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Silica Scaling in Boilers

Silica, in amounts ranging from less than 1 to over 100 ppm, is found in all natural water supplies. In rain hail and snow, silica content range from 1 to 2.8 ppm. In the analyses of various surface and ground waters, silica content range from 1 to 107 ppm. This refers to soluble silica content and not to the silica that may be present in the suspended matter. Suspended matter may be removed from a water supply by coagulation and filtration; such processes have little effect in reducing the soluble silica content.

Silica is the only boiler water salt which vaporizes at pressures below 2400 psig. It can vaporize at pressures as low as 400 psig. This has caused deposition problems in numerous turbines. The solubility of silica in steam increases with increased temperature: silica becomes more soluble as steam is superheated.
Deposits are formed when steam is cooled by espansion. Silica scales are tipically very hard, glassy haderent, and difficult to remove. Thermal conductivities are usually very low and tube failures can occur with even very thin silica scales.

The conditions under which vapour silica carryover occurs have been investigated and documented. It has been demonstrated that for any given set of boiler conditions using demineralized or evaporated quality make up water, silica is distributed between the boiler water and the stream in a definite ratio. This ratio depends on water pressure and boiler water pH as shown in the picture on the right.
The values of the ratio increases with increasing pressure and decreases with increasing pH. The effect of the pH becomes greater as higer pH values (from 11.3 to 12.1).

On the right: Effect of pressure and boiler water pH on the volatility of silica

To minimize this problem, the quantity of silica in the steam must be controlled. Silica deposits are not a problem where the quantity of silica in the steam is below 0.02 ppm. The proper boiler water silica level necessary to mantain less than 0.02 ppm silica in the steam is shown in the figure below. 

On the left: Maximum boiler water silica allowable to mantain less than 0.02 ppm silica in the steam

The most significant factor is to maintain low silica concentrations in the boiler water. This can be gained through external treatment equipment for make up water or monitorning and controlling the condensate contmination.
After silica enters the boiler water, the usual corrective action is to increase boiler blowdown and then to correct the condition that caused the silica contamination.

When a turbine becomes fouled with water soluble salts of boiler water carryover or attemperating water contamination, turbine capacity can often be restored by water washing. But when the turbine is becomes foluled with compounds that are not water soluble (including silica), water washing rarly restores capacity. Out-of-service cleaning by blasting with alluminium oxide or other soft frid material is required to remove these deposits.

Source: Idustrial water conditioning, Betz Dearborn, 1991

Find information about the other main problems occurring in boilers: foaming and priming, corrosion, scaling.
For a description of the characteristics of the perfect boiler water click here.
Check also our web page about boiler water treatment, in particular through deaeration (deaerating heaters or membrane contractors).

Relate topic: scaling and antiscalants

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