Crystalline scale

Crystalline scales and other minerals may form deposits that reduce the effectiveness of the boiler as a heat transfer device or cause fouling that can impede the flow of water and cause overheating by reducing the level of furnace cooling below design specifications. The presence of process contaminants also leads to fouling. 

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. 

Although the presence of hardness is reported as calcium carbonate, in reality, for most water supplies the most common major contributors to total dissolved solids are calcium and magnesium bicarbonates. These dissolved solids most readily produce crystalline scales and thus predominantly contribute to boiler system deposits unless removed by some form of pre-boiler, external treatment process. 

The precipitation of calcium carbonate in the boiler involves a number of discrete processes, including nucleation, followed by the formation of microcrystals which compete to grow into larger crystals (accretion) that eventually form layers of dense, crystalline scale on the heat exchange surface. 

At pH levels above 9.0, there is an increased tendency for the soluble silica to form silicate anions and react with magnesium (and to a lesser extent with calcium) to form insoluble forms of magnesium silicate that precipitate to form crystalline scales and sludge. 

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

Sodium cycle softening (base-exchange softening) is used primarily to remove the risk of calcium- and magnesium-based crystalline scale formation and deposition. The general reaction is as shown in Figure 9.2. 

All designs claim to treat the water for control of crystalline scales. Some designs claim to also control corrosion. There does not appear to be any claims to nonchemically treat water so as to... 

These condensed tannins and their derivatives, all of high molecular weight, function as anionic polyelectrolyte sludge conditioners, tending to sequester hardness salts and hinder their precipitation as crystalline scales. In addition, when precipitation does occur, the condensed tannins coagulate the particles, resulting in a mobile sludge that can be easily blown down. 

The use of electric powered steam generators, steam-to-steam heat exchangers, or reboilers to provide steam to steamtables, humidifiers, autoclaves, and for direct injection into the process. Where this type of approach is employed, it generally is necessary to provide a high quality water source to avoid rapid internal deposition of crystalline scales and foulants. 

Any of a number of crystalline or non-crystalline scales and other insoluble materials laid down in a boiler system by a variety of mechanisms. Usually on a heat-transfer surface. 

Any sedimentary deposit or foulant that fails to form a crystalline scale. Often the result of supersaturation or the binding of biological or other organic material with dust, sand, or other mineral deposits. Also, sludge is not always deposited at point of origin and can additionally bake onto heat transfer surfaces. 

Accdg to Davis (Ref 2), Fourcroy, by digesting red oxide of mercury (HgO) in ammonia water for 8 to 10 days, prepd a material which became white and finally assumed the form of crystalline scales. The dried product exploded loudly from fire, but underwent spontaneous de-compn when left alone. At slightly elevated temp it gave off ammonia and left a residue of mercury oxide... 

A current of nitrous acid, as disengaged by the -deoxidation of nitric acid when heated in contact with starch, through pure oleic acid at a low temperature, and avoiding the use of an excess of acid, The1 liquid after a short time oongaals into a crystalline mass. This 1b washed with hct water to remove nitric acid, and then dissolved in its own volume of hot alcohol, On cooling, elaidic acid is deposited as a mass of crystalline scales, whioh may be purified by pressure, re solution, -and recrystallization, from alcohol ... 

Meconic acid forms beautiful pearly crystalline scales, of a harsh sour taste, sparingly soluble in cold, but more readily In hot water and in alcohol. The anhydrous acid consists of Clt nOa, and is denoted by the. symbol Me the acid dried at 212° contains in addi-... 

Sodium Vanadite, Na2V409.4H2O, forms golden-brown needles, and is prepared similarly to the corresponding potassium salt.6 Brown, crystalline scales of a heptahydrate, Na2V409.7H20, have also been reported.7... 

If the tetrahydrate is kept for some time in a superfused state, the (3, d)-hydrate is formed as a hard white crystalline scale,5 and this hydrate may also be obtained 6 by evaporating a solution of arsenic acid in an open vessel at temperatures from 50° to 100° C. or under increased pressure at 150° C. 

Diamminotrimethylplatinic iodide, (CH3)3PtI(NH3)2, consisting of white crystalline scales, occurs when trimethylplatinic iodide is heated with a mixture of benzene, alcohol and concentrated ammonia on a water-bath, then evaporated to dryness. It yields free ammonia on heating with potassium hydroxide, is slightly soluble in water, moderately soluble in benzene or ether, and easily soluble in alcohol, ethyl acetate or acetone. 

White crystalline scales M.P. 113° odourless possesses bitter taste a valuable analgesic and antipyretic. (D.R.P., 69883, 26429.)... 

Figure 6. Heat of fusion (A W). in J/g of the polymer matrix, of the silane-modified composites against the number of silane layers. The crystallinity scale on the right-hand side was obtained by assumine AH for a polyethylene crystal to be 290 J/g.

Each of the dissolved minerals in any given source of makeup water has a potential for causing difficulties to a greater or lesser extent. For most water supplies, the commonest and highest levels of dissolved solids are the temporary hardness salts, calcium and magnesium bicarbonates. These are also the dissolved solids that most readily produce crystalline scales and thus contribute to cooling system deposits. Bicarbonates and carbonates provide the total alkalinity (M alkalinity) in most cooling waters, and normally there is no free hydroxide alkalinity. 

All cooling water treatment programs, whether designed in-house or via a water treatment service company, continue to focus on the minimization of hard water crystalline scales and sludges in the system as a major criteria for success. Program techniques employed are either pretreatment processes, such as lime-soda softening or ion exchange, the use of sulfuric acid or polymer-based chemicals that operate in an alkaline environment, or combinations of some or all of these processes. 

The limit of solubility is typically around 150 to 175 ppm. This limit should not be exceeded in a cooling system otherwise a glassy, crystalline scale can be laid down, which is an extremely efficient barrier to heat transfer and requires special and expensive techniques for its removal. 

The principal reason, however, why sulfuric acid is used as part of many cooling water pretreatment programs (as when dosed to the makeup line) or treatment programs (as when dosed to the tower basin) is to produce salts that are more soluble than calcium carbonate. Thus the point at which calcium salt saturation is reached is extended and the risk of fouling and crystalline scale on heat transfer surfaces is minimized. 

Deposition involves the formation and precipitation of crystalline scales and the throwing down, within critical parts of the cooling system, of silt, sand, muds, and sediments all these deposit components have the effect of reducing the rate of heat transfer. Corrosion can also occur under deposits. 

This term refers to the crystalline growth of a variety of insoluble salts or metal oxides, which are usually deposited as a hard adherent layer, or layers, on heat-transfer surfaces. This is extremely undesirable because all crystalline scales reduce the rate of heat transfer and therefore seriously reduce the efficiency of the overall cooling system. 

Fig. 4.2 Calcium carbonate deposits can occur as layer-upon-layer of scale inside a pipe or on a metal surface, reducing both the flow of water and heat-transfer rates. The deposition is often a combination of both amorphous scale and crystalline scale (as depicted in the sketch)...

Analysis of most crystalline scale deposits and mineral foulants taken from the waterside of cooling systems shows, not unexpectedly, considerable variation in composition. The analysis normally reveals the presence of mixtures of several salts and metal oxides, originally derived from water impurities or corrosion processes. 

As part of the overall scale-inhibiting threshold effect, the mechanism of crystal growth retardation is complemented by crystal distortion, whereby the crystal fails to successfully agglomerate into a larger crystalline scale. 

These properties are still desired today in modem water treatment formulations. The phosphonates and newer organic polymers exhibit some or all of these effects, but modern inhibitors have considerably more hydrolytic stability, show a greater effectiveness in these properties, and have extended abilities, beyond that of controlling calcium carbonate crystalline scales. 

The deposition of crystalline scales, air-borne contaminants, biofilms, etc. tends to be higher on previously fouled or corroded surfaces than on clean surfaces. Also, deposition and fouling affect both the rates and mechanisms of corrosion on a clean metal surface. It can therefore be useful to obtain subjective information on deposition and fouling tendencies in a cooling system, provided that the methods are simple and produce results quickly. The use of blank coupons inserted in a bypass corrosion rack can often provide this support information. 

If the sample is a hard crystalline scale taken from a heat exchanger, some heating of the deposit in a test tube may be necessary before fizzing occurs. 

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