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  • Silica in Condensate vs main steam

    Have a query regarding how come silica in condensate water is higher than in steam silica? Want to know if this could be a problem or is it normal? Find responses from experts here.

    Why silica in Condensate water is always higher i.e 14 PPB as compared with main steam silica i.e 11 PPB? Its a high pressure boiler with turbine having capacity of 75 megawatt. The water used is de mineralized having mixed bed. Is this a problem or it's normal?
  • Answers

    3 Answers found.
  • Generally softened and demineralised water will be used in a boiler to keep the impurities like Silica and others at the minimum which have a potential to corrode or damage the boiler and its associated pipes etc from inside. You have mentioned that you are finding the Silica more in the condensate water than the working water in the boiler. Normally condensate water which we get after a cycle of boiler process should have less impurities and in fact it is reused for many purposes. So, one thing that we have to check is whether we are replacing the working water timely or not and the place where the condensate water is collected or accumulated is free of impurities or not. This is a very general but vital consideration.

    The main water supply to the boiler is used for making up the steam and some part is then condensed back and returned. If the main steam is having 11 PPB Silica then the condensate cannot have more than that. Here we have to check the inlet Silica also properly because if the demineralisation is not working properly and our sample itself is faulty then it requires to be taken care at that grassroot level itself.

    Anyway this is a highly technical matter and I would suggest you to refer to the free 'Pdhonline' courses (specifically PDH course M165) for understanding and resolving this issue.

    Knowledge is power.

  • Pressure in the silica in the boiler water is critical for steam turbine and this phenomenon leads to deposition of silica on the internal surface of the turbine. Scale formation is caused due to its accumulation in the steam turbine in the varying concentrations.
    The solubility of silica ( sio2) increases with the increase of the temperature and in case of super heated steam under the influence of high steam, formation of silica accelerates with the decrease of temperature and density, the solubility of silica drops. Since the pressure affects steam density having a strong bearing on steam temperature , it can impact the solubility of silica in the steam.
    There exists a relation between the solubility of silica at a high pressure lying at 22,0000 KPA and the content of silica in the condensate after this procedure should not exceed beyond 500 mg / kg. This simple to use formula can be of immense practical value for the engineers to ascertain the content of silica content at varying conditions without indulging in any experimental measurement.
    Hence at all the costs, silica in the condensate should remain much below the specified values so as to avoid the corrosion of the boilers. This needs periodical check up of feed water to avoid the silica contamination in the turbine.

  • Silica is having the property of getting vaporized at pressures below 2400 PSIG. Silica may vapourize even at a pressure of 400 PSIG. This will result in deposition problems in many turbines. . Higher the temperature of the steam higher the solubility of Silica. Silica solubility will be very high if the steam is superheated. This silica vapour will be distributed between the boiler water and the steam in a definite ratio. There are many papers published on this. You can understand this if you go through those papers.
    Water pressure and boiler water pH are the two important parameters which will decide the ratio of silica in water and steam. The silica content in the steam should be controlled. This can be done by controlling the silica in feed water. Another action is to increase the frequency of boiler blowdown. If the turbines are having deposits of non-water-soluble deposits like silica turbine capacities can't be attained back by simply doing water wash. Blasting with aluminium oxide during out of service may be required to clean these deposists.

    always confident

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