As the name suggests the BlowDown Valve is used to blow off the hot water which contains both soluble and insoluble solids. This valve helps in maintaining the level of dissolved solids in the boiler water. Now, if we focus on impurities, some of them are already present in the water, the time it was fed to the boiler for steam generation.
So, now the point to be considered is the Boiler feed water quality, some boiler rooms have a water softener unit installed in the feed water line but that doesn’t affect the TDS level that much. Also when the water comes in contact with the surfaces of Boiler tubes and drum plates, corrosion occurred adds up in magnifying the impurities present. Carry over in the steam must be removed as it may hamper the process’s output or even damage it after a period of time.
Only a De-mineralization plant can maintain the desired TDS levels of feed water. Otherwise the impurities present in the feed water leads to scaling of tube’s surface and this layer of scaling has an insulating characteristics which reduces the heat transfer from tubes to the water present outside or vice versa. Also because of this scaling, Boiler consumes more fuel for desired pressure levels.
So, these impurities must be removed from time to time but keeping in mind the blowdown calculations. With every blowdown, there is a significant reduction in the Boiler pressure, so to increase the pressure again to the desired level, lots of fuel gets wasted. Thus blowdown calculations help in determining the number of blowdowns required per day. These calculations depend completely upon water quality being fed to the Boiler. Quick removal of impurities from the Boiler ensures long run operation of the Boiler. This process though manual but can be automated too, the instrumentation involved works on the levels of TDS levels present in the water.