Objective 1: Boiler Scale Flashcards
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?
No.
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?
- Calcium
- Magnesium
They generally exist in relatively large quantities and deposit quite readily from the water
What 4 other mineral deposits are of less concern?
- Iron
- Copper
- Aluminum
- 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?
- Scale restricts heat transfer and causes metal to overheat
- Scale traps water beneath it and encourages corrosion
- 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?
- Precipitating the scale-forming solids from the water by chemical means and then discharging them from the boiler
- 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?
Phosphates
What operating pressure is suitable for Phosphate Treatment?
All operating pressures
What is the result of Phosphate Treatment?
- Chemically, the scale-forming minerals remain in solution as less harmful solids that don’t deposit on the metal surfaces.
- These suspended solids either fall to the bottom of the water space or remain dispersed throughout.
- 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?
- Having feed water that contains alkalinity
2. Adding caustic soda (sodium hydroxide) to the boiler water
Why is it preferred to have sodium hydroxide present in the water?
- It forms magnesium hydroxide precipitate which is much less sticky than the magnesium phosphate precipitate.
- It forms a mixed phosphate/hydroxide compound which is also much less sticky than the calcium phosphate precipitate
How is Phosphate controlled in a boiler?
Injecting sufficient chemical to maintain a slight residual of excess phosphate.
Why is excess phosphate in the boiler good?
It suggests that all calcium and magnesium have been reacted with.
Why must excess phosphate in the boiler be kept to a minimum?
Large excesses, over time, will create a phosphate sludge or soft scale in the boiler
What are 3 causes of loss of phosphate residual in the boiler?
- Underfeeding phosphate chemical
- Sudden breakthrough of excess hardness from the pretreatment system
- Contamination in the condensate return system
Chelate Treatment
Chelates (or chelating agents) hold scale-forming impurities in solution, not allowing them to precipitate out of the boiler water to form scale and sludge
What is the chemical makeup of Chelates
Weak, organic acids which have been neutralized into sodium-based salts
What are the 2 most common chelates?
- EDTA: Tetrasodium salt of Ethylenediaminetetraacetic acid
2. NTA: Trisodium salt of Nitrolotriacetic acid
Describe the process of Chelate Treatment
Chelates enter the boiler water and hydrolyze into strong organic ions. These ions then attract the ions of calcium, magnesium and iron that already exist in the water and absorb them. Once absorbed, the calcium, magnesium and iron will no longer form a deposit and can be removed by blowdown.
What are 3 advantages of Chelate Treatment?
- It can remove existing scale from a boiler without the boiler being removed from service
- More forgiving with changing feedwater conditions
- Can handle sudden hardness breakthroughs either directly or by subsequent absorption of resulting scale
What are 4 disadvantages of Chelate Treatment?
- Requires great care as if scale is removed too quickly it can lead to sloughing and collection of sludge into locations that can restrict water circulation.
- Extreme overfeeding (>100 ppm) can lead to the loss of protective magnetite layers on boiler metal and even corrosion
- Needs complete removal of oxygen to be effective
- More expensive (esp. if feedwater hardness >2 ppm)
How must the Chelate feeding system be designed?
With non-corrosive materials, usually stainless steel, for tanks, injection pumps, lines, and nozzles
Sludge Conditioners/Dispersants
Treatment Chemical that conditions precipitates to stay well dispersed and not adhere to the boiler surface
How are Sludge Conditioners injected into boiler water?
Either separately or in mixture with other treatment chemicals
What do modern Sludge Conditioners consist of?
Synthetic polymers; most commonly Sulphonated and Carboxylated polymers.
How do Sludge Conditioners work?
Their polymers have an ionic, negative charge which has the ability to attract positively charged ions from the water and condition the sludge to be more crystalline. Therefore sludge particles are prevented from attracting each other to form large particles and attracting to the metal surface of the boiler.
What kind of Dispersant is used in Chelate Treatment?
Dispersants specifically designed for iron. There is very little calcium or magnesium present and therefore the dispersants can concentrate on iron and copper suspended solids.
