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A Deaerator is a device for air removal and is used to remove dissolved gases (an alternate would be the use of water treatment chemicals) from boiler feedwater to make it non-corrosive. A deaerator typically includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater tank.
Additional recommended knowledge
Necessity for deaeration
A steam generating boiler requires that the circulating steam, condensate, and feed water should be devoid of dissolved gases, particularly corrosive ones, and dissolved or suspended solids. The gases will give rise to corrosion of the metal. The solids will deposit on the heating surfaces giving rise to localized heating and tube ruptures due to overheating. Under some conditions it may give rise to stress corrosion cracking.
The adjacent diagram depicts a typical conventional trayed deaerator. The actual construction details will vary from one manufacturer to another depending on their specific design requirements.
This is generally a horizontally mounted cylindrical steel vessel with dished ends with a dome on the top and with internal and external fittings. The size of the same depends on the unit capacity it is associated with. The cylindrical vessel portion acts as storage for boiler feed water supplying to the suction of the boiler feed pumps from a pipe connected to the bottom of the tank, generally in the mid portion.
During a cold start of the unit, it is possible that the water in the feed tank may be cold. At that time, the water has to be heated to bring it up to normal operating temperature so as to expel the dissolved gases. For this, a sparger pipe is provided inside the tank at the bottom level. Low pressure steam supplied to the sparger pipe provides the small amount of heat needed to increase the temperature of the feedwater.
The storage tank is also provided with connections for the injection of chemicals, level sight glasses, a drain, etc.
Since the feed tank is always hot, sufficient external insulation (known also as lagging) is provided to minimize the heat loss.
Domed deaeration section
At the top and in the mid portion of the feed tank an inverted domed vessel of sufficient size as dictated, is attached which is called the deaerator. This portion has internals something like a perforated tray to breakdown the down flow of condensate water from the top into fine globules to separate dissolved gases. The heating steam, which is fed at the lower level of the dome, passes upwards to give good intermixing. A small vent pipe at the topmost point of this dome is provided for venting out the dissolved gases. Some designs may have a vent condenser to trap and recover any water particles escaping through this vent. The typical vent rate is in the range of 0.1% to 0.25% of total deaerator capacity.
The deaerator dome therefore has connections for condensate water inlet (at one side of the dome near the top end) from previous LP feed heater and also a connection for the deaerating steam from the bottom of the dome (which also incidentally heats the feed water). This steam is generally from an extraction point of the turbine to improve the cycle efficiency. The deaerator therefore is also termed as one of the feedwater heaters in the turbine cycle. Flash steam can also be used here. Since the deaerator is always hot, sufficient insulation is provided to minimize the heat loss.
The feed tank is mounted horizontally at a sufficient height above the boiler feed pump to provide the necessary positive head (NPSH) to the boiler feed pump under all conditions of the system operation.
Controls and monitoring
Normally, all the control and monitoring equipment for startups, normal operation and alarms for out of parameter operations are provided at the operators' console.
Deaerator level and pressure must be controlled by adjusting control valves - the level by regulating condensate flow, and the pressure by regulating steam flow.
If operated properly, most deaerator vendors will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Deaerator". A list of authors is available in Wikipedia.|