TECHNIFAX Selective silica carryover - Steam Forum

1 NALCO CHEMICAL COMPANY ONE NALCO CENTER NAPERVILLE, ILLINOIS 60563-1198 SUBSIDIARIES AND AFFILIATES IN PRINCIPAL LOCATIONS AROUND THE WORLDR egistered Trademarks of Nalco Chemical Company 1970, 1997 Nalco Chemical CompanyAll Rights Reserved Printed in 6-97TF-5 TECHNIFAX silica CARRYOVERCAUSES TURBINEDEPOSITSMost boilers making Steam forturbines rarely have excessivecarryover of boiler water in thesteam. Good operating practices,improved Steam separators, andproper chemical control mini-mize this. silica deposits inturbines, however, can occureven when boiler watercarryover is negligible. Thereason: Steam selectively picksup silica from the boiler water,dissolves it, and carries it to theturbines, where it show that the keyto minimizing silica carryover isin keeping the boiler water silicacontent below certain levels, theconcentration depending onoperating SOLUBILITYR esearch on the solubility ofsilica in water and Steam showsseveral important points.

2 Steam is a solvent for silica For any particular steamdensity and temperature,there is a definite saturationsolubility of silicaSelective silica carryoverFigure 1 silica distribution ratioFigure 2 Maximum boiler water silicapermitted at various pressures to keepsteam silica at or below ppm The maximum solubility ofsilica in Steam is a direct func-tion of both the Steam densityand temperature. As steamtemperature or density de-creases, the silica solubility pressure affects steamdensity and has a bearing onsteam temperature, it has animportant effect on the solubilityof silica in CARRYOVERI nvestigation of the problem ofsilica carryover in a laboratoryexperimental boiler revealed twoimportant facts: With constant pressure andboiler water pH, silicacarryover is directly propor-tional to the amount of silica inthe boiler water.

3 This was trueover a wide range of boilerwater silica concentrations. The ratio of silica in the steamto silica in the boiler waterincreases rapidly as boilerpressures two points are illustratedby the graph in Figure experts and utilityoperators determined empiricallythat ppm of silica in thesteam is a practical maximumlimit for boiler water pH > Steam silica contents ppm or less, appreciableturbine deposits would notnormally occur. A survey ofoperating experience with utilitytype boilers confirmed that ppm figure is a valid andrealistic maximum limit. Figure 2shows boiler water silica limitsneeded to keep Steam silica at orbelow extensive study of silicacarryover in a laboratory boilerconfirmed the results shown inFigures 1 and 2. In addition, thestudy demonstrated that:1.

4 The presence of calcium andmagnesium sludge, antifoam,organic treatments, and combi-nations of these do not signifi-cantly affect silica High ratios of hydroxidealkalinity to silica in the boilerwater reduce the ratio of silicain the Steam to silica in theboiler water, thereby reducingpotential silica deposits These two curves apply to highalkalinity (pH > ) boilerwater new boiler installationshave high purity makeup sys-tems that allow acceptable boilerwater pH control at pH 9 vaporous silica carryoverincreases as boiler water pHdecreases, Figures 3 and 4 shouldbe used for these high addition, some new turbineinstallations require a silica insteam limitation of 10 ppb asopposed to the traditional 20 ppblimit. This control curve (10 ppb)is also shown in Figure SILICASOLUBILITY ANDCARRYOVERThe conditions under whichsilica carryover occurs had beenthoroughly investigated anddocumented.

5 What was neededat this point was a means ofdetermining the conditions underwhich silica is redeposited in theturbines. To determine thisexperimentally would be diffi-cult. Reasonably valid calcula-tions could be made, however,based on the prior investigationsof silica solubility and have calculated theapproximate relationship be-tween boiler pressure, boilerwater silica , silica carryover ,and silica solubility in saturatedand superheated Steam . Theserelationships are combined inFigure BOILERWATER AND STEAMSILICA VALUESIn Figure 5, silica in the Steam isplotted against boiler diagonal line represents aparticular silica concentration inthe boiler water. The amount ofsilica expected in the Steam froma particular boiler is shown by theintersection of the boiler pressureline and boiler water silica example, a boiler operating at1400 psi with 10 ppm silica in theboiler water would show ppm in the 3 Ratio of silica in Steam tosilica in water vs boiler pressureFigure 4 Boiler water silicaconcentration vs boiler pressureThe solid curved line in Figure 5represents the calculated maxi-mum solubility of silica insaturated Steam .

6 If the steamcontaining ppm silica in theabove example is allowed toexpand without being super-heated, the graph shows thatsilica will start to precipitate outwhen the pressure drops below300 point is found by followingthe ppm line from the boilerpressure and boiler water silicaintersection across until it inter-sects the silica solubility dotted lines in Figure 5indicate calculated maximumsolubility of silica in superheatedsteam. To illustrate how thesesuperheat lines are used, supposethe Steam containing ppmsilica has 100 F superheat. Fol-lowing the line across until itintersects the 100 F superheat lineindicates that silica will start toprecipitate out as the pressure isreduced to about 200 5 silica in Steam vs boiler pressure for pH = CUSTOMERSATISFACTION THROUGH QUALITYN alco Chemical Company s mission is to enhance the profitability of our customers business by providing programs and services that add value to their operations and enhance their return on are committed to a quality management philosophy that strives for continuous improvement.

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