Chapter 3 — Waterside Care and Requirements
Part No. 750-184
3-13
substances. Regardless of their high efficiency, some solids will be
present in the boiler feedwater.
Solids become less soluble in the high temperature of the boiler
water and tend to accumulate on heating surfaces. Therefore
blowdown and internal chemical treatment are required to prevent
the solids from forming harmful scale and sludge.
Scale has a low heat transfer value and acts as an insulation barrier.
Scale retards heat transfer, which not only results in lower operating
efficiency, and consequently higher fuel consumption, but more
importantly, can cause overheating of boiler metal. Over heating of
boiler metal can result in tube failures or other pressure vessel metal
damage and lead to boiler down-time 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 the 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 thickened with the solids. If the
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 the concentration requires the use of
boiler water blowdown.
1. TYPES OF BLOWDOWN
There are two principal types of blowdown: intermittent manual
blowdown, and continuous blowdown.
Intermittent 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 in order to lower the dissolved solids in the pressure
vessel water, and to remove a por tion of the sludge that
accumulates in the lower part of the vessel.
Equipment generally consists of a quick opening valve and a shut-
off valve. The valves and 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.
Continuous Blowdown
Continuous blowdown is used in conjunction with a surface blow-
off tapping and is the continuous removal of concentrated water.
The surface blow-off opening, when furnished, is on the top center
line of the pressure vessel. It is provided with an internal collecting
pipe terminating slightly below the working water level for the
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