Introduction

The previous chapters have outlined the importance of feedwater and boiler water chemistry. Although control of chemistry in these systems is critical for protection of the systems themselves, feedwater and boiler water chemistry have a direct effect on steam purity. Deposition of solids and corrosion in superheaters and turbines have been common problems. Even in industrial plants that have no turbines, proper control of steam chemistry is still very important. Some of the most intensive research in the steam generating industry is being directed towards steam chemistry.

Steam produced in the boiling process is never completely pure, and even at its best contains trace amounts of solids. The process by which solids are transferred to steam is known as carryover. Carryover is influenced by the following:

• Virtually all solids are at least slightly soluble in steam. Solids become more soluble as boiler pressure increases, and some, particularly silica, carry over extensively as a vapor.

• Even with the best steam separating devices, moisture droplets still enter the steam.

• Sulfate and chloride will enter steam as ammoniated salts.

• Improper drum level control, poor drum design, or excessive solids buildups in drum water will increase carryover.

Contaminants also enter steam via attemperator systems. Even if the drum is operating properly and minimizing carryover, dissolved solids still have this direct path to the steam system. Attemperator contamination is greatly exacerbated during upset conditions such as a condenser leak.

Another mechanism for steam contamination is exfoliation of superheater and reheater tube walls, which introduces...