United Brass Bottom Blowdown Valves:

Boiler blowdown is water intentionally wasted from a boiler to avoid concentration of impurities during continuing evaporation of steam. The water is blown out of the boiler with some force by steam pressure within the boiler. Bottom blowdown used with early boilers caused abrupt downward adjustment of boiler water level and was customarily expelled downward to avoid the safety hazard of showering hot water on nearby individuals.

Some boiler water treatments cause precipitation of impurities as insoluble particles anticipating those particles will settle to the bottom of the boiler before they become entrained in water circulating past the heat exchange surfaces. These water treatments often include compounds forming a sludge to entrap such particles; and boilers intended for such water treatment include a structure called a mud drum at the lowest part of the boiler. Bottom blowdown involves periodically opening valves in the mud drum to allow boiler pressure to force accumulated sludge out of the boiler. Similar blowdown connections at the bottom of water wall headers are blown down less frequently. Several short blowdown events remove sludge more effectively than a single continuous blowdown. Shorter blowdown events cause less significant changes in boiler water level, and are safer during periods of high steam demand.

Bottom blowdown piping drains the lowest parts of the boiler so it can be used to drain the boiler for servicing. Bottom blowdown piping must be of a diameter large enough to minimize the risk of being plugged with baked sludge. Modern boilers discharge bottom blowdown to a blowoff tank where the blowdown can flash and vent steam upwards without entraining water which might cause burns. A pipe near the bottom of the blowoff tank maintains a water level below the blowdown entry point and allows cooler water remaining from earlier blowdown events to drain from the tank first.