Every boiler plant has a deaerator that generally gets little or no attention until it “burps,” jeopardizing the entire steam system. These incidents can be expensive to correct and affect the integrity of other critical auxiliary equipment as well as the careers of those charged with technical oversight of utility systems.

Deaerators are large, insulated tank-car-shaped pressure vessels. They produce preheated boiler feedwater to help ensure efficient steam-plant operation. Deaerators come in numerous sizes and shapes, depending on boiler-plant design, capacity, and system pressure rating. The units are produced in spray (Figure 1), tray, spray-tray, vacuum, and atmospheric pressure designs.

Benefits

Deaerators provide many benefits to a steam-plant system, including:

• Serving as a surge collection tank for process condensate return.

• Blending hot return condensate and cold makeup water.

• Preheating feedwater uniformly to improve boiler efficiency.

• Removing oxygen (O₂) and carbon dioxide (CO₂) from feedwater to mitigate corrosion.

• Holding 13 to 15 min of treated hot water in reserve storage for load change.

• Providing a location for feeding and blending chemicals and testing protocol.

• Providing a constant, pressurized supply of water to boiler feed pumps.

• Providing a convenient location for instrument controls and feedwater test data.

• Helping to ensure precise drum water level and pressure inside of a boiler.

Most deaerators are American Society of Mechanical Engineers- (ASME-) certified pressure vessels that operate at 5 to 15 psig by utilizing a low-pressure steam supply. A safety valve must be installed to ASME International Boiler and Pressure Vessel Code requirements. Quality pressure and temperature sensors are essential to ensuring precise pressure control and achieving a water temperature of 225°F (the minimum temperature required to remove dissolved O₂ and CO₂ from feedwater) in the deaerator's storage section through mechanical evaporative scrubbing.

At this elevated temperature, feedwater pumps, piping, and economizers are subject to severe O₂ corrosion attacks even though only a minute level of O₂ is dissolved in the water. When properly operating, a deaerator should be able to scrub O₂ to 5 to 10 ppb residual.

Typically, a ½-in. vent pipe in a deaerator's stainless-steel-dome degassing chamber removes noncondensable gases. A steam plume of 18 to 24 in. should be visible at the end of the vent pipe.

The addition of sulfite or hydrazine chemically ensures a “zero” level of dissolved gas in the feedwater. (The chemical-reaction equation is: 2 Na₂SO₃ + O₂ = 2 Na₂SO₄, which represents a soluble salt removed in blowdown.) A ¼-in. stainless-steel pipe should be used to distribute the liquid sulfite or hydrazine inside the deaerator storage section about 6 to 12 in. below the normal water level (NOWL). Daily testing of boiler blowdown water should confirm a sulfite residual of 25 to 40 ppm, ensuring zero O₂ residual in the feedwater system (Figure 2).