January 12, 2022
Author: E. S. Beardwood, The Analyst, 2021 Fall Edition, Volume 28, Number4, pages 22-32.
The management and control of boiler feedwaters and boiler waters are dependent on the control of steam purity, corrosion and deposition within the steam generating system. Practices or guidelines have been developed to minimize the penalties of severe corrosion, or deposition, frequent cleaning requirements, or unscheduled outages in steam generating systems and their auxiliary steam users. Other guidelines are available for pure utility-based steam generation systems operating above 1,000 psig to critical (i.e., > 3,200 psig). While some manufacturers of waste heat and specialty boilers their own guidelines, which are typically hybrids of those already available. These are not part of the scope of the paper and will not be discussed.
This paper will briefly discuss steam purity and steam generation system corrosion control. The overall emphasis will be on deposition control within steam generators and the application and expectations of the various types of internal chemical treatment programs that are available. Typically, as the operating pressures of the steam generators increase, the impurity loadings allowed decrease, which require either improvements of feedwater and make-up water purity and or increased blowdown rates to lower the carrying capacity of the steam generator. External pretreatment and contaminant removal is always more reliable and usually more economical than higher levels of internal treatment chemicals and higher blowdown rates. That is, internal chemical treatments are used as polishing agents to minimize corrosion and deposition within the steam generating system. Table 1 provides a general illustration operating pressure and the potential for occurrence of internal boiler corrosion or deposition. The corresponding hydroxide alkalinities and pH for these corrosion and deposition categories can be found in Table 2. Equivalents per million of OH (hydroxide) alkalinity is denoted by EPMOH, and can be used to determine vaporous carryover of silica.