Corrosion of iron or steel in boilers or boilers feed water piping is caused by three t hree fundamental factors
•
Feedwater Feedwa ter temperature temper ature
•
Feed water ph value
•
Feedwater Feedwa ter oxygen content conte nt
Why Deaeration
Deaerator Principles Deaeration is the mechanical removal of dissolved gases from the boiler feedwater. There are
principles that must be met in the design of any deaerator.
Deaerator Principles The incoming feedwater must be heated to the full saturation temperature, corresponding to the steam pressure maintained inside the deaerator . This will lower the solubility of the dissolved gases to zero.
Deaerator Principles The heated feedwater must be mechanically agitated. This is accomplished in a tray deaerator by first spraying the water in a thin film into a steam atmosphere. Creating a thin film reduces the distance the gas bubble has to travel to be released from the water. Next, the water is cascaded over a bank of slotted trays, further reducing the surface tension of the water. This allows for the removal of any gases not liberated by the initial spraying.
Deaerator Principles Adequate steam supply must be passed through the water, in both the spray section and the tray section to sweep out the gases from the water.
Using Henry’s law of partial pressures, the principle behind Deaeration can be explained
The quantity of a gas dissolved in a given quantity of liquid is directly proportional to its partial pressure surrounding the liquid. Therefore, by reducing the partial pressure of the unwanted gasses in the surrounding atmosphere, the gasses are diminished. These partial pressures are reduced by spraying the liquid into a counter current flow of steam.
The steam, which is free of non-condensable gasses, is the liquid’s new atmosphere and Henry’s law prevails. Using steam is advantageous in that the solubility of a gas in a liquid decreases with an increase in the temperature of that liquid. The liquid is sprayed in thin films in order to increase the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations.
With these principles in mind, Sterling Deaerator employs a two-stage system of heating and deaerating feedwater
This system reduces oxygen concentration to less than 0.007
ppm, and
completely eliminates the carbon dioxide concentration when tested by the APHA method. Testing for oxygen concentration shall be done in accordance with ASME
Performance Test Code 12.3.
R PERATIONS O T A R E A E D
Deaerator Operation -
The prime element in our vent condenser zone is the self-adjusting spray valve that allows incoming water, which is to be deaerated, to discharge as a thinwalled, hollow cone spray (See Figure IA).. Because steam flows countercurrent, intimate water to steam contact occurs with consequent latent heat transfer. As the falling water reaches the tray stack (tray type Deaerator), or the collection basin (spray type Deaerator), its temperature is within 2°F (1ºC) of the counterflowing saturated steam temperature.
Deaerator Operation -
Most of the dissolved oxygen and free carbon dioxide have been removed at this point. Since nearly all of the steam has been condensed, the non-condensable gasses and the small amount of “transport” steam exits through the vent piping
Deaerator Operation -
The partially deaerated water enters the tray stack at saturation temperature. The heated water flows down over the trays, zigzagging as shown in Figure IB through counter flow steam. This arrangement provides additional retention time to allow a final oxygen strip by the purest steam.
Deaerator Operation -
The two-stage tray deaeration technique is the most reliable method for meeting critical performance over a complete load range.
Deaerator Operation -
Water from the collection basin flows down the vertical down comer and into the scrubber section where it comes in contact with upcoming steam. Through carefully sized orifices, the steam and water violently mix, heating and removing the remaining gasses from the water. The mixture moves to the top of the scrubber housing and there the steam separates from the water and gasses and continues to flow up into the spray area and the vent condensing zone (Stage One of our Deaerator). See Figure I.
Deaerator Operation -
Water from the collection basin flows down the vertical down comer and into the scrubber section where it comes in contact with upcoming steam. Through carefully sized orifices, the steam and water violently mix, thus setting off heating and flashing of the water as it propels the mixture to the top of the scrubber housing. At this point steam separates from the mixture and continues to flow up into the vent condensing zone, or Stage One of our Deaerator. See Figure I.
Tray Type Deaerator
Spray Type Deaerator
10
10 12
9 6
11
7
7
2
1
9
8
5 6 6
4
4
3
3
1
Feedwater Storage Tank
4
Liner Plates
7
Globe Valve, Stud B/2N & Gasket
10
Vent Silencer
2
Deaerator
5
Platform & Ladder
8
Vent Orifice
11
Safety Relief Valve
Vent Silencer Support, Hex
Outlet Piping, Stud B/2N &
STERLING MODELS
Horizontal Type With No Storage Capacity. Spray/Tray Technology
Vertical Type With No Storage Capacity. Spray/Tray Technology
Dome Type With Horizontal Storage. Spray/Tray Technology
Vertical deaerator horizontal storage. Spray/Tray technology
Horizontal deaerator with horizontal storage. Spray/Tray type
Dome type with integral horizontal storage. Spray/scrubber technology
Horizontal type with integral horizontal storage. Spray/scrubber technology
Upright with integral storage. Spray/tray technology
