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. 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.

2 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. 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.

3 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. 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.

4 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. 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 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

5 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). 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. Sec-tion C illustrates the condition that actually develops.

6 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. 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.

7 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. WATER circulatingFIG. Normal structure of low-carbonboiler steel. The dark is formed by an alterna-tion of cementite (Fe3C) and ferrite platelets,collectively called pearlite, in the larger matrixof light colored Above 80O0F, the carbon begins toform spheroids and grain growth psi280,000 Btu/ft2/hrTube metal temperature increase, 0 FDeposit thickness, Deposits on the WATER -side of a BOILER tube insulatethe metal from the cooling effect of WATER flow.

8 The metal on thefireside may then become overheated. (Adapted from Jacklin,C.: "Deposits in Boilers,"Ind. Eng. Chem., May 1954.)FIG. A typical tube failure caused may be scale, precipitated in situ on a heated surface, or previouslyprecipitated chemicals, often in the form of sludge. These drop out of WATER inlow-velocity areas, compacting to form a dense agglomerate similar to scale, butretaining the features of the original precipitates. In the operation of most indus-trial boilers, it is seldom possible to avoid formation of some type of precipitateat some time. There are almost always some particulates in the circulating boilerwater which can deposit in low-velocity sections, such as the mud drum. Theexception would be high-purity systems, such as utility boilers, which remain rela-tively free of particulates except under conditions where the system may becometemporarily second major WATER -related BOILER problem is corrosion, the most commonexample being the attack of steel by oxygen.

9 This occurs in WATER supply systems,preboiler systems, boilers, condensate return lines, and in virtually any portionof the steam cycle where oxygen is present. Oxygen attack is accelerated by hightemperature and by low pH. A less prevalent type of corrosion is alkali attack,which may occur in high-pressure boilers where caustic can concentrate in a localarea of steam bubble formation because of the presence of porous feed WATER TREATMENT chemicals, such as chelants, if not properly applied,can corrode feed WATER piping, control valves, and even the BOILER the elimination of oxygen from BOILER feed WATER is the major step incontrolling BOILER corrosion, corrosion can still occur. An example is the directattack by steam of the BOILER steel surface at elevated temperatures, according tothe following reaction:4H2O + 3Fe - Fe3O4 + 4H2t (1)This attack can occur at steam-blanketed BOILER surfaces where restricted boilerwater flow causes overheating.

10 It may also occur in superheater tubes subjectedthrough the tubes normally conducts heat away from the metal, preventing thetube from reaching this range. Deposits insulate the tube, reducing the rate atwhich this heat can be removed (Figure ); this leads to overheating and even-tual tube failure (Figure ). If the deposit is not thick enough to cause such afailure, it can still cause a substantial loss in efficiency and disruption of the heattransfer load in other sections of the This instrument is used to monitor corrosion-pro-duced hydrogen concentrations in salts, such as silica and sodium compounds; or it may be caused by foam-ing. Carryover is most often a mechanical problem, and the chemicals found inthe steam are those originally present in the BOILER WATER , plus the volatile com-ponents that distill from the BOILER even in the absence of are three basic means for keeping these major problems under External TREATMENT : TREATMENT of WATER makeup, condensate, or both,before it enters the BOILER , to reduce or eliminate chemicals (such as hardnessor silica), gases or Internal TREATMENT : TREATMENT of the BOILER feed WATER , BOILER WATER , steam,or condensate with corrective Slowdown.