750-96 (revised 2010)
Model CB Packaged Boiler Manual
2-11
2.8 — Blowdown: Steam Boiler
overheating of boiler metal. This can result in tube failures or other pressure vessel metal damage causing boiler
downtime and costly repairs.
Scale is caused primarily by calcium and magnesium salts, silica, and oil. Any calcium and magnesium salts in the
boiler water are generally precipitated by the use of sodium phosphate, along with organic materials, to maintain
these precipitates or “sludge” in a fluid form. The solids such as sodium salts and suspended dirt do not readily
form scale, but as the boiler water boils off as relatively pure steam, the remaining water is thicker with the solids.
If this concentration is permitted to accumulate, foaming and priming will occur and the sludge can cause harmful
deposits that bring about overheating of the metal.
The lowering or removal of this concentration requires the use of boiler water blowdown.
There are two principal types of blowdown: Intermittent manual blowdown and continuous blowdown.
2.8.1 — Manual Blowdown
Manual or sludge blowdown is necessary for the operation of the boiler regardless of whether or not continuous
blowdown is employed.
The blowdown tappings are located at the bottom or lowest part of the boiler so that in addition to lowering the
dissolved solids in the pressure vessel water, it also removes a portion of the sludge which accumulates in the
lower part of the vessel.
Equipment generally consists of a quick opening valve and a shutoff valve. These, along with the necessary piping,
are not normally furnished with the boiler, but supplied by others. All piping must be to a safe point of discharge.
Piping must be properly supported and free to expand.
2.8.2 — Continuous Blowdown
Continuous blowdown is used in conjunction with a surface blow-off tapping (furnished on units 60” in diameter or
larger) and is the continuous removal of concentrated water.
The surface blow-off opening, when furnished is on top center line of the pressure vessel and is provided with an
internal collecting pipe terminating slightly below the working water level for the purpose of skimming surface sed-
iment, oil, or other impurities from the surface of the pressure vessel water.
A controlled orifice valve is used to allow a continual, yet controlled, flow of concentrated water.
Periodic adjustments are made to the valve setting to increase or decrease the amount of blowdown in accor-
dance with the test analysis.
The flow control valve and piping are generally provided by others. All piping must be to a safe point of discharge.
2.8.3 — Frequency of Manual Blowdown
When continuous blowdown is utilized, manual blowdown is primarily used to remove suspended solids or sludge.
The continuous blowdown removes sediment and oil from the surface of the water along with a prescribed amount
of dissolved solids.
When surface or continuous blowdown is not utilized, manual blowdown is used to control the dissolved or sus-
pended solids in addition to the sludge.
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