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What is a phosphate reserve? Why is it important in high-pressure boilers?

A reserve of phosphate should be present in the boiler water to neutralize any hardness 
salts, which may enter. These salts would deposit as a scale on the heating surface if reserve  were too low while, too high a reserve leads to foaming and possible excess production of  sludge. It also gives alkalinity.
At high pressure and temperature, reaction from left to right will be more. Therefore, it is
very important in high pressure boilers to keep the reserve level up to required concentration
to avoid excessive caustic alkalinity and thus caustic cracking.

In high pressure boilers where there is a risk of caustic concentration and subsequent caustic  Attack it is common to apply a coordinated or congruent phosphate control programme.
These control methods are based on the hydrolysis of tri-sodium phosphate (TSP) and
disodium Phosphate (DSP) in the boiler water.

The objective is to maintain a desirable pH without the presence of free OH alkalinity. The
desired conditions are obtained by maintaining the relationship of the pH to phosphate
concentration in the boiler water at less than that of the equivalent stochiometric solution of
Na3PO4 (<3:1). This is achieved by the equilibrium reaction above. The congruent phosphate
approach utilizes mixtures of TSP and DSP to further ensure the absence of free OH alkalinity
and usually run with a Na:PO4 ratio of <2.8.

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