Technifax TF-5

Silica Carryover CausesTurbine DepositsMost boilers making steam for turbinesrarely have excessive carryover ofboiler water in the steam. Goodoperating practices, improved steamseparators, and proper chemicalcontrol minimize this. Silica deposits inturbines, however, can occur evenwhen boiler water carryover isnegligible. The reason: steam selectively“picks up” silica from the boiler water,dissolves it, and carries it to theturbines, where it redeposits. Investiga-tions show that the key to minimizingsilica carryover is in keeping the boilerwater silica content below certainlevels, the concentration depending onoperating pressures.

Silica SolubilityResearch on the solubility of silica inwater and steam shows severalimportant points:

• Steam is a solvent for silica

• For any particular steam density andtemperature, there is a definitesaturation solubility of silica

• The maximum solubility of silica insteam is a direct function of boththe steam density and temperature.As steam temperature or densitydecreases, the silica solubility alsodecreases.

Since pressure affects steam densityand has a bearing on steam tempera-ture, it has an important effect on thesolubility of silica in steam.

Silica CarryoverInvestigation of the problem ofsilica carryover in a laboratoryexperimental boiler revealed twoimportant facts:

• With constant pressure and boilerwater pH, silica carryover isdirectly proportional to theamount of silica in the boiler water.This was true over a wide range ofboiler water silica concentrations.

• The ratio of silica in the steamto silica in the boiler waterincreases rapidly as boilerpressures increase.

These two points are illustrated bythe graph in Figure 1.

Turbine experts and utility operatorsdetermined empirically that 0.02 ppmof silica in the steam is a practicalmaximum limit for boiler water pH>10. With steam silica contents of0.02 ppm or less, appreciable turbinedeposits would not normally occur.A survey of operating experiencewith utility type boilers confirmedthat the 0.02 ppm figure is a valid andrealistic maximum limit. Figure 2shows boiler water silica limitsneeded to keep steam silica at orbelow 0.02 ppm.

Selective silica carryover

Figure 1 — Silica distribution ratio

Figure 2 — Maximum boiler water silicapermitted at various pressures to keepsteam silica at or below 0.02 ppm

An extensive study of silica carryoverin a laboratory boiler confirmed theresults shown in Figures 1 and 2. Inaddition, the study demonstrated that:

1. The presence of calcium andmagnesium sludge, antifoam, organictreatments, and combinations ofthese do not significantly affect silicacarryover.

2. High ratios of hydroxide alkalinity tosilica in the boiler water reduce theratio of silica in the steam to silica inthe boiler water, thereby reducingpotential silica deposits in turbines.

3. These two curves apply to highalkalinity (pH > 10.5) boiler watersystems.

Many new boiler installations have highpurity makeup systems that allowacceptable boiler water pH control atpH 9–10. Since vaporous silica carry-over increases as boiler water pHdecreases, Figures 3 and 4 should beused for these high purity systems.

In addition, some new turbine installa-tions require a silica in steam limita-tion of 10 ppb as opposed to thetraditional 20 ppb limit. This controlcurve (10 ppb) is also shown inFigure 4.

Correlating Silica Solubilityand CarryoverThe conditions under which silicacarryover occurs had been thoroughlyinvestigated and documented. Whatwas needed at this point was a meansof determining the conditions underwhich silica is redeposited in the

turbines. To determine this experimen-tally would be difficult. Reasonably validcalculations could be made, however,based on the prior investigations ofsilica solubility and silica carryover.

Researchers have calculated theapproximate relationship betweenboiler pressure, boiler water silica,silica carryover, and silica solubility insaturated and superheated steam.These relationships are combined inFigure 5.

Interpreting Boiler Waterand Steam Silica ValuesIn Figure 5, silica in the steam is plottedagainst boiler pressure. Each diagonalline represents a particular silicaconcentration in the boiler water. Theamount of silica expected in the steamfrom a particular boiler is shown bythe intersection of the boiler pressureline and boiler water silica line. Forexample, a boiler operating at 1400 psiwith 10 ppm silica in the boiler waterwould show about 0.04 ppm in thesteam.

Figure 4 — Boiler water silica concentrationvs boiler pressure

Figure 3 — Ratio of silica in steam to silica inwater vs boiler pressure

NALCO and the logo are Registered Trademarks of Nalco Company©1970, 2004 Nalco Company All Rights Reserved 8-04

NALCO COMPANY OPERATIONS

North America: 1601 West Diehl Road • Naperville, Illinois 60563-1198 • USAEurope: Ir.G.Tjalmaweg 1 • 2342 BV Oegstgeest • The NetherlandsAsia Pacific: 2 International Business Park • #02-20 The Strategy Tower 2 • Singapore 609930Latin America: Av. das Nações Unidas 17.891 • 6° Andar 04795-100 • São Paulo • SP • Brazil

www.nalco.com

The solid curved line in Figure 5represents the calculated maximumsolubility of silica in saturated steam.If the steam containing 0.04 ppmsilica in the above example is allowedto expand without being super-heated, the graph shows that silicawill start to precipitate out when thepressure drops below 300 psi.

This point is found by following the0.04 ppm line from the boilerpressure and boiler water silicaintersection across until it intersectsthe silica solubility line.

The dotted lines in Figure 5 indicatecalculated maximum solubility ofsilica in superheated steam. Toillustrate how these superheat linesare used, suppose the steam contain-ing 0.04 ppm silica has 100°F (38°C)superheat. Following the 0.04 lineacross until it intersects the 100°F(38°C) superheat line indicates thatsilica will start to precipitate outas the pressure is reduced to about200 psi.

Figure 5 — Silica in steam vs boiler pressurefor pH = 11.3