Deposits in Boilers

Deposits in boilers can be considered in two major categories sludge and scale. The usual way to tell the difference between them is by the nature of their adherence. Scale is commonly thought of as being tightly adherent to the metal, while sludge may be dispersed in the boiler water, can be spread on the metal surface, from which it is easily removable, or else it can possibly serve as a binding 

Oil contamination of feed water causes a sludge that adheres to the boiler walls and is difficult to remove. The formation of sludge balls can be encountered when the binder is a corrosion inhibitor, a paint residue, a fuel oil, or a lubricant. These sludge balls can become very large under some turbulent conditions. Severe attack by sludges may result on carbon steel and even on Monel. 

A chemical analysis of the scales wiU only identify the chemical composition, so, for positive identification of the crystalline nature of consituents. X-ray diffraction must be employed. Table 7.1 shows scale constituents of deposits from high operating-pressure boilers that have been identified by X-ray diffraction. 

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Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Ash fusion characteristics are important in ash deposition in boilers. Ash deposition occurring on the furnace walls is termed slagging, whereas accumulation on the superheater and other tubes is termed fouling. A variety of empirical indexes have been developed (60,61) to relate fouling and slagging to the ash chemical composition through parameters such as acidic and basic oxides content, sodium, calcium and magnesium, and sulfur. 

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. 

Copper (Cu) is deposited in boilers either by direct exchange with iron or by the hydrogen reduction of copper oxide during the corrosion of steel. 

Fouling and deposition in boiler sections may occur unintentionally through the application or misapplication of chemical water treatment programs. Internal treatment-related boiler section problems may... 

Hydrogen embrittlement is commonly associated with problems of excessive waterside deposition in boilers operating at over 1000 psig. Where HE takes place, the primary form of corrosion generally is either ... 

High levels of silica in the raw water supply can lead to serious risks of deposition in boilers, especially if cycles of concentration (COC) also are high. The incoming silica can be reduced by adsorption on magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes, or by the addition of magnesium hydroxide in a reaction tank, followed by filtration. 

When large amounts of copper deposits in boiler mbes cannot be removed with hydrochloric acid due to the relative insolubility of copper, ammonia-based oxidizing compounds have been effective. Used in a single separate stage, the ammonia sodium bromate step includes the introduction into the boiler system of solutions containing ammonium bromate to rapidly oxidize and dissolve the copper. 

Reid, W. T. External Corrosion and Deposits in Boilers and Gas Turbines, American Elsevier, N.Y. (1971). 

It is also soluble in acid solutions containing thiourea, which stabilizes Cu1 as a complex acid thiourea solutions are also used to dissolve copper deposits in boilers. 

Figure 10. Influence of slagging In lower furnace half on predicted surface temperatures of deposits in boiler combustion chamber fired with coal at a rate of 590 MWf...

Alexander, P.A., Fielder, R.S., Jackson, P. and Raask, E., 1960, An air cooled probe for measuring acid deposition in boiler flue gases. J. Inst. Fuel, 33, 31-37. 

Because silica is volatile (Si(0H)4 from Reaction R42 in Table 4.1) at high temperatures in high-pressure steam, it is now excluded from catalysts for steam reforming [85] [389], unless it is combined with alkali. For the same reason, silica-free materials are applied for the brick-lined exit gas collector and in autothermal reformers. Silica would be slowly removed from the catalyst (or brickwork) and deposited in boilers, heat exchangers and catalytic reactors downstream of the reformer. 

Use is being made of polymers alone in controlling deposits in boilers up to ISOOlb/in. The treatment solubilizes Ca "", Mg" " , Al" " " and maintains silica in solution. It removes scales from boilers. If polymers are to be used, oxygen must be strictly controlled. The effectiveness of a polymer is determined by its molecular weight and concentration. For example, polyacrylic acid (molecular weight = 20 000) addition reduces scale formation of only 52% compared to polymaleic acid (molecular weight = 5000) which reduces scale formation by 97%. Some typical polymers used are polycarboxylic acid, polymethacrylic acid, styrene and maleic add. 

Water that contains certain salts, such as those of calcium or magnesium, which form insoluble deposits in boilers and form precipitates with soap. 

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