Problems Associated with the Final Feedwater Blend

4 PROBLEMS ASSOCIATED WITH THE FINAL FEEDWATER BLEND 

Boiler FW is comprised of variable ratios of returned condensate and a MU water source. The MU requirement is proportional to the sum of nonrecoverable steam and BW blowdown that is lost from the plant. 

Apart from the waterside problems discussed earlier, which are due to impurities introduced with either the MU or the returned condensate, additional problems may develop both in the pre-boiler system and further downstream. These problems are associated with the final blended FW and are primarily related to oxygen corrosion of various items of pre-boiler equipment or the deposition of scales and metal oxides on waterside surfaces. 

Some additional very specific types of corrosion may also occur in the economizer, feed pumps, and other items of pre-boiler equipment, the mechanisms of which must be firmly understood if they are to be properly controlled. Some of the waterside problems affecting these items of equipment are discussed in the following sections. 

Inadequate FW deaeration is a common cause of serious corrosion problems that affect many hundreds (if not thousands) of boiler plants of all sizes around the world. Despite the abundance of available literature advising the need to eliminate oxygen from boiler FW, inadequate deaeration continues to cause permanent waterside damage. 

---