The dissolved oxygen in the boiler feed water needs to be removed as much as possible to prevent boiler corrosion. The Deaerator is mechanical devices that are used to reduce the concentration of dissolved oxygen and carbon dioxide to a level less than the content of dissolved oxygen of 7ppb in the feedwater.
Construction of Deaerator
It consists of below mention parts:
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Shell
The upper part of the Deaerator shell serves as deaerating zone and the lower part serves as a water storage zone. The water storage zone is designed to accumulate the Feedwater and serves to balance the Feedwater required during the plant operation.
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Spray valve
The feed water, fed in to the Deaerator evenly through each spray valve from the feed water pipe at the upper part of the Deaerator. The spray valve is designed so as to spray the feed water thinly and horizontally and maximize the surface area where the feed water makes contact with steam. The spray valve is adjusted by means of a spring so that water can be sprayed evenly at any capacity.
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Water sprinkling box
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Deaerating tray
The feed water, coming out from the water sprinkling box, drops into the V-shaped trays in sequence while being sprinkled evenly and finely in sequence and stirred, and reaches the lower part of the Deaerator. The V-shaped trays are installed facing up with proper deviation provided between each stage so that the feed water certainly drops from upper tray to lower tray on the way to reach the storage tank.
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Water level Indictor
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Safety relief valve
Working of Deaerator
The feed water is sprayed through the spray valve continuously in an almost film-like form onto the water sprinkling box. From the water sprinkling box, the water is sprinkled evenly on the trays.
In this process, the water is heated considerably by the steam coming up from the lower tray, and the water turns into very fine water drops in sequence while the water drops from the top tray down to the bottom trays, these are stirred, spread and deaerated while being on contact with steam coming up from the lower part in the process until when the water drops reach the bottom of the Deaerator and Feedwater.
On the other hand, the steam goes up to the upper part from the lower part of tray group while passing through the V-notch of each tray stage and heating it. In the meantime, the deaerated gas is sent up to the upper part by the steam. Low temperature feed water with coming from the spray nozzle is deaerated by steam from underneath the spray nozzle. As a result, the steam is condensed and flows down to the lower part together with the feed water, while the deaerated gas and a small amount of the steam are vented into the atmosphere through the vent pipe after passing through the spray valve feed water diaphragm.
Normally the quantity of gas and steam vented is about 0.5 % of the heating steam. The Deaerator is used at the atmospheric or higher pressure as a pressure type. The exhaust is discharged through the orifice and is gathered in the vent pipe and then vented into atmosphere.
Preparations for Operation of Deaerator
(1) Fill the Deaerator with water to normal level. For time saving at water filling, open vent orifice bypass valve.
(2) Check the water level control device.
(3) Check the alarm system.
(4) Check the function of auxiliary steam pressure reducer.
Operation of Deaerator
(1) When water level in the Deaerator come up to normal water level, feed the auxiliary steam into the Deaerator gently.
(2) When the temperature and oxygen content of stored feed water has become adequate, start the boiler feed water pump. Feed the heating steam gently. At this time, warm the piping adequately.
(3) After the greater part of the air in the deaerator has been vented, close the air outlet bypass valve and vent the air through the orifice.
(4) After the start-up, check the pressure and water level and see whether the control unit is working properly
In operating the Deaerator, attention shall be paid to the following points:
- Do not change the inlet drain flow rapidly. To allow the Deaerator to perform optimally, it is essential to keep internal temperature at the saturation temperature corresponding to the internal pressure. For this purpose, take care not to change the flow of drain and others flowing into the Deaerator rapidly, as this will cause the internal temperature to change suddenly.
- Water level control device to be kept in good condition. As water level of Deaerator controls and adjusts water feed flow with use of automatic regulator. These accessories need to be always kept in good condition.
Maintenance of Deaerator
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Maintenance of Spray Valve
Spray valves are hardly ever out of order, if the feed water condition is satisfactory. Spray valve can be taken out by removing the top cover. During Inspection/checking the valve, examine whether it can move up and down freely and whether the sheet surface is not damaged.
In case of disassembling the valve, check in advance, how the spring is set and ensure that it can be reassembled with the original set dimensions. The spray valve spring is set in such manner that proper load is applied on the sheet surface when no water is fed. It is so designed that the loss of head is minimized even at the maximum flow rate.
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Maintenance of Tray
Trays need to be periodically inspected and cleaned. At the cleaning of the trays, they are to be taken out from the manhole one by one. Trays made of stainless steel shall be easily cleaned by rubbing or pickling process. Check the insert direction of each tray before reinstalling them.
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Maintenance of Shell
The debris or accumulation in the Deaerator should be removed periodically.
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Maintenance of Accessories
The Deaerator is equipped with Safety valves, water level indicator, and water level alarm device. These devices should be checked daily and periodically to see whether they are functioning properly.
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