Co-ordinated and congruent Phosphate treatments
Sodium Hydroxide NaOH under certain conditions works to dissolve boiler metal.
Co-ordinated Phosphate treatment helps to avoid this by relying on the fact that NaOH and di-sodium phosphate combine to form tri-sodium phosphate. However, a small proportion of the NaOH always remains.
Congruent treatment substantially eliminates this by ensuring 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
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 correct treatment reserve levels within the control area is achieved by the addition of tri-sodium phosphate based and di-sodium phosphate based treatment chemicals.
The tri-Sodium Phosphate based treatment is an alkaline product and in water initially decomposes to NaOH and Na2HPO4 but with increasing concentration will recombine.
The di-Sodium Phosphate is much less alkaline.
By the careful attention to pH and PO4 reserves correct treatment is managed by the addition of the tri and/or di-sodium based treatment chemicals depending on measure conditions.
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 water level 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.
In high pressure boilers 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 phosphate at temperatures above 250'C
When phosphate hide-out occurs there is a risk of permanent scale deposition and/or evolution of free caustic which in turn could lead to severe corrosion due to caustic attack
Treatment using volatile solids free chemicals such as hydrazine, Eliminox and neutralising amines should be considered. This is termed All Volatile Treatment (AVT)
Recommended ranges( Co-ordinated phosphate treatment for w/t boiler )
pH - 9.6 to 10.3
PO4 - 4 to 20 ppm
N2H4 - 0.01 to 0.03 ppm
TDS - < 150 ppm
Cond pH - 8.6 to 9.0
Cl - 20 ppm
O2 - 10 ppb
Si - 10 ppb
Alternately Vanado-molybdnate test
Alkalinity Methyl orange
Bi carbonates do not show up in the phenolpthalein sample as they have a pH < 8.4. Bi carbonates can not occur in boiler but if suspected in raw feed then the following test.
Ammonia in feed
For pressures below 20 bar dissolved O2 in the feed does not cause any serious problems
so long as the water is kept alkaline
However cold feed should be avoided as this introduces large amounts of dissolved O2 are present, for pressures greater than 18.5 bar a dearator is recommended