DAERATOR

DAERATOR

A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feedwater to steam-generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic 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.

There are two basic types of deaerators, the tray-type and the spray-type:

    The tray-type (also called the cascade-type) includes a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.

    The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.

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.

   Tray type daerator

The typical horizontal 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 above the perforated trays and flows downward through the perforations. Low-pressure deaeration steam enters below the perforated trays and flows upward through the perforations. 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 and 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.

   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 syste

     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 go away. 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 properly diluted and for locations using volatile programs is hydrazine (N2H4).

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