Dear Sir As above 270 o C inside drum Colloidal Silica Converts to Reactive Silica (forming ...

There are basically three guidelines for water and steam quality control that are being followed by the industries. These are from CEGB, EPRI and ABMA. Then there is the International Association for the Properties of Water and Steam (IAPWS), who have developed water and steam quality standards for power station equipment of different types on the basis of theoretical studies and long-term experience in the operation of power plants in various countries.

The boiler water silica limits for various pressure systems are more or less similar in these guidelines. But if your OEM decides on a limit of 1.2 for your system, then you would be bound by that as this limit is not too far away from the international guidelines. After all, can you ensure zero mechanical carry over?

I have seen in some systems with less than 100 ksc pressure, they are operating blow down the moment drum silica exceeds 0.5 ppm which they are maintaining as limit. In such cases, the management may think of changing the practice to save unnecessary blow down loss.

Your second question is not quite clear to me. You open the CBD valve as required. IBD on the other should not be used unless you suspect sludge in your system. It is quick open and close operation and not for continuous operation as this would affect the circulation in the evaporator circuit.