In any thermal power plant, there are mainly three cycles i.e. Steam Cycle, Condensate cycle and Feed water cycle. We have already discussed the steam cycle in thermal power plant and now we will discuss about the Condensate cycle & Feed-water cycle. These two cycles are very important in efficiency point of view. As we know Regeneration (water preheating before sending it to boiler) increases the efficiency of our plant. If we send preheated water to boiler drum then lesser heating of water is required in the drum which decreases the heat-rate and the fuel consumption will be reduced and it will be very economical for the plant.
1.The Condensate cycle includes the flow of water from Condenser to Deaerator. The equipments involved are Condenser, CEP (Condensate Extraction Pump), Gland steam cooler, LP heaters (Low Pressure Heaters).
2.The feed water cycle includes Deaerator, FST (Feed Storage Tank), Booster Pump, BFP (boiler Feed Pump).
So basically after the turbine, the steam goes to the condenser where this steam condenses and water formation takes place. This amount of water is less in comparison to steam which condenses because we know that the specific volume of water is much lesser than the steam, because of that only vacuum is generated in the condenser. We take cooling water from outside to cool that steam in a shell & tube type heat exchanger. The steam after condensing becomes water and goes to the hot-well for storage. This hot-well is just below the condenser for the storage of water/condensate. Then to maintain the flow and send this water to boiler we use a pump called Condensate extraction pump (CEP). After this CEP (condensate extraction pump) the water goes to Gland steam cooler, where gland steam (used for end side sealing of turbine) comes and this steam is used to heat that water which we call Condensate water. After gland steam cooler that condensate water goes to LP heaters. The LP (Low Pressure Heaters) heaters are also shell & tube type of heat exchangers. The Number of LP heaters is varying from plant to plant and also depends upon the capacity and requirement of the plant. For the Heating of condensate water in LP heaters, we take steam extractions from turbines. Mostly for the LP heaters we take the steam extraction from LP turbine stages. The steam used to heat condensate water get condensed after the heat exchange and becomes water, we call it Drip.
We send this drip to condenser again through Flash tank or directly. In condensate cycle, after LP heaters this condensate water goes to Deaerator. In Deaerator, we again heat that water, this is a direct type of heat exchanger, and in this we heat the water as well as remove the dissolved gasses, Non-condensable gases and oxygen if any in condensate cycle. That’s why we give it a name, called Deaerator because it de-aerates the dissolved gasses from the condensate. In Deaerator, we give steam from IP turbine extraction for heating and that condensed steam after heating the water become condensate and goes to a tank which is just below the Deaerator called FST (feed storage tank). This tank stores the water which goes to boiler. In this tank we supply chemical like; hydrazine for removing the dissolved oxygen in our water cycle otherwise there will be a chance of internal corrosion of tubes in our water cycle, which will deteriorate or damage the equipment or its performance.
After this the Feed water cycle is started which includes the water path from Deaerator to Boiler drum. This cycle includes Deaerator, Feed storage tank (FST), Boiler feed pump (BFP), HP heaters (high pressure heater), and FRS (feed Regulating Station) to boiler drum. After FST this feed-water goes to boiler feed pump. This boiler feed pump include booster pump, motor, hydraulic coupling and a main pump. Here booster pump take water from FST tank. This booster pump boost or increase the pressure of water and send it to main pump. This booster pump is also important in the point of NPSH (net positive suction head). In pump we require minimum NPSH otherwise there is a chance of churning and cavitation phenomenon. That cavitation damage our BFP (boiler feed pump). That`s why initially we give recirculation from BFP to FST to remove the cavitation phenomenon and insure minimum flow through pump. If we not maintain that minimum flow through pump then heating start in pump and water droplets start change its state from water to vapor, and when this vapor pocket burst then it also remove some part of material of pump which damage the pump. This phenomenon is called cavitation. To restrict the cavitation phenomenon, height factor between FST and BFP is also important.
After this, the BFP sends the feed water to drum with high pressure. This outlet pressure we require must be above the drum pressure for feeding the feed water in boiler. Then this feed water goes to HP heaters (high pressure heater). Like LP heaters, the number of HP heaters is also depended on the requirement and capacity of plant. For HP heaters in order to heat the feed water we take steam extraction from HP turbine stages, CRH (cold reheat line) or IP turbine stages as per the requirement. This heat exchanger is also shell & tube type of heat exchanger. After exchanging heat in heater, this steam condensed and this water (drip) goes to Deaerator. After the heaters, this feed water goes to FRS (feed regulating station). This feed regulating station design also change plant to plant. But there in FRS minimum two rows of valve line is there.one is for 100% and other is for lesser flow (flow at the time of starting and shutdown). This FRS station regulates the flow according to requirement in boiler drum through control valve. After this FRS station feed water directly goes to drum.