Process Control Instrumentation

As water evaporates within a boiler, dissolved solids in the water are left at the bottom of the boiler, resulting in buildup of sludge and scale. This buildup is alleviated by discharging some boiler water through a valve at the bottom of the boiler. High concentrations of solids can also lead to foaming near the top of the boiler and result in water hammer, which may damage piping, steam traps, and process equipment. Skimming or surface blowdown removes the dissolved solids near the surface of the liquid and is usually a continuous process. This flow is routed through a heat exchanger. The amount of blowdown necessary depends on boiler operating pressure, amount of makeup water, impurity levels in the makeup water, and the dissolved solids concentrations that a given boiler can tolerate.The existing boiler blowdown system to recover the energy from the continuous blowdown stream from the boiler blowdown flash tank. Modifications included installing a plateand- tube heat exchanger and associated piping (Figure 1). The 300° F water from the flash tank drain was routed to the “hot side” of the heat exchanger to preheat boiler feedwater passing through the other side of the heat exchanger.