Boiler blowdown

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.

Water is a capable solvent, and will dissolve small amounts of solids from piping and containers including the boiler.

Precipitation would be expected to occur in the form of scale deposits on those heat exchange surfaces.

Scale deposits thermally insulate heat exchange surfaces initially decreasing the rate of steam generation, and potentially causing boiler metals to reach failure temperatures.

Similar blowdown connections at the bottom of water wall headers are blown down less frequently.

Shorter blowdown events cause less significant changes in boiler water level, and are safer during periods of high steam demand.

Bottom blowdown piping must be of a diameter large enough to minimize the risk of being plugged with baked sludge.

Surface blowdown is removed from the steam drum at the top of the boiler and bottom blowdown is removed from the mud drum at the bottom of the boiler.
Steam locomotive boiler blowdown.