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Phosphate Hideout
When the concentration of phosphate in the boiler water is too high, phosphate will react with boiler scale forming iron and iron sodium phosphates or precipitate to form a solid phase on the hot boiler tube surfaces and elsewhere.  This phenomenon is called phosphate hideout and it can result in changes of pH, as well as the concentrations of phosphate and other chemical species in the boiler water.  Hideout usually first occurs in boiler tubes with the highest heat flux and low mass flow, where departure from nucleate boiling (DNB) or under deposit concentration occur.  Surprisingly, hideout and boiler tube corrosion can also occur in HRSGs where the heat flux is much lower than in coal- or oil-fired units.  This is because there may be accumulation of corrosion products or inadequate mass flow through some generating tubes; particularly during duct burner firing.
Because the solubility of phosphate decreases as boiler pressure increases, hideout is typically only noticed during load changes.  Phosphate hideout usually results in an increase of PO4 concentration and a decrease of pH during load reduction and in an increase of pH and decrease of PO4 concentration during the load increases.  The severity of phosphate hideout is boiler, heat flux, temperature, pressure, and boiler cleanliness specific.  In clean, conservatively designed and operated boilers, these swings do not result in any boiler tube corrosion problems.
When using phosphate boiler water treatment, the concentration of phosphate should not exceed the equilibrium concentration.  This is the maximum concentration of PO4 in the boiler water that the boiler can tolerate under the highest heat flux conditions (full load).  This concentration can be experimentally determined by increasing the phosphate feed to the point where there is no further increase in the PO4 concentration in the boiler water.  This is the equilibrium concentration and the normal boiler water phosphate maximum limit should be approximately one-half of this concentration.  
The equilibrium phosphate concentration should be determined once per year because it changes with the boiler cleanliness.  In addition, a phosphate balance should be performed once per year using several weeks of chemistry data to determine the amount of hideout that is occurring.  The total amount of phosphate fed into the boiler and the amount being removed through blowdown, plus sampling, should be approximately equal.  If the feed is greater than the removal, hideout is occurring.
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