How to remove a Silica Scale in Boiler Tubes?

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These days I am facing an issue with a Silica Scale in Boiler Tubes. Scale thickness is around 2 mm.

This is a Combined (Water Tube & Fire Tube) boiler. 

How to remove a Silica Scale in Boiler Tubes?

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10 Answers

  1. (1) you could use cryogenic blast to cool the scale material, and easily shed it from the tubes.

    (2) replace boiler tubes

    (3) Use low heat to boiler (by-pass controls), then "shock" the boiler tubes with cold water, and hope you do not break the tubes.

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  5. Many people do not measure silica in water supplies.   Many boreholes have silica present but it is much lower than the hardness often as much as 1/10.  There are waters where the hardness levels are relatively low but the silica levels are proportionately high i.e. 20-25% of the hardness.  Scales can form including some calcium and magnesium but silica and iron and these form heat resistant scales. It doesn't need much hardness slippage to allow these complexes to occur.  To control silica in a boiler, a minimum of 2.4 times of free caustic is required.  Phosphate programs are better than chelant to aid removal.  I have also used organic sequestrants in conjunction with phosphate polymers and caustic levels 3 times the silica content.  This can aid removal, by causing cracking of scale deposits.  Care must be taken because lumps of scale can bridge the tubes causing localised overheating.  Once the scale is breached normal acid cleaning using inhibited hydrochloric acid can be used.  If the scale has not cracked then hydroflouric is the answer.  It is extremely nasty to carry out and should only be done by a specialist company under strictly controlled conditions.  

    I disagree with the comment that often chemical suppliers motive is to push as much chemical as they can.  Reputable companies, of which there are many, dose the required amount of chemical to do the job in question.  There is a British Standard for the treatment of steam boilers, which is a useful guide for the correct treatment.

     

    2 Comments

    1. By probably not staying in control of how much residual hardness finds its way into the boiler where alkalinity is high, and there is also a cycled up residual of silica that may easily exceed 100 mg/L, the source of scaling is clear.  Getting rid of the mistake is what our topic is here.  Chemical cleanings may be effective, but I feed that cryogenic treatment directly to the scale results in differential contraction that removes this silicate scale as dust.

    2. I totally agree! 

      There is EU standards "Water-tube boilers and auxiliary installations - Part 12: Requirements for boiler feedwater and boiler water quality" (12952-12) which is has good approach, even for silica and [OH] ratio. 

  6. Good Morning Asanka;

    Often boiler and cooling tower chemical suppliers' motive is to push as much chemical as they can.   And the companies' reps fail to periodically redo water analyses that may result in changing chemicals and dosages. 

    Pretreatment softening greatly reduces chemical consumption and deemed a conflict of interest by the chemical suppliers.

    Aquathin has long experience working with boilers and cooling towers...and chemical companies.

    Regarding silica;  Aquathin reduces the effects of silica by pretreating the source water with a Colloidal Silica Scavanging Resin.

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    1 Comment

    1. Thanks for your comment.

      Yes, we may need better Silica remover in makeup water. My worry is how to remove the scale which already formed in the tube surface.

      1 Comment reply

      1. You also need a more eficient water treatment specialist to advice for your water treatment, by performing more accurate analysis and mass equilibrium for silica ion, in order to verify silica or other ion precipitation  

  7. Silica scale in a firetube boiler is a pretty amazing accomplishment. Chances are better than great that the scale is actually magnesium silicate. As pH increases, Silica solubility increases primarily do to the formation of silicate ion. You can purchase sodium silicate where it is 35% concentration. The complication occurs when you have high magnesium concentrations at higher pHs. At that point, you get Magnesium hydroxide forming and it reacts with the silicate to make magnesium silicate (talc powder).

    The fact that you have scale would indicate that your boiler feedwater is not what it should be and if you have a water treatment supplier, they are not paying attention to what needs to be done to prevent these issues. Sometimes the causes are not so obvious or easy to find. However, in your case, you have a problem result that you can see. I agree with the earlier answer that you need to make a change but changing the chemical is not always the solution. You need to discover what is causing the problem and change that.  If you have an alien contamination leak, adding a boat load of a dispersant is like taking an aspirin after drinking all night. You might get rid of the headache but you are killing your liver.

    Things happen from cause. Identify the cause. Remove the cause.  

    1 Comment

  8. I agree with the previous answers.

    If you do not change the chemical treatment you will face the same problem again. You need to add special polymer dispersant to prevent silica scaling. . 

  9. Use concrete remover products, they actually work, are far safer to use on metals than HF, safer for personnel, and you will be pleasantly surprised by the results.  The best ones apparently have a product that is a by-product of sugar cane, not necessarily glycolic acid.  I have witnessed such a product literally peel the combine alkaline-earth silicate scale from a stainless steel tube at a thickness of 1.5 mm within a few minutes when the tube was suspended vertical in the liquid.

    You just need to circulate the material through your boiler, then immediately re-passivate the metal with DEHA or sulfite.  I expect nothing short of a miracle in your case.

    1 Comment

    1. What are these concrete removing Products generically and by brand? I would like to know more about them.

      1 Comment reply

      1. Typically they traditionally used some form of inhibited hydrochloric acid, however, the new products are "greener".  One of the best utilized a by-product of sugar cane, glycolic acid, I believe.  Along with certain polymers, these products will work to help maintain cement trucks, finishing equipment, etc.  I have used with good effect on thick silica scale to at least 50% clean metal.  One company in Frisco, TX, as I recall:

        https://www.romixchem.com/romix_cart/products.php?cat=10  

        I have good experience with their "Platinum" product.

        a generic search on the web for "concrete remover products" brings up many competitive products.