Objective 1: Boiler Scale Flashcards Preview

3B1-10: Internal Water Treatment of Boilers > Objective 1: Boiler Scale > Flashcards

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Boiler Scale

Relatively hard layer of mineral deposit that forms on the water side of boiler metal.


Which areas of the boiler are most susceptible to boiler scale?

The hotter areas, particularly the steam generating sections


Which tubes are most susceptible to boiler scale?

Generating tubes, including those in the generating banks of risers and in the waterfall sections of watertube boilers


Are firetube boilers exempt from boiler scale?



What is the primary cause of boiler scale?

The presence of undesirable minerals (dissolved solids) in the boiler water.


Which minerals are the chief culprits of boiler scale and why?

1. Calcium
2. Magnesium

They generally exist in relatively large quantities and deposit quite readily from the water


What 4 other mineral deposits are of less concern?

1. Iron
2. Copper

3. Aluminum

4. Silica


How does the boiler scale form?

A porous scale forms on the waterside of heating surfaces when the water boils


What are the 3 chief effects of scale deposition?

1. Scale restricts heat transfer and causes metal to overheat
2. Scale traps water beneath it and encourages corrosion

3. Scale sloughs off and causes flow restrictions


How does boiler scale restrict heat transfer?

It acts as an insulator that restricts heat transfer to the water which would have a cooling effect on the tubes.


What can happen to a boiler tube if scale is allowed to thicken?

Can overheat and thus the metal can weaken to the point of rupture.


How does scale encourage corrosion?

Water can be trapped in the scale and upon boiling, the remaining water can be highly concentrated with a high pH.


At what general pressure is underdeposit corrosion most frequent?

High-Pressure Boilers


Why is Scale-Induced Corrosion dangerous?

It is visually undetectable and can cause sufficient loss of metal to the point of tube failure.


What causes scale to slough off?

Extreme temperature changes in the metal as occurs when the boiler is shut down and is restarted


Why is Scale-Induced Flow Restriction dangerous?

Restricted water flow reduces the cooling in a tube and can reach the point of the becoming overheated until it is weakened to the point of rupture


What is the first line of attack in preventing boiler scale?

Remove the scale-forming minerals before they enter the boiler through various pre-treatment systems.


Why is internal treatment necessary?

Pre-treatment is not 100% efficient.


What are the 2 Main Methods used to control Scale Formation?

1. Precipitating the scale-forming solids from the water by chemical means and then discharging them from the boiler
2. Using the special chemical Chelate to react with the scale-forming minerals to prevent them from coming out of solution with the boiler water.


What is the primary treatment chemical used to precipitate scale-forming solids from the water?



What operating pressure is suitable for Phosphate Treatment?

All operating pressures


What is the result of Phosphate Treatment?

1. Chemically, the scale-forming minerals remain in solution as less harmful solids that don't deposit on the metal surfaces.
2. These suspended solids either fall to the bottom of the water space or remain dispersed throughout.
3. Excess suspended solids are removed by blowdown.


What operating pressure is suitable for Chelate Treatment?

<4000 kPa


What is the Phosphate Treatment process?

Sodium phosphate is injected into boiler water which reacts with the calcium and magnesium carbonates present. The calcium and magnesium carbonates combine with the phosphate and precipitate out as the new calcium and magnesium phosphates that are virtually insoluble in water.


What is the byproduct of Phosphate Treatment

Sodium carbonate which does not form hard scale and can be removed by blowdown.


What are the common chemicals used for Phosphate Treatment?

Orthophosphates, including monosodium, disodium, and trisodium phosphates and more complex polyphosphates. In all cases, they convert to trisodium phosphate upon entering the water.


What is problematic about forming magnesium phosphate?

It's a very sticky, adherent precipitate. If silica is present however, the magnesium usually precipitates as magnesium silicate.


What does the success of a Phosphate Treatment program depend?

Sufficiently high alkalinity


What is the ideal pH for Phosphate Treatment?

Between 11.0 and 12.0. Below 9.5, the calcium will not precipitate efficiently.


What are two ways of adding alkalinity to a boiler system?

1. Having feed water that contains alkalinity
2. Adding caustic soda (sodium hydroxide) to the boiler water