Phosphate Treatment

Alkalinity

NaOH under certain conditions dissolves boiler metal.

Co-ordinated treatment tries to avoid this by relying on the fact that NaOH and disodium phosphate combine to form trisodium phosphate. However, a small proportion of the NaOH always remains. Congruent treatment tries to avoid this by making a mix so that the Na⁺ to PO^-3₄ ion ratio never exceeds 2.6 : 1 ( Na₃PO₄ has a sodium to phosphate ratio of 3 : 1 )

Co-ordinated phosphate treatment

• Purpose
  • to maintain a phosphate reserve to react with incoming hardness
  • To maintain sufficient alkalinity to minimise corrosion and aid in forcing the reaction with hardness salts.

For boiler pressures above 40bar problems of caustic alkalinity caused by addition of caustic soda as part of a feed treatment occur.
With caustic alkalinity, free hydroxides concentration in a thin film at the tube surface destroying the protective magnetite layer and attacking the metal to form caustic gouging craters, and intercrystaline cracking as it attacks the iron carbide in the iron grain boundaries.
Avoiding the prescience of free hydrides is the only prevention from this form of attack 

Co-ordinated phosphate-pH control

Maintaining within the control area is achieved by the addition of Co-phos III tri sodium phosphate based, and co-phos II, disodium phosphate based.

Co-Phos III is an alkaline product and in water decomposes to NaOH and Na2HPO4 increasing concentration recombine.

Co-Phos II is much less alkaline.

Falling into the area on the co-ordinated phosphate pH diagram below the lower dotted line means that normal treatment to bring back into the target area is impossible.

The only way of recovering the situation is by blowing large quantities of water out of the boiler

The ideal time for this is during trip testing when the boiler is isolated from the plant (and hence feed water flow to the boiler is much reduced). The flames are extinguished on the low low foxboro trip

It is also beneficial to blow down the headers at this time

Problems may arise when the mixed bed demineralisation plant is allowed to remain in need of regeneration for a long time. The make up water is so effected so as to lower the alkalinity of the boiler without a comparable drop in phosphate. This becomes particularly troublesome during periods of heavy make up, say during trip testing or heavy steaming.

Hide-out

It can be seen in high pressure boilers that as the steaming rate increases the levels of certain salts, particularly phosphate salts, does not raise in line with others. When the load is reduced the concentrations return to normal.

This is termed hide-out and is due to the reduced solubility of sodium phsophate at temperatures above 250'C

When phosphate hide-out occurs there is a risk of permenant scal deposition and/or evolution of free caustic which in turn could lead to severe corrosion due to caustic attack

vTreatment using volatile solids free chemicals such as hydrazine, Eliminox and neutralising amines should be considered. This is termed All Volatile Treatment (AVT)