UFC 3-240-13FN
25 May 2005
3-2
BOILER WATER TREATMENT AND CONTROL. References to boiler
water treatment in the late nineteenth century relate that the process of removing the
scale from a boiler required much less effort and time if the boiler operator had forgotten
to remove cooked potatoes from the boiler water. It was determined that the starch in
potatoes causes a soft sludge, rather than a hard scale, to be formed, and this sludge
was easier to remove than hard scale. Other natural organics, including lignin and
tannins from wood, plant matter, plant extracts, and even manure and coffee grounds,
produced similar results. It was determined that addition of phosphate by itself produces
a manageable sludge, although starch, lignins, and tannins have been used to
supplement the phosphate. Phosphate is still very commonly combined with other water
treatment chemicals. Early use of alkaline materials included lime, soda ash, and
caustic soda. Oxygen scavengers, such as sodium sulfite, were found to be effective for
preventing oxygen corrosion. Many of these materials are still used today. The wide
range of water treatment chemicals that is currently available allows for development of
a comprehensive approach to industrial boiler water treatment, including using specialty
chemicals such as chelants, polymers, and amines.
3-2.1
Boiler Deposit Formation. Dissolved solids in boiler feed water become
more concentrated in the boiler water as steam is generated. Some of the dissolved
solids can come out of solution (precipitate) and form scale in the boiler tubes. Some
dissolved solids can form sludge (mud) in the boiler and form adherent deposits on
boiler tubes. These deposits reduce heat transfer.
3-2.1.1
illustrated in Figure 3-10. When a steam bubble forms on a heated surface, a thin film of
water situated between the bubble and the tube wall becomes more concentrated with
the dissolved materials. This thin layer can be as much as 17 oC (30 oF) hotter than the
average boiler water temperature. These local conditions can cause precipitation of the
dissolved solids and local formation of scale.
47
Previous Page
