instrument, test instrument etc

3.      Power Tools such as hammer, drill, screwdrivers, impact wrenches, grinder,  drill presses, etc

4.      Tube expansion tool and removal tool, such as tube expander, tube puller, hydraulic tube cutter, etc

5.      Welding equipments such as welding machines, electrode, cutting torch, regulator, welding gage, etc.

6.      Hand tools such as punches, pipe threader, pipe wrenches, sockect, knives, etc

7.      Pressure washer tools such as hot water pressure washer and its accessories, vapor steam cleaner, etc

Classification of Maintenance Steam Boiler

Maintenance is required to perform and maintain steam boiler in good condition. Maintenance can be classified into two types as the following:

1.      The routine and preventive maintenance

The routine and preventive maintenance is maintenance that be made periodically whether daily, monthly and annually. This maintenance is made before equipments of steam boiler damaged, so equipments can work continuously and ready for use any time. The life of steam boiler equipment can be long life and in excellent condition for the best efficiency, safety and reliability if maintenance is well controlled. This maintenance may include cleaning pressure part, draining header, check equipments like valve and the others, burner maintenance, check water level in the steam drum, flame safeguard, refractory, condensate equipment and the like. 

2.      The forced maintenance

The forced maintenance is maintenance that be made if there is equipment of steam boiler damaged suddenly. The cost of forced maintenance is usually higher than the routine and preventive maintenance. The forced maintenance is commonly caused by overloading of fuel which be burned in the furnace, so steam boiler working extra that could cause the steam boiler to be fatigue and over heat; there are too much slag attached on surface of pressure part; feedwater entering the steam boiler is not pure, there are still solid particle and oxygen in the feedwater, so steam boiler will contain sludge too much and more corrosive; and improper lubrication in the equipment.

Maintenance Program of Steam Boiler

Steam boiler plant should running well, efficiently and safety. The owner must have plan or program to control maintenance of steam boiler plant to avoid unscheduled shutdown occurs. Regular and routine maintenance can make the steam boiler operates continuously and prevent breakdown which cause high cost for repair and loss profit in production downtime.

Fig.1 : Qualified Personnel Doing
Maintenance Program of Steam Boiler

The maintenance should be done by qualified personnel who familiar with all equipment of steam boiler. The personnel can be engineer, operator, or technician. The personnel must have maintenance program to keep steam boiler from possibility of damage and shutdown suddenly. The program which has been prepared must include the following:

§         Routine maintenance

Routine maintenance is performed daily, weekly, monthly and annually. This program include cleaning pressure part, combustion chamber, etc; draining sludge to continuous and intermittent blowdown; protection from erosion and corrosion, inspection of equipments such us valve, burner, etc.

§         Preventive maintenance

Preventive maintenance is performed to prevent possibilities of accident and anticipate if there are unusual something can be happen. This program includes observation of water level in steam drum; check the supply of fuel and feedwater, check equipment for safety like safety valve, flame detection devices; test and perform all of equipment in steam boiler.

§         Stock of spare part

The owner of steam boiler should have stock of spare part and list it and then to be used if there are emergency repair.

§         Shutdown maintenance

Shutdown maintenance can be down annually to check all parts of steam boiler and replace if there are parts that are not working anymore. Procedure of shutdown should be done properly.

§         Personnel training

The qualified personnel who operate and maintain the steam boiler are absolutely required because they are understand about condition and can make solving problem on steam boiler. So, training boiler should be done to each personnel.

§         Upgrading boiler

Upgrading boiler is an effort who need deep analysis to make steam boiler more efficient and reduce cost for production.

Design Spray Desuperheater of Steam Boiler

In steam boiler, desuperheater can be known as attemperator which has function reduce and control temperature of superheated steam. Superheated steam is derived from saturated steam which be heated again through superheater. Temperature of superheated steam must suitable for turbine generator. If temperature of superheated steam is overheated, desuperheater with type spray will sprays amount of water from steam boiler feedwater pump into steam flow to reduce its temperature.

Amount of water will be injected into superheated steam must have high purity, if not it can leave some troubles like deposit on the superheater tubes and can cause erosion on turbine blade. Design spray desuperheater type must include thermal sleve in pipe line desuperheater, to avoid thermal shock by water droplets which are sprayed through the nozzle strike the hot surface of pipe desuperheater.

Fig. 1: Design Spray Desuperheater of Steam Boiler

Steam Boiler Feedwater Pump

Fig. 1: Steam Boiler Feedwater Pump

A steam boiler feedwater pump is rotating device applied to serve feedwater which taken from the feedwater heater to be supplied to the steam boiler. Ideally a power plant need at least two steam boiler feed pump with each pump should has independent source of drive power, if one of them suffered damage, the steam boiler can still be operated with the other standby pump, so the supply of steam to turbine generator is not stopped. Based on the cycle of feedwater, the steam boiler feedwater pump may get its suction from deaerating heater, condensate pump discharge, or from the makeup water sources.

The steam boiler feedwater pump should have discharge pressure which is greater than the operating pressure on the steam boiler. The pressure of boiler feedwater pump is designed to recover all of pressure loss because of friction in feedwater piping and valve, economizer and its connection tube, feedwater control valve, feedwater heater, safety valve, and static head pressure. The volume capacity of the pump when installed in power plant must be same amount of total evaporated water in steam boiler coupled with application of continuous blowdown and spray water into desuperheater.  

The outside source of pressure must be provided either form static head if the suction souce is above the boiler feedwater pump or atmospheric pressure if below the pump. Pressure pushes the feedwater up into the pump suction. The pump creates a vacuum by movement of the tight fitting piston or action of the impeller into which the water rushes.  Design of steam boiler feed pump must consider pressure and temperature feedwater, so it can recover all of requirement of steam boiler.

Mechanism of Combustion Air Supplied into Steam Boiler

There are some mechanisms of combustion air will be supplied into steam boiler:

§         Natural Draft

If the height of stack is higher from sea level, so pressure above stack will be lower than one atmosphere. So a steam boiler’s exit pressure being less than the steam boiler furnace pressure. Pressure tends to move from higher space to lower space. Beside that difference temperature between the outside air and inside flue gas also creating suction in the furnace of steam boiler. Parameter of height stack, pressure, and temperature can generate natural draught.

§         Forced  Draft

Force draught is created by Force Draft (FD) Fan blowing air which taken from atmosphere to the furnace.  FD Fan can force air to furnace until the pressure will be positive.

§         Induced Draft

Induced draught is created by Induced Draft (ID) Fan located at the steam boiler outlet which sucks air from the furnace and discharge them at the stack exit. Steam boiler is maintained in negative pressure to prevent flames leaking out and make certain steam boiler is operated safely with avoid fire of insulation and casing.

§         Balanced Draft

Balanced draught is created by employ FD Fan and ID Fan together. In Balance draft operation, the furnace pressure is maintained near to the atmospheric pressure. FD Fan force air to make combustion take place, in turn ID Fan pull flue gas (product of combustion) from steam boiler through to stack.

Composition of Air Combustion in Steam Boiler

The steam boiler must have sufficient supply of air to make combustion properly. Air is one of three important parts for combustion to take place, Air-Fuel-Heat. The air for combustion contains based on volume, 21% Oxygen (O₂) and 79% Nitrogen (N₂). The Nitrogen does not have any role and is not chemically active in the process of combustion steam boiler, through the furnace without any changes and carries away the heat.

All fuels commonly contain three substances, carbon, sulphur and hydrogen. Combustion of pure carbon in air has three possibilities:

1.      If combustion is not supplied by enough air, some of the carbons will burns incompletely and form unburned fuel carbon monoxide (CO) blowing out of the stack and probably smoke in the combustion product of steam boiler.

2.      If combustion is supplied by enough air, oxygen will be fused with the carbon to be burned together completely to form carbon dioxide (CO₂).

3.      If combustion is supplied by too much air, it will cools the furnace, the flame is cooler, short and clear also there is not enough heat will be produced. Too much excess air give possibility that at given firing rate could result in unstable fires in steam boiler.

Sufficient air must be provided to the furnace of steam boiler to make perfect combustion. Air should be supplied under the right control and not to leak into the steam boiler through holes, cracks, and the like. 

Air System Devices of Steam Boiler

Steam boiler needs auxiliaries and appurtenances to perform it working in good condition efficiently and safely. One of steam boiler’s auxiliaries is air system devices. Air system provide sufficient air in combustion process of steam boiler and as medium heat transfer in pressure parts like water wall tubes, superheater, generating bank, economizer, air heater, and the like.

Air system consists of devices as follow:

• Force Draft (FD) Fan
FD Fan force primary air to furnace steam boiler through burners or grates at pressure above atmospheric pressure (positive pressure).
• Induced Draft (ID) Fan
ID Fan produces induced draft to suck flue gas from the furnace
• Secondary Air (SA) Fan
SA Fan supply air to controls combustion efficiency by supervising how completely the fuel is burned
• Air Heater
Air heater is heat exchanger device to heat air from FD Fan to be supplied to furnace steam boiler to increase efficiency of combustion process
• Air Compressor
Air compressor is a device to press air and as storage of air. This device is used to supply cooling air at peed door or peep hole (an opening used to observes furnace) and supply for energize instrument air.
• Air Dryer and Filter
This is a device to remove water from compressed air and filter solid particulates such as dust.
• Air Piping & Ducting System
Air piping and ducting system is a collection of pipes and ducts which in routing to deliver air in the steam boiler system 

Evaporation in Steam Boiler



Evaporation is the process changing water into steam. In this evaporation process requires heat transfer that be obtained from combustion process in the furnace steam boiler. Evaporation occurs in the water wall tubes of steam boiler. Water from steam drum is flown through downcomer into lower header, and the proceed to water wall tubes either in the left side, right side, front side and rear side steam boiler.



Fig. 1: Evaporation Process in Steam Boiler



Surface water wall tubes get heat transfer by radiation, and then heat is transferred in entire of surface water wall tubes of steam boiler by conduction. When water in the water wall tubes is heated, reduction of density occurs, so water becomes lighter and tends to move upward. Then water which has heavy density will flow downward and replace the heated portion of the water. So there is heat transfer between particle of water which has density lighter and heavier. This process is called convection.



The quantity of water that be evaporated into steam is known as Evaporation Rate and usually expressed in pounds of steam per hour, or pounds of steam per hour per square feet heating surface, or pounds of steam per hour per cubic volume of furnace steam boiler. Enthalpy of evaporation or laten heat evaporation is the amount of heat energy required to convert water into steam.

Steam which be generated in the water wall tubes of steam boiler is saturated steam (still mixture between steam and water where temperature of steam is same with the water from which it was converted). The mixture goes to steam drum, where steam is separated from water. The steam goes from the drum to superheater to be heated again, so steam will be superheated steam. The water is recirculated through downcomer, lower header, and water wall tubes again. 

Classification of Furnace Steam Boiler

Furnace of steam boiler is the space in which combustion of fuel take place. Furnace can be classified in many types. The general classifications of furnace steam boiler are as follow:



§         Based on wall construction:

1.      Air cooled refractory setting

2.      Solid refractory setting

3.      Water wall / water cooled



Fig. 1: Furnace Room of Steam Boiler



§         Based on pressure maintained:

1.      Balance draft furnace



2.      Pressurized furnace

3.      Supercharged furnace

§          Based on fuel:

1.      Coal furnace

2.      Oil furnace

3.      Gas furnace

4.      Baggase furnace, etc



§         Based on firing method:

1.      Hand fired furnace

2.      Stoker fired furnace

3.      Pulverized coal fired furnace

§         Based on heat transfer:

1.      Radiation furnace

2.      Convection furnace

§         Based on waste heat recovery:

1.      Recuperative furnace

2.      Regenerative furnace

Stack of Steam Boiler

Fig. 1: Construction of Stack in Steam Boiler System

Stack is a equipment to take out the product of combustion at a sufficiently high elevation to prevent disturbance caused by ash, low flying smoke, and soot and to disperse the combustion gases. To perform flue gas flow through steam boiler room such as furnace, superheater, steam boiler bank, economizer, air heater, dust collector until be discharged to atmosphere, requires a certain amount of draft.  Amount of draft can be generated by fan either force draft fan or induced draft fan to overcome the pressure drop (draft loss) that are developed in the steam boiler. Beside that, height and difference between the outside air and inside flue gas temperature of the stack can produce the draft.

Excessive heat temperature of stack is something should be avoided because it show that there are heat loss and efficiency reduction of steam boiler. The following formula may be used to calculate draft generated by a stack:

Stacks are designed in the worldwide steam boiler system are commonly completed with caged ladder, walk way, and instrument nozzle to measure the concentration of either excess O2 and CO2 to determine the operating excess air level. CO or carbon monoxide is also measured to determination of incomplete combustion in flue gas of steam boiler. 

Classification of Soot Blower

Fig.1: Long Retractable Soot Blower

Soot blowers that are installed in steam boiler commonly can be classified as:

1.      Long Retractable Soot Blower

This type of soot blower is located in area superheater and reheater. There is not temperature limitation in its use. Has a pair (2) of nozzle in lance tube located on both side in exact opposition (angle 0^o and 180^o) with diameter approximately 22 or 25 mm. The steam flow through soot blower is approximately 4 to 20 tph. Need more space to place long retractable soot blower on the sides of steam boiler.

Fig. 2: Wall Soot Blower

2.      Wall / Short Retractable Soot Blower

This type of soot blower is located in area furnace. There is not temperature limitation in its use. Has 1 or 2 nozzle in lance tube with diameter approximately 25 mm. The steam flow through soot blower is approximately 3.5 tph. Mostly used in high temperature furnace of Pulverized Fuel Boiler. One soot blower is required to blow furnace area in every 10 m².

3.      Rotary Soot Blower

Fig 3: Rotary Soot Blower

This type of soot blower is located in area boiler bank, economizer, and air heater. Its use is limited to temperature 1100^o C. Have many nozzles in lance tube with diameter approximately 8 mm. The steam flow through soot blower is approximately 3.5 tph. Maximum lance length is limited until 7 m, if the steam boiler larger than 7 m, so soot blower will be installed in both sides and can be operated by motor or manual.

4.      Rake Soot Blower

Fig. 4: Rake Soot Blower

This type of soot blower is located in area fin tube economizer and vertical tubular air heater. Its use is limited to temperature 530^o C. Have many sets of nozzle. The steam flow through soot blower is approximately 3.5 – 4.5 tph. Dimension of rake soot blower is limited to 5 m length and 3 m width.

Soot Blower of Steam Boiler

Fig. 1: Soot Blower of Steam Boiler

Soot blowers is mechanical device  automatically controlled to deliver intermittent jet of compressed steam or air to clean ash, soot, slag  from heat absorbing surfaces by blowing on the surface. Soot blower is used eliminate slag, so maintain efficient flue gas temperatures and generally promote more desirable operating conditions. Determination of location soot blower is important to design new steam boiler.

Many installtions of steam boiler are now designed to use more boilers and they depend on the type of steam boiler size, firing, fuel type, ash in fuel, slag potential, previous experience, etc. Soot blowers remove soot and ash deposits from the fire sides of heating surfaces so that they remain optimally clean and heat transfer is maintained at the original design levels. Slagged area can be cleaned periodically and one at a time.

Steam boiler tubes and heating surfaces get dirty because of accumulation of slag. With excessive slag, the waterwall absorb less heat because slag is a good insulator and reduce the effectiveness of heating surface, so decrease load and increase steam temperature beyond control limit. Every effort must be made to avoid slagging passes in the boiler. The increase in steam boiler maintenance resulting from slgging at high furnace exit temperature is well known.

For water tube boiler, soot blowers are commonly used and permanently installed but not very effective for firetube boiler. Soot blower has element that are located in the direct path of the products of combustion. Steam is blown from the nozzle in the elements at high velocity between the rows of tubes. 

Classification of Air Heater

Air heaters in a steam boiler system are classified according to their principle of operation as:

§         Tubular Air Heater

Fig. 1: Tubular Air Heater

In a tubular air heater, energy is transferred from the hot flue gas flowing inside many thin walled tubes to the cold combustion air flowing outside the tubes. The unit consists of a nest of straight tubes that are roll expanded or welded into tubesheet and enclosed in a steel casing. The casing serves as the enclosure for the air passing outside of the tubes and has both air inlet and outlet opening. In the vertical type tubes are supported from either the upper or lower tubesheet.

The tubular air heater is simple to fabricate and does not increase the existing auxiliary power or erosion problems (in case of coal-fired boilers), because there are no leakages from air to gas side such as in the case of an Rotary Air Heater and no maintenance issues because the assembly is static.

§         Rotary Air Heater

Fig.2: Rotary Air Heater

Rotary air heater can be known as Ljungstrom type which features a cylindrical shell plus a rotor which is packed with bundles of heating surface elements and is rotated through counter flowing air and gas streams. The rotor is enclosed by a stationary housing which has duct at both ends. Air flows through one half of the rotor and gas flows through the other half. The most prevalent flow arrangement has the hot gas entering the top of the rotor as cold air enters the bottom in counter flow.

The rotary air heater needs regular adjustment to the seals and is also maintenance prone but the real advantage lies in its compactness and simplified duct layout. With several shop fabricated subassemblies, the site erection takes much less time. Also, with enameled baskets, lower gas-exit temperatures can affect boiler efficiency favorably.

Air Heater of Steam Boiler

Fig. 1: Air Heater of Steam Boiler 

Air heater has an important role to improve the efficiency of the high thermal of the steam boiler by utilizing the thermal energy with low temperature flue gas from the steam boiler before being discharged into atmosphere. Air heater doing heat transfer between flue gas with combustion air. Air heater cooling the flue gas air for every 40^o F (22^o C)will increase the overall steam boiler efficiency is about 1%. With the utilization of heat from flue gas combustion air will be hotter so it can save fuel requirement needed.

Air heater requires a large amount of heat transfer surface per unit of heat recovered because of the relatively small difference between the flue gas temperature and temperature of combustion air. Air heater is typically located behind steam boiler, where it receives hot flue gas form the economizer and cold combustion air from the force draft fan. The hot air produced by air heater to enhance combustion of all fuel and is needed for drying fuel like coal in stoker steam boiler.

The air heater is usually the last heat trap in the steam boiler. Air heater exit gas temperature should be higher than the corrosion limit of the AH internals and the downstream equipment. Air heaters generally operate in a temperature range of 450–120°C on the gas side and 150–450°C on air side. Design and operation of air heater should consider as follow:

§         Fouling and plugging of air heater

Fouling is the deposition of gas-entrained ash. The deposition at low temperatures occurs due to the adherence of ash to the acid-moistened surfaces at the low end of the air heater. Pressure drop increases as the deposits grow in size. If unchecked, gas passages become progressively choked or plugged. Regular soot blowing can reduce the fouling considerably. Offload water washing is carried out in many installations for deposit removal, as they are mostly water soluble.

§         Erosion in air heater

Employing conservative gas velocities is a design-stage precaution against gas-side erosion, particularly for dusty fuels. Gas velocities have to be high to economize on the surfaces and evolve decent layouts and erosion has to be minimized. Normally erosion is at the entry to the bank where the velocities are the highest, and the maximum velocity limit should be applied at this point. For a tubular air heater it is normal to provide the top row with thicker tubes of, for example erosion shields of typically 3 mm. Heat transfer in tubes covered with erosion shields suffers because there is no direct contact of the tube and gas.

Air heater requires a large amount of heat transfer surface per unit of heat recovered because of the relatively small difference between the flue gas temperature and temperature of combustion air. Air heater is typically located behind steam boiler, where it receives hot flue gas form the economizer and cold combustion air from the force draft fan. The hot air produced by air heater to enhance combustion of all fuel and is needed for drying fuel like coal in stoker steam boiler.

The air heater is usually the last heat trap in the steam boiler. Air heater exit gas temperature should be higher than the corrosion limit of the AH internals and the downstream equipment. Air heaters generally operate in a temperature range of 450–120°C on the gas side and 150–450°C on air side. Design and operation of air heater should consider as follow:

§         Fouling and plugging of air heater

Fouling is the deposition of gas-entrained ash. The deposition at low temperatures occurs due to the adherence of ash to the acid-moistened surfaces at the low end of the air heater. Pressure drop increases as the deposits grow in size. If unchecked, gas passages become progressively choked or plugged. Regular soot blowing can reduce the fouling considerably. Offload water washing is carried out in many installations for deposit removal, as they are mostly water soluble.

§         Erosion in air heater

Employing conservative gas velocities is a design-stage precaution against gas-side erosion, particularly for dusty fuels. Gas velocities have to be high to economize on the surfaces and evolve decent layouts and erosion has to be minimized. Normally erosion is at the entry to the bank where the velocities are the highest, and the maximum velocity limit should be applied at this point. For a tubular air heater it is normal to provide the top row with thicker tubes of, for example erosion shields of typically 3 mm. Heat transfer in tubes covered with erosion shields suffers because there is no direct contact of the tube and gas.

Design Superheater of Steam Boiler

Superheater is a device for producing superheated steam, typically a group of tubes exposed to heat through which steam is passed. Its purpose is to add additional heat to the steam. Thus a superheater is employed in the steam boiler to add additional energy into steam and raise its temperature. The function of superheater are increase the capacity of the steam boiler, eliminates erosion of the steam turbine, reduces steam consumption of the steam turbine.

Superheater is the tube banks that attain the highest temperatures in a steam boiler and consequently require the greatest care in the design, fabrication, and construction to ensure that the permissible metal temperatures are never exceeded. The final sections of superheater must be placed in the highest gas temperatures, which calls for adopting the most appropriate high-temperature alloy for the tubing from considerations of metal temperatures, fouling due to ash compounds and corrosion due to salts in ash.

The most important aspects of the design of superheater are: uniform distribution of steam and gas across all the sections to minimize unbalance of flows, optimally high steam velocity in all the tubes to keep the metal temperatures as low as possible, and minimum steam pressure losses. Superheater pressure drop is normally limited to ∼8% of outlet pressure to reduce the pumping load.

The superheater is usually drainable along a single loop of an inverted U. As the required

superheat increases or a larger control range is desired, a double inverted U-type superheater may have to be adopted, making the heater nondrainable. The superheater in a package steam boiler is largely convective. The degree of control range is also not as large as in field-erected steam boilers. Occasionally, a loop is pushed into the furnace space to achieve higher temperature.

Water Drum of Steam Boiler

Fig.1: Water Drum of Steam Boiler

If the surface heat transfer in steam boiler may not be enough to produce steam saturation for specific end use, so require additional tubes to increase heat transfer surface. These additional tubes connecting between steam drum and water drum were called steam boiler bank. This is necessary for industrial steam boiler that has low pressure. Steam boiler bank usually consist of steam drum which be typically located on the top, a series of bent connecting tubes on the middle, and water drum on the bottom.

Water drum is a pressure chamber of a drum type located at the lower extremity of a watertube-steam boiler convection bank. The steam drum internals and tube sizes are arranged so that subcooled water travels down the tubes into water drum. From water drum, water is distributed by downcomer to lower drum and then distributed again to the tubes of furnace wall where water is partially converted to steam and returned to the steam drum 

Water drum is present only in the bidrum steam boilers and play a less important role such as: a water drum acts like a large header connecting the riser and downcomer tubes of the bank, and it feeds the downcomers to the various evaporator sections. The water drum is always smaller than the steam drum as there are no important internals.

Steam Drum of Boiler

A steam drum is a pressure chamber of a steam boiler located at the upper extremity of steam boiler circulatory system. It is a reservoir of water/steam at the top end of the water tubes. The steam drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture. Steam is separated from the water and then discharged at a position above the water level maintained there.

Fig.1: Steam Drum of Boiler

In natural circulation of steam boilers, the steam drum occupies a position of importance

because of: it is the heaviest PP and often the heaviest component, the layout and erection of the steam boiler revolve around it, It takes the maximum time for manufacturing and needs the costliest machinery, moving it demands a high level of logistics, it needs the heaviest crane or lifting tackle during both fabrication and erection.

Thermodynamically, however, a steam drum is merely a surge tank in the circulation

system to which the following parts are connected: Economizer outlet and Superheater inlet pipes on one side, downcomers and risers on the other side.

Under steady operating conditions when the flows are balanced, the steam drum produces

a steady water level. Drum water level is one of the most important parameters, of a steam boiler control system. The other important function of the steam drum is to house the drum internals, which perform the following functions: saturated water from evaporator circuits, which contain varying amounts of steam, is separated by the steam separators. Water-free steam to the SH and steam-free water to the downcomers are required of steam separators and purifiers, chemical dosing to maintain the chemistry of the circulating water, continuous blowdown of drum water to keep the carryover under specified limits.