CHAPTER 39 BOILER WATER TREATMENT

1 CHAPTER 39 BOILER WATER TREATMENTOf the many uses for energy in the United States today in industry, in trans-portation, in homes and commercial buildings the largest portion of total use isdirected toward producing steam through the combustion of fossil fuels. Utilitiesaccount for the greatest share of this, but industrial plants also produce enormousquantities of steam for process uses, often generating electric power through tur-bines as a by-product (Cogeneration).The TREATMENT of WATER for steam generation is one of the most sophisticatedbranches of WATER chemistry.

2 An understanding of the fundamentals of boilerwater chemistry is essential to the power engineer who continually strives toincrease the efficiency of the boilers and steam-using pressure and design of a BOILER determine the quality of WATER it requiresfor steam generation. Municipal or plant WATER of good quality for domestic useis seldom good enough for BOILER feed WATER . These sources of makeup are nearlyalways treated to reduce contaminants to acceptable levels; in addition, correctivechemicals are added to the treated WATER to counteract any adverse effects of theremaining trace contaminants.

3 The sequence of TREATMENT depends on the typeand concentration of contaminants found in the WATER supply and the desiredquality of the finished WATER to avoid the three major BOILER system problems deposits, corrosion, and , particularly scale, can form on any WATER -washed equipment surface especially on BOILER tubes as the equilibrium conditions in the WATER contactingthese surfaces are upset by an external force, such as heat. Each contaminant hasan established solubility in WATER and will precipitate when it has been the WATER is in contact with a hot surface and the solubility of the contaminantis lower at higher temperatures, the precipitate will form on the surface, causingscale.

4 The most common components of BOILER deposits are calcium phosphate,calcium carbonate (in low-pressure boilers), magnesium hydroxide, magnesiumsilicate, various forms of iron oxide, silica adsorbed on the previously mentionedprecipitates, and alumina (see Table ). If phosphate salts are used to treat theboiler WATER , calcium will preferentially precipitate as the phosphate before pre-cipitating as the carbonate, and calcium phosphate becomes the most prominentfeature of the the high temperatures found in a BOILER , deposits are a serious problem,causing poor heat transfer and a potential for BOILER tube failure.

5 In low-pressureboilers with low heat transfer rates, deposits may build up to a point where theycompletely occlude the BOILER modern intermediate and higher pressure boilers with heat transfer rates inexcess of 200,000 Btu/ft2/n (5000 cal/m2/hr), the presence of even extremely thindeposits will cause a serious elevation in the temperature of tube metal. Thedeposit coating retards the flow of heat from the furnace gases into the boilerwater. This heat resistance results in a rapid rise in metal temperature to the pointat which failure can occur.

6 The action that takes place in the blistering of a tubeby deposit buildup is illustrated by Figure For simplification, no tempera-Section A Section B Section CFIG. Temperature profile across clean tube and tube having a Expected Composition of BOILER SludgeConstituentCalcium carbonateCalcium phosphateCalcium silicateCalcium sulfateCalcium hydroxideLoss on ignitionMagnesium phosphateMagnesium hydroxideMagnesium silicateSilicaAluminaOilIron oxideSodium saltsCopperOther metalsCoagulation-typetreatmentHighUsual ly less than 15%Usually less than 3%NoneNoneUsually less than 5%NoneModerateModerateUsually less than 10%Less than 10%NoneUsually less than 5%Usually less than residual

7 TreatmentUsually less than 5%HighTrace or noneNoneNoneUsually 8-12% except higher invery pure feed watersUsually less than 5% except insome high-pressure boilersModerateModerateUsually less than 10%Usually less than 10%NoneUsually less than 5% except inhigh-purity feed watersUsually less than lowLowTube wallScale drops through gas or WATER films have been shown. Section A shows a crosssection of the tube metal with a completely deposit-free heating surface. There isa temperature drop across the tube metal from the outside metal (T2) to the metalin contact with BOILER WATER (T1).

8 Section B illustrates this same tube after thedevelopment of a heat-insulating deposit layer. In addition to the temperaturedrop from T2 to T1, there would be an additional temperature drop through thedeposit layer from T1 to T0. This condition would, of course, result in a lowerboiler WATER temperature T0. However, BOILER WATER temperature is fixed by theoperating pressure, and operating conditions require that the same BOILER watertemperature be maintained as before the development of the deposit layer.

9 Sec-tion C illustrates the condition that actually develops. Starting at the base boilerwater temperature of T0, the increase through the scale layer is represented by theline from T0 to T3. The further temperature increase through the tube wall is rep-resented by the line from T3 to T4. The outside metal temperature T4 is now con-siderably higher than the temperature T2, which was the outside metal tempera-ture prior to the formation of deposit on the tube surfaces. If continued depositiontakes place, increasing the thickness of the heat-insulating deposits, furtherincreases will take place in the tube metal temperature until the safe maximumtemperature of the tube metal is exceeded.

10 Usually this maximum temperature is900 to 100O0F (480 to 54O0C). At higher heat transfer rates, and in high-pressureboilers, the problem is more severe: at temperatures in the 900 to 135O0F (482 to7320C) range, carbon steel begins to deteriorate. Figure shows the normalstructure of carbon steel BOILER tubes, and Figure illustrates the spheroidiza-tion of carbon and successive changes in structure, which begin to take placeabove 80O0F (4270C), weakening the metal. Temperatures within the BOILER fur-nace are considerably above this critical temperature range.