Boiler water treatment – Controlling scale formation: Which measure most directly reduces scale formation in boilers supplied by natural waters?

Introduction / Context:
Scale forms when sparingly soluble salts (e.g., CaCO3, CaSO4, Mg(OH)2) and silica precipitate on heat-transfer surfaces, increasing fuel use and causing overheating. The cornerstone of scale control is managing the constituents that form scale.

Given Data / Assumptions:

• Natural feedwaters often contain calcium, magnesium, silica, and suspended alumina-bearing clays.
• Boiler conditions promote precipitation.

Concept / Approach:
Softening (lime-soda, zeolite, ion exchange) and silica reduction (lime softening, magnesium dosing, demineralization) remove scale formers before the boiler. Proper alkalinity and phosphate programs in high-pressure units further condition residual hardness to produce non-adherent sludge for blowdown.

Step-by-Step Solution:
Target the root cause: remove hardness and silica to reduce deposit precursors.Maintain appropriate internal treatment and blowdown to control any remaining solids.

Verification / Alternative check:
Boiler performance data show lower heat-flux penalties and tube metal temperatures when hardness and silica are minimized.

Why Other Options Are Wrong:

• Preheating alone: aids deaeration and some precipitation but insufficient by itself.
• pH just below 7: incorrect; boilers typically operate alkaline (pH > 8.5) to prevent acid corrosion.
• Eliminating H2S: addresses odor/corrosion, not principal scale formation.
• Adding oxygen: increases corrosion risk.

Common Pitfalls:
Overlooking silica’s role in high-pressure units; relying solely on blowdown instead of proper pretreatment.

Final Answer:
Reducing hardness, silica, and alumina in the feedwater