Steam Boiler: Deaerator

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Design Deaerator or Feed Water Storage Vessel

The design daerator should consist of the dearating unit and a feed water storage vessel. A feed water storage vessel should be provided within the feed water heating system. The dissolved oxygen content in the feed water effluent from the heater should not be more than 0.007 mg/liter at any load condition, measured in accordance with the "Method and Procedure for the Determination of Dissolved Oxygen" of the Standards of the Heat Exchange Institute. The feed water storage vessel should be integrated with a deaerating unit to fully de-aerate the feed water, if an alternative water chemistry regime is proposed the design should substantiate his provisions for control of dissolved oxygen in the feed water system.

The main functions of the feed water vessel should be to:

Condition feed water for start-up
Provide a reserve to compensate for fluctuating feed flows
Ensure boiler feed pump suction requirements are met at all times
Removal of oxygen via a steam heating/de-aerating process

Whichever arrangement of feed water / deaeration vessel in the design deaerator or feed water storage vessel to provide a full description of the normal function, including:

Level indication and level control
Conditioning of feedwater prior to start-up (cold start)
Flows of condensate, bled steam, auxiliary heating steam
System responses to transient conditions
Any specific arrangements proposed for part load operation
Disposal of scrubbed or vented non condensable gases

The design deaerator or feed water storage vessel should state the provision for deaeration and heating the stored water on plant starts when the LP heater is out of service. The description should also describe the provision included to monitor and control the condensate level within the storage tank and any recirculation system if necessary to ensure homogenous conditions in the stored water.

The feed water storage vessel should be located at or as close to the turbine operating floor level as possible, consistent with satisfying the feed pump net positive suction head (NPSH) requirements. The tank should store a minimum quantity of feed water corresponding to 7 minutes of rated (MCR) feed water flow or that quantity of feed water which should permit a controlled and safe shut down of the boiler, whichever is greater and assuming that the condensate is initially at the normal working level.

The feed water storage vessel should be designed to operate with freedom from condensate surging and vessel vibration. The freeboard above the top of the working level range should be sufficient to accommodate the total condenser hot well content with margin.

Boiler feed pump leak-off returns should be introduced into the feed water storage tank in a controlled manner to prevent damage from high velocity evolved steam or water impingement. At all other points when steam or water enters the deaerator / feed water storage vessel, suitable precautions such as baffles or diffusers should be provided to prevent direct impingement on the tank plates, internals or water surface, internal baffles should be arranged within the feed water storage tank to prevent surging of the condensate.

Provisions to protect the steam turbine from the risk water induction arising from any bled steam pipe work connecting the deaerator / feed water storage vessel should be as stated in Section "Bled Steam supply Lines".

The deaerator should be of the spray/tray type and should include storage tank, supports, vent condenser and fittings. The design should be to the Heat Exchange Institute standard and suitable for full vacuum.

The deaerator should be designed and arranged for the efficient removal of non-condensable gases from the feed water under all conditions of operation, including the admission of auxiliary steam during starting and low temperature condensate under fault or restart conditions.

If a part load deaerator is offered then the design should include a full description of the start-up and operation with increasing load up to full load on the steam turbine-generator.

The design deaerator or feed water storage vessel should describe features of the deaerator head which facilitate the removal of non-condensable gases from the circulating feed water and the provision if any, for recovering heat from the vented gases and vapor. Deaerator level indicators and alarms should be provided in the CCR, these alarms should be fully functional at all times when the plant is available for operation, including periods when the plant is on standby duty.

Safety valves should be provided to protect the deaerator and feed water storage vessel from over pressure from any source. All parts of the deaerator exposed to oxygen or corrosive gases should have an adequate corrosion allowance or be of corrosion resistant materials.

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.

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

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.

Deaerator Feedwater Tank

Steam boiler has some systems which running together to produce high quality steam. One of the systems is feedwater system. One of the main parts of feedwater system is deaerator feedwater tank. The mass balance between steam produced by boiler and the feedwater should be maintained in a fluctuating load conditions based on firing rate and steam demand. Furthermore feed water system should be controlled to supply enough water into steam boiler.

The required water must faces water treatment system before supplied into steam boiler, so the water has required parameter contents about silica, scaling, conductivity, PH, dissolved oxygen and so on. The main consideration is how to remove oxygen to prevent corrosion and remove scaling which can isolate heat transfer process in the pressure parts of steam boiler (water wall, header, economizer, etc).

The main functions of deaerator feedwater tank are:

1. Remove oxygen and non condensable gases.

2. As storage tank to maintain and supply enough feedwater into steam boiler

3. Increase temperature feedwater until saturated temperature.

Water which is treated from demineralization process and from condensate is sprayed into through nozzle deaerator. Low pressure steam is injected to deaerator to remove gases content and dissolved oxygen and then discharge them through deaerator venting. In addition to remove oxygen content, low pressure steam is also used to increase temperature up to 130 140 C. The water which gas content and dissolved oxygen had been removed enters to feedwater tank. Moreover chemical contents such as sodium sulfite (Na2SO3) or hydrazine (N2H4) are injected to reduce oxygen content in feedwater.