As steam leaves, dissolved solids stay behind and concentrate. Surface or continuous blowdown bleeds water from just below the waterline to hold total dissolved solids in range, while bottom blowdown opens valves at the lowest point in short bursts to dump sludge. Bottom blowdown lines carry a quick-opening valve next to the boiler and a slow-opening valve after it. Water columns, gauge glasses and low-water cutoffs get blown down too.
Bottom blowdown on two, open the quick-opening first, then the slow one, a few seconds, and close them in reverse.
Never leave a boiler during bottom blowdown; a valve that sticks open can drop the water below the glass in a minute.
Also heard as
- blow down
- BD
- boiler blowdown
Related terms
TDS
Total dissolved solids: the concentration of dissolved minerals and salts in boiler water, held in range by blowdown.
Cycles of concentration
How many times more concentrated dissolved solids are in the boiler water than in the feedwater.
Scale
A hard mineral crust that forms on boiler heat transfer surfaces from calcium, magnesium and silica in the water.
Water column
A vertical chamber piped to a steam boiler's steam and water spaces that carries the gauge glass and level controls.