The Formation of Scale

Water has a tendency to dissolve everything it contacts. Some materials have the limit of their solnbility set, primarily, by the temperature of the water and the concentration of other materials 

Bringing together barium ions (Ba ) and sulfate ions (804 ) results in the formation of barium sulphate, which has an extremely low solubility in water. The barium sulfate, consequently, precipitates from the water as crystals that we call scale. 

The precipitation of solid material that may form scale will occur when the temperature, composition, and pressure of the water changes to produce a solubility limit that is lower than the present concentration of the solid, and when ions react with one another to form a new material that has a lower solubility than the ions in solution. 

Scale frequently deposits in the oil formation near the well bore, in the perforations, or even on the face of the formation. Scale can form over the inlet ports of a rod pump or a Reda pump, starving them of fluid and possibly causing the Reda to get hot and bum out. Scale can form in the pump itself, even though the velocity of fluid movement is high. Fire tubes in aU types of heaters fail prematurely when scale formation results in overheating. Corrosion is often more severe under a scale deposit. 

Because of these problems, scale control should be of primary concern in the production of oil and the injection of water. 

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Pretreatment Feed water is pretreated to remove gross objects that could plug the stack. Additives that inhibit the formation of scale, frequently acid, may be introduced into the feed. 

Silica SiOj Results in the formation of scale in boilers and cooling water systems, can produce insoluble scale on turbine blades due to silica vapori2ation in high pressure boilers (usuallu over 600 psi). 

In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly ehosen. Thus, the addition of normal phosphates to an unsoftened feed water ean eause a dangerous eondition by elogging the makeup line with preeipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the eombination of ealcium or magnesium with negative ions of soap stearates. 

Magnetic devices A magnet(s) is fixed onto, or plumbed into a system, along the parallel axis of water flowing in a pipe. It is claimed that, with careful sizing and fitting, these devices inhibit the formation of scale. 

The formation of calcium carbonate (CaCOs), calcium sulfate, and barium sulfate scales in brine may create problems with permeability. Therefore it is advantageous that newly made fractures have a scale inhibitor in place in the fracture to help prevent the formation of scale. Formulations of hydraulic fracturing fluids containing a scale inhibitor have been described in the literature [1828]. 

The total deionisation of mineral water can be achieved by a combination of a cation exchanger resin and an anion exchanger resin. This has found application in the deionisation of boiler waters to avoid the formation of scales of the dissolved salts on the walls of boilers and pipes. This is also used in the deionisation of water to be used in domestic steam irons. 

Temperature Level. As the formation of scale will be influenced by the kinetics... 

A pipe with accumulated mineral deposits, (left) The lengthwise section shows the formation of scale, (right) The cross section clearly indicates the reduction in pipe capacity. 

In the original conventional manufacturing process, the crystallization from aqueous solutions tended to produce needles with diameter of 10 p,m or less. The crystals were very fine with large specific volume. These characteristics led to many problems in the filtration and drying processes, the formation of scale on reactor surfaces, and the high dustability and hygroscopicity of the final product, which made it difficult to handle and unsuitable for use as a direct (i.e. table top) sweetener (Ajinomoto 1983 Kishimoto et al. 1989). 

To evaluate the feasibility of forming these higher melting silicates and minimizing the formation of scale in the MS-FBC system, limestone (CaC03)was added to the DSS feed in Run 216 to form devritite (MP=1030 C) the reaction is ... 

The corrosion resistance of stainless steels and nickel-based alloys in aqueous solutions can often be increased by addition of chromium or aluminum. " Chromium protects the base metal from corrosion by forming an oxide layer at the surface. Chromium is also considered to be an important alloying metal for steels in MCFC applications. Chromium containing stainless steel, however, leads to the induced loss of electrolyte. Previous studies done to characterize the corrosion behavior of chromium in MCFC conditions have shown the formation of several lithium chromium oxides by reaction with the electrolyte. This corrosion process also results in increased ohmic loss because of the formation of scales on the steel. Aluminum additions similarly have a positive effect on corrosion resistance. " However, corrosion scales formed in aluminum containing alloys show low conductivity leading to a significant ohmic polarization loss. 

Hardness caused by bicarbonates is called temporary because it disappears on boiling. It is a contributory factor in the formation of scale in boilers, which is a deposit, amongst other things, of calcium carbonate and magnesium hydroxide. The latter is formed by the decomposition, in the boiler, of magnesium carbonate ... 

In selecting metals and alloys as materials of construction, one must have knowledge of how materials fail, for example is, how they corrode, become brittle with low-temperature operation, or degrade as a result of operating at high temperatures. Corrosion, embrittlement, and other degradation mechanisms such as creep will be described in terms of their threshold values. Transient or upset operating conditions are common causes of failure. Examples include start-ups and shutdowns, loss of coolant, the formation of dew point water, and hot spots due to the formation of scale deposits on heat transfer surfaces. Identification and documentation of all anticipated upset and transient conditions are required. 

A common area of concern in the SO2 scrubbing field is the formation of scale in the system. This was not a major problem at Scholz, although some scale formation took place in the reactors and in the scrubber. 

The formation of scale on the tubes of an evaporator adds a thermal resistance equivalent to a fouling factor. 

The dramatic inhibitory effect of even trace amounts of PMA on the precipitation of calcium carbonate has long been recognised [249] and it has thus found considerable commercial application as an inhibitor of the formation of scale in hard water systems [251]. However, the mechanism by which this species inhibits the precipitation reaction remains unclear. Furthermore, to our knowledge, there have been no kinetic or morphological studies on the effect of polymaleic acid on the dissolution process. 

One of the most serious problems encountered in the recovery of fresh water from sea water (and other saline waters) is the formation of scale within the equipment used in some of the processes. The scale consists of insoluble compounds of calcium, magnesium, iron, and other metals which are gradually deposited on the vessel walls. After a time, the scale formation builds up to a point where the operation must be interrupted so that the deposit may be removed. This is costly in both labor and loss of production. 

The temperature and flow conditions within the heat exchanger will determine the location at which these various stages occur. For instance the supersaturation and crystallite formation may occur in the bulk fluid with the growing crystals moving towards the wall to form the deposit. The movement of foulant will under these circumstances, follow the processes described in Chapter 7 for particulate deposition. It is possible that due to the level of turbulence within the system, that some (or possibly many) of the crystallites formed are swept into re ons where the solution is not supersaturated. Under these conditions the particles will redissolve. On the other hand crystallisation may occur near or at the heat transfer surface. The presence of nucleation sites on a solid surface may encourage the formation of scale on the surface. Under these circumstances the process is largely governed by the mechanics of the crystallisation process. 

The three forms of sodium phosphate have somewhat different uses. Monobasic sodium phosphate is used as a food additive to maintain proper acidity and in baking powders, as a food supplement to provide the phosphorus needed in a person s daily diet, in the treatment of boiler water to reduce the formation of scale on the inner surface of the boiler, and as a feed supplement for cattle and other farm animals. 

On heating a solution of calcium/magnesium bicarbonates, carbon dioxide is evolved and calcium/magnesium carbonates precipitate (this mechanism accounts for the formation of scale in kettles and boilers in hard water areas). 

It is the cause of temporary hardness of water, because the calcium ions react with soap to give scum. Calcium hydrogencarbonate is unstable when heated and decomposes to give solid calcium carbonate. This explains why temporary hardness is removed by boiling and the formation of scale in kettles and boilers. 

Web question—answer will vary depending on site fiaund. The key idea is that calcium carbonate is less soluble in water at higher temperatures, leading to the formation of scale in equipment used to boil or heat water. 

Most evaporators employ steam-heated, tubular heating surfaces, over which the liquid to be vaporized is circulated. The efficiency of an evaporator may be reduced considerably by the formation of scale on the heating surface, which may be removed by chemical or mechanical means. Another problem is the formation of foam which may seriously restrict the capacity of an evaporator, but this may be controlled by the use of special chemical defoaming agents or gas or air jets. 

Removal of scale deposits can be accomplished either chemically or mechanically, but treatments are expensive and often ineffective. For this reason, prophylactic treatment to inhibit the formation of scale deposits is an attractive approach, especially... 

Figure 4.40 shows the quantity of metal lost due to corrosion as a function of exposure time from a steel pipe filled with water containing sulfate ions. The mass loss is not a linear function of time because the corrosion rate decreases due to the formation of scale on the pipe wall. On the other hand, the higher corrosion rate observed at higher Reynolds number indicates that mass transport plays a critical role. 

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