In our previous articles, we have explained you the Water Cycle of thermal power plant. This Water cycle consists of Feed Water Cycle and Condensate Water Cycle. Condensate water cycle is from Condenser to the Deaerator and feed water cycle is from Deaerator to the Boiler Drum.
Impact of Dissolved gasses in Feed-water
It is an utmost necessary to remove the dissolved gases from the feed-water, as the presence of Dissolve Oxygen in the water leads to pitting corrosion in the high pressure pipes/metal tubes and the presence of CO2 gas reduces the pH level of the water by generating Carbonic Acid. So to avoid these two consequences, we are removing these gasses by the help of Deaerator.
Why Deaerator ?
Deaerator is one of the most effective & thermal efficient device to remove the dissolved gasses. It works on two principles; one is Hennery’s Law and the other one is Temperature & Gas Solubility Relation.
The mass of a dissolved gas in a given volume of solvent at equilibrium is proportional to the partial pressure of the gas.
As in the Deaerator, the pressure of the feed-water is low due to the surface distribution of the water through the nozzles in it. As the pressure of the fluid is reduced so the solubility of the gasses decreases.
Temperature & Gas Solubility Relation:
According to this relation, the solubility of the dissolved gasses in the water reduces as the temperature of the water increases. That is the reason we maintain high temperature of the feed-water and Due to the counter flow of Water and steam in the Deaerator, scrubbing action takes place and the steam carries away the gasses with it.
Working & Construction
Deaerator as the name signifies, it deaerate the air or remove the air from the condensate and feed water cycle. Generally Deaerator is placed after LP (Low Pressure) heaters and before BFP (Boiler Feed Pump).After LP heaters, the condensate water comes to the Deaerator. In the Deaerator tank, the water is sprinkled through the Nozzles to increase its surface area for better scrubbing by steam and to reduce the pressure of the fluid as the surface area has increased. Simultaneously the steam from the Turbine Extraction line is also supplied from the bottom of the deaerator for a counter flow arrangement of Steam and Condensate. Steam used for the de-aeration of feed water is called pegging steam and its pressure is around 7 kg/cm2
(Steam flow from below and water flow from upper nozzle)
After the removal of non-condensable and dissolved gases through the deaerator vent. This water is stored in a tank named FST (Feed Storage Tank). This feed storage tank is just below the Deaerator. Deaerator can have a dome or tank type structure in which various connection and pipes are arranged. As we have already explained that Inside Deaerator, there are spray nozzles and tray type structure is present. From this spray nozzle water is sprayed and this water converts in small droplets of water so the contact area of water is increased. From below the steam comes in the opposite or counter flow direction of water.it heats the water and condenses and by these non-condensable and dissolved gases out of the water and after that water is stored in the FST. Here the heat transfer is of direct contact type. So it is having higher efficiency than other heat exchangers. Below mentioned are few details of De-aerator in 135 MW Unit in terms of Flow, Temperature and Pressure.
Auxiliaries and Connections
Apart from the main FST and Deaerator, there are many connections made to Deaerator and FST tank. There are manholes provided in the Deaerator and FST for maintenance work. There is Safety Valve provided in the FST and Deaerator to provide safety from excessive pressure in the vessel. Also air venting provision is there for venting of dissolved gases out of the Deaerator. In Deaerator there is opening of water inlet coming from LP heaters. There is connection of steam in the Deaerator. The source of steam for Deaerator is varying from plant to plant but usually we take steam from Auxiliary Steam Header and from Cold Reheat Line (CRH) after HP turbine outlet. The steam that is coming for the heating of the feed-water in HP heaters gets condensed after heat transfer and after condensing it’s drip/drain comes to the Deaerator, we call it Cascading Drip. In FST there is also provision for filling of FST for maintain water level for the BFP (Boiler Feed Pump). There are also lines for BFP recirculation to maintain minimum recirculation at the time of starting of BFP. The water after FST outlet goes to the Booster Pump of BFP and there is also connection for the overflow and drain of water. When plant is not running there is also Nitrogen Filling Line connection in the FST for preservation of FST and Deaerator. In some plant there is also a provision of gland sealing leak off steam line to Deaerator.
During the initial stage when the plant is not running then we uses auxiliary steam from auxiliary steam header for initial heating of FST water and to pressurize it. After turbine rolling when the parameters (pressure, temperature & flow) have reached, we take steam directly from CRH line and extract steam for pressurizing the Deaerator and to remove dissolved gases from Deaerator.
In the air removing process we have to ensure total removal of air, non-condensable gases and dissolved gases from the system otherwise there may be chances of corrosion in feed water line for this we dose Hydrazine (N2H4). This hydrazine reacts with water and removes oxygen and make nitrogen gas which is inert in nature. This hydrazine dosing position varies from design to design. In some plants dosing of Hydrazine is done in FST and in some plants at the outlet of FST.
In some design there is also provision for the vent condenser. This vent condenser placed above the Deaerator. In this vent condenser upcoming feed water is heated before going to Deaerator by steam which leaving from Deaerator to venting. This serves two purposes; first it heats feed water before going to Deaerator and second it utilizes the left energy of steam which going out to atmosphere.
Commonly Deaerator is found at very high level in power plant because it gives the NPSH (Net Positive Suction Head) to booster pump and BFP. If this NPSH is not available then there is chance of churning and cavitation in BFP and there is a chance of damage of BFP material. This NPSH is nothing but head or pressure required at the suction of BFP otherwise there will be flashing of water or steaming of water. This phenomenon occurs when, the pressure at the suction side of inlet is below its vapour pressure and its temperature is high. To avoid this, NPSH available is always higher than NPSH required. Deaerator is placed above the BFP and there is provision of booster pump which give sufficient pressure to BFP for its smooth operation.
**P.S – Ingress of Oxygen and other dissolved gasses occur mainly through the Glands of CEPs and Drip Pumps. So necessary precautions should be taken for the Sealing.