Method for Predicting the pH of Boiler Water at Actual Temperatures

ABSTRACT

Normally, the pH of boiler water is controlled based on the pH at 25°C. However, since the dissociation constants of water, acid components, and alkaline components vary with temperature, the pH behaviour at high temperatures differs significantly from that at 25°C. The author developed a calculation program using the Microsoft Excel^® macro function Visual Basic for Applications (VBA) to theoretically predict the pH of boiler water at actual temperatures (pH_T).

As an application example of this program, the relationship between temperature and pH_T for various alkalising agents as well as the relationship between the dosage of each alkalising agent and pH_300°C were shown. The results indicated that ammonia and alkalising amines have lower alkalising capabilities at high temperatures compared to Na₃PO₄ and NaOH. Additionally, the example shows the relationship between the concentrations of acidic substances (HCl, acetic acid, formic acid, and orthosilicic acid) and pH_300°C when using each alkalising agent.

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