Steam Boiler: Feed Water System

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Feedwater Treatment, Boiler Water Conditioning and Steam Purity

Feedwater treatment, boiler water conditioning and steam purity is important to be known. It is very important to monitor and control boiler water chemistry to prevent failures due to corrosion. The single largest cause of pressure part failures is waterside corrosion.

The successful operation of steam generating equipment depends upon a firm control of feed water and boiler water to assure free from scale formation and corrosion due to water and steam in the boiler.

Scale formation in boilers is prevented by providing good make-up water to the feedwater system and by avoiding condensate contamination. For higher pressure of boiler, the make-up must be of condensate quality such as provided by evaporation or by demineralization.

Corrosion of metal surfaces in contact with water and steam constitutes the major maintenance expense to the power industry. The condensate and feed water must be free of dissolved corrosive gases and the pH of the water must be properly adjusted to prevent the attack of metal surfaces.

The oxygen content in the feed water must be reduced to low levels by effective deaeration in the condenser and in deaerating heaters. Care must be taken to prevent the introduction of air into heater drips which may be added to the condensate. Make-up water from storage tanks should not be added directly to the feed water system without deaeration. Minimum forced boiler outage time has been realized where the oxygen content has been reduced and held below 0.01 ppm. It is important to maintain a chemical reducing environment in the boiler water and chemicals such as hydrazine have been used effectively to achieve this.

Low pH feed water readily dissolves iron and copper in the pre-boiler equipment. These corrosion products when introduced into the boiler will contribute to the corrosion of boiler steam generating surfaces. Oxides of iron and copper may permit the diffusion of boiler water to the heated surfaces of the unit and cause locally high concentrations of boiler water salines that result in the attack of the tube metal.

The pick-up of metals from pre-boiler surfaces can be minimized by the addition of volatile alkaline chemicals that raise the pH of the feed water. Ammonia or various amines, added Lo maintain a pH range of 8.5 - 9.0 have produced the best results.

The philosophy of control of boiler water pH varies widely. The caustic-based treatment, the coordinated phosphate treatment and the volatile treatment have all been successfully applied however the latter two methods of control are preferred in higher pressure boilers.

For this boiler, the coordinated phosphate treatment is adopted with using sodium-phosphate (Na3PO4) and hydrazine (N2H4).

Recommended Water Quality

The recommended limitation value of feed water and boiler water is as follows:

		Feed Water	Boiler Water
pH at 25 degC		8.5 – 9.0	9.3 – 9.8
Hardness (CaCO3)	ppm	0.0	-
Micro-Mhos/cm at 25 degC		-	< 200
Dissolved solids	ppm	-	< 100
Oxygen	ppm	< 0.007	-
Silica	ppm	-	< 1.5
Al	ppm	< 0.03	-
Fe	ppm	< 0.03	-
Cu	ppm	< 0.01	-
Hydrazine (N2H4)	ppm	> 0.02	-
Phosphate (PO4)	ppm	-	2 - 6

Boiler Feedwater Pumps

Boiler feedwater pumps purpose as the supply process to steam boiler, supplying pressurized water into steam boiler. Boiler feedwater pumps perform that by using suction via feedwater heater in deaerator and moving feedwater directly into steam boiler by way of the heater of pressurized feedwater. There are a lot of techniques offered to supply adequate suction head to boiler feedwater pumps and control volume of feedwater.

One technique to manage volume is to utilize a adjustable part of boiler feed pump such as motor of speed drive. A different technique is to utilize electric motor operating on consistent speed, combined to adjustable speed drive of hydraulic. In which the expenditure of a adjustable speed drive just isn't validated or desired, volume could be managed through throttling valve mounted among boiler and pump. A determination varies according to whether the funding expenditure of adjustable speed drive is countered through savings within electric power. Control valve of feedwater needs high pressure drop to get correct operation; for that reasons the boiler feedwater pumps should get added in head capacity.

In the following kind of procedure, condensate in deaerator is going to be condensed, needing deaerator for being heightened as far as doable on top of boiler feedwater pumps to produce adequate NPSH (Net Positive Suction Head) with regard to pump. That elevation may perhaps need more cost of piping isntallation and structural that may be countered by the assembly of enhancer pumps which give suction head extra pertaining to boiler feedwater pumps.

At least 2 feedwater pumps are generally established, according to the requirement to keep capability in case of pump malfunction. This installation enables one pump stop to do regular maintenance, for example fixing electric driving motor, changing packing or bearing seals.

Feedwater recirculation returning to deaerator is essential if one speed motor is applied and stream reaches or under the pump’s lowest stream. Steam turbine is frequently utilized to generate feedwater pumps since the velocity of pump and turbine might be different such as a adjustable speed motor. Recirculation of programmed control valve is managed through the operation’s dispersed control program could execute this purpose. Pumps must be in recirculation function as few as doable to increase energy benefits.

Oxygen Scavenger

Nearly all of oxygen in feedwater of steam boiler is eliminated through deaerator nevertheless some amounts of oxygen is continue to current that may, eventually lead to corrosion in steam boiler. To avoid that, oxygen scavenger is included to boiler water, ideally in deaerator storage space tank thus the scavenger can get optimum time period to start up by using residual oxygen. Within specific conditions, for example when feedwater of boiler is employed for attemperation to decrease steam temperature, different places are more suitable.

The best generally applied oxygen scavenger can be sodium sulfite. It is low-cost, more efficient and quickly reacts by using the trace quantities of oxygen. It might be simply calculated in boiler water.

Generally oxygen scavenger is good solution to eliminate oxygen. There are situations in higher pressure boilers (commonly preceding 900 psig), in which several of sulfite might decompose and get into steam, leading to problems in condensing steam turbines and condensate systems. In this instance, replacement (generally organic-dependent) oxygen scavengers could be utilized.

Innovative oxygen scavengers had been presented nowadays. The determination to utilize oxygen scavengers or depend on sodium sulfite must solely be designed by these experienced to generate water treatment of boiler conclusions. In all conditions the modern product must be properly included and its performance considered relating with working procedures.

HP Feedwater Heater

HP (High Pressure) feedwater heater is a heat exchanger that serves to heat boiler feedwater before it is distributed by way of heat exchange between steam extractions of HP (High Pressure) turbine with feedwater from the feedwater pump.

HP feedwater heater is arranged in graduated with direction opposite between feedwater and steam. As a result of heat exchange between steam and feedwater, condensation event will occure that produce condensate. Condensate still has such high temperatures are not discarded but used to help warm up next heaters.

The level of condensate in each of heater is maintained at position set point of 20%. As shown in Figure 1, the control of HP feedwater heater is still relying on feedback control system that regulates the output of heater. The system is only intended to control each heater with no connection to each other. So when one of heaters that has an input steam and condensate (7A and 6A) increased steam load, the heater is still also get the input of large quantities of condensate heater before.

This condition finally will make the level of heater exceeds the limit of 40%, which means turn on Level Switch High (LSH) to open the blowdown valve. In this condition the control system is otherwise not able to keep the level in a safe condition, so that the safety system worked.

Figure 1: Control Feedwater Heater

Boiler Feed Water Treatment

Water used in boilers is water which the content of mineral had been released and filtered (demin water) because the mineral content in water can damage the parts of boiler. To eliminate the mineral content in water, can use the flocculation and clarification steps. Flocculation is the process of removal of particles suspended in water either large or colloidal compounds are suspended in water. Content levels of these particles are expressed as turbidity. The flocculation steps are:

Filtration process

Filtration is the process of filtering large-sized dirt. The filtered impurities are organic compounds, fine particles, color compounds and microorganisms.

Coagulation

Coagulation is done to remove dirt impurities form compounds that are ionic.

Water generated from the process above is called with demineralized water (demin water). But in demin water still contained dissolved gases such as carbon dioxide and oxygen can cause corrosion of pipes and tubes. To eliminate the solution of these gases is performing deaeration process. Deaeration process is performed in the deaerator process on the two stages, namely:

Mechanically

Deaeration process mechanically is done by stripping process with low steam (LS). This can eliminate the solution of oxygen and carbon dioxide by up to 0.007 ppm.

Chemically

Deaeration processes chemically are carried out by injecting solution of hydrazine (N2H4). The end result of deaeration process is called Boiler Feed Water (BFW) which is then used as feed water to the steam boiler.

Economizer as Supporting Instrument in Steam Boiler

Economizer has similar function with deaerator as supporting instrument in steam boiler, though only enhancements, the usefulness of economizer can make working process of steam boiler will be more efficient. Where we know the burning water in economizer is only utilizing exhaust gases from combustion in the boiler by not adding fuel to heat water in it. It is not only deaerator and economizer alone that working as supporting heater, but many other heaters can also be used in an industrial system that can convert the water into steam in the power plant.

Figure 1: Graph the Using of Economizer

The graph above shows the advantages and disadvantages of using the economizer for preheating. Clearly seen without using a boiler economizer so work efficiency of steam boiler will decrease, in the sense that without heating which is assisted by the economizer, the boiler must work longer in the production of steam and in addition boiler will require more fuel to reach the hot temperature steam that has been determined. In addition, if the boiler still forced to work much more quickly it will damage the tubes within the boiler itself. Once this occurs then a plant will experience huge losses in the boiler operation because the use of too much fuel and the resistance of a tool will quickly decline and need to replace equipment.

However, if a boiler using the economizer and some other auxiliary heater to heating water in the heating process before it burned, it will further enhance the work efficiency of the boiler itself, because the water temperature before being burned in the boiler is already quite high, it means that heating water into steam inside the boiler does not take long time and do not use that much fuel to achieve a predetermined standard temperature. Furthermore the operating costs can be more efficient and can indirectly benefit to power plant performance. Besides maintenance or maintenance of equipment or replacement equipment can be carried out much longer.

It is apparent that by using a boiler economizer can increase the capacity of boiler and can also make efficient in the process of combustion, convert water into steam in the boiler, so it will doing more fuel savings far enough difference if compared to the boilers which running without economizer.

Deaerator as Supporting Instrument in Power Plant

In this deaerator water will be heated to a temperature of 100^oC -105^oC, temperature of water is initially 30^oC - 50^oC. After going through the process of preheating the feed water then flowed into the economizer to be heated back up to the level of temperature 150^oC - 160^oC in which the heating in the economizer using exhaust gases from combustion in the steam boiler or chain grate before the gas was discharged through a chimney or stack.

After to be heated up in economizer, water flowed into the drum boiler before the water is burned in water wall tubes boiler. Then the water inside the boiler to be burned at a temperature of 400^oC - 459^oC, at this form of water has been turned into full steam. But at this level the water can not be used to turn turbines, and therefore at this level after the water turns into steam, steam will be distributed into the superheater to raise the temperature of the steam itself to the level of 500^oC - 600^oC.

Steam at this level is ready to turn turbines and generators to produce electricity play. The remaining steam turbines were going to play back streamed to the deaerator in order to preheat the water in it, that's so deaerator and economizer cycles of use as a supporting instrument in warm water until it becomes steam. We know the function of deaerator is to remove the gases contained in the boiler feed water, after the purification process of water (water treatment).

In addition deaerator heater also serves as the initial water filling the boiler before it is inserted into the boiler. Deaerator works based on the nature of oxygen solubility in water decreases with an increase in temperature. If water from water treatment directly burned in a boiler, it will cause severe corrosion because the water still contains gases that can cause corrosion and so on.

Likewise, if the water is burned directly in steam boilers will not rule out going to use fuel that is not less, because water from water treatment temperature is 30^oC - 50^oC and burned in a boiler with a target temperature of the water into steam at 400^oC and above. From the small sample above shows clearly that the pre-heating the water is very useful for saving fuel.

Deaerator-Economizer as Feedwater Heater in Steam Boiler

Use of deaerator and economizer as auxiliary instruments in warm feedwater before the feedwater is burned in a boiler. Water is obtained from the raw water that has been in treatment to conform to the standards set supplied to deaerator with the aim of separation of the gases dissolved in water and separating minerals contained in water in order to keep all the tubes that pass through can avoid corrosion. In addition, in the deaerator water went through the process of preheating the steam heated by the rest coming from turbine generator. The function of the deaerator is as a gas separator-gas dissolved in water and heats the boiler feed water before it was burned in the boiler.

Economizer is shaped tubular heat transfer equipment used to heat boiler feed water before entering the steam drum. The term economizer is taken of the usefulness of such tools, namely to conserve fuel by taking the hot flue gas before being discharged into atmosphere. An economizer can be used to utilize the exhaust heat to preheat boiler feed water. Any reduction in exhaust gas temperature through the economizer or preheater is 1% saving of fuel in the boiler. Any increase in temperature of feed water through the economizer or the air temperature rise through the combustion air preheater, there is a 1% fuel savings in the boiler.

Figure 1: Mechanism of Deaerator and Economizer

Economizer performance is determined by the fluid having a low coefficient of heat transfer gas. Heat transfer speed can be improved by increasing the total heat transfer coefficient by regulating the composition of tubing / fin properties and increase the contact area of heat transfer. The response generated by the economizer is heat transfer effectiveness and operating costs. Effectiveness of heat transfer is the amount of energy that can be drawn from the total amount of energy that can be absorbed. The greater efficiency of heat transfer in the economizer, heat the remaining gas that is picked will be many more.

The greater effectiveness of heat transfer that occurs, then the tool is more efficient. Economizer operation costs are determined by fan power and pump power. Fan used to flow combustion air to the boiler through the economizer. The more loops and more complex arrangement of economizer tubing on the fan power required increases. Pumps used to drain the boiler feedwater to the steam drum through the economizer. The longer and more loops in the economizer, the required pump power increases. The optimum response is obtained using the design factors that affect the performance of economizer as follows:

Outside diameter tubing, the diameter of the tube size used in preparing the economizer. The larger the diameter of the tube will result in diminishing the effectiveness of heat transfer.
Transverse spacing, which express the distance between the tubes parallel to the direction of the width of economizer. The wider spacing between the tubes resulted in the induction process of economizer heat decreases, thus decreasing the effectiveness of heat transfer.
Fin density, the number of fins per inch that can be structured to incorporate some of the tubes in the economizer. The more structured fin will result in heat transfer is not effective because the distance between the tube will be farther.

Heating Process in Steam Boiler

With rising temperatures and water near its boiling conditions, some molecules gain enough kinetic energy to reach the speed that made him any time off from the liquid into the space above surface, before falling back into the liquid. Further heating causes greater excitation and the number of molecules with enough energy to leave the liquid increases. Taking into account the molecular structure of liquid and vapor, it makes sense that the density of steam is smaller than water, because the steam molecules are far apart from one another. Space that suddenly occurs above the water surface becomes filled with dense steam molecules.

If the number of molecules leaving the liquid surface is larger than the re-entry, then the water evaporates freely. At this point the water has reached boiling point or saturation temperature, which is saturated with heat energy. If the pressure is fix, adding more heat does not cause further increase in temperature but causes water to form saturated steam.

The temperature of boiling water with the saturated steam in the same system is same, but the heat energy per unit mass is larger in the steam. Saturated temperature at atmospheric pressure is 100 ° C. However, if the pressure increases, then there will be the addition of more heat that the increase in temperature without phase changes. Therefore, the increase in pressure will effectively increase the enthalpy of saturated water and temperature. The relationship between saturation temperature and pressure are known as saturated steam curve (Figure 1).

Figure: Saturated Steam Curve

Water and steam can exist simultaneously on a variety of pressure on this curve; both will be at saturation temperature. Steam saturation curve in the above condition is known as superheated steam / steam through the saturated:

- Temperature above saturation temperature is called the degree of saturated steam.

- Water on the conditions under the curve is called sub-saturated water.

If the steam flows from the boiler at the same speed with which it produces, the addition of heat will further increase the rate of production. If the same steam leaving the boiler is not retained, and the amount of incoming heat is maintained, the energy flow to the boiler will be greater than the energy that flows out. This excess energy will raise the pressure, which in turn will lead to saturation temperature increases, because the temperature is related to pressure saturated steam.

In this case the water in a boiler is combustion, water through the economizer which has flowed through the heating inside the drum boiler (steam shelter) and then burned in a boiler to be heated further up to a wet steam. The temperature inside the boiler is approximately 400^oC - 459^oC. Combustion of fuel coal and assisted with air to maintain combustion stability in combustion system. Combustion control system connects the control of heat input to the boiler with the ratio of air / fuel entering the combustion chamber. This control system must be able to ensure sufficient amount of air available for combustion of a fuel efficiently without causing smoke and with minimum particulate discharge from the chimney. After this process within the boiler, steam flow and then proceed to the superheater to be his dry steam, steam temperature was about 520^oC - 600^oC and ready to turn turbines.

Feed Water Process of Boiler

A boiler is a closed vessel where the combustion heat flowed into the water until it forms a hot water or steam. Hot water or steam at a certain pressure and then used for transferring heat to a process. Water is a useful and inexpensive medium for transferring heat to a process. If water is boiled into steam, its volume will increase by about 1,600 times, producing a force that gunpowder is explosive, so the boiler is a device that must be managed and maintained very well.

Boiler system consists of: feed water systems, steam systems and fuel systems. Water system provides water to the boiler automatically as needed steam. Various valves are provided for purposes of maintenance and repairs. Steam system collects and controls the production of steam in the boiler. Steam is directed through a piping system to the user's point.

The entire system, steam pressure is set using a valve and monitored with a pressure monitor. The fuel system is all the equipment used to provide fuel to generate the necessary heat. Equipment required on the fuel system depends on the type of fuel used on the system. Water supplied to the boiler is converted into steam is called feed water. Two sources of feed water are: condensate or steam is returned from the process and make up water (treated raw water) which must come from outside the boiler room and plant processes. To obtain a higher efficiency boiler, an economizer for heating feed water using waste heat in flue gases.

The raw material used to make steam is clean water. Water from the Reverse Osmosis (RO) that has been processed in the stream using a pump to the deaerator tank to the level specified. Warming in the deaerator is to use the residual steam from the turbine playback. In this case there is some stage or stages of the circulation of steam to preheat deaerator.

Phase 1

Residual steam from the steam turbine rotating returned directly to the deaerator to reheat the water contained in the deaerator tank. The remaining steam is directly due to differences in pressure and flow density of water and steam, because of differences in the density of the steam was inclined towards a greater density of water. Circulation at this stage continuously like that.

Phase 2

The remaining steam turbine of the player falls into the condenser (cooling process). At this stage the remaining pedinginan steam assisted by seawater. After going through this cooling process, the steam turns into water again later in the stream to the LPH (Low Pressure Heater) to be heated again. Once the water is almost hot LPH earlier in the stream again to the deaerator for heating up. when heated in hot water deaerator was not directly in the stream to the economizer, but the water in the stream prior to HPH (High Pressure Heater) to be heated more and after that then flowed into the economizer. Help some heaters on stage 2 this is just a step in the maintenance of the instrument where it has be set in such a way as to guard. It also can be used as a safety if any of the one-stage system of the stage had been damaged, other than that it's step by step depending on the type of turbine used

Deaerator Instrument

Deaerator is installed in power plant system to treat feedwater as per boiler requirement. Working principle of deaerator should be known for engineer and operator who have responsibility to design and operate the boiler. Deaerator types should be also known to choose the best deaerator type to be operated to get suitable performance and highest efficiency.

The following below are instruments which are installed in deaerator:

Vent Condensor

Vent condensor steam serves to condense the gases and collect these gases prior to release into the atmosphere. The inside of the vent condenser is made of stainless steel. The gases that have been separated from water go out into atmosphere through the vent. Valve in this pathway must be opened slightly so that the expenditure of gas can be seen with the release of smoke from the vent line.

Tray (barriers)

Tray is installed in deaerator serves as heater, filter and also as a place to expand room of the condensing steam.

Liquid level glass / glass probe

The glass probe is used to determine the level of surface water in deaerator tank. The working principle of this tool is associated with the vessel. Its center line approximately 20 mm and length 300 mm.

Thermometer

The thermometer is placed in storage tanks of the deaerator. The thermometer in the storage tank will be consistent with the operating pressure of steam. If it takes a thermometer can also be added to steam intake path.

Pressure gauge

The reading on the pressure gauge shows amount of steam pressure inside the unit. Pressure gauge is placed on the intake line which is equipped with a steam valve.

Transmitter electro

Transmitter electro has same function as thermometer to measure temperature. But there are differences on the way of reading this instrument.

Control Valve

Control valve is also called the faucet / valve control. This tool is widely used in water pipes and serves to control pressure or flow of fluid. Control valves can be classified into two types, namely analog and digital.

Deaerator Type for Boiler

Deaerator is one of equipment in power plant. Deaerator is used to eliminate corrosive gasses such oxygen in feedwater before supplied into steam boiler. Deaerator has certain working priciple and some types. Classification of deaerator type can described as following below:

Spray Type Deaerator

Spray type deaerator is used when feedwater need to be heated in advance by using steam as heater. As the Figure 1 below, the steam flow into spray type deaerator will break water into small pieces resulting in gases that dissolve in water being forced out so that the concentration of oxygen in the water decrease.

Figure 1: Spray Type Deaerator

Vacuum Type Deaerator

Working mechanism vacuum type deaerator is gases which dissolve in water is removed by using steam ejectors or vacuum pump. Mechanism vacuum type deaerator can be seen in Figure 2 below until obtain required vacuum. The amount of vacuum depends on water temperature, but usually 730 mm Hg.

Figure 2: Vacuum Type Deaerator

Description:

1 = Steam inlet

2 = Water cooler

3 = Steam ejector

4 = Disposal

5 = Steam and air outlet

6 = Hot water tub

7 = Cool water outlet

8 = to feedwater pump

9 = Water level inlet controller

10 = Float

11 = Water inlet distribution

12 = Steam and Air Outlet

Tray Type Deaerator

Tray type deaerator can be shown in Figure 3 below. Tray type deaerator maximize tray or barriers as a medium to enlarge the space to fall for water so that water molecules will separate each other. Furthermore the working principle of tray type deaerator is to force water molecules to spread out making it easier for air release.

Figure 3: Tray Type Deaerator

Condensate Recovery in Power Plant

Condensate is essentially distilled water which is originally taken from steam out from turbine generator. Steam will be condensed into condensate form in condenser. Condensate has ideal quality as boiler water, so condensate recovery is performed to be supplied into steam boiler again. But if the condensate is contaminated then condensate should not be used for further processing.

Condensate is the result of heat exchanger process. Steam should be totally converted into condensate (liquid form). If there is steam contain in condensate, steam should be trapped with equipment which is called steam trap. Pressure and temperature of steam from turbine generator after pass condenser will be reduced by flash steam process. Flash steam is a process to discharge steam into atmosphere. The amount of steam which is discharged into atmosphere is approximately 10% - 15%.

Condensates will losses its mass about 13% if flash steam is performed. The quality of condensate should be maintained so can be used again and distributed to deaerator to get further treatment. The following below is example figure that show circuit of steam and condensate.

Figure 1: Circuit of Steam and Condensate

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.

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

Feedwater Regulator

Feedwater is the water to be supplied into steam boiler. Requirement of feedwater is regulated by ABMA (American Boiler Manufacturing Association). Water treatment is one of step in this requirement. Temperature of feedwater influence efficiency of steam boiler, higher temperature will increase the efficiency, so feedwater heater is required in the feedwater system.

Amount of feedwater is maintained to keep steam boiler on the range of NOWL (Normal Operation Water Level). Maintain water level is totally important to keep steam boiler safe from both short term overheating or long term overheating and also damage turbine generator.

There are three types of feedwater regulators commonly used in the power plant system. The type of feedwater regulator is generally used based on function, size, and the principal working. The following below are the feedwater regulator type:

1. Thermostatic Expansion Regulator.

This type is feedwater regulator which uses inclined tube and connects it between feedwater control valve and boiler. If the water level in the boiler indicates low level, the tube will get expansion, so the feedwater control valve will supply the feedwater.

2. Hydraulic Regulator

This type is feedwater regulator which consist tube, fin, and jacket. This regulator use tube which be connected between water column in the steam drum and feedwater control valve. When water level show low level which means that amount of steam steam is higher than water, the steam will enter the tube, at the certain temperature and pressure, the valve will open to supply feedwater into steam boiler.

3. Float Regulator

This type is feedwater regulator which uses floating equipment like a ball and connects it to the boiler. The floating ball will move up and down parallel with water level in the boiler. A connecting system must be set up between this equipment and the feedwater control to maintain the steam boiler water level in the range NOWL.

Condenser in Power Plant

Steam boiler has main function to convert water to high quality steam. The steam is then supplied into turbine generator. Before supplied into turbine generator, steam should apply the requirement for both pressure and temperature. Condenser is used in power plant to make back pressure and temperature of steam from turbine generator is lower and then converts steam into condensate.

Condenser can be classified into two types: contact condenser and surface condenser. Condenser with contact type is no longer used in power plant, because its working principle is making contact each other between condensing water and steam. Therefore, possibility of steam getting contamination from condensing water is highly occuring. So the condensate is not suitable for the requirement of steam boiler feedwater, it will be high disadvantage for designer or owner.

Condenser with type surface has different working principle from contact condenser type. It never making contact between steams and condensing water, so the purity of condensate is maintained from contamination of condensing water. The working principle of this condenser is similar with economizer, steam from turbine generator pass through around the tubes (outside tubes or can be called the shell side) and condensing water flow in the inside tubes.

The shell condenser is not designed for under pressure but for vacuum pressure. So the relief valve is placed on the condenser to release pressure to the atmosphere if higher than vacuum.