Scales and Deposits

As bacteria often attach themselves to pipe walls or under scale, culture of deposits or scrapings can frequently detect bacterial contamination, when culture of water samples has provided inconclusive results, but there is field evidence of bacterial problems. 

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Scale and deposits are controlled through the use of phosphates, chelants, and polymers. Phosphates are precipitating treatments, and chelants are solubilizing treatments. Polymers are most widely used to disperse particulates but they are also used to solubilize contaminants under certain conditions. 

For most HP boilers, the sublayer film temperature gradient is more important than the bulk boiler water chemistry in determining the risk of deposition of salts. If a local overconcentration of salts occurs, waterside scaling and deposition are the inevitable results. 

Hard crystalline scale and deposition also may result from the interaction of silica, residual hardness, and inorganic coagulants carried over into the treated water. Also, pre-boiler system damage by erosion may occur. 

Silica and many of the metals commonly found in scales and deposits can form various permutations of oxides and salts, depending on the specific boiler area location and operating circumstances. Each mineral possesses distinct characteristics and may exert different effects. 

Thus, although the analysis of scales and deposits typically is carried out by a combination of wet chemistry techniques and atomic absorption spectroscopy, an identification of the specific nature of the crys-... 

Also, basic factors such as the transport of materials, residual hardness, ion leakage, soluble iron, colloidal silica and clays, and other contaminants, which can produce scales and deposits in the FW lines and other parts of the pre-boiler section, may also produce similar detrimental effects in the boiler section. In the boiler itself, however, the buildup rate may be quicker and the results may be more devastating. 

Although any given source of water typically has a wide range of dissolved minerals present, and each of these has a potential for causing difficulties to a greater or lesser extent, it is the alkaline earth salts (.hardness salts) that are always present to some degree and generally are the most troublesome in a boiler. This section discusses these salts, their presence in natural makeup (MU) water sources, and their contribution to hardness scales and deposition in boiler plants. 

Magnesium salts exhibit properties similar to those of calcium, except that they generally are slightly more soluble and normally are present in MU water sources at a lower level of concentration, so that the overall scaling and deposition effect of magnesium salts is lessened. 

Apart from calcium and magnesium bicarbonates, most natural sources of MU water commonly contain some small amounts of silica and other dissolved minerals, salts, and contaminants. Under a wide variety of operational circumstances, every one of these common materials may contribute to complex boiler scales and deposits, especially the silicates. Thus, it is necessary to ensure that water chemistries are properly balanced and controlled. 

When sulfates, carbonates, and other dissolved BW salts exceed their individual maximum solubility limits, they form sludges, scales, and deposits. This situation may arise either from a general overconcentration of the BW TDS (high COC) or from the deliberate precipitation of salts of selective ions, as occurs when using phosphate precipitation programs. 

As a general rule, boiler section deposits initially are often soft and porous but are prone to become harder and more adherent as they age. Thus, the presence of hard, dense scales and deposits in a boiler indicates that deposition has been occurring for a considerable time. The deposition of scales often produces a layered effect similar to the rings of a tree, indicating periodic changes in the water chemistry and perhaps providing some clues as to the history of the deposits. 

Sulfate, chlorides, and phosphates are common contaminants and contribute to boiler scales and deposits. 

Table 7.4 Summary notes silica and silicate crystalline scales and deposits affecting boiler section waterside surfaces.

In addition, the surface tension of the water molecules passing through the chamber is reduced, which allows the water to absorb greater quantities of these scale- and deposit-forming minerals and carry them away in suspension. As a result, scale that has been formed within a system will gradually return to solution and be carried away with the water flow. ... 

Scale and deposit control. This is primarily control over the deposition of hardness salts and other scale-forming minerals. 

Within these two extremes comes a wide range of cleaning programs that generally must be tailored specifically to the individual boilers and boiler components in question, after taking into consideration the types and volume of scales and deposits or corrosion products that must be removed. 

Carbonate in boilers usually is present as a hard, dense, white to tan or brown calcite scale (CaC03). A tan to brown color usually indicates the presence of iron. Samples of scales and deposits normally fizz when tested with concentrated muriatic acid (hydrochloric acid, HC1) if carbonate is present, although some preliminary heating may be required. 

Most chemical treatment programs have not in the past specifically focused on controlling silica levels in cooling water, and as a consequence almost all analyses of scales and deposits taken from the waterside of cooling systems, especially from heat exchangers, have shown the presence of small percentages of silica. Research over the last five years or so has led to the introduction of silica-specific deposit control polymers and has also led, with some success, to the reevaluation and promotion of some established calcium carbonate polymer products for effective silica control. 

Additional applications of complexing agents include removal of mineral scales and deposits from industrial processes, oil reservoirs, and vapor generation that often become sources of in-plant production problems as well as safety concerns. 

Amersperse. [Drew Ind. Div.] Evapora-tor/vacuum pan scale and deposit inhibitor. 

Belros. [Ciba-Geigy] Scale and deposit contrd additive. 

Scaleclean F. [Stewart H ] Scale and deposit inhibitor for potable water systems. 

Protection of cooling system metals involves complex interrelated problems of scale and deposit prevention, control of microbiological growths and other fouling sources, in addition to corrosion mitigation by inhibitors. Corrosion control programs may be expected to vary considerably between systems, since each installation presents different environmental factors and operating problems which affect response. 

Uses Scale and deposit control agent for water treatment, soil removal, antiredeposition aids, water conditioners in detergents and cleaners... 

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