Drum Water Level Control - Continued.

Here are some key points about the CBD system:

a. Highest concentration of dissolved solids exists just below the water surface. As water leaves the water surface in form of steam, it leaves its dissolved solids behind. Therefore it stands to reason that the highest concentration of solids be just below the surface. To make use of this fact, the CBD is drawn from a location just below the water surface.

b. In many boilers, the conductivity of the boiler water is continuously monitored through automatic instrumentation. The value of conductivity is used to control the CBD flow by an automatic flow controller.

c. An increase in the CBD flow has two effects. First, it reduces the solids concentration. Second, it forces a lot of heat energy to go to waste. Therefore a balance has to be found between the need for low solids versus energy loss to waste.

d. In all cases at least two valves are used on the CBD line. This ensures better isolation if necessary.