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Flash tank in power plant is designed to collect condensate from boiler drain line and generate flash steam, which could be used for feedwater deaeration by thermal process.
Boiling point of liquids will go up or down as the pressure increases or decreases. When water is heated under atmospheric pressure, 100℃ is the highest temperature it could be. More heat can’t increase the temperature of water, but only to turn water into steam. Heat that is absorbed by water before boiling point is called sensible heat. Heat that turns water into steam is called latent heat. If pressure goes higher, boiling point will rise. If pressure goes down, boiling point will drop, and some sensible heat will be released. Such heat is absorbed as latent heat, and some water is vaporized into flash steam.
Temperature of condensate entering flash tank is higher than its boiling point under the low pressure inside tank. Some water will flash into steam, and then gets separated. This flash steam can be used as heat source for deaerator or heat exchanger.
Boiler steam blowdown tank is designed to dispose blowdown water at saturated temperature from boiler. When this hot and pressured water enters low pressure flash tank, it will boil again. Part of the water will form flash steam, while the rest is cooled down and reduced to atmospheric pressure. Flash steam can be released through vent on the top of flash tank, or utilized for heat recovery.
To make boiler run safely and efficiently, feedwater is strictly processed by chemical or physical methods to maintain its specified quality parameters, but it is not enough. To avoid concentration of impurities during boiler operation, blowdown water is discharged by two ways, and replaced with feedwater:
Continuous surface blowdown: Blowdown water is continuously removed from steam drum on the top of boiler at a low speed. Normally a pipe will be put 80mm below the water level, where salt concentrate is the highest level due to water evaporation into steam. Dirty water is removed continuously through this pipe to prevent scale buildup ensure high steam quality.
Intermittent bottom blowdown: Blowdown water is removed from the lowest header pipe or mud drum of boiler to clean dissolved solids and sediments that accumulate and settle down here. It is done once per shift. It is better to do intermittent bottom blowdown when boiler is running at a low load, and water level in steam drum should be 30-50mm higher than normal.
Parameter of flash tank for continuous surface blowdown:
Parameter of flash tank for intermittent bottom blowdown:
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