Figure 1-30. - General laboratory equipment.
complete treatment of feedwater must be considered. The ideal water for boilers does not form scale or deposits. does not pit feedwater systems and boiler surfaces, and does not generate appreciable CO2 in steam. However. such raw makeup water is impossible to get in the natural state from wells or surface sources. Does the advantage of treatment make up for the cost of treatment?
Feedwater of 20- to 25-ppm hardness as calcium carbonate (CaCO3 ) need not be treated externally to reduce hardness if enough alkalinity is present to precipitate the hardness in the boiler as CaCO3 , or if hardness reducers, such as phosphates, are introduced to combine with and precipitate the hardness. Precipitation of this hardness in a low- or medium-pressure boiler generally does not cause wasteful blowdown. When the mixture ofcondensate and makeup in a medium-pressure steam plant has a hardness greater than 20 to 25 ppm as CaCO3 , the hardness should be reduced to a level of 0 to 2 ppm as CaCO3 .
Feedwater of a hardness in excess of 2 ppm as CaCO3 should be treated to bring it within the range of 0 to 2 ppm as CaCO3 . This small remaining hardness can be precipitated in the boiler by secondary treatment and removed by continuous blowoff equipment.
The test for hardness, as presented here, uses the calorimetric titration method. This test is based on finding the total calcium and magnesium content of a sample by titration with a sequestering agent in the presence of an organic dye sensitive to calcium and magnesium ions. The end point is a color change from red to blue, which occurs when all the calcium and magnesium ions are separated.
The following equipment is used for the hardness test:
One 25-ml buret, automatic, complete
One 210-ml casserole, porcelain
One 50-ml cylinder, graduated
One stirring rod, glass
The reagents for the test are as follows:
Hardness indicatorContinue Reading