Deaerators are mechanical devices that remove dissolved gases from boiler feed-water. Deaeration protects the steam system from the effects of corrosive gases. It accomplishes this by reducing the concentration of dissolved oxygen and carbon dioxide to a level where corrosion is minimised. A dissolved oxygen level of 5 parts per billion (ppb) or lower is needed to prevent corrosion in most high pressure ( >13.79 barg) boi lers. While oxygen concentrations of up to 43 ppb may be tolerated in low pressure boilers, equipment life is extended at little or no cost by limiting the oxygen concentration to 5 ppb. Dissolved carbon dioxide is essentially completely removed by the deaerator.
The design of an effective deaeration system depends upon the amount of gases to be removed and the final gas (O2) concentration desired. This in turn depends upon the ratio of boiler feed- water makeup to returned condensate and the operating pressure of the deaerator.
Deaerators use steam to heat the water to the full saturation temperature corresponding to the steam pressure in the deaerator and to scrub out and carry away dissolved gases. Steam flow may be parallel, cross, or counter to the water flow. The deaerator consists of a deaeration section, a storage tank, and a vent. In the deaeration section, steam bubbles through the water, both heating and agitating it. Steam i s cooled by incoming water and condensed at the vent condenser. Non-condensable gases and some steam are released through the vent. However, this should be optimised to provide satisfactory stripping, with minimised steam loss ( see Operational data, below).
Sudden increases in free or 'flash' steam can cause a spike in deaerator vessel pressure, resulting in re-oxygenation of the feed-water. A dedicated pressure regulating valve should be provided to maintain the deaerator at a constant pressure.
Applicable to all sites with deaerators on steam systems. Optimisation is an ongoing maintenance measure.
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Energy Efficiency (2009) 3.2.8