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A Deaerator is a device for air removal. In industry it is used to remove dissolved gases (an alternate would be the use of water treatment chemicals) from water to make it non-corrosive.
With reference to a thermal power station, the word deaerator generally implies not only the deaerator but also the feed water tank below where deaerated water is stored and fed to the suction of boiler feed pumps. The description herein is mainly with reference to its use in thermal power stations. See also feedwater heating.
Necessity for deaerationEdit
Practical considerations demand that in a steam boiler/steam turbine/generator unit 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 in contact thereby thinning them and causing rupture. The solids will deposit on the heating surfaces giving rise to localised heating and tube ruptures due to overheating. Under some conditions it may give rise to stress corrosion cracking.
Position in the turbine cycleEdit
The above diagram shows the deaerator's position in the turbine cycle diagram.
The diagram shows the construction of a typical deaerator and feed tank of about 250 MW unit.
The actual construction details may vary from manufacturer to manufacturer. It also depends on the size of the unit and their own design to suit the system including the steel structures erection.
This is generally a horizontally mounted cylindrical steel vessel with dished ends 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 cold start of the unit, it is possible 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 to expel the dissolved gases. For this, a provision of a heater pipe inside the tank longitudinally and at the bottom level is provided. A few vertical pipes on this line are provided with holes to distribute the heating steam uniformly to avoid water hammer in the initial stages of heating. For this normally a connection from auxiliary steam header is provided, since the auxiliary steam is available first after startup of the boiler.
A small bore connection with a pipe line to the full length of the feed tank at the bottom is also provided for injection of chemical liquids.
Generally a direct reading gauge glass is provided on each end for absolute level indication.
Since the feed tank is always hot, sufficient insulation covering (known also as lagging) is provided to minimize the heat loss.
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 of smaller sizes may have a vent condenser to trap and recover any water particles escaping through this vent.
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.
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 boiler feed pump level to give the necessary positive head (NPSH) to the boiler feed pumps under all conditions of the system operation. The mounting arrangement is such that one end of the dished end is able to move or expand due to hot boiler feed water storage where as the other end is fixed.
The moving end is supported on steel rollers to give it frictionless movement whereas the other end is bolted to the girder support underneath.
Controls and monitoringEdit
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 generally guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).
Kernt’s Mechanical Handbook, Design, Shop Practice. Eleventh edition, rewritten by Robert Thurston Kent. M.E., Editor in Chief, Wiley Engineering Handbook Series. New York; John Wiley & Sons, Inc; London: Chapman & Hall Ltd
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