As part of the internal treatment series, this article covers congruent phosphate, the evolution of coordinated phosphate.
The coordinated phosphate program didn't achieve the expected success because it started from the premise that Na₃PO₄ underwent hydrolysis generating NaOH, and that this reaction would reverse once evaporation concentrated the water at the tube surface. In theory, the system was self-regulating.
In practice, the different sodium phosphate salts (Na₃PO₄, Na₂HPO₄, NaHPO₄) have different solubilities at the temperatures and pressures a high-pressure boiler operates under. When water concentrates locally, the less soluble compound precipitates first and allows free NaOH to be present.
The Congruent Phosphate Program
In 1964, Marcy and Halstead proposed the Congruent Phosphate program. Its logic differs from Coordinated Phosphate: this program narrows the operating range to values where the compound that eventually precipitates itself has a Na/PO₄ ratio similar to that of the liquid phase. If the solid and the liquid have similar compositions, local concentration doesn't shift the equilibrium toward generating free NaOH. It's this equivalence between the precipitated phase and the dissolved phase that gives the program its name — the phosphate precipitates "congruently".
With a Na/PO₄ ratio in solution exactly equal to 2.85, the Na/PO₄ ratio in the precipitated phosphate solids is identical — that is, congruent. However, later studies showed that at higher temperatures, the congruent point could occur at a Na/PO₄ ratio as low as 2.6. To account for this and other uncertainties, congruency control programs typically operate within "safe" Na/PO₄ molar ratios between 2.3 and 2.6.
As with the coordinated phosphate program, control relies entirely on the relationship between boiler water pH and PO₄ readings, aiming to keep the plotted coordinates within the appropriate zones on a control diagram.
The Hideout Problem
This approach solved much of the boiler internal treatment problem, but the progress that allowed higher boiler pressures also brought new challenges: Hideout.
Coordinated and congruent phosphate programs can be difficult to control because of phosphate "Hideout". As phosphate concentrates and reacts or precipitates locally, it's removed from the boiler water, dropping to very low residual equilibrium levels. Boiler water PO₄ can thus fall from a normal value of 5 ppm to 2 ppm or less during Hideout.
This revision came with Equilibrium Phosphate, the subject of the next article in this series.