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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 redeposits.Investigations show that the keyto minimizing silica carryover isin keeping the boiler water silicacontent below certain levels, theconcentration depending onoperating pressures.

SILICA SOLUBILITYResearch on the solubility ofsilica in water and steam showsseveral important points:

• Steam is a solvent for silica

• For any particular steamdensity and temperature,there is a definite saturationsolubility of silica

Selective silica carryover

Figure 1 — Silica distribution ratioFigure 2 — Maximum boiler water silicapermitted at various pressures to keepsteam silica at or below 0.02 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 alsodecreases.

Since pressure affects steamdensity and has a bearing onsteam temperature, it has animportant effect on the solubilityof silica in steam.

SILICA CARRYOVERInvestigation 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. 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 increase.

These two points are illustratedby the graph in Figure 1.

SM

Turbine experts and utilityoperators determined empiricallythat 0.02 ppm of silica in thesteam is a practical maximumlimit for boiler water pH >10.With steam silica contents of0.02 ppm or less, appreciableturbine deposits would notnormally occur. A survey ofoperating experience with utilitytype boilers confirmed that the0.02 ppm figure is a valid andrealistic maximum limit. Figure 2shows boiler water silica limitsneeded to keep steam silica at orbelow 0.02 ppm.

An extensive study of silicacarryover in a laboratory boilerconfirmed the results shown inFigures 1 and 2. In addition, thestudy demonstrated that:

1. The presence of calcium andmagnesium sludge, antifoam,organic treatments, and combi-nations of these do not signifi-cantly affect silica carryover.

2. 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 inturbines.

3. These two curves apply to highalkalinity (pH > 10.5) boilerwater systems.

Many new boiler installationshave high purity makeup sys-tems that allow acceptable boilerwater pH control at pH 9–10.Since vaporous silica carryoverincreases as boiler water pHdecreases, Figures 3 and 4 shouldbe used for these high puritysystems.

In 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 4.

CORRELATING SILICASOLUBILITY ANDCARRYOVERThe conditions under whichsilica carryover occurs had beenthoroughly investigated anddocumented. 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 silicacarryover.

Researchers have calculated theapproximate relationship be-tween boiler pressure, boilerwater silica, silica carryover,and silica solubility in saturatedand superheated steam. Theserelationships are combined inFigure 5.

INTERPRETING BOILERWATER AND STEAMSILICA VALUESIn Figure 5, silica in the steam isplotted against boiler pressure.Each 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 line.For example, a boiler operating at1400 psi with 10 ppm silica in theboiler water would show about0.04 ppm in the steam.

Figure 3 — Ratio of silica in steam tosilica in water vs boiler pressure

Figure 4 — Boiler water silicaconcentration vs boiler pressure

The solid curved line in Figure 5represents the calculated maxi-mum solubility of silica insaturated steam. If the steamcontaining 0.04 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 psi.

This point is found by followingthe 0.04 ppm line from the boilerpressure and boiler water silicaintersection across until it inter-sects the silica solubility line.

The dotted lines in Figure 5indicate calculated maximumsolubility of silica in superheatedsteam. To illustrate how thesesuperheat lines are used, supposethe steam containing 0.04 ppmsilica has 100°F superheat. Fol-lowing the 0.04 line across until itintersects the 100°F superheat lineindicates that silica will start toprecipitate out as the pressure isreduced to about 200 psi.

Figure 5 — Silica in steam vs boiler pressure for pH = 11.3

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