DEAERATOR IN THERMAL POWER PLANT

DEAERATOR IN THERMAL POWER PLANT 

TABLE OF CONTENT 

1:- Deaerator in thermal power plant

2:- Reason for use

3:- Deaerator working principle

4:- Working process 

5:- Types of deaerator

6:- Condition must be fulfilled for successfull operation of the deaerator

7:- Causes to increase concentration of oxygen and carbon dioxide 

8:- Some importan point related to deaerator

DEAERATORS IN A THERMAL POWER PLANT

1:- Deaerators is a large, insulated tank, pressure vessels.

2:- One type of feed water heater is a contact type open heater.

3:- Purpose of deaerating the feed water (removal of carbon dioxide and oxygen from feed water).

4:- Mechanical deaeration is coupled with other external treatment aspects (chemical process), it provides the highest quality feed water for boiler use.

5:- Raise the temperature of the water (heating cold water to protect the boiler from thermal shock and prolong the boiler life).

6:- Economizing overall operation with strong reduction in the chemical treatment costs.

7:- Utilize low pressure exhaust and flash steam (CBD flash steam).

8:- Cmmon collection point for high energy drips (HP heaters drips) & low temperature condensate water.

9:- Serving as a surge collection tank (FST) for process condensate return.

10- Blending condensate and steam.

11:- Storage section volume holding 5 to 15 min of treated hot water in reserve storage for load change (Store treated water in a reserve storage for load change).

12:- Providing a constant, pressurized supply of water & NPSH to boiler feed pumps.

13:- Increases efficiency from preheating feed water.

14:- Most deaerators are utilizing a low-pressure steam supply.

REASON FOR USES (WHY DEAERATOR USED)

1:- The make-up water storage vessel is contact with the atmosphere, so that dissolved gases can enter the boiler water here.

2:- Another source of dissolved gases is leakage into the condensate or through seals.

3:-  Water fed to the boilers consists of raw make-up water and condensate returns, most commonly a mixture of the two.

4:- Both contain oxygen and carbon dioxide in solution in varying amounts.

5:- Water used in high-pressure boilers must be very pure (other wise corrosion can rapidly destroy the materials of boiler).

6:-  Depending on the design and size of your plant, the amount of feed water required can be small or exceptionally large.

7:- Certain applications allow the steam condensate to be recycled back into the boiler and be used as a component of the feed water.

8:- Dissolved oxygen in feed water will cause corrosion damage in a boiler metals (form iron oxide).

9:- Used for removes oxygen and other dissolved gases from feed water (before it can be used in the boiler system).

10:- Dissolved carbon dioxide (dissolved in water resulting low pH value and the production of corrosive carbonic acid) combines with water to form carbonic acid that causes further corrosion.

11:- Low  pH levels in feed water cause acid attack on the boiler tubes.

12:- Dissolved gases and low pH levels in the feed water can be controlled or removed by the addition of chemicals.

13- The rest of the water comes in as makeup water.

14:- The amount of condensate returns, you produce the makeup water must be appropriately conditioned before it is processed as feed water.

15:- While oxygen and carbon dioxide could be removed by the addition of scavenging chemicals to the feed water alone, this practice is extremely costly.

16:- Althoug Deaerators are capable of reducing oxygen and carbon dioxide to very low levels, they should not be considered as a substitute for proper internal boiler feed water treatment. They will, however, greatly reduce the amount of "scavenging chemicals" which must be fed to maintain proper residuals with the boiler.

17- 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.

18:- It is more economical and thermally efficient to remove these gases mechanically.

19:- Dissolved gases are removed mechanically in deaerator, this mechanical process is known as deaeration.

DEAERATOR WORKING PRINCIPLE

Henry's Law

Gas solubility in a solution decreases as the gas partial pressure above the solution decreases (The solubility of the gas in a liquid is directly proportional to the partial pressure.)

Therefore if we decrease the partial pressure of the dissolved gas by adding steam in Deaerator, its solubility decreases and the gas is removed from water.

Inverse Solubility of Water

When the temperature of water is increased, the dissolved oxygen content in the water is decreases (become zero at saturation temperature).

So the water temperature is increased by adding steam in Deaerator, the dissolved gas solubility is decreased and the gases are removed from water.

Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature.

WORKING PROCESS

1:- Gases are removed in deaerator, where water is heated to saturation temperature by the steam extracted from the turbine or from CRH (extracted from partial work done).

2:- Water enters through the inlet water connection.

3:- The water is sprayed from the top so as to expose large surface area and bled steam is feed from the bottom and allowing it to become quickly heated to saturation.

4:- Spraying water in thin films increases the surface area of the liquid in contact with the steam, by contact the steam condensed and feed water is heated to the saturation temperature.

5:- The water temperature rises, which releases most of the dissolved gases in it including oxygen and carbon dioxide.

6:-  The results of solubility of a gas in water decreases as the water temperature increases and approaches saturation temperature more rapid oxygen removal and lower gas concentrations.

8:- This process reduces the solubility of all dissolved gases and removes them from the water

9:- Water heated (close to saturation temperature) with a minimum pressure drop and minimum vent.

10:- The liberated gases are then vented from the deaerator

11:-  After deaeration water is collected in to the FST below the deaerator.

12:- To neutralize the effect of residual dissolved oxygen and carbon dioxide gases in water, hydrazine is injected to the suction of the BFP.

TYPESOF DEAERATORS

There are two basic types of deaerators.

1:-:Spray & Tray Type

2:- Stork Spray-Type

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

2:- 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.

CONDITIONS MUST BE FULFILLED FOR SUCCESSFULOPERATION OF THE DEAERATOR

1:- The temperature of deaerated water must be equal to the saturation temperature of water corresponding to the pressure at which the deaerator operates.

2:- Sufficient heating steam must be delivered to the deaerator to ensure continuous boiling of water undergoing deaeration.

3:- The water charged to the deaerator must be break into fine droplets to ensure better heat transfer between the heating steam and water.

4:- Deaerator must be provided with sufficient venting to purge all the non-condensing gases out of the system and to ensure minimum partial pressures of these gases in the upper part of the deaerator

5:- Reduction of partial pressure will enhance desorption of dissolved gases

6:- If two or more deaerators are operated in parallel, all must be operated at same pressure.

CAUSES TO INCREASE CONCENTRATION OF OXYGEN AND CARBON DIOXIDE

1:-  Inadequate deaeratorvept teading to accumulation of non-condensing sases and increase in the partial pressures.

2:-  High water flow rate inadmissible at the given water temperature causing hydraulic impacts.

3:-  Frequent pressure drop in the deaerator

4:-  Faulty deaerator internal.

5:-Delivery of overheated condensate on to the tray.

6:- Delivery of relatively cold flows with higher oxygen content to deaerator.

IMPORTANT POINT RELATED TO DEAERATOR

1:-  Achfeving a water temperature of 225°F(107°C) minimum temperature required to remove dissolved oxygen and carbon dioxide from feed water  in the deaerator's storage section through mechanical evaporative scrubbing.

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

3:- Typically, a vent pipe in a deaerator's dome degassing chamber removes noncondensable gases.

4:- The addition of hydrazine chemically ensures a "zero" level of dissolved gas in the feedwater.