Objective

How to avoid “phosphate hide out” in the boiler drum water in power plants.

Boiler Drum water treatment

In order to maintain the water steam cycle chemistry in power station boilers, a numbers of chemical treatment are adopted.

In the power station boilers when the unit is synchronized concentration of minerals increases in the boiler drum water. Increasing in minerals is concentration due to increase in "cycles of concentration (COC)".

This can lead to carry over of salts to steam and gets deposited on steam turbine blades.

In order to control the “carry over” of salts in the steam & to avoid water steam contamination related problems chemical additives are used.

Mechanical and Chemical method are adopted to control the carry over. In the chemical method tri sodium phosphate (TSP) used as a scale inhibitor.

 Tri sodium phosphate reacts with dissolved minerals in boiler drum water and forms a non-sticky movable sludge. This non-sticky movable sludge is easily removed by operating the blow down in the boiler drum.

Why phosphate hide out takes place in the boiler drum water?

Power station are operating at varying electrical loads as per demand in the grid. Load can be increased or decreased within a short span of time, depending on the grid requirement & this change in load can lead to phosphate hide out in the boiler drum water.

What is Phosphate hide out phenomenon?

Before adaptation of phosphate treatment for the boiler drum water it is necessary to maintain the relationship between phosphate level & pH.

At elevated temperature there is every possibility that solubility of trisodium phosphate decreases. Decrease in solubility of TSP leads to from free sodium hydroxide. Due to free sodium hydroxide in the boiler water “caustic gauging” or “caustic embrittlement” takes place in the boiler drum& boiler tubes.

“Phosphate hide out” is nothing but the apparent loss of residual phosphate in the boiler drum water with decreasing boiler drum water pH and increasing dissolved minerals.

 This phenomenon makes boiler water to become corrosive.

As per Canadian electrician association phosphate react with magnetite layer to form a corrosion product called “maracite”.

How free sodium hydroxide is formed ?

Na3PO4.12H2O = Na2HPO4 + NaOH +12 H2O (Unbalanced equation, for representation only).

The presence of sodium hydroxide makes the water alkaline. This alkaline water enters in the minute cracks present in the inner walls of the boiler tubes by capillary action inside the cracks.

Water evaporates and the amount of hydroxide keeps increasing progressively.

The concentrated area with high stress works as anode and diluted area works as cathode. At anode, sodium hydroxide attacks the surrounding material and then dissolves the iron of the boiler as sodium ferrite rust.

This causes embrittlement of boiler parts like rivets, bends and joints, which are under stress.

How to remove the phosphate hide out from the system?

1. Decrease the power station load up to 50%.
2. Increases the trisodium phosphate dosing for 5 to 6 hour.
3. Increase the power station load
4. Operate heavy blow down in the boiler water system

All the hidden out phosphate will come back into the soluble form in the boiler drum water.

Analyze residual phosphate in the boiler drum water. When the residual phosphate is normal, close the blow down in the system.

Phosphate hide out problem in the boiler water will be resolved completely.

Advantages of removal of Phosphate hide out

1. Prevents corrosion in the boiler drum and boiler tubes.
2. Decrease in the blow down rate in the system which leads to saving the fuel finally.
3. Reduction in makeup water requirement with saving water treatment cost.
4. Enhancement of the boiler drum & boiler tubes life