4- 7.1 Steam-Generator Feedwater Specifications

Feedwater conditioning is required to maintain opera­tional capability of the steam generator The steam­generating surfaces remain clean and heat-transfer capa­bilities are favorable if good water quality is maintained The minimum standards given in Table 4 19 should be maintained for satisfactory feedwater quality.

4- 7.2 Usual Impurities in Water Supply

Table 4.20 lists the impurities usually found in water supplies and indicates their properties, effects, and methods for treatment and removal.

4- 7.3 Effect of Impurities in Steam-Generator Feedwater

(a) Total Solids (Dissolved and Undissolved). The total solids in the feedwater is a general indicator of how much material is collecting in the steam generator. Insoluble materials are deposited on the steam-generator surfaces. The soluble material (e. g., NaCl, NaOH, and Na2S04) is carried over in the steam with the remaining material and tends to collect on the steam-generator tubes. The types of constituents in the feedwater depend on the preboiler characteristics. The quantity of soluble material should be larger than the quantity of insoluble material. Turbidity is a measure of the undissolved constituents, and electrical conductivity is a measure of the dissolved constituents

(b) Dissolved Oxygen. Dissolved oxygen promotes cor­rosion in the steam generator and therefore should be kept as low as possible. Deaeration removes much of the dissolved oxygen. Hydrazine, a good oxygen scavenger, can eliminate the remaining oxygen. A high level of dissolved

oxygen could indicate a malfunctioning deaerator or an air leak in the area of the condenser.

(c) Total Silica. Silica should be maintained at a low level for two reasons (1) silica can concentrate in the steam generator and subsequently plate out on heat-exchange surfaces thereby reducing steam generation and (2) silica may carry over and plate out m the turbine causing turbine inefficiency. A higher than allowable silica concentration implies a demineralizer breakthrough. Switching to the spare demineralizer while the exhausted resin is regenerated or changed can probably eliminate the high silica level.

(d) Total Iron. Iron tends to build up in the steam generator and reduces its efficiency by degrading the heat-transfer characteristics. The level of iron in the feedwater affords some measure of the degree and rate of corrosion in the system.

(e) Total Copper. Copper should be avoided where possible in the feedwater system. Equipment in contact with the feedwater should be ferritic or austenitic stainless steel. Copper in the feedwater system can be carried into the steam generator in solution and plate out there. The copper plate can then make it necessary to clean the steam generator in a two-stage process one for copper and another for iron. Copper carryover in the steam can plate out on the turbine and lower its efficiency Copper alloy tubes in the condenser should be satisfactory because the temperatures and pressures are reduced and the dissolution of the copper is less likely.

(f) Total Lead. Lead in the feedwater concentrates in the steam generator This can result in problems with Inconel in oxygenated water containing lead Satisfactory instrumentation for monitoring traces of lead is not available at the present time

(g) Conductivity. Cation (positive-ion) conductivity cells can be used to monitor the feedwater Measurements

•From R. T. Kent, Mechanical Engineers’ Handbook, Power, 12th ed, p. 7 51, John Wiley & Sons, Inc, New York, 1950

should be made after removal of the ammonia that is used to regulate the pH

(h) Corrosion. The principal accelerators of corrosion are dissolved oxygen, acids, surface deposits, especially those electronegative to steel, dissimilar metals in contact, and electrolytes.

Common methods to prevent corrosion are removal of dissolved gases, especially oxygen, neutralization of acids and maintenance of desirable alkalinity and pH, periodic mechanical cleaning, and avoiding excessive salt concentra­tions.