blowdown

Blowdown is the controlled discharge of liquid from a closed or semi-closed system to manage liquid quality, pressure, or operability. It is commonly used in power plants, boilers, cooling-water systems, and other process installations to remove accumulated impurities, dissolved solids, or sludge, and to purge lines during maintenance or commissioning. The term covers both routine continuous discharges and periodic drains.

In boiler and steam systems, blowdown refers specifically to removing water from the boiler to control the concentration of dissolved solids and to flush out impurities. Continuous or surface blowdown typically removes a small amount of water from the steam drum, while bottom blowdown drains water from the boiler bottom to remove settled sludge. The blowdown rate is chosen to balance water and energy costs against the risk of scaling, corrosion, and decreased heat transfer efficiency. Improper blowdown can lead to reduced efficiency, tube damage, or unexpected boiler shutdowns.

In cooling-water and other circulating systems, blowdown serves to control mineral buildup by discharging highly concentrated water and replacing it with make-up water. The rate depends on the cycles of concentration and the target total dissolved solids, aiming to prevent scaling and corrosion while minimizing water waste. Blowdown is also used in maintenance and purging of pressurized pipelines and vessels, where venting or draining through blowdown valves helps depressurize and purge contents before repairs. Safety and environmental considerations, such as proper discharge routing and treatment, are important in all blowdown applications.