Deaerator

Deaerator plant

A deaerator is a device that removes oxygen and other dissolved gases from water, such as feedwater for steam-generating boilers. Dissolved oxygen in feedwater will cause serious corrosion damage in a boiler by attaching to the walls of metal piping and other equipment and forming oxides (rust). Dissolved carbon dioxide combines with water to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less, as well as essentially eliminating carbon dioxide.[1][2]

Deaerator Working Principle

Deaeration relies on the principle that the solubility of a gas in water decreases as the water temperature increases and approaches saturation temperature. In the deaerator, water is heated up to close to saturation temperature with a minimum pressure drop and minimum vent. Deaeration is done by spraying the feedwater to provide a large surface area, and may involve flow over multiple layers of trays. This scrubbing (or stripping) steam is fed to the bottom of the deaeration section of the deaerator. When steam contacts the feedwater, it heats it up to saturation temperature and dissolved gases are released from the feedwater and vented from the deaerator through the vent. The treated water falls to the storage tank below the deaerator.[3][1]

Types of deaerators

There are many different horizontal and vertical deaerators available from a number of manufacturers, and the actual construction details will vary from one manufacturer to another. Figures 1 and 2 are representative schematic diagrams that depict each of the two major types of deaerators.

There are two basic types of deaerators, the Termochimica spray&tray-type and the Stork spray-type:[2][4][5][6]

  • The Termochimica spray&tray-type (also called the cascade-type) includes a vertical or horizontal domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.
  • The Stork spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the storage tank for boiler feedwater.

Termochimica Spray&Tray-type Deaerator

Figure 1: A schematic diagram of a typical tray-type deaerator.

The typical spray&tray-type deaerator in Figure 1 has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical deaeration section through spray valves above the perforated trays and then flows downward through the perforations. Low-pressure deaeration steam enters below the perforated trays and flows upward through the perforations. Combined action of spray valves & trays guarantees very high performance (as confirmed by HEI std ) because of longer contact time between steam and water. Some designs use various types of packed bed, rather than perforated trays, to provide good contact and mixing between the steam and the boiler feed water.

The steam strips the dissolved gas from the boiler feedwater and exits via the vent valve at the top of the domed section. Should this vent valve not be opened sufficiently the deaerator will not work properly, causing high oxygen content in the feed water going to the boilers. Should the boiler not have an oxygen-content analyser, a high level in the boiler chlorides may indicate the vent valve not being far enough open. Some designs may include a vent condenser to trap and recover any water entrained in the vented gas. The vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small visible telltale plume of steam.

The deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam generating boiler system. Low-pressure heating steam, which enters the horizontal vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored boiler feedwater warm. External insulation of the vessel is typically provided to minimize heat loss.

Stork Spray-type deaerator

Figure 2: A schematic diagram of a typical spray-type deaerator.

As shown in Figure 2, the typical spray-type deaerator is a horizontal vessel which has a preheating section (E) and a deaeration section (F). The two sections are separated by a baffle (C). Low-pressure steam enters the vessel through a sparger in the bottom of the vessel.

The boiler feedwater is sprayed into section (E) where it is preheated by the rising steam from the sparger. The purpose of the feedwater spray nozzle (A) and the preheat section is to heat the boiler feedwater to its saturation temperature to facilitate stripping out the dissolved gases in the following deaeration section.

The preheated feedwater then flows into the deaeration section (F), where it is deaerated by the steam rising from the sparger system. The gases stripped out of the water exit via the vent at the top of the vessel. Again, some designs may include a vent condenser to trap and recover any water entrained in the vented gas. Also again, the vent line usually includes a valve and just enough steam is allowed to escape with the vented gases to provide a small and visible telltale plume of steam.

The deaerated boiler feedwater is pumped from the bottom of the vessel to the steam generating boiler system.

Deaeration steam

The deaerators in the steam generating systems of most thermal power plants use low pressure steam obtained from an extraction point in their steam turbine system. However, the steam generators in many large industrial facilities such as petroleum refineries may use whatever low-pressure steam is available.

Oxygen scavengers

Oxygen scavenging chemicals are very often added to the deaerated boiler feedwater to remove any last traces of oxygen that were not removed by the deaerator. The type of chemical added depends on whether the location uses a volatile or non-volatile water treatment program. Most lower pressure systems (<650psi) use a non-volatile program. Most higher pressure systems (>650psi) and all systems where certain highly alloyed materials are present, are now using volatile programs as the old phosphate-based programs are phased out. Volatile programs are further broken down into oxidizing or reducing programs [(AVT(O) or AVT(R)] depending whether the waterside environment requires an oxidizing or reducing environment to reduce the incidence of flow-accelerated corrosion (FAC) which is a highly debated topic within the industry today. FAC-related failures have caused numerous accidents in which significant loss of property and life has occurred.

The most commonly used oxygen scavenger for lower pressure systems is sodium sulfite (Na2SO3). It is very effective and rapidly reacts with traces of oxygen to form sodium sulfate (Na2SO4) which is non-scaling. Another widely used oxygen scavenger is hydrazine (N2H4), used for locations using volatile programs.

Other scavengers include 1,3-diaminourea (also known as carbohydrazide), diethylhydroxylamine (DEHA), nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), and hydroquinone.

See also

References

  1. 1 2 "The Deaerating Principle". Sterling Deaerator Company.
  2. 1 2 "Deaerators". Stork.
  3. ↑ "Deaerator working principle". Boilers Info.
  4. ↑ Robert Thurston Kent (Editor in Chief) (1936). Kents’ Mechanical Engineers’ Handbook (Eleventh edition (Two volumes) ed.). John Wiley & Sons (Wiley Engineering Handbook Series).
  5. ↑ Babcock & Wilcox Co. (2005). Steam: Its Generation and Use (41st ed.). ISBN 0-9634570-0-4.
  6. ↑ Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering (2nd ed.). McGraw-Hill Professional. ISBN 0-07-019435-1.
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