Sir 1) As observed Silica increases  in Drum > 1.2ppm - owing to colloidal silica (&...

You have three questions, so let me answer these in sequence.

1. A bit of both I should say. If colloidal silica is present in FW, it would gradually increase the boiler water silica due to the COC (Cycle of Concentration) in the evaporator. The silica inclusion in the steam through the attemperation spray would be negligible as the spray quantity is considerably less as compared to the steam flow. The silica measurement also doesn’t have the level of accuracy to pick up that increase. But you would not be wrong if you worry about more silica reaching the Turbine if you are using high amount of spray to control the MS temperature, particularly if you suspect colloidal silica is present in your FW.

2. As dissolved silica is weakly ionized, it can be removed in SBA and not WBA. The MB ensures the removal of silica ions to less than 0.01 – 0.02 ppm. The slippage occurs due to improper regeneration of these equipments. Fouling of ion exchange resins is one of the major reasons.  It may sound odd, but the practice of regeneration of SAC on the basis of OBR may sometimes lead to silica slippage from SBA.

3. Ions like sulphate and chloride are pretty strong ions and therefore a WBA can remove these and thus reduce the ionic load for the SBA, which has a more difficult job of capturing the weak ions like nitrate, phosphate and particularly silica. One would use a WAC if the hardness of raw water is more.