QUESTIONS AND ANSWERS
What is phosphate hideout?
Phosphate hideout is a phenomenon that causes boiler water control difficulties. It is defined as the disappearance, byprecipitation or absorption, of the ionic phosphate (PO4) species, in the boiler water under high heat transfer or high load conditions. When the high heat flux or high load condition is reduced, thespecies (phosphate) returns from hideout, creating even more control difficulties. The magnitude of phosphate hideout and the variation in load required to cause the phenomenon vary from one boiler toanother. Phosphate hideout is primarily a result of a temperature-dependent interaction of sodium phosphate compounds with iron oxide, which creates a low solubility reaction product that precipitatesin high heat flux areas or under high load conditions. At lower loads and lower heat fluxes, the precipitates return to solution.
Figure 1 — Normal congruent phosphate-pH boiler chemistry.Figure 2 — Phosphate hideout caused by lower combustion zone.
During phosphate hideout, little or no change in phosphate and/or pH values occurs when chemical charges are added to the system sincefresh phosphate may precipitate along with that which has already occurred. The readings for pH and phosphate are typically out of control during hideout when the load is increased, and then again when itis decreased. While it seems that the phosphate has mysteriously disappeared, and that additional charges are unsuccessful in increasing the phosphate reading, the actual phosphate inventory in theboiler has not decreased. It has combined with oxide deposits to form a slightly lower Na/PO4 ratio solid (approximately 2.1:1), causing an increase in alkalinity in the liquid phase (bulk water) —hence the boiler water pH increases. When the load is decreased again, these combinations return to the soluble form and the phosphate returns to the bulk water, causing higher phosphate readings and...