Steam Boiler: June 2011

Modification of Rankine Cycle

Modification of Rankine cycle aims to improve the efficiency of the cycle, in this case made the extraction of steam from turbine generator to heat feedwater, so the steam boiler can work easily and reduce requirement of fuel. Practically, turbine generator with high initial pressure usually uses number of extraction 5 until 7 extraction. Number of extraction can be added 8 – 9 extraction if turbine generator has parameter of critical hot steam. For steam turbine which has middle pressure, the number of extraction is usually limited only 1 until 4.

One of modification of Rankine cycle can be seen in the figure below:

Figure 1: Rankine Cycle with 1 Extraction

Steam heat up from boiler is distributed to turbine generator, after going through several levels of turbine blades, some of steam are extracted to the deaerator, while the remaining steam goes to condenser and to be condensed there. Furthermore, water from condenser is also pumped to deaerator. Steam from turbine will mix with water from condenser in the deaerator. From deaerator process, water will be supplied into steam boiler to be converted into steam then will be distributed again into turbine generator.

The purpose of steam is extracted to deaerator is discharge gases that are not condensed, so the heating process in the boiler will be effective, prevent corrosion and increase efficiency. To simplify thermodynamic cycle analysis, the processes mentioned above is simplified in the form of following diagram:

Figure 2: Diagram T-S of Rankine Cycle with 1 Extraction

Deposit Formation on Boiler

Deposit is the occurrence of clumping substance in the boiler water which is caused by suspended solid such as copper oxide, iron oxide and others. Deposit is also caused by steam which is contaminated in the production process. Sources of deposit in the feedwater are dissolved salts and suspended solid which can affect particles that can make lower solubility are deposited. There is different definition between crust and sludge. The crust is a form of deposits that remain on the metal surface of boiler, while sludge is a form of deposits that are not settled or can be called soft deposits.

In the high pressure boiler, silica can settles with steam so can make deposit on the turbine blade and potential to damage occur. Prevention actions can be done as following below:

- Minimize the entry of minerals in the feedwater that can lead to deposits such as iron oxide, copper oxide and the others on the water treatment plant.

- Prevent corrosion by doing neutralization process such as adjust PH 8.2 to 9.2 and can be also performed by the occurrence of air leakage in the condensate system.

- Prevention contaminated steam can be performed by using chemical to disperse the minerals that causing deposits.

- Reduction of existing deposits can be done by acid cleaning, online cleaning and mechanical cleaning.

Centrifugal Pump Classification

Centrifugal pump can be classified based on some parameters. The following below are the parameters of centrifugal pump classification:

Based on discharge pressure:

Low pressure pump, the operating pressure less than 5 kg/cm²

Medium pressure pump, the operating pressure approximately 50-50 kg/cm²

High pressure pump, the operating pressure higher than 50 kg/cm²

Based on capacity:

Low capacity pump, the capacity less than 20 m³/h

Medium capacity pump, the capacity pump approximately 20-60 m³/h

High capacity pump, the capacity higher than 60 m³/h

Based on direction of exit flow of impeller:

Radial flow pump

Axial flow pump

Mixed flow pump

Based on number of suction

Single suction pump

Double suction pump

Based on shaft position:

Vertical shaft pump

Horizontal shaft pump

Based on number or composition of impeller and level:

Single stage pump, consists of one impeller and one casing.

Multiple stage pump,

Multiple impeller pump, consists of several of impellers which is arranged parallel in a single casing.

Multiple impeller – multiple stage pump, the combination of multiple impeller pump and multiple stage pump.

Pump Definition

Pump is the equipment to move fluid (liquid) from one point to other point by increasing pressure of that liquid. Increasing the pressure of liquid is used to overcome flow resistance. The flow resistances can be caused by difference pressure, different height or friction resistance. Pump can be generally classified as two types, namely: positive displacement pump and non positive displacement pump. One of non displacement pump type is centrifugal pump which has working principal to convert kinetic energy of liquid to potential energy (dynamic) through an impeller which rotates in the casing.

In the steam boiler, type pump which is mostly used is centrifugal pump type because its simplicity and low cost. Steam boiler need feedwater pump to supply water into steam drum. Centrifugal force is a force which occur because a particle move through circle way. The advantage of centrifugal pump than type positive displacement pump is the impeller can run continuously so the flow will be soft and do not produce bubble. Other advantages of centrifugal pump are can operate in high speed rotation, can be coupled electric motor or small scale steam turbine so only need small and light room, and low cost for installation and maintenance.

Separation Process in Steam Drum

Steam drum is a collector and separator equipment for water and steam from water wall tubes. According to separation concept of water and steam in steam drum, separation process can be classified in three types such as natural gravity driven separation, baffle assisted primary separation and mechanical primary separation.

- Natural Gravity Driven Separation

In the process of natural separation, mixture of water and steam is separated due to differences in density. Steam has less density than water so steam will go to the top of drum and water will fall to bottom of drum. This separation process depends on the location out of steam and water, the speed and position of the incoming steam, the quality of steam and so on. But the natural separation process has some disadvantages such as Figure 1 below:

Figure 1: Natural Gravity Driven Separation in Steam Drum

In the figure shown, if the steam entered through the bottom of drum, the steam will mix with boiler water so reduce temperature of steam and increase temperature of water which will be distributed to downcomer pipe or convection wall tubes. If velocity of steam is low, steam will not be able to pass water and its quality will be reduced while if the velocity of steam is too high, some steams will go to downcomer pipe so the water level in steam drum will rise, thus disturbing the accuracy of water level gauges on drum.

If steam is entered to the center of drum, the water level in steam drum will be uneven and some steams will enter to downcomer pipe. If the steam is entered from the top of drum, steam will affect to water level in steam drum.

- Baffle Assisted Separation

In this separation process, a mixture of steam and water coming out of the water wall tubes will be separated by baffle with redirecting the flow into baffle, so water separate from steam and the flow of steam will be directed so it does not mix with water in the steam drum as shown in Figure 2 below.

Figure 2: Baffle Assisted Separation in Steam Drum

- Mechanical Primary Separation

The working principal of mechanical primary separation is use separation process due to centrifugal force and radial force by passing the mixture of steam and water in cyclone shaped equipment. Steam will be separated from water because centrifugal force and radial force occur when the mixture flow this cyclone. The figure below is sample of cyclone equipment such as conical cyclone, curved arm cyclone and horizontal cyclone.

Figure 3: Type of Cyclone in Steam Drum

Component of Economizer

Economizer has a role as a heat exchanger to increase temperature of feedwater before to be supplied into steam drum. Economizer consists of similar component with other heat exchanger but on the certain parts has different task and function. The following below are the component of economizer

- Shell, is one of parts in economizer which be like a house for tubes and header. Between tubes and header, there are fluids that can receive and release heat in accordance with occurred mechanism.

- Tubes, is the most important component of economizer in the process of heat transfer. Fluids flow both inside and outside tube. The flowing fluid has different capacity, temperature, density and pressure. Position of tube must be maintained to prevent vibration, so on certain design, tubes is supported with baffle. Tube should also be able to transfer heat between the fluids inside tube and outside tubes.

Determination material of tube should follow these requirements; tube should have capability to transfer high heat, has resistance to heat, corrosion, and erosion and has well other properties. Capability to receive and release heat is also affected by amount of surface area.

- Baffle, is one of component in economizer that has some roles such as to support tube and maintain its position, baffle also has function to prevent vibration on the tube and control and direct the flow of fluid outside tubes.

Baffle can be classified based on construction as follow: segment baffle, rod baffle, horizontal baffle and impingement baffle. These types of baffle are usually used independently but in the special needs, one type and other type can be combined.

- Header, is the component of economizer as a water collector and distribute water to each tubes evenly. Header has two types; upper header and lower header. Tubes are connected into header, so header has size and mount of hole in accordance with tubes condition.

- Tie rods and spacer, are one of part economizer to maintain position one baffle and other baffle so vibration can be prevented.

Electrostatic Precipitator as Flue Gas Boiler Filter

Electrostatic precipitator is one of flue gas filter equipment in boiler. There are some choices to determine flue gas filter equipment. There are multi cyclone, bag house and electrostatic precipitator. Basic working principle of electrostatic precipitator is passing the exhaust gas or flue gas (product of combustion from furnace boiler) to the room which containing the electrode plates, made of copper or brass.

Attached electrodes in the construction of electrostatic precipitator is energized by direct current electric (DC) with positive and negative charges. Between the electrode trunks which has negative charge and plate dust collector which has positive charge is energized by voltage 70-90 KV. The grains of dust that pass through the electrode trunk will be induced by negative charge and so now the grains of dust have negative charge. Because their charge is negative, the electrode plate which has positive charge attracts the grains of dust so the grains of dust stick to the plate.

Presence of vibration (rapping) that touch the plate dust collector will cause dust fall into dust hopper, then the dust will be separated from flue gas in the electric precipitator. By passing static electric current will precipitate dust effectively so the impact of air pollution can be eliminated. This method can reach efficiency above 90% and can capture small particle dust until less than 2µC. Electrostatic precipitator can be applied to cement plant, pulp and paper plant and power plant.

The factors that determine the selection of flue gas filter equipment in boiler are as follow:

- The efficiency to filter dust particle

- The size of captured dust particle

- The fuel used in the combustion furnace

- Capacity of fuel in the combustion furnace

- Development and maintenance cost

Thermodynamic Analysis in Steam Boiler

Rankine cycle is the theoretical cycle which includes basic working principle of steam power plant. Rankine cycle is different from air cycle in term of working fluid that had changed its phase during cycle especially at evaporation and condensation time. Therefore, the fluids that work in Rankine cycle are water and steam.

Rankine cycle ideally does not involve some internal irreversibility problems. Irreversibility is resulting from friction fluid, throttling, and mixing. Irreversibility can also occur in steam turbine generator and pump and will lead to heat loss and pressure loss in heat exchanger equipment, pipes, bends, and valves.

Figure: Simple Diagram of Rankine Cycle

Figure 2: Simple TS Diagram of Rankine Cycle

In the figure 1: Simple diagram of rankine cycle above, feedwater to be supplied into steam boiler is heated either with feedwater heater or deaerator. At condition 2, water is compressed by boiler feed pump into steam drum or boiler. In the boiler, water is converted into steam at condition 3. Steam is heated again until to be superheated steam which has available temperature and pressure for steam turbine generator. Superheated steam at condition 3 is distributed and expanded isentropic to turbine generator and perform rotation shaft which is connected to electric generator.

During on the turbine generator process, temperature and pressure of steam will decrease and reach point 4 where steam is distributed to condenser. Steam which enter the condenser is saturated steam and has high quality. Condenser is one of heat exchanger equipment where steam is condensed there and its temperature is decreased by cooling medium in the condenser.

Air Pollution from Steam Boiler

Parameters of air pollutants resulting from the combustion furnace of the steam boiler are:

1. Sulfur Dioxide

Air pollution by sulfur oxides is mainly caused by two components which in the form gas and colorless, namely sulfur dioxide (SO2) and sulfur trioxide (SO3). Both are called as sulfur oxides (SOx). Sulfur dioxide has a characteristic sharp odor and not flammable in the air while triode is not reactive component.

The problem posed by the pollutants created by humans are in terms of its distribution is uneven, so concentrated in certain regions. Pollution from natural sources is usually more spread out evenly. Source of SOx pollution can be found in the burning of fuels in industry such as fuel coal. SOx pollution will give bad impacts on humans and animals and also damage plants if occurs at levels of 0.5 ppm. The primary effect of SOx pollutants on humans is respiratory system irritation. To suppress the emission of SOx gases, a unit of FGD (Flue Gas Desulfurization) is always used.

2. Carbon Monoxide

Carbon monoxide is a compound that has characteristic such as odorless, tasteless, and at normal air temperature will form colorless gas. CO compounds have the potential to be dangerous because the toxins are capable to forming strong bonds with blood pigment.

Artificial sources of CO include motor vehicles, especially those that use gasoline, while the source does not move like the burning of coal, oil from industrial and domestic waste burning.

The impact of CO varies depending on a person's health status; the influence of high CO levels is against central nervous system. To suppress the emission of CO, the unit of scrubber is used in the chimney or stack.

3. Nitrogen Dioxide

Nitrogen oxides (NOx) are a group of nitrogen contained in the atmosphere consists of nitrogen monoxide (NO) and nitrogen dioxide (NO2). Nitrogen monoxide is colorless and odorless gas while nitrogen dioxide is reddish brown and pungent.

The largest source of NOx pollution is from human activities in urban and industrial activities. The impact of NOx pollution is as toxic material so will be danger for human’s lung. To suppress the emission of NOx is steam boiler should use fuel with low content of NOx.

4. Dust Particle

The combustion product especially for dust can be divided into two types namely bottom ash and fly ash. Effect of liquid or solid form of dust particulate in the air depends on its size. The size of dust particulates are harmful to health has range generally between 0.1 microns to 10 microns. To suppress this emission, steam boiler can be equipped with multi cyclone, bag house or Electrostatic Precipitator (ESP).

Electric Boiler Definition

Electric boiler is one of type boiler which is classified by the heat source to generate steam with using a heating element. Boiler generally has main function to convert water into steam. Steam is obtained by giving amount of heat to water. Heat can be obtained from heater element. In other word, boiler is the equipment to steam which is then distributed to industrial process or turbine generator to generate electrical energy, this type of boiler can be found in power plant area.

Electric boiler basically consists of a pressure vessel which contain network of heating elements which is rinsed by the electric current. Electric boiler is very simple steam generator and it is limited to low steam pressure.

Figure 1: Electric Boiler

Mechanism of Economizer

Economizer is one of steam boiler’s equipment which is used to heat feedwater before it is supplied into steam drum. Economizer is the heat exchanger equipment to increase boiler efficiency by absorbing heat recovery of flue gases. The lower temperature of flue gas out from stack, the heat loss will be less and the fuel which is needed to convert water into steam will be also less in certain circumstances. So it can be said that economizer can save the fuel efficiently. Economizer will make temperature of feedwater higher, so steam boiler can produce steam easily.

The heating process of water in the economizer is like an heat exchanger, water is distributed inside tubes and flue gas outside tubes. The hot flue gas outside tubes flow and making contact with outside surface tube, so the heat transfer occur between hot flue gas and surface metal tubes by convection. Water inside tubes receives heat transfer from metal tubes, so temperature of water is higher than before enter economizer. Contact of flue gas flow to feedwater flow is regular and perpendicular. It is controlled and regulated by setting the flue gas flow.

Furthermore, feedwater that has been heated in economizer is sent to steam drum through connecting pipes. In the steam drum, feedwater will be boiler water and be heated again in water wall tubes to be saturated steam and superheater will make it as superheated steam.

The advantages of using economizer as initial heater are as follow:

- The metal wall of steam drum will not easy to be contracted, shrivel and broken because the incoming water is not cold condition so the cost maintenance will be smaller.

- Increase steam boiler efficiency and decrease heat loss.

- Save the fuel needed to perform combustion process

- The size of furnace can be smaller because the requirement heat surface is less than without using economizer.

Deaerator Working Principle

Feedwater to be supplied into steam boiler should pass deaerator equipment. Deaerator has main functions as follow: to get rid of the gasses contained in feedwater, in this case feedwater had been processed and purified in water treatment plant and has function as initial heater before supplied into steam boiler. Deaerator works based on the nature of oxygen solubility in water decrease with an increase in temperature.

Deaerator consist of two drums where smaller drum is prelimenery heating and exhaust gases from feedwater, while the larger drum is a shelter for feedwater that falls from a smaller drum on it. On the smaller drum contained spray nozzle that serves to spray feedwater into fine droplets of water so the heating process and exhaust gases from feedwater can be performed perfectly.

The main element in determining the success of this process is physical contact between feedwater and the hot steam. Some things that must be considered in the deaerator process are:

- The pressure in the deaerator

- The water level in deaerator

- The amount of condensate water flow

- The amount of feedwater flow

Figure 1: Deaerator

If the deaerator can not work properly can adversely affect to the quality of feedwater, condensate system and also increase the use of higher chemical in the next process.

To achieve good efficiency of deaerator, there are few things to be noted as follow:

Maintain temperature and pressure as high as possible in accordance with the design
Make sure gases such as oxygen can be discharged out from deaerator.
Perform inspection of the inside of the deaerator to ensure all components are not damaged.

Deaeration Process

Deaeration process is the process to treat water by removes dissolved gases in the water. Deaeration process is one of step in water treatment to be ready feedwater and can be supplied into boiler. The soluble gases in water are:

- Hydrogen (H2S)

- Carbon dioxide (CO2

- Oxygen (O2)

Water can be acidic if containing high amount of CO2 in the water. When the gas is contained in water, the water will become corrosive to metal pipe or tube that will form soluble carbonate. In the water contained 2-50 ppm of CO2, water can be categorized as corrosive water. Gas which can accelerate corrosion is oxygen, corrosion will make hole or pit in material surface. Mechanical and chemical method can be used to remove oxygen contain in the water.

Deaeration methods can be divided into two parts as follow:

- Deaeration method with heating system

Deaeration heating process is separation proses which is performed by using mechanical equipment that has been designed and used as expected. The basic working principle of this method is increase temperature of water so solubility of gases in the water can be reduced and dropped. So the terms of maximum deaeration is highly dependent on temperature.

- Deaeration method with addition of chemical system (chemical treatment)

Deaeration with ddition of chemical system is by injecting a chemical solution into water.

Crust or Scale Formation in Steam Boiler

Crust or scale formation in the surfaces of pressure parts steam boiler such as water wall tubes, header, water drum and economizer are the result of minerals which are collected to form crust or scale. These minerals are hardness ions like magnesium (Mg) and calcium (Ca). Crust or scale formation can be also caused by water evaporation and heating boiler. The types of crust or scale are common in the steam boiler is calcium sulfate, compounds of carbonate and silicate. Silica is precipitated along with magnesium and calcium so as to make hard crust or scale. These substances can form a hard and dense crust or scale. If this condition is long left without maintenance, the crust or scale will be difficult to remove.

Crust or scale that surrounds the steam boiler surfaces affects the heat transfer surfaces. There two main consequences because of slag formation. They are heat transfer reduction from hot combustion in the furnace to water and decrease efficiency of steam boiler.

To reduce the occurrence of scaling or crust in the steam boiler is perform prevention action as follows:

- Reduce the amount of minerals with softener equipment

- Conducting regular blowdown

- Providing anti-crust or scale chemicals

Suspended and dissolved substance found in natural water can be eliminated or reduced on the pretreatment which proved economical. Crust or scale existing countermeasures can be done by:

- Mechanical cleaning with scrubs, chisel, brush, etc.

- Off-line cleaning or acid cleaning which dissolves the old crusts or scales with special acid but steam boiler must be shut down.

- On-line cleaning which softening old crust or scale with chemical injection during operation of steam boiler

Fuel and Ignition System

Steam boiler needs fuel and ignition system to perform combustion in the furnace. Fuel selection and design ignition system is the most important parameter to do firing perfectly. The following is type of fuel and ignition system which must be considered:

1. Solid fuel such bagasse, coconut shell and wood. It usually uses stoker to carry out the combustion system. The firing process needs perforation and grate to put fuel for combustion. Perforation is used to flow primary air or combustion flow through the slits of grates.

2. Fuel powder for pulverized coal boiler. The fuel is coal powder that be transported by combination of primary air and secondary air and inject it to furnace.

3. Liquid fuels such as fuel oil or diesel fuel. Fuel oil and diesel oil need atomize steam or atomize air to split particle of oil or diesel into very small scale to make firing process easily. Atomize steam or air is injected through nozzle gun in the burner system. Combustion use liquid fuel needs some equipment such as atomizer either for air or steam, pressure system to spray oil, steam and air, and heater system to warm up oil to decrease viscosity of oil.

4. Fuel gas such as natural gas is sprayed into the furnace which is helped by combustion air. Transportation gas fuel need compressor to give pressure in the gas piping.

Cause of Failure in Fire Tube Boiler

Fire tube boiler is the type boiler which is classified based on tube type. The combustion occurs and flow through inside tube and boiler water is distributed outside tubes. When fire tube boiler is running, there are possibilities of failure occur. Failure should be prevented to avoid damage in boiler itself and environment. Failure can be caused by water level in boiler exceed minimum low water level. This condition may be caused by operator who careless to maintain fire tube boiler keep in normal operating water level (NOWL). In modern boiler, fire tube boiler is designed automatic. If water level reaches minimum low level, fire tube boiler will sound alarm and automatically shut down.

Another cause of failure in fire tube boiler is the boiler age is very old. Material of boiler is fatigue so cause material strength can not resist the pressure. This condition will make pressure part such as tubes and header explodes. Furthermore other reasons for failure are may be safety valve does not work if the pressure exceeds maximum allowable working pressure. Safety valve does not open to vent steam so the pressure will be accumulated and lead to failure.

Too much scale in boiler water so heat is not transferred perfectly. This condition is similar with too much slag or smoky on inside tube or flue gas way. Heat transfer does not perform as expected condition. When metal temperature increases up to 600 F, the metal will be shrinkage and possibility of failure will be highly occur.

Advantage & Disadvantage Fire Tube Boiler

Each steam boiler type has its advantages and disadvantage as well as with the type of fire tube boiler. The following are advantages and disadvantages if the owner of power plant choose fire tube boiler as his boiler.

Advantages of fire tube boiler:
1. The water is supplied in shell and outside tubes while hot gas is supplied inside tubes so the water volume can not be shaken easily when the fire tube boiler is running.
2. Fire tube boiler is so easy to use, operate, clean and maintain
3. Fire tube boiler can be used in small scale industries.
4. Fire tube boiler is relatively cheaper than water tube boiler.

Disadvantages of fire tube boiler:
1. From the furnace combustion side, required time to fill water is longer than to increase temperature and pressure.
2. The efficiency of heat transfer (heat transfer efficiency) is bad enough because of the heat exchanger does not use thermal radiation.
3. In case of bombers fire tube boiler would be very dangerous if a large amount of hot water and steam have been accumulated inside (leakage occur).
4. The fire tube boiler can not produce steam at a pressure higher than 250 pounds per square inch.
5. Capacity of generated steam is limited.

Fire Tube Boiler Based on Configuration

Fire tube boiler is one of type boiler which is classified based on tube type. Fire tube it self can be classified based on configuration as Horizontal Fire Tube and Vertical Fire Tube. The following below is its classification:

1. Vertical Fire Tube Boiler

Figure 1: Vertical Fire Tube

Verictical fire tube consists of a bundle of tubes, pipe, shell and head. The heat from gas combustion flows inside tubes. Combustion chamber and nozzle may be located on the bottom side or middle of boiler. The fire combustion burn in the middle of the shell boiler with a fire hose. Volume water is maintained approximately 80% of theigh of shell.

The most suitable fuel for this boiler is gas because it is cheaper than the price of oil fuel and burn cleaner than diesel fuel. The range capacity of the boiler is between 10-1250 kg/hr and maximum allowable working pressure (MAWP) is not more than 1 MPa. It is ideal to use as a small boiler to produce steam, up to 800 kg / hr.

2. Horizontal Fire Tube

Figure 2: Horizontal Fire Tube

Horizontal fire tube boiler is type of fire tube which is configured as horizontal model (see figure 2). The firing flows inside tube and the water outside tubes. This type can generate evaporation 100-12000 kg/hr and pressures up to 1-2 MPa (150-300 psig). Regarding to the large diameter of the shell boilers, structural support is designed to enough strong to resist load of boiler. If the steam pressure increases, so steam temperature will be higher. The size of this boiler also has a steam capacity between 500-1200 kg/hr and pressures up to 1-2 MPa (150-200psig).

Reason of Corrosion in Steam Boiler

Corrosion can occurs in every point of location in steam boiler. Each location of occurrence, corrosion can be caused by some reasons depend on type of corrosion. The following below is the classification of corrosions in steam boiler based on reason for occurrence:

1. Inter granular Corrosion
Inter granular corrosion occurs in the metal of chromium which may be starved by formation of chromium carbide so chromium oxide as its protective layer is not produced.

2. Fatigue Corrosion
Fating corrosion is caused by stress which working cyclic and fluctuating.

3. Pitting Corrosion
Reason of occurrence this corrosion is the presence of pitting because of electrochemical reaction which can break protective oxide layer

4. Caustic Attack
Caustic attack is caused by high concentration PH which can break protective layer of iron oxide.

5. Acid Attack
Acid attack is caused by low concentration PH in the water wall tubes which can accelerate occurrence of corrosion.

6. Stress Corrosion
Stress corrosion commonly occurs because of combination of stress, environment and material.

7. Galvanic Corrosion
Electric potential will be generated on two dissimilar material such section superheater and reheater tubes and cause particle of metal move from one to other (anode to cathode).

Location of Corrosion in Steam Boiler

There are some types of corrosions which possible occur in steam boiler. Each type has reason for occurrence and location of occurrence. Base on them corrosion can be classified as pitting corrosion, fatigue corrosion, hydrogen embrittlement, caustic attack, acid attack, galvanic attack, inter granular, and stress corrosion. The following below is the description of corrosion based on location of occurrence in steam boiler area:

1. Pitting Corrosion
Pitting Corrosion usually occurs in economizer and tube which is not drained during maintenance.

2. Fatigue Corrosion
This type always causes cracking in location near welded area of water wall tubes.

3. Hydrogen Embrittlement
Hydrogen embrittlement generally occurs in water wall tubes

4. Caustic Attack
This corrosion often occurs in the grooving area under deposit on water wall tubes.

5. Acid Attack
Acid attack commonly occurs in water wall tubes area.

6. Galvanic Attack
Galvanic attack corrosion usually occurs in joint welding between two materials of tube both in superheater and reheater

7. Stress Corrosion
Stress corrosion commonly occurs in austenstic steel such as superheater tubes and reheater tubes.

Advantage Fluidized Bed Boiler

Fluidized bed boiler has more some advantageous than other type of boiler such as pulverized coal boiler. The followings below are the characteristic of fluidized bed boiler which has more attractions:

1. Manage emission of sulfur dioxide efficiently because the temperature of furnace can be maintained uniformly so the non combustible particle can be mixed perfectly. Therefore sulfur dioxide will compound with CaCo3 to be calcium sulfate (CaSO4) and then can be discharged into hopper or bin.

2. Less emission of NOx than other type of boiler because the combustion temperature is low (800 – 900 C)

3. Fluidized bed boiler does not have a flame.

4. Easier to operate fluidized bed boiler

5. Has more stability operation than pulverized coal boiler.

6. Eliminate corrosion because fluidized bed boiler generates soft ash which can melt in the furnace temperature relatively low.

7. Does need pulverizer so the preparation of fuel is simpler and less maintenance.

8. Need short time to start up the boiler

9. During start up, fluidized bed can be burnt without oil fuel and can be operated in low load. It is different with pulverized coal boiler.

10. he owner of power plant has more flexibility to choose fuel for combustion in the fluidized bed boiler with some quality of coal.

11. Because of low temperature of furnace, the alkali metal in the ash can not be evaporated, so the possibility of fouling is less because it can not be condensed.

Fluidized Bed Boiler

Fluidized bed boiler use non combustible materials such as sand or ash as medium for taking heat transfer process with fluidize fuel hydro dynamically. A bed is required to burn fuel and combustion air is supplied in high pressure. Fluidized bed boiler can be classified as two types; circulating fluidized bed boiler and bubbling fluidized bed boiler.

1. Circulating fluidized bed boiler
This type supplies higher amount of combustion air than bubbling fluidized bed boiler type through the grid to the furnace. Therefore the combustion produces a bed with lower density and longer dimension up to the top furnace. Loop seal is used to catch solid non combustible particle and recycle it into furnace again. This condition increase boiler efficiency and low emission.

2. Bubbling fluidized bed boiler
This type requires less amount of combustion air than type circulating fluidized bed boiler. The combustion air is supplied through the grid or can be also called as distributor and maintain sand (non combustible particle) at the low level of furnace. This condition is different with the circulating type. Bubbling fluidized bed boiler is usually applied to burn fuel with high volatile matter and low quality such as sludge, waste and wood.

General Thermal Design Steps

Thermal design of steam boiler is the process to design furnace, heat transfer surface, control steam and water temperature, and select suitable burner or without burner method. Design furnace is calculation process to reach the best size and configuration of furnace to perform combustion completely. Furnace can be designed based on some parameters; wall construction, pressure, heat transfer, waste heat recovery, fuel and firing methods.

After design furnace, the next step to design thermal in steam boiler is calculate heat transfer surface in wall tubes, superheater, reheater, economizer, and air heater. The optimum heat transfer is the objective of this step to determine the requirement a number of tubes needed for the listed pressure parts above.

After design furnace and heat transfer surface, the next important step is control water and steam temperature. Control water temperature is used in economizer while steam temperature is performed in steam outlet temperature in superheater and reheater. Control steam temperature is responsible by desuperheater or attemperator.

Then design burner equipment for selected fuel (gas, oil or pulverized coal) and design without burner if steam boiler use stoker firing is the other necessary step to do thermal design process.

Pulverized Coal Firing

Steam boiler can be classifiedbase on fuel. One of them is solid fuel such as bagasse, wood and coal. Methods to burning coal can be distinguished by underfeed stoker, overfeed stoker and pulverized coal firing. Pulverized coal firing is generally used for bigger scale of power plant than use stoker firing. The best classification of coal to be used in this method is bituminous type.

The size of coal to be burned should be small and ground so grinding step must be performed by pulverizes coal. Pulverized coal will make combustion occur easily and fast when reach ignition temperature. Pulverized coal should be designed dry by use warm air. Warm Air and pulverized coal are mixed flow though exhauster to furnace and to be burned on temperature about 3000 F. Warm air is taken from heat exchanger process in air heater. Primary air is supplied by force draft fan into pulverizer while secondary fan flow through winbox.

Multi Cyclone in Steam Boiler

There are three type of dust collector; multi cyclone, baghouse, and electric static precipitator. The steam boiler which has stoker type, usually use multi cyclone to collect dust. Dusts from flue gas are separated by centrifugal and gravitation force.

Figure 1: Multi Cyclone in Steam Boiler

Multiple cyclone has some cyclones to separate dust from flue gas. Multi cyclone usually receives flue gas from air heater ducting, collect dust from flue gas flow then distribute flue gas into stack. Top section of cyclone is used for impact effect to flue gas flow, so debris or dust can be filtered and fallen down into hopper. The multi cyclone is connected by two flanges of ducting, outlet and inlet. Flue gas flow hit cyclone and make its flow to be centfrifugal flow. Because of gravitation, dust will fall down into hopper and separation of dust from flue gas happen.

Maximum Allowable Working Pressure Calculation

The maximum allowable working pressure (MAWP) of a steam boiler is an absolute restriction of pressure within psig unit for a steam boiler is allowed to run. The ASME BPV Code (American Society of Mechanical Engineers Boiler and Pressure Vessel) says that absolutely no steam boiler should be run at a pressure above the MAWP with the exception of the safety valve is operated to discharge excessive pressure. Steam boiler has some technical terms that be generally used. One of them is MAWP (Maximum Allowable Working Pressure) which means that all of working pressure at the pressure part shall not exceed MAWP. MAWP on the header or drum can be calculated based on the equation below:

MAWP = (t x E x TS) / (R x SF)

Abbreviation of the equation above is:

MAWP = Maximum Allowable Working Pressure in the drum or header.

E = Joint efficiency of welding in the drum or header

t = Thickness of header or drum

TS = Tensile strength of the material, can be looked at ASME BPV Section II D

SF = Design safety factor, the value which is commonly used is five

R = Inside radius of header or drum either steam drum or water drum