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Boiler feedwater treatment- marine boiler requirement

The use of boiler feedwater
Modern high-pressure, high-temperature marine boilers with their large steam output require very pure feedwater. 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 scaleforming 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.

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