Boiler water conditioning—role of trisodium phosphate (Na3PO4) In internal boiler water treatment, trisodium phosphate is primarily used to reduce which specific risk or problem?

Introduction / Context:
Internal boiler water treatment aims to prevent scale, corrosion, and embrittlement by controlling alkalinity and precipitating hardness inside the boiler where it can be removed by blowdown. Trisodium phosphate (Na3PO4) is a cornerstone chemical in the phosphate treatment program.

Given Data / Assumptions:

• Phosphate treatment substitutes strong caustic (NaOH) with phosphate to maintain alkalinity safely.
• Calcium reacts with phosphate to form non-adherent calcium phosphate sludge.
• Embrittlement is associated with very high local caustic concentrations at metal surfaces.

Concept / Approach:
Using Na3PO4 moderates alkalinity to the phosphate/pH window, minimizing the risk of caustic embrittlement and promoting scale-forming ions to precipitate as soft sludge. Oxygen removal requires chemical scavengers (e.g., sodium sulfite, carbohydrazide) or deaeration—functions distinct from phosphate chemistry.

Step-by-Step Solution:

Verification / Alternative check:
Boiler treatment guidelines specify coordinated phosphate control or congruent phosphate control to both capture hardness and avoid free caustic conditions that cause embrittlement.

Why Other Options Are Wrong:

• Turbidity: managed by clarification/filtration upstream, not by Na3PO4.
• Suspended silica: addressed via external softening or demineralization.
• Dissolved oxygen: removed by deaeration or oxygen scavengers, not phosphate.

Common Pitfalls:
Overdosing caustic and assuming high pH alone is beneficial; excessive free caustic promotes embrittlement and caustic gouging.

Final Answer:
Caustic embrittlement of boiler metal