Page 109 - Introduction to Marine Engineering

96 Boilers

Common impurities
Various amounts of different metal salts are to be found in water. These
include the chlorides, sulphates and bicarbonates of calcium, magne-
sium and, to some extent, sulphur. These dissolved salts in water make
up what is called the 'hardness' of the water. Calcium and magnesium
salts are the main causes of hardness,
The bicarbonates of calcium arid magnesium are decomposed by heat
and come out of solution as scale-forming carbonates. These alkaline
salts are known as 'temporary hardness'. The chlorides, sulphates and
nitrates are not decomposed by boiling and are known as 'permanent
hardness*. Total hardness is the sum of temporary and permanent
hardness and gives a measure of the scale-forming salts present in the
boiler feedwater.

Feedwater treatment
Feedwater treatment deals with the various scale and corrosion causing
salts and entrained gases by suitable chemical treatment. This is achieved
as follows:

1. By keeping the hardness salts in a suspension in the solution to
prevent scale formation.
2. By stopping any suspended salts and impurities from sticking to the
heat transfer surfaces.
3. By providing anti-foam protection to stop water carry-over.
4. By eliminating dissolved gases and providing some degree of
alkalinity which will prevent corrosion.

The actual treatment involves adding various chemicals into the
feedwater system and then testing samples of boiler water with a test kit.
The test kit is usually supplied by the treatment chemical manufacturer
with simple instructions for its use.
For auxiliary boilers the chemicals added might be lime (calcium
hydroxide) and soda (sodium carbonate). Alternatively caustic soda
(sodium hydroxide) may be used on its own.
For high-pressure watertube boilers various phosphate salts are used,
such as trisodium phosphate, disodium phosphate and sodium
metaphosphate. Coagulants are also used which combine the scale-
forming salts into a sludge and stop it sticking to the boiler surfaces.
Sodium aluminate, starch and tannin are used as coagulants. Final
de-aeration of the boiler water is achieved by chemicals, such as
hydrazine, which combine with any oxygen present.