Water is called the universal solvent. The purer the water, that is, the lower its dissolved solids content, the greater the tendency to dissolve its surroundings. Pure water, if stored in a stainless steel tank after a short contact time, has a very small amount of iron, chromium, and nickel from the tank dissolved in it. This dissolving of the tank does not continue indefinitely with the same water. The water, in a sense, has satisfied its appetite in a short time and does not dissolve any more metal. Pure water, if exposed to air, immediately absorbs air and has oxygen from the air dissolved in it. A glass of tap water at 68F contains 9.0 ppm of oxygen. Tap water heated to 77F contains 8.2 ppm of oxygen, and some oxygen is driven out of the water. The higher the temperature of the water, the less dissolved oxygen it can hold. Conversely, the higher the pressure imposed on the water, the greater the dissolved oxygen it can hold. Water, when boiled, produces steam. The steam contains some liquid water. There is never a perfect separation of pure steam from the boiling water. The steam above the boiling water always has entrained with it some boiling water. The foregoing three ideas: water is a universal solvent, water dissolves oxygen when in contact with air, and boiling water is always entrained with steam should help you understand the nature of feedwater.
The feedwater, as it enters the boiler steam drum, is now considered boiler water. Complete understanding of the nature of boiler water can be gained by temporarily making the assumption that no water treatment, chemical addition, or blowdown is applied to the boiler water. The character of the boiler water continually changes as the boiler steams. The dissolved and suspended solids, contained in the feedwater, concentrate in the boiler water at the rate of eightfold every hour if the boiler is producing steam at 50 percent of its normal capacity. Three damaging conditions arise in the boiler as the boiler water continues to steam wit bout treatment. Scale formation on the steam generating surfaces, corrosion of the boiler metal, and boiler water carry-over with the steam due to foaming are the three results of untreated boiler water.
To prevent scale formation on the internal water-contacted surfaces of a boiler and to prevent destruction of the boiler metal by corrosion, you must chemically treat feedwater and boiler water. This chemical treatment prolongs the useful life of the boiler and results in appreciable savings in fuel since maximum heat transfer is possible when no scale deposits occur.
The method of using chemicals may take the form of external treatment, internal treatment, or a combination of both. The principal difference between these forms of treatment is that in external treatment the raw water is changed or adjusted by chemical treatment outside of the boiler so a different type of feedwater is formed. In internal treatment, the water is treated inside the boiler by feeding chemicals into the boiler water, usually through the feed lines. Again, in external treatment the main chemical action takes place outside the boiler, while in internal treatment the chemical action takes place within the boiler.
At many Navy installations, the boilers are not large and do not operate at high pressure. When the makeup water is not too high in hardness or dissolved solids, good operation is possible with only internal treatment. Under this condition, external treating equipment is unnecessary. Chemical treatment covered in this chapter applies primarily to internal treatment.
When water evaporates in a boiler, the hard components that were in the water, such as calcium salts, magnesium salts, and other insoluble materials, form deposits on the tubes and other internal surfaces. These deposits are known as scale. Actually, the temperature of the water determines how well the different salts dissolve and how long they remain dissolved. Some salts are such that the hotter the water, the better they stay dissolved. Other salts stay dissolved while the water is at a relatively low temperature but formContinue Reading