Silica scale in a firetube ...

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.