Common Boiler Problems, Part 2: Improper Blowdown Techniques
The concentration of undesirable solids in a boiler is reduced through proper feedwater treatment and the proper operation of a continuous purge (blowdown) system, and by performing intermittent blowdowns on a regular basis.
The sodium zeolite water softening process is the predominant method of water treatment for boilers operating at low pressures with saturated steam. In this ion exchange process, harmful scale-producing calcium and magnesium ions are exchanged for sodium ions. The resultant water has a total dissolved solids concentration equal to the previous combined total of sodium, magnesium and calcium concentrations.
The main purpose of blowdowns is to maintain the solids concentration of the boiler water within certain acceptable limits. The blowdown rate can be determined by several factors which include dissolved solids, suspended solids, silica, and alkalinity. Maximum recommended concentration limits in the water of an operating boiler should be maintained in accordance with the recommendations of the American Boiler Manufacturer’s Association (ABMA).
As the operating pressure increases, the limits become substantially more stringent. This can potentially require an extremely high blowdown rate, if sodium zeolite softening is the feed- water treatment method. To substantially lower the blowdown rate and control the concen- tration of silica, a total demineralized water treatment system should be used. A deminera- lized water treatment system removes the anions and cations instead of substituting them for other ions.
The continuous blowdown rate is set to control the boiler water within these ABMA-recom- mended acceptable limits. A well-designed continuous blowdown system will constantly monitor boiler water conductivity (solids concentrations) and adjust the blowdown rate to maintain the control range. If the boiler water exceeds the recommended limits, potential problems can occur which include scale and sludge formation, corrosion and moisture carryover due to foaming and poor steam drum separation equipment performance. When this occurs, solids and silica are carried over in the steam. This results in silica and scale formation on the superheater and other process equipment, including steam turbine blading. This foaming phenomena associated with high conductivity can also cause drum level instability leading to nuisance water level alarms and potential boiler trips.
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