Why is calgon conditioning better than phosphate conditioning? Explain the following:
a. Scale and sludge formation and their disadvantages
b. Caustic embrittlement
c. Boiler corrosion
What is hardness of water? How is it determined by EDTA method?
T he Correct Answer and Explanation is :
Why Calgon Conditioning is Better than Phosphate Conditioning
Calgon conditioning (using sodium hexametaphosphate) is better than phosphate conditioning because:
- Calgon prevents scale formation by forming soluble complexes with calcium and magnesium ions, keeping them in solution.
- It works at both high and low temperatures and doesn’t form precipitates.
- Phosphate conditioning leads to the formation of soft sludge, which can still deposit under high heat or flow restrictions.
- Calgon is more efficient in preventing scale and maintaining boiler efficiency over a wider range of operating conditions.
(a) Scale and Sludge Formation and Their Disadvantages
- Scale is a hard, adherent deposit (mainly of CaCO₃, CaSO₄, etc.) formed on boiler walls due to precipitation of dissolved salts at high temperatures.
- Sludge is a soft, loose precipitate (e.g., Mg(OH)₂, Ca₃(PO₄)₂) that settles at cooler areas of the boiler.
Disadvantages:
- Reduced heat transfer efficiency
- Overheating and tube damage
- Increased fuel consumption
- Risk of boiler explosion
(b) Caustic Embrittlement
- It’s a type of corrosion caused by the concentration of NaOH in stressed parts of the boiler.
- High concentrations cause intergranular cracks in steel, making it brittle.
- Occurs when Na₂CO₃ (used for water softening) decomposes to NaOH under high pressure.
Prevention:
- Use of phosphate or lignin-based treatments instead of Na₂CO₃
- Adding inhibitors like tannin or sodium sulfate
(c) Boiler Corrosion
- It’s the chemical or electrochemical deterioration of boiler metal.
- Caused by dissolved oxygen, CO₂, acids, or salts in water.
- Leads to pitting, weakening of boiler material, and ultimately failure.
Prevention:
- Mechanical deaeration
- Use of oxygen scavengers (e.g., hydrazine, sodium sulfite)
Hardness of Water
Hardness is the concentration of calcium and magnesium ions in water.
EDTA Method for Determining Hardness
- EDTA (ethylenediaminetetraacetic acid) forms stable, colorless complexes with Ca²⁺ and Mg²⁺.
- Water sample is buffered to pH 10 and treated with Eriochrome Black T indicator.
- Indicator forms a wine-red complex with Mg²⁺.
- EDTA is titrated until the color changes from wine-red to blue (endpoint).
- The volume of EDTA used is proportional to the hardness.
Formula: Hardness (mg/L as CaCO3)=Volume of EDTA (mL)×1000Volume of sample (mL)\text{Hardness (mg/L as CaCO}_3) = \frac{\text{Volume of EDTA (mL)} \times 1000}{\text{Volume of sample (mL)}}
This method is precise and widely used in water treatment labs.
